Chapter 2: State of the Climate

About this document

Lead Authors: Gregor C Leckebusch, Mark McCarthy

Contributing Authors: Jess Baker, Natasha Barlow, Lucy Bricheno, Chantelle Burton, Wilson Chan, Mat Collins, Daniel Cotterill, Daniel Cubbon, Natalie Garrett, Ruth Geen, Helen Hanlon, Angela Heard, Gabi Hegerl, John Hillier, Svetlana Jevrejeva, Abdullah Kahraman, Douglas Kelley, Elizabeth Kendon, Jason Lowe, Ben Maybee, Amanda Maycock, Francis Pope, Carol McSweeney, Jeff Polton

Additional Contributors: Alexander Askew, Daniel Bannister, Luke Bateson, Lindsay Beevers, Hannah Bloomfield, Gemma Coxon, Laura Devitt, Hayley Fowler, Neil Gunn, Christopher Jackson, Zoe Jacobs, Elizabeth Lewis, Richard Millar, Anna Murgatroyd, Kelvin Ng, Matt Palmer, Chris Parker, Chris Short, Magdalena Szczykulska, Emily Wallace, Daniel Williams, Richard Wood

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2.1 Chapter summary

The global climate is rapidly changing. Human activities have increased the concentrations of greenhouse gases in the atmosphere to levels not experienced for over a million years, trapping more heat within the atmosphere and oceans. Globally, temperatures, precipitation and extreme weather phenomena such as heatwaves and downpours are rising and breaking historic records by bigger margins. The rate of global sea-level rise has increased, with significant contributions from melting glaciers and ice sheets, as well as from the thermal expansion of warmer sea water. In the 2024 calendar year, global mean surface temperature (GMST) exceeded 1.5 °C above pre-industrial temperatures for the first time since observational records have existed.

National temperature records around the world are now frequently broken, in some cases by several degrees Celsius. The heatwave in July 2022 broke the UK temperature record, set only in 2019, by 1.6 °C; the heat exceeded 40 °C and contributed to approximately 2,200 excess deaths from 10 to 25 July. A warmer atmosphere can also hold more moisture, enabling larger precipitation and hydrological extremes. In 2015, Storm Desmond broke 48hr UK rainfall records and caused record breaking floods over a broad swathe of Northern England. The Environment Agency estimates the winter floods in 2015 caused £1.7-2.5 billion in economic damages (Flood and Coastal Erosion Risk Management Research and Development Programme, 2021). In 2023, the storm series Babet, Ciarán, and Debi caused insured losses of about £570 million in the UK (Association of British Insurers, 2025). In 2023 the sea level was the highest on record at the Newlyn tide gauge, one of the longest available sea level records in the UK, recording data since 1915. This continues the long-term increase in sea level around the coast of the UK. Further increases in temperature, changes in rainfall and increases in sea level are projected to occur over the coming decades. Some future changes are already locked-in, and the magnitude of these changes is dependent on greenhouse gas emissions.

Observed changes in the UK climate

The UK has become warmer, wetter, and sunnier. It is warming at a rate of approximately 0.25 °C per decade. This is sufficient to push the UK climate outside the range of historical observations and greatly increase the frequency and intensity of impactful weather and climate hazards. Recent warming has far exceeded any temperature observed in data sets spanning more than 300 years. Rainfall has increased over the winter half year. For example, the period October to March 2023-2024 was the wettest such period for England, and second wettest for the UK, in a series from 1836 (Kendon, M., et al., 2025).

Changes in UK temperature-dominated hazards

• All the UK’s top ten warmest years, in a series from 1884, have occurred since 2000; the most recent four years (2022, 2023, 2024, 2025) are all in the top five warmest years.
• Extreme high temperatures are occurring more frequently across the UK. The temperature of the hottest summer day is increasing at more than double the rate for mean temperature.
• Without climate change, the record breaking 40 °C heat in 2022 would have been extremely unlikely. The likelihood of such events is already six times higher than in the 1980s, and there is a 50% chance of UK temperatures exceeding 40 °C again within the next 12 years.
• Hotter, drier weather that can fuel wildfires is becoming more common, but it is unclear if wildfires are happening more often due to climate change.
• Extreme cold events are occurring less often and for shorter time periods.
• Attribution studies indicate that human-induced climate change has increased the likelihood of extreme high temperatures and reduced the likelihood of extreme low temperatures.
• Near-coastal UK ocean temperatures are steadily rising. In the decade from 2015-2024 they were 0.9 °C warmer than in 1961-1990. In June 2023, an intense 2-week heatwave caused ocean temperatures of 5 °C warmer than average, contributing to a record-breaking warm June for the UK.
• The urban heat island effect in major towns and cities increases the severity of hot weather and reduces the severity of cold weather, especially at night.

Changes in water-dominated hazards

• The UK’s climate is getting wetter. The most recent decade (2015-2024) was 10% wetter than 1961-1990. The winter half-year (October to March) was 16% wetter, with little change for the summer half-year (April to September). Since 2000, five of the top 10 wettest years, but none of the 10 driest, have occurred, in a dataset collected since 1836.
• The likelihood of wet winters has increased by at least 1.5 times, with winter daily precipitation extremes now 1.4 to 2.6 times more likely.
• The influence of human-induced climate change was detected in the record wet February 2020, extreme daily rainfall in October 2020, and in heavy rainfall leading to the record wet winter of 2023/24.
• Widespread snow events are less frequent and less severe than in 1961-1990.
• Some parts of the UK are experiencing lower water levels in rivers and reservoirs, and drier soils in summer since the 1960s, but the relative contributions from climate change, natural variability, and water management are uncertain.
• Sea level rise around the UK is accelerating. Tidal gauge records show a rise of 19.5 cm since 1900, with two-thirds of this occurring in the last 30 years. Since 1993 the rate of UK sea level rise has tripled to 4.2 ± 1.0 mm per year, compared to the long-term average since 1900. This is much faster than any rise in the past 3,000 years.

Changes in wind-dominated hazards

• There is no clear long-term pattern in UK severe storms, with decadal and year to year variability. The 1990s were especially stormy, with fewer damaging storms since 2000.
• Fewer maximum gust speeds have exceeded 40 knots in the last two decades compared to the 1980s and 1990s. This has not been attributed to specific drivers but is likely to relate, at least in part, to long-term natural variability in storminess.
• Sequences of several multi-hazard storms, producing extreme wind and heavy rainfall, have become more common.
• Sustained absences of wind from summer hot-dry high-pressure periods have become more common.
• Sea-level rise, combined with multi-hazard storm sequences, is increasing the frequency of extreme coastal flooding and intensifying coastal erosion.

Future changes in the UK

Unprecedented extreme events will continue to occur with increasing frequency and severity over the coming decades. The severity of climate risks we will experience depends on levels of global greenhouse gas emissions. Current global policies are estimated to result in a 66% chance of limiting warming to below 2.8 °C (United Nations Environment Programme, 2025) but pathways to limiting warming to well below 2 °C are still plausible if climate action is increased around the world. Sea-level rise will continue well beyond the 21st century in all plausible scenarios, but lower emission scenarios slow this rate of rise.

In the UK in the future there is a greater chance of hotter, drier summers and wetter, warmer winters, with rising sea levels and increases in some extremes such as heatwaves, summer droughts, intense rainfall events.

The future of temperature-dominated hazards

• As greenhouse gas concentrations increase, the UK will experience more frequent hot summers and more extreme high temperatures, with more severe heatwaves reaching higher temperatures and lasting longer.
• Hot days that exceed 30 °C are expected to increase in frequency from 1.3 days now to over 20 days per year for the 2070s high scenario at a Global Warming Level (GWL, as described in section 2.2.1) of 3.5 °C, with the greatest increases across south and east England.
• Wildfire is an emerging risk for the UK. High fire weather days, which provide conditions conducive to wildfire activity, double at a GWL of 2 °C and increase fivefold at a GWL of 4 °C relative to the 1850-1900 baseline.
• Summer heat will likely extend to later in the year, with a longer wildfire season projected into late summer.
• Cold winters and extreme low temperatures are expected to become less frequent and less intense, but cold extremes will still be a hazard.

The future of water-dominated hazards

• Short-duration (1-3 hours) extreme rainfall will become more intense and will likely cause increased flash flooding.
• Daily rainfall extremes are increasing, and this is projected to continue, with increasing risk of flooding. The latest climate science indicates greater increases in peak river flows than previously thought.
• Hot, dry summers are expected to occur more frequently, reducing river flows and worsening drought impacts.
• Centuries of sea level rise (SLR) are already committed due to past emissions. The specific rate and amount of future SLR depend on greenhouse gas emissions. By 2100, global sea levels will likely rise by 0.28-0.55 m under a low emission (likely range, SSP1-1.9) and 0.63-1.02 m under a high emission (likely range, SSP5-8.5) scenario, respectively.

The future of wind-dominated hazards

• Extreme damage-producing windstorms will continue to be driven by natural cycles.
• Recent high-resolution climate modelling studies indicate that human-induced climate change is expected to intensify both the jet stream, which drives UK storm systems, and future near-surface wind speeds.
• There is increased potential for combined impacts from extreme winds and extreme rainfall (wet-windy) in future windstorms, with a doubling of storms with both high wind and river flow risks at 3 °C GWL.
• Severe convective storms are expected to increase in the warm season, with increases in lightning occurrence. Hailstorms are expected to decrease in frequency but may produce more large hailstones. Little is known about future changes to tornadoes or wind gusts within thunderstorms.

In addition to the above, the likelihood of very severe hot-dry summer (heat, drought, fire, hail, tornado, wind, subsidence) and wet-windy winter (wind, flooding, landslide, storm surge) seasons containing multiple, linked, high-intensity events (e.g., a heatwave or storm) is expected to increase.

Tipping points of the global climate system and impact on the UK

Tipping points in the climate system are defined as critical thresholds beyond which the climate system reorganises, often abruptly and/or irreversibly. Tipping points in the climate system may cause rapid changes in oceanic and atmospheric circulation, in ice sheets, the Amazon rainforest or boreal forests, and permafrost. Changes in climate may also cause tipping points in regional climate or ecological systems, such as permafrost, mountain glaciers or tropical corals.

• Our current understanding of the effect of triggering specific tipping points is very uncertain, but they may pose major risks.
• Reaching a tipping point may be significantly earlier, in some cases several decades or more, than experiencing impacts from this tipping point transition.
• Some tipping points could affect the UK directly, such as a shutdown or severe slowdown of ocean circulation in the North Atlantic, which would cool the UK relative to the rest of the world. (Nevertheless, this is still considered unlikely in the 21^st century, although some emerging literature suggests an earlier slowdown is possible.
• As global warming continues, the probability of crossing tipping points increases, with some tipping points already possible, but uncertainties in these estimates are very large. This implies an urgent need for improved monitoring of tipping points and planning of potential adaptative responses.
• When tipping processes have started the changes can be rapid, they can change the regional trajectory of climate change and can be irreversible on human timescales.

Scientific advances and knowledge gaps

This report outlines key scientific advances since the Third Climate Change Risk Assessment – Independent Assessment Technical Report (CCRA3-IA TR) and identifies remaining gaps in our understanding of future climate change. Observational data and model projections now provide robust evidence that the UK climate has already shifted, with some further change inevitable. Increasingly frequent and severe extreme weather events, both globally and within the UK, pose escalating risks to economic, social and ecological stability. Many knowledge gaps do remain, including around the precise amounts by which the climate will change in the future, and especially the conditions that could trigger tipping points.

Visual summary:

The following table and maps provide a set of summary statistics to illustrate UK climate projections at 2 °C and 4 °C GWLs. These values, drawn from bias-corrected UKCP climate models^[1], represent plausible scenarios that could occur at the indicated GWLs, and are compared to the equivalent observed extremes for the period 2005-2024 from the Met Office HadUK-grid dataset (Hollis et al., 2019). The metrics are calculated for each of the 20 years within the respective GWL and the value for the highest 10% of the region encompassing each of the cities provided (meaning these values should not be directly compared to point observation measurements). The ‘Average year’ is the value that would be expected to be reached or exceeded in 50% of all years, and the ‘Extreme year’ is the value that would be expected to be reached or exceeded in 5% of all years. These values are included to illustrate the nature of changing climate hazards for the UK. They have been produced specifically for this chapter. These values are consistent with the wider body of evidence presented in this chapter, but the values do not synthesise the wider literature and are not intended to be used in isolation for adaptation or policy decisions, as they do not account for the full range of potential outcomes. For more comprehensive climate projections, including a full assessment of the associated uncertainties in future climate change, readers are directed to the range of products provided through the UK Climate Projections (UKCP).

The maps below show the ‘change’ in climate between a 1981-2000 baseline compared to the recent climate (2005-2024) and for the 2 °C and 4 °C GWL scenarios.

Table 2.1: Summary of selected climate hazard metrics for recent past along with estimates under 2 °C and 4 °C Global Warming Level scenarios. Where calculations result in a fractional number of days, these have been rounded to the nearest number of days. For Windy days there is not a comparable metric available from the HadUK-Grid observational dataset, so the baseline values are from the 1 °C GWL from the climate model ensemble. The ‘Extreme Year’ for summer rainfall is the extreme low value, for all other metrics it is the extreme high value.

Figure 2.1: Maps showing current (2005-2024) and future changes in hottest day of the summer and winter rainfall relative to a 1981-2000 baseline. Panels are provided for the CCRA4-IA TR ‘Average Year’ and ’Extreme Year’ for UK administrative regions. Temperature change is presented in °C and rainfall changes are presented as a % of the baseline value.

2.2 Introduction

This chapter provides an overview of observed changes and model projections for several climate hazards for the Technical Report of the Fourth Independent Assessment of UK Climate Change Risk (CCRA4-IA TR). The chapter provides a review of relevant literature and key datasets, a summary of major scientific advances, and details of remaining knowledge gaps.

Observational data form the foundation for examining past climate change; however, the short durations of many records and their incomplete spatial coverage present notable challenges. Model simulations can enhance our understanding of historical trends and variability. Climate model projections further extend this insight by providing information on how climate might change under a range of alternative global greenhouse gas emission scenarios. They also allow exploration and quantification of uncertainties arising from future emissions, model representations of climate processes, and natural variability. Their results are often presented as time series for different future emission scenarios. Figure 2.2 shows global mean surface air temperature (GMST) anomalies relative to the 1995-2014 average (left axis), and to the 1850-1900 baseline (right axis). The two axes are offset by 0.82 °C, representing the multi-model mean and closely matching the observed best estimate.

Figure 2.2: Global mean surface air temperature (GMST) anomalies relative to the 1995-2014 average (left axis) and to the 1850-1900 baseline (right axis) from the CMIP6 climate models for historical (black) and a range of future emission scenarios (blue, yellow and red). The shaded areas represent the 5%-95% range for the low- through high-emissions scenarios SSP1.2,6 and SSP3-7.0. Vertical grey shading highlight 20-year segments centred on the 2030s, 2050s and 2090s. The two axes are offset by 0.82 °C, representing the multi-model mean and closely matching the observed best estimate. The colours represent different Shared Socioeconomic Pathways (SSP) Reproduced from Figure AR6 WG1 | Climate Change 2021: The Physical Science Basis. doi: 10.1017/9781009157896.006

Climate models are used to explore how changes in atmospheric greenhouse gas concentrations affect the climate system. These computer-based tools represent our current understanding of climate processes and can reproduce many of the patterns observed in the real world. They are used to create climate projections; estimates of how the climate might change in the future. While climate models provide a credible basis for understanding possible future weather and climate conditions, their results are best interpreted alongside other sources of evidence, including expert judgement and observations of past climate and extreme events. The latter is also provided by the suite of products in the latest UK national climate projections (UKCP18) (Lowe et al., 2018). This integrated approach is important for understanding potential changes in specific atmospheric circulation patterns, where some evidence suggests that models underestimate future changes, particularly for extreme events.

Global climate models (GCMs) simulate the whole Earth’s climate, but their resolution is quite coarse, with each grid cell representing an area of around 60 km across. We can get more detail feeding global climate model results into regional models. These smaller-scale models focus on specific regions and work at much higher resolutions (typically 2-5 km), similar to the models used for national weather forecasts, and are included in UKCP18. Although climate models are valuable tools that can simulate many features of the current and past climate, they have limitations and may not capture all aspects of future changes.

2.2.1 Climate framing

The climate framing adopted within the CCRA4-IA TR considers changes to UK climate risks for specified levels of global warming (GWLs), as described in the Methods chapter. Since the reviewed literature adopts several different framings, where possible we interpret these in the context of the GWL framing for CCRA4-IA TR. Limitations of the GWL approach are summarised in the Methods chapter.

To quantify how climate has already changed, and how it might change in future, we compare it against a baseline or reference climate. The published literature uses several baseline time periods. Where these are relevant to the conclusions, this will be stated. The most common baselines are summarised below:

• Global Warming Levels (GWLs) typically use a pre-industrial baseline of 1850-1900, i.e., a 2 °C GWL is equivalent to the global mean surface temperature (GMST) being 2 °C higher than the GMST in the 1850-1900 reference period.
• The World Meteorological Organization (WMO) defines the climatological standard normal as the most recent 30-year period ending in a year that ends with zero. Currently, this period is 1991-2020. The WMO uses this baseline for routine climate monitoring and for international comparison, as it represents the average climate of the recent past.
• The WMO also maintains a standard reference period of 1961-1990 for assessing long-term climate change. This baseline is particularly important for observational studies, as reliable records may not be available before this time.
• The UK Climate Projections (UKCP18) uses a 20-year baseline of 1981-2000 as a common reference period.

2.2.2 Drivers of UK climate change

Climate change in the UK is ultimately driven by global emissions of greenhouse gases such as carbon dioxide, methane and nitrous oxide. These are well mixed in the atmosphere and directly warm the global (and UK) climate. Emissions of atmospheric aerosols have a more regional effect, and different aerosols can offset some warming from greenhouse gases or add to their warming effect. Processes in the climate system, known as feedbacks, can amplify or reduce the warming. For example, a warmer atmosphere holds more water vapour, which leads to further warming as water vapour is itself a greenhouse gas. Changes in clouds can warm or cool local temperatures depending on the type of cloud, time of year, and other factors.

UK climate change is also strongly affected by changes in ocean and atmospheric circulation, especially in the Atlantic. In particular, the Atlantic jet stream drives UK weather variations: climate change will likely change its strength, position and variability. UK weather is also affected by other more distant phenomena, including weather in the tropics, and processes linked to the El Niño Southern Oscillation.

Local feedback effects can also impact UK weather phenomena and are important in estimating climate risks. For instance, together rainfall drought and enhanced evaporation of water from the land surface can lead to strong localised warming, enhancing heatwaves, due to insufficient soil moisture for further evaporative cooling.

Climate models reproduce many processes that control UK climate risks. However, there is still uncertainty around processes driving the jet stream, and some processes are simply not simulated or realistically represented within models. The latter is especially the case for coarse GCMs which cannot directly simulate local processes and feedbacks.

2.3 UK climate hazards

2.3.1 Temperature-dominated hazards

2.3.1.1 Heat

This section considers hazards relating to increases in UK annual mean temperature, summer mean temperature, hot days, and heatwaves. The table below identifies the risks to which this hazard is particularly (but not exclusively) relevant.

Table 2.2: Summary of annual average hot summer days and maximum summer temperature for recent past (2005-2024) and under GWL central and high scenario. See also Table 2.1. These values are derived from observations and UKCP climate models. They are included to illustrate the nature of changing climate hazards for the UK. They have been produced specifically for this chapter. These values are consistent with the wider body of evidence presented in this chapter, but the values do not synthesise the wider literature and are not intended to be used in isolation for adaptation or policy decisions, as they do not account for the full range of potential outcomes.

Figure 2.3: Maps showing current (2005-2024) and future changes in hot summer days (daily maximum above 30 °C) and hot summer nights (daily minimum above 20 °C) relative to a 1981-2000 baseline. Panels are provided for change in the ‘Extreme Year’ for UK administrative regions. For a fuller description see Table 2.1 and associated text.

Introduction

In 2024 global mean surface temperature (GMST) exceeded 1.5 °C above the 1850-1900 baseline for the first time (WMO, 2025). For the most recent decade (2015-2024), GMST has been 1.24 °C above the 1850-1900 baseline, with human-induced warming contributing to about 1.22 °C of this (range from 1.0 to 1.5) (Forster et al., 2025).

The UK has warmed at a higher rate than the global average. Observed data in the Central England Temperature series shows the most recent decade (2015-2024) was 1.6 °C warmer than the 1850-1900 baseline. Warming has occurred in all seasons, with the UK’s warmest year, warmest summer, and extreme heat events all made more likely by climate change (Ciavarella and McCarthy, 2024; Kay, G., et al., 2025; Logan et al., 2025)

Observational data indicates that both hot and cold extremes of temperature are changing more than average temperatures. Hot days, hot spells and heatwaves are major climate hazards for the UK. This can have direct impacts from the heat and can also have compounding affects especially when heat is also associated with drought conditions. Heat and drought can affect water resources for example through increased demand, reduced availability and impacts on agriculture and the environment. Table 2.2 and Figure 2.3 summarise the changing hazards of hot days and hot summer nights. The frequency of hot days increases in all regions, rising from an average of four days in London in the recent past to an average of over 30 days per year at 4.0 °C GWL. The increase is even greater for an ‘Extreme Year’ and largest in south and east England. Hot summer nights, which are relatively rare in the current climate, also increase, particularly in London, with increases across many regions of England and Wales.

Observed change

UK weather and climate records continue to be broken. Four of the top ten hottest summers in a dataset from 1884 have occurred since 2015 (2025, 2018, 2022, and 2023). The average temperature associated with the record-breaking 2025 summer is now around 70 times more likely than in a pre-industrial climate and has a 20% chance of being exceeded in the near-future (Logan et al., 2025). An extreme summer (with a roughly 1% probability) in the near-future could be as much as 1 °C warmer than summer 2025 (Kay, G., et al., 2020).

The number, duration, and extent of UK hot days and heatwaves have increased in observational data. The average daily maximum temperature was 1.2 °C higher from 2015-2024 than during 1961-1990, whereas the hottest day of the year (the highest daily maximum temperature) has increased even faster, by 2.7 °C. The frequency of very hot (higher than 30 °C) and extremely hot (higher than 32 °C) days has more than trebled (Kendon et al., 2025).

Extreme high temperature records have been broken in many European countries in recent years. In the UK, the probability of temperatures exceeding 40 °C in any year in the current climate is about 4%, a six-fold increase compared to the 1980s (Kay, G., et al., 2025). The likelihood of exceedance is increasing rapidly, and there is an estimated 50-50 chance of reoccurring 40 °C heat in the UK within the next 12 years.

Extreme heat – Summer 2022

Summer 2022 was the UK’s fifth warmest summer in the Met Office national series (1884-present) cooler only than the summers of 2025 (Logan et al, 2025), 2018 (McCarthy et al., 2019), 2006 and 2003. Notably, the summer of 1976 is now only the sixth warmest. The high ranking of summer 2022 was in part a consequence of two significant heatwaves in mid-July, and mid-August as shown in Fig 2.4. The hottest of these heatwaves was substantially stronger than previously recorded heatwaves (Yule et al., 2023).

In July 2022, high pressure across western Europe caused persistent hot and dry conditions, with UK temperatures regularly exceeding 30 °C. By mid-July heatwave temperatures exceeded 40 °C in Spain, France, Germany and the UK, exacerbated by dry heat drawn north from Africa. On 18 and 19 July 2022, the UK-wide average daily maximum temperature exceeded 30 °C. Overnight, temperatures remained high, setting a new UK record for highest overnight minimum temperature of 26.8 °C in Oxfordshire. These high temperatures and drought caused widespread damage to crops and water resources and extensive wildfires across Europe (Tripathy and Mishra, 2023) as well as high excess heat-related deaths (Ballester et al., 2023).

Figure 2.4: Time series of UK daily maximum temperature during summer 2022.

The July 2022 event was also remarkable for the heatwave’s extent. The maximum temperature of 40.3 °C was recorded at Coningsby (Lincolnshire), but six more stations recorded temperatures exceeding 40 °C in a band from London to Nottinghamshire (Kendon, M., 2022). Moreover, 30 stations exceeded 39 °C, and some 46 stations broke the previous national temperature record of 38.7 °C. New national temperature records were set in Wales (37.1 °C at Hawarden Airport on 18^th July) and Scotland (34.8 °C at Charterhall on 19^th July). The England temperature record was broken by 1.6 °C, while both Wales and Scotland broke their records by 1.9 °C. Many long-running temperature stations in the UK network recorded their hottest day on record by extraordinary margins, including Durham (maximum temperature: 36.9 °C, exceeding the previous record by: +4.0 °C, in a 155-year dataset), Sheffield (39.4 °C, +3.8 °C, 138 years), Bradford (37.9 °C, +4.0 °C, 134 years).

The temperature during extreme heat events in the UK is rising faster than average temperatures, as shown in Figure 2.5. This may be partly due to changes in wind patterns, which some climate models do not fully capture (Vautard et al., 2023). UK heatwaves often occur when high pressure over Scandinavia and low pressure over the Atlantic bring hot air from the south and are therefore coupled with heatwave events affecting continental Europe more widely.

Climate change is increasing the likelihood of extreme temperatures, such as during summer 2022. Heat extremes are more severe than in the past for comparable large-scale weather patterns, and heatwaves are projected to continue to become more frequent and more severe in the future.

Figure 2.5: Station observations, showing UK maximum temperature (in °C) for each year from 1910 to 2024. The UK national maximum recorded temperature through time increases from 36.7 °C in 1911, 37.1 °C in 1990, 38.5 °C in 2003, 38.7 °C in 2019, and 40.3 °C in 2022.

Future change

Hot summers are expected to continue to become more frequent in the UK, with an increasing chance of hotter, drier summers (Lowe et al., 2018). Heatwave-drought events with a 1% chance of occurring in the current climate are estimated to become five times more frequent by the 2040s in a high emissions scenario (Kendon, E., et al., 2024), which would be equivalent to a 2.5 °C GWL.

In summer, greater increases in maximum temperatures are projected over the southern UK where the most extreme high temperatures are generally experienced now. The frequency and intensity of heatwaves are also expected to increase. For example, heatwaves producing 40 °C temperatures in the UK could occur every few years by 2100 for a 4 °C GWL (Christidis et al, 2020).

The rising temperatures will increase heat-related hazards. Tropical nights, where temperatures do not fall below 20 °C, are currently rare in the UK climate, but are expected to become more common. High temperatures overnight can be a hazard to health, making it hard to cool buildings, especially in urban areas. Climate models indicate multiple occurrences annually in the London area under a 2 °C GWL and more widely across England at 4 °C GWL (Hanlon et al., 2021).

2.3.1.2 Fire

This section considers hazards relating to wildfire in the UK. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to.

Introduction

High risk fire weather occurs on days with high temperatures, low humidity, and strong winds, creating dry conditions that promote ignition and fire spreading. In the UK, wildfire is defined as ‘any uncontrolled vegetation fire which requires a decision, or action, regarding suppression’ (Fire and Rescue Service: wildfire operational guidance, 2013). Currently, UK fire frequency peaks in April with a secondary peak in summer. In spring, the ground is still largely covered by dry plant matter from the previous growing season. If dry air and winds prevail, this creates ideal conditions for fires to spread following accidental ignitions from springtime influxes of visitors to the countryside. Later in the summer, new plant growth can dry out in warm weather, becoming more flammable. Drought can exacerbate high fire risk by drying fuel further, as well as increasing leaf-drop and creating additional dry, dead biomass. UK wildfires primarily affect grasslands, broadleaved woodlands, and heathlands, especially near to the rural-urban interface where human activity frequently ignites wildfires. Larger wildfires occur in moorlands and peatlands, where fuel accumulation and slower reporting of wildfires contribute to their spread.

Weather, vegetation, and land management shape UK wildfire risk. High fire risk weather quantifies the contribution to the hazard from the meteorological conditions. Wildfire hazard is also affected by land management and human activity (e.g., escaped fires). Drought, heatwaves and fires represent compound hazards, where soil and vegetation drying combine with extreme heat, resulting in larger and more intense fires (Seneviratne et al., 2021).

Observed change

The number of fire weather days in the UK has increased due to climate change, with hotter, drier conditions exacerbating wildfire risks (United Nations Environment Programme, 2022). In 2022 there was a significant increase in fire activity, with 185,437 fire incidents in the UK, a 28% rise from the previous year. This increase is attributed to the hot dry summer that year (Home Office, 2023). While long-term wildfire data is limited, recent trends suggest important changes, shown in Figure 2.6. Since 2002, the total UK area burned by wildfires has generally declined, but the number of fires has increased, and fire seasons are becoming longer. This shift points to more frequent but smaller fires, with trends potentially influenced by land management changes as well as by climate change. However, extreme fire events, with large impacts on people and property, are now occurring. The 2022 heatwave, when temperatures exceeded 40 °C, produced an unprecedented number of fires. Research suggests that climate change made fires six times more likely in 2022 (Burton et al., 2025). In 2025 the UK saw more than 46,000 ha of land burned, the highest area ever recorded (moorlandassociation.org). Scotland also experienced it’s largest ever wildfire, burning carbon-rich moorlands and leading to record-high emissions (Carbon Brief, 2025). This evidence highlights the growing risks from wildfire in a warming UK.

Figure 2.6: Annual UK burned area from the Global Fire Emissions Database, https://www.globalfiredata.org/index.html (GFED5, top) and GFED500 active fire counts (bottom) for 2002 onwards. While the total burned area has generally declined, the number of active fires has increased, suggesting a shift toward more frequent but smaller fires.

Case study: The Saddleworth Moor Fires (2018)

In June 2018, the Saddleworth Moor fire in Greater Manchester became one of the UK’s largest wildfires, burning for several weeks. Triggered by human activity, the fire was worsened by a prolonged dry spell and extreme high temperatures. The fire spread rapidly, burning large areas of peat and heather, emitting an estimated 24,000 tonnes of carbon (S. J. Baker et al., 2025). The fire was particularly difficult to control due to smouldering peat fires that burned underground.

During the fire, PM2.5 levels – fine particulate matter that can harm human health – were four to five-and-a-half times higher than normal in the Greater Manchester area, significantly increasing the risk of respiratory problems and excess mortality. The health impact was considerable, with an estimated 4.5 million people exposed to levels of PM2.5 higher than health-based guidelines for at least one day and a rise in hospital admissions for respiratory issues (UK Health Security Agency, 2023). The fire caused major financial losses, estimated at £21 million (UK Health Security Agency, 2023), including the cost of firefighting, damages to the environment, and disruption to local communities.

Future change

Climate projections suggest the UK is likely to experience warmer, wetter winters and hotter, drier summers, increasing the risk of wildfires (Lowe et al., 2018; Perry et al., 2022; UK Health Security Agency, 2023). Spring and autumn are also expected to become warmer, potentially extending the fire season beyond its usual spring peak (Belcher et al., 2021). At a GWL of 2 °C, the number of days with “very high” fire danger could double, and increase fivefold at a GWL of 4 °C (Perry et al., 2022). Spring fire risk is also likely to rise, with a 1.5 times increase in England at a GWL of 2 °C and a doubling at a GWL of 4 °C (Perry et al., 2022). This suggests a potential extension of the wildfire season, particularly in late summer and early autumn, depending on fuel availability. Regions in the south and east will experience the greatest increase in fire risks from increased conditions conducive to wildfire, with the average number of high-fire-risk days rising 3-4 times by the 2080s (Arnell, Freeman and Gazzard, 2021). Fire risks will also increase in the north of the UK. Warmer summers may also increase the risk of human ignitions from increases in outdoor recreational activities (Belcher et al., 2021).

As housing developments near to wildfire-prone areas increase, exposure to wildfires and potential ignitions may also increase (UK Health Security Agency, 2023). New work from Hetzer et al. (2024) shows that fire weather in Europe, including the UK, will likely become more severe this century under high emissions scenarios, with projections showing an increase of 25% in climate conditions conducive to wildfire in temperate regions by the end of the 21st century, relative to 1996-2016 (Senande-Rivera et al., 2022).

2.3.1.3 Cold events

This section considers hazards relating to winter mean temperature, cold snaps, cold extremes, snow and ice. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to:

Table 2.3: Number of frost days annually for current and future scenarios. As Table 2.1. These values are derived from observations and UKCP climate models. They are included to illustrate the nature of changing climate hazards for the UK. They have been produced specifically for this chapter. These values are consistent with the wider body of evidence presented in this chapter, but the values do not synthesise the wider literature and are not intended to be used in isolation for adaptation or policy decisions, as they do not account for the full range of potential outcomes.

Figure 2.7: Maps showing current (2005-2024) and future changes in frost days relative to a 1981-2000 baseline. Panels are provided for the ’Extreme Year’ for UK administrative regions. For a fuller description see Table 2.1 and associated text.

Introduction

Globally, the frequency and intensity of cold extremes have decreased since 1950, with human-induced greenhouse gas forcing the main driver (Seneviratne et al., 2021).

UK winter mean temperature has increased significantly: the Central England Temperature winter mean temperature for the most recent decade (2015-2024) is 1.7 °C higher than the 1850-1900 baseline. Cold spells in Europe and the UK are caused by high pressure systems with winds from the north or east. The frequency of these weather patterns is strongly influenced by the North Atlantic Oscillation (NAO), which varies from year-to-year and decade-to-decade.

Cold days, cold spells and snowfall constitute a significant climate hazard for the UK. Table 2.3 and Figure 2.7 summarise changes in frost days for 2 °C and 4 °C GWL scenarios. Reductions are projected across the country, with the largest reductions in northern and eastern regions, for example Edinburgh, which has an average of 82 frost days in the current climate for a typical year, reducing to 63 and 34 days at 2 °C and 4 °C respectively

Observed change

In recent decades, UK winters have become warmer and wetter. The frequency of air frosts and ground frosts have reduced annually by more than 14 and 30 days respectively, when compared to a 1961-1990 baseline.

The number of Heating Degree Days (HDD), days where heating of buildings is required for comfortable indoor temperatures, has reduced by 14% relative to the 1961-1990 baseline. Conversely, Growing Degree Days, with conditions suitable for plant growth, have increased by 21% (Kendon, M., et al., 2025).

Extreme cold temperatures have reduced in both frequency and severity. The top ten coldest months, seasons, and years recorded in UK regions predominantly occurred in the late 19^th and early 20^th centuries. In contrast, the top ten warmest months, seasons, and years are predominantly within the most recent decade.

Although widespread and substantial cold and snow events occurred in 2022, 2021, 2018, 2013, 2010 and 2009, their frequency and severity have generally declined since the 1960s. The cold winter of 2009/2010 and the cold spring of 2013 were particularly extreme for the recent climate and consequently high impact events. Human-induced climate change has halved the likelihood of a 2010 winter (Christidis and Stott, 2012).

Hazards co-occurring with cold weather includes strong winds, snowfall and blizzards, snow drifts, and icy surfaces. Snow-covered surfaces reflect more sunlight and insulate the air from the Earth’s surface; this negative feedback can further reduce air temperatures, particularly during extended cold spells normally associated with high pressure systems.

The winter of 1963 is the only year, in a series from 1884, with a winter (December to February) mean temperature below zero (−0.27 °C). The exceptional cold affected much of mainland Europe, and resulted in huge negative impacts on people, society and infrastructure, including transport disruption, power outages, and shortages of food and fuel. If similar weather patterns to the extreme cold winter of 1963 were to occur today, it would still lead to an extreme cold anomaly. This would be more severe than recent cold winters such as 2010, or the ‘beast from the east’ cold event in 2018. Although a winter as severe as 1963 is still physically possible in Europe, it is very unlikely (Sippel et al., 2024). However, if preparedness and adaptation to cold extremes reduce in response to a warming climate this may cause less severe future cold waves to be more impactful than past events (Pinto et al., 2024).

Future change

The UKCP18 regional climate models indicate that the number of frost days (daily minimum temperature below 0 °C), icing days (daily maximum temperature below 0 °C) and Heating Degree Days are all projected to decrease across the UK. The largest reductions are in Scotland (Hanlon et al., 2021). Intense cold spells, where the local daily average surface temperature is less than −2 °C for two or more days, are also expected to become less frequent, but will still occur (Kendon, E., et al., 2021). Decreases in the amount of snowfall and snow cover are expected across the UK, especially in mountainous regions; in low-lying areas snowfall is already more intermittent (Met Office, 2025b).

2.3.2 Water-dominated hazards

2.3.2.1 Rainfall, flooding and further hydrological hazards

This section looks at how extreme rainfall in the UK affects flooding. It covers both short bursts of heavy rain that cause surface flooding and longer events that lead to river flooding. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to.

Table 2.4: Maximum 5-day precipitation in winter for current and future scenarios. See Table 2.1. These values are derived from observations and UKCP climate models. They are included to illustrate the nature of changing climate hazards for the UK. They have been produced specifically for this chapter. These values are consistent with the wider body of evidence presented in this chapter, but the values do not synthesise the wider literature, and are not intended to be used in isolation for adaptation or policy decisions, as they do not account for the full range of potential outcomes

Figure 2.8: Maps showing current (2005-2024) and future changes in winter wet spells relative to a 1981-2000 baseline. Panels are provided for the CCRA4-IA TR an ‘Extreme Year’ for UK administrative regions.

Introduction

Although rainfall in the UK varies greatly from day to day and year to year, there is strong evidence that the climate is becoming wetter overall, particularly during the winter half-year (October to March). Several recent record-breaking rainfall events have been linked to climate change, and evidence suggests that rainfall extremes are becoming more frequent and intense. Climate projections indicate a general shift toward wetter winters and drier summers, but with heavier bursts of summer rainfall. New high-resolution climate model projections available since CCRA3-IA TR have produced new findings that are reported here.

Table 2.4 and Figure 2.8 provide illustrative figures for winter 5-day wet spells. The observed ‘Extreme Year’ values in Table 2.4 relate to periods during the extreme wet winters of 2013-14 (Cardiff), 2019-2020 (London and Edinburgh) and 2023-2024 (Belfast) which were all winters that caused disruptive flooding and have evidence for aspects of the rainfall during those winter being made more likely or more intense by human-induced climate change (Christidis and Stott, 2015, Davies et al., 2020, Kew et al., 2024).

Observed change

Observations show that both annual average rainfall and the frequency of heavy rainfall events in the UK have increased. Between 2015 and 2024, the UK was 10% wetter than during 1961-1990, with winters 21% wetter and smaller changes in other seasons (Kendon, M., et al., 2025). Five of the ten wettest years on record have occurred this century, while none of the 10 driest years (since records began in 1836) have occurred during this period. The likelihood of wet winters has increased by at least 1.5 times compared to a climate without human-induced climate change (Christidis, Ciavarella and Stott, 2018), while extreme winter daily precipitation is now 1.4 to 2.6 times more likely than without human-induced climate change (Christidis et al., 2021; Cotterill et al., 2024).

There is growing evidence that rainfall extremes are changing (Hawkins et al., 2020) and human-induced climate change has increased the likelihood of several recent record-breaking rainfall events. These include the record wet February 2020 (Davies et al., 2021), extreme daily rainfall in October 2020 (Christidis et al., 2021), and heavy rainfall leading to the record wet winter in 2023/24 (Kew et al., 2024). However, detecting trends in sub-daily rainfall (e.g., rainfall measured over minutes to hours rather than days) remain challenging. This is due to sparse and short sub-daily rainfall observations as well as significant decadal variability in the UK’s climate (Kendon, E., Blenkinsop and Fowler, 2018). Recent studies have made use of new km-scale climate model projections to explore convective rainfall events (Tett et al., 2023) which estimated that the probability of an intense downpour that affect Edinburgh on 4 July 2021 increased by around 30% due to human-induced climate change. Attribution studies for extreme rainfall are limited by small ensemble sizes. Overall, attributing trends in rainfall at regional to local scales is difficult, as natural variability plays a major role in driving extremes.

Attribution studies now extend beyond rainfall to include hazards such as flooding, revealing that human-induced climate change has increased the severity of several recent UK floods. This includes the winter 2023/24 floods, where England and Wales experienced their highest winter flows on record (Chan, W., et al., 2025a), estimated as 13.5% higher than analogue periods back to the 1850s.

Extreme wet month – September 2024

A sequence of storms in the last ten days of September 2024 bought 150 to 200 mm of rain to parts of the Midlands, almost a quarter of the expected annual rainfall for the region. Oxfordshire and Bedfordshire had their wettest calendar month on record in a series from 1836. Oxford recorded its wettest calendar month for 250 years, and the two-day rainfall total from 22 to 23 September was 118.9 mm, breaking the previous record of 98.1 mm from 9 to 10 July 1968. Some locations received more than 400% monthly average rainfall from 21-30 September.

Figure 2.9 shows rainfall totals and rankings (by county) for September 2024. A large region in central England exceeded 300% of average monthly rainfall. Gloucestershire, Wiltshire, Oxfordshire, Berkshire, Warwickshire, Leicestershire, Northamptonshire, Buckinghamshire, Bedfordshire, and Rutland recorded their wettest September on record (shown in dark blue on the right-hand panel).

Figure 2.9: (left) September 2024 rainfall as a percentage of the 1991-2020 monthly average. (right) Ranking of September 2024 rainfall within the monthly series from 1836-2024.

Torrential downpours from thunderstorms caused disruption and damages to transport and infrastructure. Surface water flooding closed roads in the west Midlands including a section of the M5 motorway, and flooded sections of the rail network.

For Oxfordshire this was a record-shattering event (see Figure 2.10). The 185.2 mm recorded was 27% wetter than the previous wettest September (1927, with 145.5mm). Some of the same areas also recorded their wettest February in 2024, experiencing two record wet months in the same year.

Figure 2.10: Time series of Oxfordshire September monthly rainfall amounts. The solid brown line indicates the September 2024 total rainfall, far outside the range of other September monthly totals in a series from 1836.

Future change

Rainfall intensity is projected to increase with further warming, mainly due to the capacity of the warmer atmosphere to hold more moisture. According to the UKCP Local projections, by 2070 under a high emissions scenario (equivalent to roughly a 4 °C Global Warming Level), the number of events exceeding 20 mm per hour, enough to trigger a flash flood, could increase 4-fold (Kendon, E., Fischer and Short, 2023). Similarly, the number of autumn days exceeding 50 mm could increase by 85% (Cotterill et al., 2021). Using the same projections, events with a 3% annual likelihood are expected to become 30% more intense for hourly extremes and 20% more intense for daily extremes (Chan, S., et al., 2023b). Overall, for every degree of regional warming, the intensity of heavy downpours is expected to rise by 5-15% (Kendon, E., Fischer and Short, 2023), meaning significant changes are likely even under lower global warming levels. Short-duration summertime rainfall extremes are projected to increase most strongly in northern parts of the UK and for the most extreme events (Kendon, E., et al., 2021). The largest increases in hourly extremes are expected in autumn, with projected rises of 30% in the southern UK and 50% in the northwest UK by the 2070s (Kendon, E., et al., 2021). This is consistent with an extension of the season for intense summer downpours.

Changes in hourly rainfall extremes may not occur gradually (Kendon, E., Fischer and Short, 2023). Instead, there may be rapid shifts from year-to-year, and high regional differences, reflecting the combined effects of natural variability and climate change. Record-breaking events are projected to increase substantially for both seasonal rainfall and local rainfall extremes (Kendon, E., Fischer and Short, 2023).

Other aspects of rainfall change are also important for understanding future impacts. Slow-moving storms, which produce high rainfall amounts and flooding, are projected to be around 14 times more frequent across Europe by 2100 (Kahraman et al., 2021). The risk of large intense thunderstorms capable of producing heavy rainfall that lasts for several hours is also increasing (Chan, S., et al., 2023a). However, these changes depend on large-scale dynamics, and are more uncertain, varying across models.

Increases in rainfall intensity may lead to increases in flooding (Rudd et al., 2020; Rudd, Kay, A., and Sayers, 2023) but other factors such as catchment characteristics, changes in land surface and pre-event soil moisture conditions also control flood characteristics.

Hydrological models indicate decreases in low river flows and increases in high flows across the UK by the end of the century (or GWL above 4 °C) (Arnell et al., 2021; Hughes et al., 2021; Kay, A., et al., 2021a; Lane et al., 2022; Hannaford et al., 2023; Parry et al., 2024; Smith et al., 2024). Increases to flood peaks are typically higher in the west and the rate of change may accelerate for some southern regions for global warming levels over 2.5 °C (Rudd, Kay, A., and Sayers, 2023). The frequency of widespread flood events in winter is expected to increase significantly, while these show a reduction in spatial scale in summer (Griffin et al., 2024). For Northern Ireland, we expect increases in mean winter river flows but decreases in mean flows in other seasons (Kay, A., et al., 2021b).

2.3.2.2 Drought in the UK

This section considers hazards related to low rainfall, river flows, soil moisture and groundwater levels. The table below identifies which risks this hazard is particularly (but not exclusively) related to.

Introduction

Drought is commonly defined as a prolonged lack of water in the environment due to low rainfall (meteorological drought) leading to drying of soils (soil moisture drought) and low river flows or groundwater levels (hydrological drought) (Shaffrey, 2023). The definition of droughts varies across different sectors and may be shaped by operational decision-making, such as water supply drought when low rainfall affects public water supplies and agricultural drought when low soil moisture impacts agricultural practices (Environment Agency, 2025).

Droughts in the UK cause widespread economic, environmental and social impacts. Droughts impact public water supplies and negatively affect water quality, ecosystem health (freshwater and terrestrial) and agricultural productivity. The effects of drought are worse when they coincide with other climate extremes like heatwaves and wildfire. For example, droughts increase fire danger due to hot, dry conditions and indirectly affect fuel availability (Thompson et al., 2023).

Periods of low rainfall in the UK often happen when high-pressure weather systems block the usual path of Atlantic storms. These systems push the jet stream north or south, keeping wet weather away from the UK (Shaffrey, 2023). Dry winters are also linked to a weaker jet stream, bringing fewer storms to the UK. Large-scale ocean patterns like La Niña can also contribute to drier conditions (Folland et al., 2015). Additionally, slow changes in sea surface temperatures over decades can cause prolonged periods of more frequent or less frequent droughts (Sutton and Dong, 2012).

Drought affects the hydrological cycle at different timescales. Changes in river flows and groundwater levels depend on climatic factors but are also influenced by physical properties of river catchments (such as soil types and subsurface water storage) and water management (such as abstractions or streamflow support). Soil moisture deficits are additionally controlled by water loss due to evaporation, transpiration by plants and subsurface drainage.

Observed change

UK winter rainfall is increasing, while no clear long-term trends are evident for other seasons. A clear reduction in summer rainfall has not yet been observed (Ossó et al., 2022). These rainfall changes are broadly reflected in river flows, with increasing winter half-year river flows particularly for Scotland and northwest England (Hannaford et al., 2025a). Some rivers in the south and east show decreasing summer flows and worsening droughts since the 1960s, while those in the north and west tend to show trends towards less severe droughts, though trends are often not statistically significant (Hannaford et al., 2025a). Spring river flows have declined across most regions since 1960s. Rising temperatures and related increases in evaporation since the 1960s have increased the frequency of springtime flash droughts, rapid-onset events that can last weeks to months (Noguera, Hannaford and Tanguy, 2025). Warm, dry springs can also lead to early cessation of groundwater recharge, increasing drought risk in the subsequent summer.

Observed trends in groundwater levels are mixed, with increases in northern England and some reductions in central England (Di Nunno and Granata, 2024). Historically, the southeast UK has experienced more spatially extensive and longer river flow and groundwater droughts than the northwest. When the southeast is under severe drought, it is rare for the northwest to also be in severe drought (Tanguy et al., 2021). Modelled estimates of soil moisture show wetting trends in October and December and drying trends in April in most UK regions (Almendra-Martín et al., 2022; Bevacqua et al., 2024).

There is low confidence in identifying the main drivers of recent trends in river flows and groundwater levels, particularly in determining if climate or human influences have had the greatest influence. Urbanisation, reservoir management and wastewater discharge all affect river flows and groundwater levels and can exacerbate hydrological droughts (Wendt et al., 2020; Van Loon et al., 2022; Salwey et al., 2023; Coxon et al., 2024). Land use change has driven significant river flow changes in some Scottish catchments (Wray et al., 2024).

The likelihood of severe combined heatwave and drought has increased (Baker, Shaffrey and Hawkins, 2021), as evident by successive droughts in 2018, 2022 and 2025. The 2022 drought had widespread impacts on agriculture, wildlife and water supplies (Barker et al., 2024). It was the most spatially extensive soil moisture drought for the past 60 years across western and central Europe (Schumacher et al., 2024). Following record wet weather over winter 2023/24, rainfall deficits over spring and summer 2025 were exceptional and amongst the driest on record for parts of northern England and eastern Scotland. Spring 2025 was England’s driest since 1893 and summer 2025 was the warmest on record for the UK (Met Office, 2025). Average spring and summer river flows were among the lowest ever recorded across central and eastern Britain (Hannaford et al., 2025b; Chan, W., et al., 2025b). Low river flows and reservoir levels led to abstraction restrictions across the UK, and water use restrictions across northern, central and southeast England.

Events like the 2022 drought have become hotter and drier than similar droughts in the pre-industrial climate (Faranda, Pascale and Bulut, 2023). Soil moisture droughts like 2022 have become more likely across western-central Europe and are projected to become twice as likely at 2 °C GWL relative to 1850-1900 (Bevacqua et al., 2024; Schumacher et al., 2024). Drier soils may also lead to atmosphere circulation changes that prolong these hot and dry extremes (Dirmeyer et al., 2021).

Future change

The UKCP18 projections indicate, on average, wetter winters and drier summers in the UK. Compound hot, dry extremes are expected to increase with warming (Bevacqua et al., 2022). Meteorological drought coupled with increased evaporation is projected to be more than three times more frequent at a GWL of 4 °C (Reyniers et al., 2023). Extreme droughts in Scotland are most likely across eastern regions and projected to occur once every 3 years in the near future (2021-2040) (Kirkpatrick Baird, Stubbs Partridge and Spray, 2021). Nine-month rainfall deficits (December-August) similar to the severe 1976 drought are expected to occur five times more frequently from the 2040s under the high greenhouse gas concentration RCP8.5 scenario (roughly comparable to a GWL of 3 °C) relative to present-day (Kendon, E., et al., 2024).

River flow droughts are projected to become more frequent and severe across the UK based on the UKCP18 projections. Most catchments are expected to experience reduced river flows and increase river flow drought severity (Lane and Kay, A 2021; Parry et al., 2023b). Changes in future droughts vary across hydrological models, but more severe changes are projected for catchments in the south and east due to surface-groundwater interactions (Smith et al., 2024). The north and west of the UK is expected to see an increase in shorter seasonal droughts (Parry et al., 2023a). For Northern Ireland, river flows are expected to decrease in all seasons apart from winter (Kay, A.L., et al., 2021b). For Scotland, catchments within key areas for agricultural and whiskey production (e.g., Spey and Tay) are projected to experience a two- to three-fold increase in hydrological drought frequency by the 2050s (Visser-Quinn et al., 2021). Systematic biases in climate models, such as underestimating persistence of atmospheric blocking, hinders confidence in future changes of multi-year droughts, such as multiple dry winters which inhibit groundwater recharge (Shaffrey, 2023).

Projections for groundwater droughts are more uncertain. Some boreholes may show modest declines but others show increases as wetter winters enhance groundwater recharge (Parry et al., 2023b). Widespread summer droughts are more likely to affect multiple UK regions simultaneously (Dobson et al., 2020; Tanguy et al., 2023), although in the southeast, river flow droughts are less likely to occur simultaneously with groundwater droughts (Tanguy et al., 2023). This has important implications for water transfers and strategies for optimizing conjunctive use of surface and groundwater sources (Murgatroyd and Hall, 2020; Murgatroyd et al., 2022). Extreme droughts found in large ensemble simulations are useful stress tests for water supply systems (Murgatroyd et al., 2022; Chan, W. et al., 2023).

Soil moisture droughts are also projected to increase in spatial extent, frequency and severity across the UK (Arnell and Freeman, 2021; Kay, A., Lane and Bell, 2022). Extreme soil moisture droughts that had about a 6% chance of occurring in a 1981-2000 climate are projected to become much more likely, with an 18% chance of occurring for a GWL of 2 °C, and a 33% chance at a GWL of 4 °C, with months between June and October being at especially high risk of soil moisture drought in the far future (Szczykulska et al., 2024).

2.3.3 Wind-dominated hazards

2.3.3.1 Strong winds from extra-tropical cyclones and low wind events

This section considers the hazard of surface windstorms and wind drought. The table below identifies which risks those two hazards are particularly (but not exclusively) relevant to.

Introduction

Severe extra-tropical cyclones (also called low pressure systems) produce windstorms that cause significant damages to public and private property and regularly lead to fatalities. Extra-tropical cyclones usually form over the western North Atlantic over several days. Their development and strengthening are driven by complex interactions between weather patterns high in the atmosphere (Uccellini and Johnson, 1979; Hoskins and Valdes, 1990; Pinto et al., 2009), exchanges between the ocean and the air (e.g., Nelson et al., 2014), and the release of heat from condensation (Fink et al., 2012).

Surface low-pressure systems form due to interactions between atmospheric waves and the temperature difference between the north and south of the mid-latitudes. These strong temperature contrasts create fast-moving air currents called the Polar-Front Jetstream, at about 10-12 km altitude, with wind speeds reaching over 300 kph (Woollings, 2019; Stendel et al., 2021).

At the surface, strong winds in extra-tropical cyclones occur along cold and warm fronts (Shapiro and Keyser, 1990), and in ‘sting jets’, narrow streams of air that descend rapidly from high altitudes to the ground (Browning, 2004; Hart, Gray and Clark, 2017; Eisenstein, Pantillon and Knippertz, 2020). Although sting jets are hard to detect, they have been linked to severe damage in storms such as the Great Storm of 1987 (Baker, 2009; Clark and Gray, 2018).

Extreme low wind speeds also present a hazard for specific applications, such as the generation of electricity from wind (kinetic) energy, for example during March 2021 in the UK (Staffell et al., 2021). Persistent, low wind episodes are mostly caused by atmospheric blocking, where a strong surface high-pressure system blocks the prevailing westerly flow. These blocks are part of the natural variability of the UK climate and a consequence of the waviness of the atmospheric flow in the mid-latitudes.

Observed change

Analysis of observed and reconstructed storms activity over the past century shows no clear long-term trend in severe storms across Europe, including the UK (e.g., Feser et al., 2015). There is substantial year-to-year and multi-decadal variability (Alexander, Tett and Jonsson, 2005; Matulla et al., 2008), with the stormiest period occurring in the 1990s (Alexandersson et al., 1998, 2000; Alexander, Tett and Jonsson, 2005). Since the early 2000s, the number of damaging European windstorms has declined (Dawkins et al., 2016) (Fig. 2.11). This variability is closely linked to phases of the North Atlantic Oscillation (NAO) (Dawkins et al., 2016), but, so far, there is no clear evidence that changes in windstorm activity can be directly attributed to human-induced climate change (IPCC, 2021). The overall impact of climate change on midlatitude windstorms is hard to evaluate due to small signals in e.g., wind speed, as well as the high climate model resolutions required in attribution studies to represent the full dynamic processes in severe extra-tropical cyclones (Ermis et al., 2024). Nevertheless, Ermis et al. (2024) simulate two climate change counterfactual realisations of storm Eunice (February 2022) and diagnose that Eunice was intensified due to climate change.

Atmospheric blocking and related low wind events occur over the UK about 5-10 times each season, lasting for variable lengths of time (Lupo, 2021). Across the Northern Hemisphere, these events have become more frequent over the past 60 years, increasing from around 30 to 40 events per year (Lupo et al., 2019; Lupo, 2021). In observations, sustained periods of low wind power generation (duration of 14 days) have a return period of five years, while the longest observed event (about 26 days) is expected to occur about once every 100 years (Potisomporn, Adcock and Vogel, 2024).

Fig. 2.11: The number of days each year when maximum gust speeds ≥ 40, 50 and 60 Kt (46, 58, 69 mph; 74, 93, 111 kph) are recorded by at least 20 or more UK stations, from 1969 to 2024. Stations more than 500 m above sea level are excluded. The counts for 2024 are 52 (40 Kt), 14 (50 Kt) and 3 (60 Kt). The three days in 2024 in which at least 20 stations recorded gusts ≥ 60 Kt were 21 and 22 January (Storm Isha) and 7 December (Storm Darragh). Source: Kendon, M., et al. (2025, Fig. 34).

Future change

Climate models suggest a possible increase in extreme extra-tropical cyclones and windstorms over the North-east Atlantic (Carnell, Senior and Mitchell, 1996; Leckebusch and Ulbrich, 2004; Lambert and Fyfe, 2006; Leckebusch et al., 2006; Zappa et al., 2013; Little, Priestley and Catto, 2023). However, the last two IPCC assessment reports (IPCC, 2013; 2021) concluded that projections of future storminess remain uncertain. Most current models show a northward extension of the North Atlantic storm track, which could lead to increased frequency of windstorms over western Europe (Harvey et al., 2020), with larger increases at higher global warming levels (e.g., Feser et al., 2015). Higher resolution ocean models also project greater increases in east Atlantic storminess (Grist et al., 2021); (high greenhouse gas concentration SSP5-8.5 scenario to 2050; roughly equivalent to a GWL of 2.5 °C at 2050). Recent research indicates that UK storm severity could increase by around 30% by 2080 (under high greenhouse gas concentration SSP5-8.5 scenarios comparable to a GWL of 3.5 °C), mainly due to storms covering larger areas (Priestley et al., 2024). However, few studies focus on changes in storminess at mid-century (Little, Priestley and Catto, 2023) as large natural variability makes it difficult to detect clear trends in storminess.

Climate change from UKCP18 simulations

• A summary of UKCP18 simulations is presented in Table 2.6, showing the number of storm days per winter and their future change.
• Storm days are defined as days with wind speeds above a damage-relevant threshold greater than the 98^th percentile, based on Klawa and Ulbrich (2003) and Leckebusch, Renggli and Ulbrich (2008).
• For comparison the 98^th percentile of daily maximum wind gust for the 2005-2024 reference period from station observations is 50mph at Heathrow (London), 55mph at Aldergrove (Northern Ireland), 57mph at St Athan (Cardiff) and 59mph at Gogarbank (Edinburgh).
• The UK currently experiences about 3 storm days per winter across all regions. No change is identified from UKCP18 for the central GWLs; results for England, Wales, Scotland and Northern Ireland are very similar.
• At higher global warming levels, towards the end of the century, Scotland shows the largest increase in storm days, with an additional 1.2 storm days per winter in an average year and about one day also for extreme years. For England, Northern Ireland, and Wales this change is smaller, about one day or less per winter.

Table 2.6: Summary of winter windy days (>98^th percentile) for recent past (2005-2024) and under GWL scenarios. See also Table 2.1. These values are derived from observations and UKCP climate models. They are included to illustrate the nature of changing climate hazards for the UK. They have been produced specifically for this chapter. These values are consistent with the wider body of evidence presented in this chapter, but the values do not synthesise the wider literature and are not intended to be used in isolation for adaptation or policy decisions, as they do not account for the full range of potential outcomes.

Increased resolution and CPM simulations

Recent advances in computing power have made it possible to run more detailed simulations of extreme windstorms. Manning et al. (2022; 2023) used a high-resolution convection-permitting model (CPM, 2.2 km grid spacing) to examine how UK windstorms might change in the future. They found that global climate models (CMIP5 and CMIP6 ensembles) tend to underestimate storm strength, whereas the finer-scale CPM results show a clear increase in the frequency of extreme windstorms by 2100. This rise is mainly due to increases in warm-core storms that contain sting jets (climate forcing similar to a GWL of 3.5 °C, high-end for 2080). However, large-scale circulation changes could also play a role. A recent kilometre-scale global model study suggests that future storms may shift further north, potentially reducing their impact on the UK (Gentile et al., 2025). Ermis et al. (2024) conclude from counterfactual 8-day forecast simulations (horizontal resolution of about 18 km) of storm Eunice (February 2022) that similar storms could become more intense in future (high-end scenario, end of century). Nevertheless, atmospheric background conditions were unchanged, meaning that long-term circulation responses to climate change are not represented in the study.

Why is there uncertainty about future storminess?

Uncertainty in future storminess is mainly due to competing effects in the atmosphere. With human-induced climate change, the lower atmosphere is warming faster at the poles, reducing the temperature difference between north and south, which could lead to fewer storms. However, higher in the atmosphere, this temperature contrast may increase, potentially leading to stronger storms (e.g., Shaw et al., 2016; IPCC, 2021) as it alters wave activity in the atmosphere and the jet stream. Also, warmer air can hold increased moisture: this may make storms more intense and bring heavier rainfall (Booth, Wang and Polvani, 2013) (see also section 2.3.2.1 on precipitation).

However, using several climate models indicates intensification of the North Atlantic jet stream in most considered storylines at GWLs of 2 °C and 4°C (Harvey, Hawkins and Sutton, 2023). Storm frequency increases across northern and central Europe (Little, Priestley and Catto, 2023), with storm severity more than doubled at GWLs of 2.5-3.5 °C in 2080. Under lower emissions scenarios, the projected increase in storm risk is lower.

What do we know about future storm damages and loss?

Future storm damages in the UK and western central Europe are expected to increase, although estimates vary (Leckebusch et al., 2007; Pinto et al., 2007; Donat et al., 2011). One study estimates a 23% rise in annual windstorm losses with 2.5 °C global warming (Ranson et al., 2014). More recently, Severino et al. (2024) found that rare damaging storms could happen every 28 years instead of every 100 under a high-emissions scenario (SSP5-8.5, roughly comparable to a GWL of 3.5 °C). However, there remain large uncertainties in these projected changes, mainly due to differences between climate models.

The future of blocking and wind droughts

Our understanding of changes to high-pressure blocking systems and related low winds (‘wind droughts’) is characterised by three main aspects:

• Bias: Climate models systematically underestimate the occurrence of blocking situations (Woollings et al., 2018), thus hindering the clear understanding of dynamical causes of blocking occurrence and how these might change.
• Frequency of blocking events: Global climate models show an overall decline in the hemispheric mean mid-latitude blocking occurrence (Kautz et al., 2022), but these projected changes are smaller in magnitude than model biases and natural decadal variability in blocking frequency (Woollings et al., 2018), so highly uncertain.
• Duration of wind droughts: In a recent study, Qu et al. (2025) found an increasing trend in wind drought duration (including in the UK) in CMIP6 models, with an increase in the duration of the 25-year event of 20% to 40%, depending on GWL and time horizon.

2.3.3.2 Severe thunderstorms, lightning and hail

This section considers hazards relating to convective (thunder)storms: tornadoes, (large) hail, lightning, damaging wind gusts and heavy rainfall leading to flash floods. The latter is detailed in section 2.3.2.1. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to.

Introduction

Severe thunderstorms leading to lightning strikes, intense precipitation, hail or even tornadoes are common in the midlatitudes. In the UK, thunderstorms occur less frequently than in continental Europe (Enno et al., 2020), but their impacts are significant and affect public safety, disrupt transport and energy systems, and damage agriculture and critical infrastructure.

Observed change

Data limitations in observations and short-term forecasts hinder our ability to fully understand and prepare for hazards from convective storms These events are short-lived, and localised in nature, which makes them difficult to monitor with existing systems. Volunteers are relied upon to report hail and tornadoes, which leaves gaps in the data. Radar often misses small tornadoes near the ground. Over the past 30 years, thunderstorm activity has slightly decreased in southern England but slightly increased in the north, in each region by about 0.25 days per year (Stone et al., 2022).

There is no clear long-term trend for hail frequency in the UK. More tornadoes have been reported since the 1990s, likely due to better awareness and reporting (Wells et al., 2024). Because the incidence of tornadoes varies a lot from year to year and records are short, it is difficult to identify trends (Mulder and Schultz, 2015). Despite their impacts, research on convective wind gusts in the UK is limited. Long-term observational trends in wind gusts related to convective activity are unknown, although there is an overall decline in high wind gust days over the UK (Fig. 2.11).

Future change

Convection-permitting climate models, such as UKCP Local, better represent thunderstorm processes and hazards (Kahraman et al., 2022). Studies with these models suggest UK lightning frequency could increase significantly under high-emission scenarios (Kahraman et al., 2022). By 2100, the number of lightning strikes in summer could double due to more unstable air stratification and extra moisture in a warmer climate (Kahraman et al., 2022). Lightning is also expected to increase over the North Sea in winter, which could threaten offshore wind farms and other marine infrastructure (Kahraman et al., 2022).

Severe hailstorms are expected to become less common in the UK by 2100 (Sanderson et al., 2015; Kahraman et al., 2025). Sanderson et al. (2015) find the number of thunderstorms producing damaging hailstones (diameter greater than 15 mm) reduce by a factor of two by 2070 for a high emissions scenario. Kahraman et al. (2025) show an increase in the number of thunderstorms with high amounts of small hailstones. However, they suggest severe hailstorms will increase by 20% in 2040s but decrease by 64% by 2100 for a high emissions scenario.

This is because multiple factors for large hail growth change in a much warmer climate. These include increases in vertical wind shear due to a weaker large-scale circulation, and stronger updrafts in the hail growth layer. It is notable that the frequency of hailstorms in the UK is much lower than other parts of Europe in all simulations, and uncertainty in e.g., wind shear changes could dramatically affect these results. However, isolated instances of very large hailstones are expected to become more common (Kahraman et al., 2025).

There is little research on how tornadoes or strong winds from thunderstorms might change in the UK, as current climate models cannot simulate these events well. This also applies to convection-related wind gusts.

2.3.4 Marine temperature and sea level

This section considers hazards relating to sea surface temperatures, waves, storms and sea level rise. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to.

Table 2.7: Sea level projections based upon UKCP18 (Palmer et al., 2018) delivered by the Met Office Projecting Future Sea Level (ProFSea) tool (Perks & Weeks, 2023) for UK capital cities based upon future emissions scenarios. Sea level projections are not well suited to the GWL approach adopted for other climate indices, so here are presented projections for a middle scenario (RCP4.5) and a high-end emissions scenario (RCP8.5). The table includes the central estimate only and users requiring more information about the underpinning science and uncertainty ranges should review Perks and Weeks (2023) and the ProFSea tool.

Introduction

The marine environment faces threats from ocean warming, marine heatwaves, acidification, changes in ocean currents, and alterations in carbon storage. These changes affect marine life and human activities. Wave conditions around the UK are influenced by local winds and incoming swell waves from the Atlantic and Arctic Oceans. Extreme waves occur during intense storms and are closely linked to storm track.

Climate risks in the UK often manifest as multi-hazard events at the coast. Extreme sea level events are driven by various mechanisms, spanning a wide range of time scales. Extreme sea levels can be generated by storms in the form of storm surges. Their heights and potential impacts are controlled by the intensity, speed, and path of the storm and its timing relative to the tide. Temporary, but potentially very damaging, individual occurrences of sea level extremes are called ‘event-scale’ extreme sea levels. These occur due to a combination of factors such as storm surges, waves, and astronomical tides. Extreme sea levels cause coastal flooding and erosion. These risks impact various sectors, including food and water security, renewable energy, and shipping.

Global sea-level rise is driven mainly by the expansion of seawater as it warms, and the melting of land ice, due to warming temperatures. The rate of global sea-level rise is increasing, with significant contributions from melting glaciers and ice sheets. Since 1900, average sea levels in the UK have risen by 19.5 cm, with two-thirds (13.4 cm) of this rise happening in just the last three decades (Kendon, M., et al., 2025). This rate of change is higher than the global estimate of 10.6 cm calculated from satellite altimetry, suggesting UK sea level is rising faster than the global average.

Observed change

The rate of ocean warming has increased consistently over the past six decades, with record warming in 2023. Near-coast sea-surface temperatures around the UK from 2015-2024 were 0.3 °C warmer than the 1991-2020 average and 0.9 °C warmer than 1961-1990 (Kendon, M., et al., 2025). Warmer oceans are contributing to marine heatwaves, and in June 2023, an intense 2-week marine heatwave caused ocean temperatures to be 5 °C warmer than average for that time of year (Jacobs et al., 2024).

With respect to wave heights, overall, a 0.1 m reduction in average wave height has been observed, leading to calmer seas. However, the height of the most extreme waves has increased by 0.5 m in some areas, associated with increasing storminess (Bricheno and Wolf, 2018; Bricheno et al., 2023). Cyclone tracks have been shifting northward since the 1990s. Historical records show the northeast UK is most impacted by long and high storm surges, while the English Channel experiences shorter and smaller events (Camus et al., 2024). There is no firm evidence linking climate change to observed trends in UK storms and waves.

Observational data provides confirmation of the accelerating rate of sea level rise. The tide gauge at Newlyn recorded the highest sea level on record in 2023 (Kendon, M., et al., 2024). Sea level rise is not uniform around the coastline of the UK. From 1991 to 2020 the rate of sea level rise at locations around the UK ranged from 3.0 to 5.2 mm per year, with the highest rates of change recorded in the Scottish Outer Hebrides and at the southeast England coast, with the lowest rates along the coast of northeast England (Kendon, M., et al., 2022). However, the relatively low density of tide gauges around the UK coastline means there are large uncertainties in both the rate local of relative sea-level rise and the impacts of storms on local extreme sea levels.

Future change

Future marine conditions

In the northwest European shelf seas, sea surface temperatures are projected to increase by about 3 °C by 2100, with a related reduction in salinity (by 1 practical salinity unit) under a high emissions scenario. Marine heatwaves are expected to become more common by the mid-21st century (Berthou et al., 2024; Jacobs et al., 2024).

Changes in air temperature will alter ocean density stratification; areas where lighter (warmer or fresher) water sits on top of denser (colder or saltier) water. There will be an increase in the strength and duration of summer stratification. Additionally, more rainfall and runoff from the land could increase stratification in coastal seas, in particular the North Sea (Holt et al., 2022). Increased stratification reduces the mixing of nutrients from deeper waters and affects the distribution of oxygen, impacting marine ecosystems and fisheries. It also affects the regulation of sea surface temperatures, which can lead to changes in weather patterns. Decreased ocean circulation strength could also affect regional climate, especially in western Europe (Tinker et al., 2024).

Future sea-level rise

Centuries of rising sea levels are already committed due to past emissions, but the rate and amount of future sea-level rise depend on greenhouse gas emissions (Fox-Kemper et al., 2021; Naughten, Holland and De Rydt, 2023). By 2100, global sea levels could rise by 0.28 to 0.55 m under low emissions and by 0.63 to 1.02 m under high emissions scenarios (Fox-Kemper et al., 2021). Since CCRA3-IA TR, Palmer et al. (2024) have developed a sea level storyline framework that more completely represents future uncertainties, including poorly understood ice sheet instability processes, and provides a comprehensive set of possible outcomes to inform coastal decision makers. Projected sea-level rise at 2300 for the UK, under three storylines based upon IPCC AR6 likely ranges, show UK capital cities could experience 1 to 4 m of sea-level rise with a marked spatial pattern in the magnitude of sea-level rise experienced across the UK (Palmer et al., 2024). The largest sea level rise is projected for the south and southwest of the UK.

Future rates of ice loss, particularly from Antarctica and Greenland, remain uncertain, potentially leading to higher sea levels than currently predicted. High-impact, low-likelihood scenarios, providing a plausible worst-case, could lead to an additional 1 m of global mean sea level rise by 2100 (Fox-Kemper et al., 2021; Van De Wal et al., 2022) and between 8 to 17 m by 2300 under a high emissions scenario (Fox-Kemper et al., 2021). Based upon the Fox-Kemper et al. (2021) global high emission scenario, Palmer et al. (2024) model this high-impact, low-likelihood scenario for 2300, resulting in regional sea-level rise at UK capital cities ranging from 16.3 to 17.0 m.

By 2100, the winter storm track over the UK could intensify (medium confidence, section 2.3.3.1), increasing the severity of waves (Morim et al., 2019; Wolf, Woolf and Bricheno, 2020). Severe storms during autumn may become more frequent if tropical cyclones intensify and their region of origin expands northwards (Bricheno et al., 2023, this report section 2.3.2). Higher sea levels will cause waves to carry greater energy to the shore, which will affect coastal defences, and increase the risk of coastal flooding and erosion (Environment Agency, 2020). The spatial pattern of high waters around the UK depends on maximum wave height, storm surges, tidal range, and sea-level change. Southern England will experience the greatest amount of sea-level rise, raising the risk of extreme water levels (Rulent et al., 2021). Retreating sea-ice will increase the height of waves from the north. Under a high emissions scenario, 1-in-100-year coastal floods are expected to occur at least once a year along most European coastlines before 2050 (Vousdoukas et al., 2018).

2.3.5 Compound hazards

This section considers how and when hazards co-occur or compound, focussing on the link through to impacts. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to.

Introduction

Extreme weather phenomena often do not occur in isolation. When hazards co-occur or compound, linked by various atmospheric processes, impacts may be amplified (e.g., Hillier et al., 2015; Zscheischler et al., 2020) and potential impacts can be larger than from individual hazards separately. Consequently, understanding the processes leading to compound events is necessary to fully quantify their overall risks.

Risk from multiple natural hazards, or multi-hazard risk, has been long recognised (e.g., Hewitt and Burton, 1971; UNEP, 1992; Kappes et al., 2012). Recently, this has evolved into the broader concept of compound risk, which refers to the combination of multiple drivers and/or hazards that together contribute to societal or environmental impacts (Leonard et al., 2014; Zscheischler et al., 2018; Hillier et al., 2020; Figure 2.14a). Simpson et al. (2021) further expanded this framework by focussing on linkages, such as shared vulnerabilities or interdependent systems, that create complex climate risks (Figure 2.14b). They identified 10 types of risk interdependency, including cascading effects (de Ruiter et al., 2019).

Fig. 2.12: Weather and climate risks often arise due to multiple, interacting factors. a) A compound event, climate-based perspective where hazards might have common modulators (e.g., storms, El Niño) and/or a related driver (e.g., wind, rain) (Zscheischler et al., 2020). b) Linkages between determinants of complex risk – modified from Simpson et al. (2021).

The term co-occurring (Hillier et al., 2015) is increasingly used (De Luca et al., 2017; Dodd et al., 2021; Pradhan et al., 2022; Bloomfield et al., 2023). It is valued for its simplicity, as it deliberately avoids implying any underlying process. Instead, it only describes an episode (Hillier et al., 2025) in which two or more events occur within the same region (e.g., a country or a city) and within a defined timeframe (e.g., an hour, a week, a year).

Observed change

Between 16 and 21 February 2022 the storm sequence ‘Dudley’, ‘Eunice’ and ‘Franklin’ caused several hydro-meteorological hazards (snow, landslides, flooding, extreme winds) at locations across the UK and Northwest Europe (Kendon, M., 2022; Mühr et al., 2022). Then, in July 2022, there was widespread drought, extreme heat (>40°C) and wildfires. These are recent examples of co-occurring extremes within a single year. However, detecting a trend from the past to current climate in these co-occurring extremes needs a longer historical record than is required when looking at one hazard alone, and recent occurrences constitute weak evidence.

CCRA3-IA TR did not explicitly consider impactful compound events (e.g., Wood et al., 2023; Sherwood et al., 2024), compounding, or co-occurring hazards except to suggest that plausible storylines and scenarios (Shepherd et al., 2018) might be useful in developing appropriate stress tests. It noted, separately, the roles of a large-scale atmospheric pattern called the North Atlantic Oscillation (NAO). Winters with a positive NAO phase (NAO+) are dominated by storms and so are warmer, wetter and windier in the UK (Murphy et al., 2019), while NAO+ summers are characterised by high pressure favouring hotter and dryer conditions (Folland et al., 2009).

During winter, inland flooding and extreme winds are the two primary hazards impacting the UK, and evidence is increasingly robust that they systematically co-occur for timescales from daily to seasonally, driven by cyclonic storms (Bloomfield et al., 2023). This co-occurrence is greatest in NAO+ winters and for the most severe events, perhaps controlled by the jet stream via its influence on storm formation and evolution (Hillier and Dixon, 2020; Hillier et al., 2020; Manning et al., 2024). Successive storms, like in winter 2013/14 (Wild, Befort and Leckebusch, 2015) can cause very wet antecedent conditions that can lead to prolonged or extreme flooding (e.g., Bloomfield et al., 2023; Fig. 2.13). Importantly, wintertime storms can also cause sequences of storm surges and high waves (Jenkins et al., 2023), combined with high groundwater levels or saturated sediments, can cause landslides impacting roads and railways, and increase rates of coastal erosion (Palamakumbura et al., 2021; Klaver et al., 2024). Likewise, snowmelt can amplify flooding (e.g., Storm Bert) and damaging wintertime tornadoes and hail are also possible (e.g., Jersey in storm Ciarán) (Wells et al., 2024; 2025).

Fig. 2.13: Timeline of a very active storm season in the UK and Ireland, winter 2023/24. Letters are the first letters of named storms (Met Office, 2024), e.g., ‘B’ is Babet.

During summer, hot-dry conditions in the UK are typically linked to stable high-pressure and an NAO+ state (e.g., Kautz et al., 2022), enhancing the chance of drought, shrink-swell subsidence (one of the UK’s most costly geohazards at £3 billion over the past decade (Jones, Banks and Jefferson, 2020)), and dangerous heatwaves if hot air is drawn north to the UK (Felsche et al., 2024). Airflow from the south, often at the end of heatwaves, can also bring severe convective thunderstorms and accompanying multiple hazards (Gray and Marshall, 1998), e.g., hail (Wells et al., 2024), tornadoes (Clark and Smart, 2015) and intense rainfall with the potential to cause flash flooding (Gray and Marshall, 1998; Sauter et al., 2023). This combination, except tornadoes, was exemplified in the unprecedented July 18-19^th heatwave in 2022, which was an NAO+ summer (Kendon, M., 2022; Kendon, E., et al., 2023).

Future change

Climate projections suggest the need to be prepared for more combined heatwave-drought summers, comparable to 1976 (Baker, Shaffrey and Hawkins, 2021; Kendon, E., et al., 2024), and for more winters with both extreme winds and flooding (Bloomfield et al., 2023; Hillier et al., 2025). Aside from these studies the extent to which multiple hazards co-occur and how this relates to larger-scale circulation patterns is poorly known, both at present and in climate models representing the future.

This section presents a ‘what if’ (Sherwood et al., 2024) or storyline (Shepherd et al., 2018) approach. It takes the idea of a persistent seasonal state (Bloomfield et al., 2023; Hillier et al., 2020) – a hot-dry summer or wet-windy winter – and combines this with an extremely intense episode of multi-hazard events.

Flooding and saturated soils can readily follow prolonged drought and heatwaves, such as the hot-dry summer of 2022 leading into the very wet winter of 2023, in the NAO+ conditions historically typical of such rapid transitions in the UK (Parry, et al., 2023a). Desiccated earth structures, such as reservoir dams and railway embankments, can be severely weakened by prolonged dryness, only to be further stressed by saturation and heavy rainfall (Holmes et al., 2022). This type of high-impact extreme weather pattern transition is considered plausible (Arnell et al., 2025) and highlights the potentially surprising nature of climate-related hazards.

2.3.6 Climate change and air quality in the UK

Both climate change and air quality issues are caused by anthropogenic emissions into the atmosphere (Pinho-Gomes et al., 2023). They have important implications for human health and the environment, with health effects of air pollution occurring rapidly, whereas effects of climate change occur over longer timescales (Akasha, Ghaffarpasand and Pope, 2021).

Climate change causes increased temperatures, altered precipitation, and shifts in atmospheric circulation, all of which influence the dispersion and chemistry of air pollution. For example, warmer conditions accelerate the formation of ground-level ozone, while increased atmospheric stagnation can trap pollutants. Several air pollutants are also climate forcing agents, such as particulate matter and ozone, hence air pollution can affect climate change both positively and negatively depending on whether they act as greenhouse gases or reflect radiation from the Earth. For example, a recent study (Samset et al., 2025) showed that the East Asian aerosol cleanup has likely contributed to the recent acceleration in global warming.

Since 2020, evidence has strengthened that climate change is already affecting air quality in the UK. For example, in recent hot summers, such as 2018 and 2022, ozone limits were exceeded, with increased health risks, particularly during heatwaves. Observed and modelled increases in stagnation events suggest a higher likelihood of pollution build-up under future climates (Horton et al., 2014). Wildfires are an increasingly important source of air pollution as they increase with climate change, both from UK fires due to hotter, drier summers and via long-range transport from Europe and North America (Burke et al., 2023).

Climate projections suggest the frequency and severity of air pollution episodes will increase, particularly for ozone (Tran et al., 2023). Future air pollution levels will depend strongly on emissions policy and technological change. Multi-hazard air pollution and heat stress events are likely to become increasingly common (Sillmann et al., 2021), which will amplify health burdens.

2.4 Global climate and anthropogenic climate change

While the focus of this chapter is on the UK, here we discuss several aspects of global climate. Firstly, this covers the large-scale drivers of UK climate to understand how they might change in future. Secondly, extremes of climate and climate change being experienced globally are discussed. In some cases, these might have a direct impact on the UK, for instance through supply chains affecting food or goods. In other cases, these can provide a pointer to what we might expect in the UK as warming continues. Thirdly, we discuss earth system tipping points, which are very large-scale, often abrupt or irreversible changes in regional climate that, if they were to occur, would produce further consequences for UK climate.

2.4.1 Large-scale drivers of UK climate variability and change

Introduction

Direct warming from increases in the concentration of greenhouse gases is an important factor in UK climate change. This is often referred to as the “thermodynamic component”. We have also described how large-scale changes in atmospheric circulation, which we experience as weather, will be important.

The UK’s weather and climate are shaped by interactions between the ocean, atmosphere, and ice in the North Atlantic. These combine to drive the North Atlantic jet stream, a band of enhanced westerly winds over the North Atlantic region. The strength and positioning of the jet, in turn, has a major impact on the type of weather the UK experiences.

Several factors play a role in determining the characteristics of the jet and the weather the UK experiences, including:

• Sea surface temperatures in the North Atlantic. Warm seas warm the air above them. As warm, moist air rises above the sea, this can cause clouds to form, and if enough energy is transferred from the sea to the air, storms can develop.
• Spatial differences in sea surface temperature. The sea surface is not uniform in temperature; there are warmer and colder areas. More energy is transferred to the atmosphere above the warmer regions. This affects the weather. North Atlantic sea surface temperatures are also influenced by the Atlantic Multidecadal Oscillation, which is a natural cycle between warmer and cooler phases of the North Atlantic Ocean, each lasting 20-40 years (Sutton and Dong, 2012).
• Arctic temperatures and sea ice cover (Barnes and Screen, 2015), as this influences the amount and position of cold air north of the jet stream.
• The behaviour of winds circulating high up in the stratosphere (the stratospheric polar vortex), up to 30 miles (50 km) above the earth (Baldwin and Dunkerton, 2001). A strong polar vortex favours a strong jet stream, which potentially leads to shorter-lived, but more intense weather systems over the UK.
• Convection causing warm air to rise over the tropical regions affects weather patterns and influences jet stream shape in the mid-latitudes.
• A simplified description of typical atmospheric flow pattern is often framed in terms of the North Atlantic Oscillation (NAO) and the East Atlantic (EA) patterns, which are the dominant patterns for weather and climate in the UK. The NAO describes how wind patterns over the North Atlantic change between linear west-east (zonal) flow and wavier (meridional) flow. The EA pattern is centred just west of the UK and is most active in winter. When the EA is in a positive phase, it often brings high pressure over the UK, which usually leads to colder and drier weather in winter.

On top of those North Atlantic regional aspects, atmospheric signals in one part of the world communicate to other regions via the propagation of atmospheric waves (e.g., Hoskins and Karoly, 1981). Prominent examples are the influence of the El Niño Southern Oscillation (ENSO; Brönnimann, 2007) or the Quasi Biennial Oscillation (e.g., Gray et al., 2018) from the tropics on weather in the UK. For example, ENSO can influence the NAO pattern and is ultimately responsible for colder and drier winters in northern Europe and wetter weather in southern Europe in a positive ENSO phase (during El Niño years).

Beyond those global- to continental-scale influencing factors, the interaction of physical processes at regional to local scales is also important for the UK. For example, Fink et al., (2004) showed that local to regional moisture deficits enhance temperature maxima, which again reduce available moisture, thus producing a positive feedback loop. In the UK, spring 2025 was extremely dry (up to 50% less than average rainfall), contributing to the very warm summer in 2025 (top 5%, according to Met Office, (2025a)).

Further important phenomena are so-called climate (or weather) whiplash events (Swain et al., 2018) as a warmer atmosphere can absorb more moisture, e.g., by evaporation from the soil, the soils get drier while the amount of moisture in the atmosphere may increase significantly. Once the atmosphere becomes unstable, rapid changes with extreme rainfall amounts could be observed. Thus, a rapid shift between extreme dry conditions and heavy rainfall with potential flood events may occur. These weather whiplash events are also connected to significant and abrupt changes in the atmospheric circulation over the North Atlantic and the UK (Francis, Skific and Zobel, 2023).

Observed changes

Atmospheric thermodynamic changes

Between 2011 and 2020, the Earth was 1.1 °C warmer than in pre-industrial times (1850-1900), mainly due to human activities (IPCC, 2021). This warming has caused an increase in heat extremes in most regions, including the UK and Europe (Seneviratne et al., 2021) Cold spells in the UK happen less often and are not as severe (Kendon, M., et al., 2024), which is also true in other parts of the world (Seneviratne et al. 2021).

Over the last 40 years, the Arctic has warmed about four times faster than the global average (Rantanen et al., 2022). This is known as ‘Arctic Amplification’, and its causes are well known. Sea ice reflects sunlight, radiating energy from the sun back into space. Melting sea ice reflects less sunlight, allowing more heat to be absorbed by the ocean. Clouds also play a role by affecting how much heat enters and leaves the Arctic, which adds to the warming (Forster et al., 2021).

Atmospheric circulation changes

CCRA3-IA TR concluded that while theories had been proposed for how climate change could affect the North Atlantic jet stream and thus weather and climate over the UK, so far there was little observational evidence to support a climate change signal was emerging. Since CCRA3-IA TR, new evidence updates this view.

Atmospheric reconstructions using a blend of observational data and weather models (so-called reanalyses) show a recent strengthening of the winter jet stream (averaged around the entire Northern hemisphere) at altitudes of 8-12 km above the surface (Woollings et al., 2023). In consequence, these datasets also show winter lower tropospheric winds over the North Atlantic have strengthened since 1950 (Blackport and Fyfe, 2022). However, it is unclear whether this is a consequence of climate change or slowly varying natural climate variability.

Future changes

Global climate models are core tools to understand and to project future climate changes. Since CCRA3, there have been advances in understanding model limitations in simulating North Atlantic atmospheric circulation. Global climate models driven by historical forcings struggle to reproduce the observed strengthening of the winter North Atlantic jet stream (Blackport and Fyfe, 2022). Several hypotheses have been proposed to explain the discrepancy, including:

• Climate models underestimate multidecadal variability in the winter North Atlantic atmospheric circulation (Simpson et al., 2018)
• Climate models struggle to capture observed sea surface temperature trends in the equatorial Pacific (Wills et al., 2022) which influence the North Atlantic jet stream blocking through teleconnections (Cheung et al., 2022)
• Climate models may underestimate the jet stream response to external climate forcings (Smith et al., 2025)

There is currently no consensus on these mechanisms, and they are challenging to test given the relatively short instrumental record. These limitations may impact on our ability to explain in detail how climate change will influence all relevant climatic factors over the North Atlantic and the UK.

Climate models also appear to underestimate observed trends in northwest European summer heat extremes (Patterson, 2023; Vautard et al., 2023), in part because they underestimate the observed trend in summertime North Atlantic atmospheric circulation towards a ‘wavier’ state (e.g., Coumou, Lehmann and Beckmann, 2015). It is unclear whether these summer circulation trends are a signal of climate change or due to internal climate variability. Circulation-driven heatwaves may also be amplified by local ‘thermodynamic’ factors such as long-term drying trends in soil moisture (section 2.3.1.1 on heat hazard).

The models used for seasonal-to-decadal forecasting, which are similar to those used for climate projections, underestimate the potential quality of multi-annual North Atlantic and UK climate forecasts in winter due to signal-to-noise errors (Eade et al., 2014; Smith et al., 2020). The reasons remain under investigation but include a poor representation of teleconnections (Williams, Scaife and Screen, 2023; O’Reilly, 2025). This could mean simulations of future climate conditions need to be carefully analysed for biases and corrected or calibrated if needed. Nevertheless, (Degenhardt, Leckebusch and Scaife, 2023, 2024) show that seasonal forecasting models can now provide skilful predictions of extreme windstorm frequency and intensity ahead of the winter season in the UK.

Atmospheric thermodynamic changes

There is high confidence that all future greenhouse gas emissions scenarios considered by IPCC (2021) result in increased global warming over the near-term (2021-2040), mainly due to increased cumulative CO₂ emissions in nearly all considered GWLs. There is a projected increase in the frequency and severity of heatwaves across many regions, including the UK and western Europe (Seneviratne et al., 2021). Atmospheric moisture is projected to increase with warming (IPCC, 2021) due to an enhanced capacity of the atmosphere to contain water vapour with higher temperatures. This will lead to further intensification of the global water cycle, including its variability and extremes.

Climate models robustly show stronger warming over land than sea under future climate change scenarios (Lee et al., 2021), including for the UK and Europe. This corresponds to larger warming trends where people live than for the global mean trend.

Chen et al., (2022) identified the global change in frequency of significant precipitation ‘whiplash’ events by means of an 80-member multi-model ensemble for a high GWL at the 2070-time horizon. They find an increasing frequency of these events mainly in several semi-arid regions of the globe, but also for the UK. For the latter, changes of about 20-30% are identified. While thermodynamical factors are mainly responsible, in specific regions circulation related factors are also present.

Atmospheric circulation changes

As introduced in section 2.3.3.1, changes in high atmosphere waves and the way they interact with the jet stream play a major role in shaping weather patterns over the UK. At times when the waves interact with the jet stream such that the ridges of the waves are enhanced, the usual west-east flow of wind can be obstructed for days or even weeks, a process referred to as ‘blocking’ (section 2.3.3).

When this happens, weather systems can get ‘stuck’ in place, leading to prolonged periods of hot, dry weather (in summer) or cold spells (in winter) under the block, and flooding either side of the block. This has caused many examples of high impact weather events, such as winter cold spells like the “Beast from the East” (February 2018), and summer heatwaves like the UK’s first recorded 40 °C day (July 2022). On the other hand, fast travelling (so-called transient) waves may occur, which lead to an increased succession of low-pressure systems over the UK, like in winter 2013/14. Those upper-tropospheric waves normally have a wavelength of 50-70 longitudinal degrees (at 50N, roughly 3,500-5,000 km) and need to be simulated correctly in any climate model to realistically simulate changes in related weather circulation patterns at the surface. It is noted that CMIP6 models, for example, show improved blocking locations in comparison to CMIP5 models but still suffer from biases in persistence and frequency (IPCC, 2021). Notably, during winter transient flow regimes, CMIP6 models still show a too-zonal pathway for those transient waves (IPCC, 2021).

On average, the CMIP6 models show weak near-term changes (2021-2040) in Northern hemisphere midlatitude circulation patterns, in all seasons and under all greenhouse gas scenarios considered in Lee et al., (2021). By the end of this century, CMIP6 models show a tendency to a more positive NAO-like flow pattern (thus more zonal, with transient waves) in all seasons except summer (Davini and D’Andrea, 2020; Fabiano et al., 2021; Lee et al., 2021; Dorrington et al., 2022; Pope et al., 2022).

Such changes imply more frequent wet and windy winters in the UK. However, climate models differ in their quantitative details of changes in frequency and persistence (Dorrington et al., 2022), meaning uncertainty remains in the future likelihood of regimes associated with extreme weather events. In summer, climate models project an increase in anticyclonic (high pressure) circulation west of the UK in August, relevant for western and central European drought (Rousi et al., 2021), leading to more frequent dry and hot conditions also in the UK.

Francis et al. (2023) conclude that weather whiplash events in the North Atlantic/European region, including the UK, are likely to increase in frequency by the end of the century under a high greenhouse gas emission scenario equivalent to a global warming level of 3.0 °C.

2.4.2 Global climate changes with relevance to the UK

This section has two parts. First it looks at climate change beyond the UK which might impact indirectly on the UK, for instance through the impact on global food production. Second it looks at events in the global climate outside the UK that might provide analogues of what we might need to prepare for in the UK in future.

2.4.2.1 Large-scale climate changes that might impact the UK

This section considers the hazards relating to global changes in precipitation, temperature and wind. For the latter, the focus here is on tropical cyclones; for European windstorms see section 2.3.3.1. The table below identifies which risks this hazard is particularly (but not exclusively) relevant to.

Introduction

Severe internationally occurring events such as floods, droughts, heatwaves and extreme storms may impact UK risks via a variety of pathways, including food availability (Ercin, Veldkamp and Hunink, 2021), trade and shipping (Freeman et al., 2024), disease transmission (Semenza and Paz, 2021), and displacement of people and humanitarian emergencies (Hermans and McLeman, 2021). This section summarises some changes in global climate hazards that may act as shocks to the UK’s economy and security, with attention to some key regions, e.g., East Asia for manufacturing, Europe and Africa for food imports. For further detail, refer to IPCC reports.

Observed change

By combining large datasets and improving methods, it is now faster to diagnose any link between recent weather events and human-caused climate change.

Heavy rainfall: Studies indicate an increasing risk of heavy rainfall events, driven by anthropogenic climate change. Events such as the heavy rainfall affecting Spain in October 2024 are estimated to have doubled in likelihood and increased in intensity by 12% (World Weather Attribution, 2024). The peak intensity of short-duration (e.g., hourly) precipitation is increasing at the fastest rates (e.g., Fowler et al., 2021).

Flooding: In regions where soils are drying, e.g., Southern Europe, flood magnitudes have decreased; where soil moisture is increasing, e.g., Northern Europe, flood magnitudes have increased (Wasko et al., 2021). Very heavy rainfall is driving increases in the rarest, strongest floods regardless of soil moisture (Bertola et al., 2021; Wasko et al., 2021).

Drought: Summertime drought intensity has increased in Western North America, Australia, Southern Europe, Eastern, Central and Southern Africa and parts of South America (Vicente-Serrano et al., 2022). There is agreement that changes result from warming over land, which drives increased evaporation, with land-use also playing a role in some regions. Drought-related European cereal losses are estimated to have increased at 3%/yr over the past 50 years (Brás et al., 2021).

Heat: Most regions on the globe show a significant increase in extreme heat (IPCC, 2021). Observations and models indicate that heatwaves can coincide in important regions to global food supply due to connections in atmospheric patterns (Meehl et al., 2022). The frequency of drought-heatwave events is increasing globally, with the fastest increases seen in low-income regions including Africa and East Asia (Zhang, Wang and Slater, 2024), driving humanitarian and agricultural risks.

Wind: Observations and model simulations show decreasing tropical cyclone frequency but increased windspeed intensity and intensification rates (e.g., Camargo et al., 2023), and a shift poleward (Studholme et al., 2022) and towards coastal regions (Wang and Toumi, 2021). The latter implies increased risk to populous regions. Recent work suggests that intense tropical cyclones are occurring earlier in the year, which may increase likelihood of events coinciding with other types of heavy rainfall events (Shan et al., 2023).

Future change

Temperature and rainfall are expected to change as the planet warms. The Arctic will warm the most (e.g., Tebaldi et al., 2021) and some areas, like the tropical Pacific and Arabian sea, will get wetter. Other areas, like subtropical regions, the Mediterranean and southern Europe, may get drier. This leads to a potential food production risk for the UK. Stronger tropical storms are also expected. At a GWL of 2.0 °C for the late 21st century, tropical cyclone lifetime maximum windspeeds are expected to increase by 5% (range 1-10%), with an additional 13% of storms reaching very intense levels (Knutson et al., 2020).

A major advance since CCRA3-IA TR has been the production of a growing number of global and regional ‘convection-permitting’ model simulations with resolutions of less than 10 km, made possible through improved supercomputer power. These simulations resolve small-scale processes such as convection, reducing model uncertainties.

Heavy rainfall: High-intensity events are expected to increase in many regions, with reduced moderate and light rainfall, such that future climates support increases in both flood and drought (Fowler et al., 2021). Convection-permitting simulations over Western Europe indicate that the most extreme short duration events increase at approximately 10-14% per °C, (e.g., 20-24% at a GWL of 2 °C, during the 2050s for a central scenario) likely leading to increased flood risk (Lenderink et al., 2021).

Flooding: Floods that historically occurred every 50 years are expected to occur with increasing frequency, e.g., every 36 years at a GWL of 1.5 °C (2030s for a central scenario), and even more frequently for warmer climates. These affect regions of UK sensitivity for food production, e.g., Northern Europe, and manufacturing/trade, e.g., China (Huang et al., 2024). Flood return periods are shorter for the same global warming level achieved under higher emissions scenarios.

Drought: Studies agree on an increased likelihood of drought over much of South and Central America, the Middle East and North Africa, southern Europe, and southern Australia, with severity and duration increasing with global mean temperature (Douville et al., 2021). Economic impacts of drought, e.g., on crop production, are projected to worsen in a warmer climate (Naumann et al., 2021).

Heat: Global projections indicate a further increase in the frequency and intensity of extreme heat (IPCC, 2021). Dangerous levels of heat that are extremely rare today will become possible across several regions in the tropics. Estimates suggest the global population exposed to severe heat stress annually would increase to 20% at a GWL of 1.5 °C (2030s for a central scenario), 30% at a GWL of 2 °C (2050s central, 2030s high scenario), and over 50% at a GWL of 3 °C (2080s high scenario) (Freychet et al., 2022). The frequency and intensity of crop growing season heatwaves increases in models, particularly at a GWL of 2 °C, with Asia, North America and Europe most affected (Chen, Zhang and Zhou, 2024).

Wind: Poleward shifts of tropical cyclone regions are expected (Li and Zhou, 2024). The speed with which tropical cyclone storm systems move from one place to another is projected to decrease, leading to longer exposure of locations to wind and rain (Camargo et al., 2023). Future changes in how often tropical cyclones occur are uncertain. The intensity of tropical cyclone windspeed is expected to increase in a warming climate (Camargo et al., 2023). Increased tropical cyclone intensity is projected to drive increased storm surges affecting a larger coastal area, including key manufacturing and shipping regions in East and Southeast Asia (Muis et al., 2023; M. Wood et al., 2023).

2.4.2.2 Extreme climate events around the world that might provide analogues of future UK events

Recently, since CCRA3-IA TR, significant and devastating extreme weather events have occurred across the world. In this section, we discuss three unprecedented extreme events which did not directly impact the UK but are examples of extreme weather now possible in our warmer climate, which is already significantly influenced by climate change. Studies support the notion that they are harbingers of extreme weather in future climates (known at least since e.g., Schär et al., 2004; Fink et al., 2004), given futures of medium to high-end GWLs to the end or the mid of this century, respectively.

These extreme events cannot be transferred one-to-one to the weather and climate of the UK. As we have discussed above, the weather and its variability in the UK are highly variable for multiple reasons. Nearly all of them will be influenced by climate change. But we can use the study of those events to inform about potential magnitudes of events to come, and to inform necessary and important disaster risk and reduction efforts, before or in the aftermath of such extreme events. With increasing evidence of climate change impacts being realised in the present day, we cannot exclude the occurrence of respectively equivalent events with comparable magnitude for the UK.

Box 2.4 The Pacific-Northwest Heatwave, June 2021

This heatwave began in late June 2021, in response to a strong high-pressure system and advection and large-scale sinking of airmasses originating from the tropics (White et al., 2023). Temperatures broke all-time maximum temperature records by more than 5 °C, and reached 49.6 °C, an all-time Canadian record, measured in the town of Lytton. Warnings of extreme heat events being possible originated both from sub-seasonal forecasts and weather forecasts. Nevertheless, due to its extreme magnitude, the heatwave caused severe consequences. It led to hundreds of attributable deaths across the Pacific Northwest, caused spikes in hospital visits and caused severe impacts on marine life (Fleishman et al. 2025). It also led to a wildfire which burnt down the town of Lytton shortly after this town broke the Canadian temperature record. As this event was clearly a “record-shattering” event (an event that breaks records by large margins, (Fischer et al., 2021)), the event was so unprecedented that it was not anticipated as physically possible if estimating return periods of annual maximum daily maximum temperature using a General Extreme Value distribution (e.g., Bartusek et al., 2022). A rapid attribution analysis supports this view and concluded that this event was virtually impossible without human influences (Philip et al., 2022). Events of such severe magnitude have the potential to surprise populations and over-challenge adaptation as it far exceeds previous records.

Fig. 3.3.1: Timing, location and magnitude of the PNW heatwave in 2021. Figure from Bartusek et al. (2021; their figure 1a)

Fischer et al. (2021) demonstrate that the probability of occurrence of those “record-shattering” events for future climate states depends on warming rate, rather than global warming level, and is thus pathway-dependent. They also highlight the role of a disturbed and abnormal atmospheric circulation. They found that slow- and fast-moving components of the atmospheric circulation interacted, along with regional soil moisture deficiency, to trigger this extraordinary heat event (five times larger than the standard deviation).

Thompson et al. (2022) conclude that many regions could see events that break records by large margins which could cause severe consequences there as well. A storyline hindcast analysis by Bercos-Hickey et al. (2022) highlights that this was a very rare event, with the unusual circulation explaining most of the extreme temperatures, which would have been about 1 °C less extreme without anthropogenic warming.

Box 2.5 Ahr-floods in Germany, July 2021

From 12^th-15^th of July, severe rain occurred over Western Europe due to the slow-moving low pressure system Bernd, falling on already wet soils. Between 12 and 19 July 2021, the low-pressure system resulted in extreme rainfall of more than 150 mm in 72 hours (Mohr et al., 2023), about five times the observed daily maximum average in July (e.g., Tradowsky et al., 2023). According to Kreibich et al. (2022) and Rhein and Kreibich (2025), this led to unprecedented flash floods, killing 184 people in Germany (134 in the Ahr valley; Roggenkamp et al., 2024), in addition to extensive damage to buildings, long term power outages and economic losses of around 33 billion EUR (Kron et al., 2022; Munich Re, 2022). The German association of insurance industry (GDV) estimates insured losses worth about €8.7 billion (GDV, 2024). Several days before storm Bernd hit western Europe, the European Flood Awareness System (EFAS) accurately predicted severe risk of flooding and issued a notification to the German authorities (Tradowsky et al., 2023). This triggered a flood warning for major rivers but small rivers like the Ahr were not sufficiently considered. Tradowsky et al. (2023) conclude “not all exposed authorities and people received the warnings and when they did, the warnings were not always understood and acted upon”. This case illustrates that warning alone is not sufficient, but the warning needs to reach decisionmakers and population in time to get out of harm’s way.

Figure 3.3.3: Accumulated precipitation over 48 h (left) and accumulated over each of the individual days of the extreme precipitation event (middle and right). Data source: Extended E-OBS dataset. Copyright of Fig.: DWD. Figure taken from Tradowsky et al. (2023, their figure 2).

Based on rainfall data, a probabilistic event attribution conducted by World Weather attribution indicates that rainfall accumulations in the Ahr, Erft and Meuse catchment exceeded previous observed records, as did discharge levels of the rivers. However, the small spatial scale and estimate far outside previous records challenges robust event attribution based on observed extreme events. Thus, World Weather Attribution focused on rainfall over a larger Western European area that encompasses the river catchments (Tradowsky et al., 2023). The analysis estimated the observed rainfall event to have about a 400-year return period, and that compared to preindustrial conditions (1.2 °C lower global mean temperature), 1-day extreme rainfall event intensity has already increased by 3-19%, thus substantially enhancing the probability of a severe wet event. The likelihood of such an event occurring today compared to a 1.2 °C cooler climate was estimated to be enhanced between 1.2-9 times, depending on analysis method and considering uncertainties. This is in line with a warmer atmosphere raining out more moisture under suitable conditions. However, comparing the observed flood levels in the Ahr valley to earlier periods, Roggenkamp et al. (2024) estimate that floods of similar magnitude occurred before, most similarly in 1804, emphasizing the value of considering past extreme events. This example highlights the risk from a combination of factors exacerbating flood levels, from saturated soils to very extreme rainfall combined with lack of preparedness and insufficient response which needs to be considered for adaptation to intensifying extreme events.

Box 2.6 Valencia-Flood in Spain, October 2024

The province of Valencia in Spain was hit by unprecedented rainfall amounts in autumn 2024. According to Faranda et al. (2024), within 24 hours on the 29^th of October, 630 mm of rain (equivalent to 630 litres per square metre) were recorded in Turis. In Chiva, some 10 km north, 491 mm rain fell in just eight hours. At many locations across the province intense rainfall exceeding 300 mm within 24 hours was reported by AEMET (Spanish Meteorological Agency; e.g., Amiri et al., 2025), which caused widespread flooding. To give context to these rainfall amounts, the previous maximum daily rainfall at Valencia airport in October was recorded on 6^th Oct 1971 at 186.9 mm, according to AEMET. Using rainfall observations from approximately 225 personal weather stations (low-cost commercial devices primarily operated by citizens) Rombeek et al. (2025) identified two bursts of extreme rainfall leading to a first flood wave in the Margo catchment triggered by about 180 mm of rain in a few hours. While this flood wave propagated downstream, a second extreme rainfall event amplified the overall event and likely contributed to the devastating consequences. Related flash floods killed over 200 people and interrupted the water and electricity supply for hundreds of thousands in the provinces of Valencia and Castellón, as well as in Málaga (Andalusia) and Albacete (Castilla-La Mancha).

Figure 3.3.5: Rainfall over Spain, 29.10-1.11.2024; from Amiri et al., (2025); their Figure 2.

World Weather Attribution (WWA) published a rapid assessment in the aftermath of the event (WWA, 2024). They do not make full use of climate model simulations formally used to quantify the anthropogenic climate influence of specific events; their results are based on observations only. By analysing three observational, long-term rainfall datasets, they conclude that “heavy 1-day rainfall events, as intense as the one observed, are about 12% more intense and about twice as likely in today’s climate, that is 1.3 °C warmer than it would have been in the cooler preindustrial climate without human-caused warming.” Resulting estimates of probabilities for the amount of river discharges as observed during this event are highly uncertain, but include e.g., the Rambla de Poyo catchment at 1 in 900 years. WWA (2024) further states that “over the past ~75 years, daily rainfall extremes in the September-December season in central and southeastern Spain have increased significantly with global warming, approximately doubling in likelihood and equivalently increasing in intensity by 12%.”

2.4.3 Tipping points in the global climate system

This section considers tipping points of the global climate system. Tipping points could cause changes that occur faster than expected from the pace of global warming or are irreversible on human timescales.

Introduction

Tipping points are defined as critical thresholds beyond which the climate system reorganises, often abruptly and/or irreversibly (Lenton et al., 2008; Chen et al., 2021), on global or large regional scales. Examples include changes in ocean circulation, accelerated ice sheet collapse, or collapse or substantial shift in forest systems. While the concept of tipping points is intuitive, its definition is vague, hence some consider it not useful (Kopp et al., 2025). A clearer definition would specify characteristics of tipping dynamics, such as changes that, once triggered, are sustained or self-amplifying, and/or irreversible and/or rapid compared to the pace of global warming and the timescale at which a system changes normally. It is also important to quantify the timescales of tipping. Once a critical threshold has been crossed, the resulting committed climate change may take years, decades, or even centuries to fully materialise.

Past climates have shown that tipping events such as rapid changes in ocean circulation are possible, yet there is large uncertainty at what warming levels and rates of warming tipping dynamics may be triggered in the future. The capability of climate models to capture processes involved in tipping is also highly uncertain, as it requires interactions that are not well resolved or represented in models (for example, input of freshwater from melting ice, or vegetation dieback and fire caused by drought and heat). Despite this, the latest generation of earth system models can provide useful insight into tipping dynamics involving the ocean, such as changes in circulation and sea ice, and the land surface, such as changes in forests. Where the models do not yet include the necessary processes, insight can be gained by driving other, so-called offline models, for instance for ice sheet changes and permafrost carbon release.

Recent review papers highlight that the risk of tipping events can be reduced by limiting warming to below 1.5 °C (Armstrong McKay et al., 2022; see also table), although the high uncertainty in tipping dynamics makes it difficult to aim mitigation efforts specifically to address them (Kopp et al., 2025). Recent literature also highlights the need for improved process understanding and monitoring (e.g., Wang et al., 2023). Despite their high uncertainty, tipping points are discussed here due to their ability to cause severe impacts, although these are presently not well understood and there is substantial further research need.

Some tipping points could affect the UK directly, such as a collapse or substantial reduction of the Atlantic Meridional Overturning Circulation (AMOC) or Sub Polar Gyre (SPG) circulation, which would cool the UK relative to the rest of the world. Tipping points in ice sheets, particularly in Antarctica, could cause a faster rise in sea level than projected otherwise (see section 2.3.4). Triggering tipping dynamics could also cause global impacts that could affect the UK indirectly, for example through changes in livelihoods of fishing communities following collapse of warmwater coral ecosystems, or of land abandonment due to sea level rise (see overview in Table 2.8); although these are presently not well understood.

Examples of tipping points are shown in figure 2.16 below and discussed in several recent review papers (Armstrong McKay et al., 2022; Lenton et al., 2023; Wang et al., 2023).

• Collapse of the ocean currents of the Atlantic Meridional overturning circulation (AMOC), and a related collapse of the Sub Polar Gyre: AMOC collapse would cool the North Atlantic sector relative to the rest of the planet and change global weather patterns. A collapse or shift of the AMOC has been observed in past climates and occurs in climate models when large amounts of fresh water interrupt the sinking of very saline water, but how much fresh water is required for this collapse varies between lines of evidence and between climate models. Current (CMIP6) climate models suggest that the AMOC will weaken over the 21st century but not collapse completely (IPCC, 2021). A recent study (Baker, J.A. et al., 2025) identifies physical mechanisms that underly that stability. However, another recent study of CMIP6 models (Drijfhout et al., 2025) shows that on longer timescales (to the year 2300) the AMOC could reach very weak values. A severe reduction or collapse of the AMOC would affect the UK, as it is in relatively close proximity to the North Atlantic and close to the most intense cooling area associated with an AMOC shutdown or severe reduction. The detailed impacts would depend on the background level of global warming at the time of the weakening; this topic is currently under-researched, but several studies are in progress. AMOC collapse would also change the pattern of sea level rise compared to scenarios without it and affect weather patterns including rainfall. Shutdown of convection in the SPG may substantially reduce the productivity of the North Atlantic Ocean ecosystem (Kelly et al., 2025).
• Collapse of the Amazon rainforest, or its transition to a seasonally dry or less forested state, driven by a combination of enhanced drought stress and deforestation: Amazon collapse would reduce carbon uptake by the forest and with it the carbon budget, and could also change how water is cycled back to the atmosphere. This could change rain availability downstream with possible consequences for tropic-wide rainfall patterns with possible impacts on food production across the tropics. Publications highlight that deforestation, particularly in the South, could decrease forest resilience (Lapola et al., 2023).
• Accelerated collapse of major ice sheets, both in Greenland and Antarctica by tipping dynamics in ice sheets: The most significant effect across the UK would occur in response to Antarctic ice sheet tipping, for example significant loss of the West Antarctic ice sheet (Bamber et al., 2009) due to gravitational pull effects (see section 2.3.4). The resulting sea-level rise over coming centuries would be considerably higher than without the accelerated loss of the ice sheets, although the full effect would still take centuries or more to be realised.
• Shifts in the boreal forest (Wang et al., 2023) could impact how much solar radiation the Earth absorbs, while permafrost thaw could have consequences for regional water cycles and the global carbon budget if this causes significant methane release.

Many changes associated with tipping points can be of global or at least subcontinental significance (see Armstrong McKay et al., 2022). However, even regional tipping points could cause impacts that affect the UK, e.g., through disruptions of food systems or agriculture which could contribute to political instability and migration, although these links are poorly understood. Once tipping points have been crossed, changes may not be reversible if the climate forcing is later reduced to below the critical level for tipping (Lenton et al., 2023). It is also possible that overshoots above climate targets may be short enough to avoid triggering some tipping points (e.g., Ritchie et al., 2023), a topic where there is substantial uncertainty and ongoing research at present.

Lastly, triggering tipping dynamics may cause cascading effects into the climate system, potentially destabilizing other regions, but those connections are highly uncertain. For instance, a collapse in the AMOC may increase the risk to parts of the Amazon forest if the rain belt moves southward, although this is presently poorly understood. The likelihood of Amazon collapse increases if considering deforestation (e.g., Lapola et al., 2023; Flores et al., 2024). Many ideas and concepts around tipping points are being explored in ongoing climate model experiments (Winkelmann et al., 2025).

The topic of tipping points has been received significant attention since CCRA3-IA TR, with multiple reviews published and underway, but significant uncertainty remaining. Tipping points may cause high impact outcomes from climate change and stretch limits to adaptation, even if their probability is presently difficult to estimate, although some of them are more likely to occur far in the future.

Figure 2.14: The location of climate tipping elements in the cryosphere (blue), biosphere (green), and ocean/atmosphere (orange; figure from Armstrong McKay, 2022). There is also an estimate of global warming levels at which these tipping points have been hypothesised to occur below 2 °C (light orange, circles); between 2 and 4 °C, i.e., accessible with current policies (orange, diamonds); and at or above 4 °C (red, triangles). Note that many of these estimates are presently highly uncertain.

Observed change

Observations support concerns about global tipping elements, and there is evidence that tipping dynamics occurred in past climates. Some studies have used proxy indicators of past AMOC change, along with simple dynamical systems ideas, to suggest that the AMOC may be getting closer to a tipping point (e.g., Boers, 2021; Ditlevsen and Ditlevsen, 2023). However, uncertainties in both the proxies and the dynamical warning indicators mean that any predictions of tipping time are subject to huge uncertainty (e.g., Ben-Yami et al., 2024). While the Amazon rainforest is not yet severely vulnerable to climate change alone (Armstrong McKay et al., 2022), analysis of its forest ecosystem highlights its degraded state due to increased drought and increasing deforestation. Coral ecosystems have been increasingly frequently bleached during marine heatwaves (see review in (Romanou et al., 2025). Mountain glaciers in many regions are in strong retreat (Fox-Kemper et al., 2021) and are already affecting flood risk and water availability for large populations depending on them.

Observations show substantial loss of Arctic sea ice over recent decades, and this is expected to continue into the 21st century. While sea ice change is reversible when global warming reduces (Wang et al., 2023), impacts on Arctic ecosystems may not be. There is also scientific debate as to what extent regional sea ice loss could have important impacts on weather patterns and with that, affect the UK weather and weather hazards.

Future change

As global warming continues, the likelihood of triggering tipping dynamics increases. Even at present warming levels and 1.5 °C global warming, several systems that may be prone to tipping are in the ‘possible’ range estimated in expert assessments (Table 2.8), and there are some signals in observations as discussed above. As the threshold forcing (e.g., warming level) is approached, tipping can be induced by internal climate variability, even before the theoretical threshold has been reached, meaning that prediction of the timing of tipping can be particularly challenging (Romanou et al., 2023; Gu et al., 2024). The rate of future warming is also important, as some research suggests that rapid warming can trigger tipping dynamics even before the critical level has been reached (Ashwin et al., 2012).

It is worth mentioning that climate models used in UKCP18, which provides the climate projections that underpin many studies of climate risk used in CCRA4-IA TR, do include the capability to simulate some types of tipping points or other large-scale shifts in the climate system. In particular, they can simulate changes in the Atlantic ocean overturning circulation (AMOC), but there remain concerns that the generation of models used might be too stable. In detail, the versions of the climate models used to provide high spatial resolution information in UKCP18 do not include detailed ice sheet or carbon cycle components directly so cannot simulate tipping points including these elements. Alongside the projections in UKCP18, a storyline of accelerated sea-level rise was provided drawing on a range of information including climate and ice sheet models, paleo observations and physical reasoning. Additional simulations are available from the wider modelling community that artificially trigger tipping points in order to investigate their consequences.

Table 2.8: Examples of tipping elements with near-term tipping potential. Global warming level estimates and examples are taken from (Armstrong McKay et al., 2022; Ritchie et al., 2023) and are there based on expert assessment. Estimates and their uncertainties are poorly known and recent work has revised some estimates. Note also that some examples from Armstrong McKay et al. (2022) are not included here either due to 1) uncertainty if they are affected by tipping dynamics, 2) estimates that likelihood of triggering them is low for warming levels near Paris targets, or 3) due to unclear links to the UK. Only lower and upper confidence limit of triggering tipping dynamics are given to reflect deep uncertainty, which renders central estimate particularly uncertain; and confidence levels are evolving and may change with further research.

As the literature is evolving quickly, there are some conflicting views emerging around tipping points. For example, when considering slowdown or collapse of the AMOC, Ditlevsen and Ditlevsen (2023) estimate a collapse of the AMOC could occur around mid-21^st century, although the emission scenario they assume is unclear and stated as “under a current scenario of future emissions”. Their estimate is derived from changes in statistical characteristics of an observed proxy derived from sea-surface temperatures. A number of articles have questioned both the statistical framing and the suitability of the proxy for AMOC strength, as well as some technical aspects of the methodology (see for example Ben-Yami et al., 2024; Terhaar, Vogt and Foukal, 2025). Other indicators have also been suggested, e.g., van Westen et al. (2024), which might offer progress in early warning of tipping events. Drijfhout et al., (2025) focus on physical mechanisms during an AMOC slowdown and find that the risk of a northern AMOC shutdown is greater in the current generation of climate models than previously thought but note that under slowly changing forcing the decline would be expected over 50-100 years, and so may not been seen in model experiments that stop at 2100. These contributions must be tensioned with other recent work, such as Baker J.A. et al. (2025) who recently identify an AMOC resilience to extreme greenhouse gas and North Atlantic freshwater forcings because of the stabilising effect of the Southern Ocean upwelling, which can balance the downwelling in the Pacific and Atlantic.

Consequently, estimates vary strongly in the literature as to at which global warming levels tipping dynamics may be triggered. Resolving these uncertainties will take time, as climate models miss processes that simulate key mechanisms. In some cases (e.g., AMOC) the impacts of the tipping event may be strongly dependent on the background global warming level. There are large uncertainties and contradictory estimates in the scientific literature on if and at what global warming levels tipping may occur, hence the values in table 2.8 based on one publication should be treated with caution. It is possible that some systems may be already close to tipping or may even have already crossed the threshold, which would lead to committed changes that play out over the following decades to centuries (Chandler et al., 2025; Drijfhout et al., 2025). Scientific evidence suggests that triggering tipping dynamics increases the impacts of climate change and makes adaptation more difficult as some tipping events, such as AMOC substantial weakening or collapse may change expected climate change over the UK and with it adaptation needs.

2.5 Research advances, gaps and looking ahead to CCRA5

Since CCRA3-IA TR, climate research has made significant advances globally and provides new and better insight into key aspects of the weather and climate of the UK. Research advances have focussed on addressing a number of key known limitations and uncertainties. For UK climate change, this has particularly seen improved representation of high impact weather and climate extremes. This has been achieved through improved resolution of the models, allowing for more adequate representation of the meteorology and physical processes that underlie a number of key climate hazards, such as intense rainfall. Alongside these modelling improvements advances have been made in methodologies for analysis, such as development of storyline approaches which have supported impact relevant assessments of climate hazards. In this section a number of key advances since CCRA3-IA TR are summarised, as are the remaining key capability gaps in climate science to be further developed looking toward CCRA5. Table 2.9 below goes on to summarise current climate modelling capabilities and limitations.

Table 2.9: Climate models, their features, uses and limitations.

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Footnotes

1. The underpinning data for the metrics are the bias-corrected convection permitting UKCP18 Local 16-member ensemble, regridded to 5km ↑