Climate Change Turns Days In England To Health Threat

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Climate Change, as the week progresses, southerly winds are drawing in warmer air, pushing temperatures in the South of England toward heatwave conditions by the weekend of the 21st and 22nd of June (Met Office, 2025). The UK officially declares a heatwave when a location records temperatures at or above a set threshold for at least three consecutive days.

These thresholds vary by region based on the local climate. In cooler areas, such as much of northern and western Britain, the threshold is 25°C. In warmer regions like Greater London and the surrounding southeastern areas, authorities set a higher heatwave threshold of 28°C (see outlined area in Figure 1) (Met Office). When the Met Office first introduced its heatwave definitions. The area for which the threshold was 28°C was limited to greater London only (McCarthy et al., 2019).

Authorities have since expanded the threshold to include the surrounding counties. This occurred when the Met Office shifted from a 1981-2010 to a 1991-2020 baseline in March 2022, to reflect the increase in temperatures that has occurred as a result of human-induced climate change (Royal Met Soc, 2022).

Heatwaves pose a serious threat to human health and have profound impacts on ecosystems. During the summer of 2022, more than 60,000 people across Europe died as a result of extreme heat. Even in the significantly cooler summer that followed, over 47,000 heat-related deaths occurred (Gallo et al., 2024). Since this current event is the first heatwave of the year, authorities and the public must take warnings especially seriously. Early-season heatwaves tend to be particularly deadly. As people have not yet acclimatised to high temperatures and are often less prepared.

WWA performed a super rapid analysis, analysing observations only on this early summer heat in the Southeast of the UK, defined (1) by the official heatwave definition of reaching 28°C in many of the so-called home counties and greater London (dark red region in figure 1, Met Office) and (2) the hottest predicted day in the same area. Although this is not a full attribution study, the results agree with those from a July 2022 study conducted in a similar region. (Zachariah et al., 2022), thus we have high confidence in the results. 

The event 

A high-pressure system over southern England has expanded to cover much of the UK since the start of the week beginning June 16th, bringing calm, sunny days and gradually rising temperatures. As the week progresses, southerly winds are drawing in warmer air. Pushing temperatures in the South of England toward heatwave conditions by the weekend of the 21st and 22nd of June (Met Office, 2025). In the UK, a heatwave is officially declared when a location experiences temperatures. At or above a specific threshold for at least three consecutive days.

These thresholds vary by region based on the local climate. In cooler areas, such as much of northern and western Britain, the threshold is 25°C. IIn warmer regions such as Greater London and the surrounding southeastern areas, the Met Office sets the heatwave threshold higher at 28°C (see outlined area in Figure 1). When the Met Office first introduced its heatwave definitions, it set the 28°C threshold only for Greater London (McCarthy et al., 2019).

The authorities have since expanded the threshold to cover the surrounding counties. This occurred when the Met Office shifted from a 1981-2010 to a 1991-2020 baseline in March 2022, to reflect the increase in temperatures that has occurred as a result of human-induced climate change (Royal Met Soc, 2022).

Heatwaves pose a serious threat to human health and have profound impacts on ecosystems. During the summer of 2022, more than 60,000 people across Europe died as a result of extreme heat. Even in the cooler summer that followed, over 47,000 heat-related deaths occurred (Gallo et al., 2024). Since this current event is the year’s first heatwave, warnings must be taken especially seriously. Early-season heatwaves tend to be particularly deadly, as people have not yet acclimatised to high temperatures and are often less prepared.

WWA performed a super rapid analysis, analysing observations only on this early summer heat in the Southeast of the UK. Defined (1) by the official heatwave definition of reaching 28°C in many of the so-called home counties and greater London (dark red region in figure 1, Met Office) and (2) the hottest predicted day in the same area. Although this is not a full attribution study, the results align with those from a July 2022 study conducted in a similar region. (Zachariah et al., 2022). Thus we have high confidence in the results. 

Key Messages

• The early summer heatwave triggered an amber heat health alert for all of England, indicating a heightened risk of death for people over the age of 65. Increased risk of overheating of indoor environments, a high demand for health services and increased power demand (UK government, 2025). Early-season heat causes more deaths because people have not yet acclimated to summer temperatures.
• Heatwaves are the deadliest type of extreme weather globally, with hundreds of thousands of people dying from heat-related causes each year. In the case of this heatwave, the forecast temperatures are largely a risk to the most vulnerable populations. Such as people over 65, and people with pre-existing conditions such as respiratory and cardiovascular disease. In past years (e.g. 2023) even yellow heat-health alerts have resulted in significant heat-related mortality (UK Government, 2025). 
• In today’s climate, southeast England crosses the heatwave threshold in June about once every five years. In other words, there is a 20% chance of experiencing a heatwave each June. Before humans warmed the climate, the chance in any given June was only 2%. This means such a heatwave would have happened about once every 50 years.  
• Climate change of 1.3°C has made the heatwave about 2–4°C more intense, turning early summer sunshine into dangerous heat. 
• The hottest day in the forecast is approximately 32oC across the region. This is a rarer event, expected in June only once every 25 years. Similar analysis of the changes in likelihood of this finds that such events have become about 100 times as likely, occurring in June only once every 2500 years before industrialisation. This shows that the chance of experiencing the most intense heatwaves is increasing most rapidly.
• The results align with WWA’s 2022 UK heatwave study in terms of the increase in intensity, which showed that the most extreme heatwaves in southern England have also become around 2°C hotter due to human-induced climate change. However, the current event is less intense than the 40°C record-breaking heatwave of 2022 and therefore has a smaller change in probability. The associated impacts this time are also expected to be less severe due to the lower peak temperatures.
• These trends are strongest in southeast England, but heat extremes are increasing across all of the UK and Ireland as the world warms.   
• High temperatures can have a secondary effect on wildfire risk. Hotter temperatures result in an increase in evaporative demand, drying out fine leaf litter and grasses. The drier vegetation conditions exacerbates the risk of wildfires, particularly in areas of continuous vegetation such as moorlands.
• High temperatures, an ageing population, poorly designed buildings, and the urban heat island effect together increase the risk of heat-related illness and death. 
• The heat-health warning system, Heatwave Plan for England, regulations in England that require new homes to consider the risk of overheating, and related efforts indicate progress towards addressing these risks. However, there is room for improvement to address the scale and urgency created by the increasing frequency and intensity of these events. Due to climate change, including by filling gaps in policies and reducing fragmentation. 

Analysis of trends in extremes

In this short observation-only analysis we examine trends in heat extremes like the one occurring in southeast England in June 2025.

Climate Change Observed extreme heat 

The unusually high temperatures observed in the southeast of England in June 2025 are studied by analysing changes in the highest temperatures for the month of June each year. As the event occurs at the start of the summer season, and temperatures are likely to increase further as a result. The study is focused on the June high temperature data rather than summer data as a whole.

In the UK, meteorologists define a heatwave as three or more consecutive days with temperatures above a set threshold. Each county sets its own threshold, usually between 25°C and 28°C, based on local climate. The highest threshold, 28oC, covers a contiguous region of southeast England, including the counties Cambridgeshire, Bedfordshire, Buckinghamshire, Hertfordshire, Greater London, Surrey and Berkshire (figure 1).

In this study, we analyse the likelihood of crossing the threshold of 28C for three consecutive days averaged across all of these counties. Three observational and reanalysis datasets are used: ERA5, EOBS, and HadUK. The methods used to analyse heat trends follow the standard WWA protocol. By using non-stationary extreme value theory, as described in Philip et al. 2020.

At the time of writing, which occurs while the high temperatures are still ongoing, only one observation-based dataset includes the event, and this is as a forecast. We therefore first use this dataset to estimate the return period of the extreme heat over the study region. The forecast intensity of the heatwave is 28.7oC, which is approximately a 1 in 7 year event.

The likelihood of a 28oC event in the present climate is approximately 1 in 5 years (in June). Given that the event is a forecast at the time of analysis (and thus the magnitude is not yet set), and that it is very close to the heatwave threshold over the region. We consider changes in the 1 in 5 year event to correspond as closely as possible to this heatwave definition. Finally, using the two other datasets, we test changes in the probability ratio and magnitude of a 5-year return period event. And combine the results into a final statement. The results of this analysis are shown in table 1. 

Table 1: Return period and magnitudes of the 3-day heat event as observed in ERA5. And magnitudes of the 1 in 5 year maximum temperature events in EOBS. And HadUK-Grid datasets in 2024 in the region over southeast England shown in figure 1. The probability ratio and change in intensity associated with a 1.3oC increase in global mean surface temperature (GMST). Are estimated for each dataset, with bootstrapped uncertainties. Finally, the synthesised results for all datasets are shown in the final row. Statistically significant (at the 95% level) results are highlighted in bold text.

Dataset	3-day event		Trend
	Magnitude (oC)	Return period	Probability ratio(95% C.I.)	Change in intensity (oC) (95% C.I.)
ERA5	28.72	7.38(4.17 – 23.88)	17.65(3.32 – inf)	4.00(1.95 – 5.79)
	28	5.05(2.99 – 12.47)	–	–
EOBS	29.31	5	8.93(3.40 – 1318)	3.73(2.03 – 5.29)
HadUK-Grid	29.23	5	4.73(1.31 – 27.45)	2.70(0.32 – 4.58)
Synthesis			9.06(1.44 – 565)	3.48(1.01 – 5.68)

All datasets show strong and statistically significant trends in June heatwaves as the world has warmed. Across the different datasets, the likelihood of crossing the heatwave threshold in June has increased by a factor between approximately 5 and 18. Equivalently, the intensity of the event has increased by between 2.7oC and 4oC. By synthesising these results, we estimate that the probability of a June heatwave in this region has increased by a factor of 9 (1.4 – 560). Or similarly the 1 in 5 year June heat event has become 3.5oC (1 – 5.7 oC) hotter as the world has warmed (table 1, fig. 3). 

The hottest day in the forecast is expected to reach approximately 32oC across the region. This 1-day event is rarer than the 3-day event described above, expected in June only once every 23 years. Using the same method and considering 1 in 25 year 1-day heat events in June. We find that such events have become about 94 (3.4 – 1000000) times as likely. Occurring in June approximately once every 2500 years before industrialisation (table S.1). The change in magnitude of such events is also slightly higher, at 4.1oC (1.5 – 6.6oC). This shows that the chance of experiencing the most intense heatwaves is increasing most rapidly. 

To test the sensitivity of these results to the event definition. We compare the trends in June with trends for the wider summer season, local-scale results, and findings from other recent studies.

Summer season, local-scale results

First, the results are similar, if slightly stronger, when considering heatwaves over all summer months (June to August, JJA). For example, one-in-five-year three-day heatwaves during JJA have become about 37 times more likely (range: 8–1,900) and about 4.7°C (2.9–6.5°C) more intense. Meanwhile, one-in-25-year single-day heatwaves during JJA have become about 85 times more likely (range: 4–312,000) and about 5.3°C (3.2–7.1°C) hotter. The relative time series for June alone and for JJA are shown in Figure S.1.

Similar results appear at the local scale. Figure 4 shows the change in three-day heatwave events per unit of global warming in three gridded datasets. It also includes data from three local weather stations in the region. All stations show an increase in intensity of 2.5–4°C per degree of warming. This slightly exceeds the larger-scale average.

In 2022, the UK hit 40°C for the first time on record. World Weather Attribution did a similar analysis then (Zachariah et al., 2022) using observations and climate models. This study covered a wider region across most of southern England and included all summer heatwaves, not only June.

The results match those here. Observed temperatures show about a 4°C rise with warming. Combined with climate models, the more conservative estimate is a 2°C increase. Even this smaller rise would push the June 2025 event above the official heatwave threshold. What might have been warm, sunny weather now becomes dangerous heat.

In summary, extreme temperatures in June in southeast England have increased due to global warming. This rise means an event classed as a heatwave today would not have crossed the heatwave threshold in the past. The current event is about 2–4°C warmer due to human-driven climate change. While a full attribution needs trends in both climate models and observations, the results in Table 1 align with past studies for the region (Zachariah et al., 2022). The findings are robust across different time frames and scales.

Vulnerability and Exposure

As the IPCC defines it, climate risk comes from the link between hazard, exposure, and the vulnerability of people, systems, and assets. Research on heat and mortality in the UK shows a clear rise in all-cause death rates once temperatures pass certain thresholds (Arbuthnott & Hajat, 2017).

Extreme heat earlier in summer often causes more deaths. This happens due to low acclimatisation, less adaptive behaviour, and harvesting effects (Gasparrini et al., 2016). In the UK, older people face the greatest risk of heat-related health impacts and death. This risk supports the UK Health Security Agency’s yellow heat-health alert issued on 11 June. The alert warned of increased pressure on health services and higher risk for people over 65 (Arbuthnott & Hajat, 2017; UKHSA, 2025).

A national survey before the 2025 heatwave found that nearly half of UK residents already endure uncomfortably hot indoor conditions during extreme heat. Yet only 15% believe future heat will likely harm their health (Jennings et al., 2025). This perception gap is worrying. Evidence shows that older adults, people with chronic illnesses or disabilities, and low-income groups face the highest risk of serious health issues during heatwaves (Åström et al., 2025; Bouchama et al., 2017; Kapur et al., 2024).

Past studies show vulnerabilities also depend on social and environmental factors like isolation, poor housing, access to cooling, and strong local social networks (Klinenberg, 2025; Chakraborty et al., 2019; Schwarz et al., 2021; Colluci et al., 2021). If high-risk groups do not see their own risk, chances for early action and adaptation are lost.

Urgent and growing public health issue

Heat-related deaths in the UK are an urgent and growing public health issue. The 2003 European heatwave caused 2,091 deaths in England alone (Johnson et al., 2005). This event pushed the UK to create national early warning systems and the Heatwave Plan for England in 2004. The plan was updated in 2012 to coordinate response across health and social care (PIRU, 2019).

These plans mark progress in recognising the health risks of heat and using evidence to reduce them. The 2022 heatwave set a new record when temperatures rose above 40°C for the first time. An estimated 2,985 heat-related deaths were recorded in England (CCC, 2025). The 2025 event is unlikely to match that record. Yet even short heat events and yellow Heat-Health Alerts still cause significant heat-related deaths, as seen in 2023 (UK heat mortality monitoring report, 2023).

Despite this progress, recent reviews show that current plans and health systems do not fully protect people most at risk. These include older adults, those with chronic illnesses, and people in poorly ventilated housing (Brimicombe et al., 2021; Bouchama et al., 2017). Without more adaptation, annual heat deaths could rise above 10,000 by 2050. This increase would come from rising temperatures and an ageing, more vulnerable population (CCC, 2025).

Research also shows that most UK homes are designed to hold heat in. This design now puts people at risk in hotter summers (Kapur et al., 2024). In cities like London, the Urban Heat Island effect makes overheating worse. This is especially true in dense inner boroughs with little green space (Goddard & Tett, 2019; Chakraborty et al., 2019).

An estimated 90% of UK homes may need structural upgrades to stay safe as temperatures rise (Kapur et al., 2024). This is critical for care homes and hospitals. These places house more vulnerable people who are more likely to face high indoor temperatures.

Official reviews for the 2025 heat event are still in progress. However, the Climate Change Committee (CCC, 2025) has already highlighted gaps in the UK’s heat adaptation progress. It warns that heat policies are fragmented and lack long-term goals. It also notes that the health system does not track how extreme heat affects care delivery and outcomes (CCC, 2025).

Even with these gaps, lessons from 2022 show a strong base for improvement. A post-event report found that clear leadership, fast decisions, a focus on vulnerable groups, good forecasting, early warnings, and the national heat alert system all worked well in 2022 (Howarth et al., 2024). These lessons offer the UK a foundation to strengthen its heatwave planning and prepare for more dangerous heat in the future.

Supplementary information

Dataset	1-day event		Trend
	Magnitude (oC)	Return period	Probability ratio(95% C.I.)	Change in intensity (oC) (95% C.I.)
ERA5	32.31	22.75(9.27 – inf)	13500(13.69 – 10^13)	4.82(2.66 – 6.87)
EOBS	33.63	25	91.35(5.57 – 400000)	4.07(2.11 – 6.17)
HadUK-Grid	33.67	25	45.13(3.07 – 143000)	3.31(1.24 – 5.40)
Synthesis	–	25	94.30(3.39 – 10^6)	4.07(1.53 – 6.61)

Table S.1: Return period and magnitudes of the 1-day event as observed in ERA5, and magnitudes of the 1 in 25 year maximum temperature events in EOBS and HadUK-Grid datasets in 2024 in the region over southeast England shown in figure 1. The probability ratio and change in intensity associated with a 1.3oC increase in global mean surface temperature (GMST) are estimated for each dataset, with bootstrapped uncertainties. Finally, the synthesised results for all datasets are shown in the final row. Statistically significant (at the 95% level) results are highlighted in bold text.

Authors

Ben Clarke, Centre for Environmental Policy, Imperial College, London, UK 

Claire Bergin, ICARUS Climate Research Centre, Maynooth University, Ireland 

Clair Barnes, Centre for Environmental Policy, Imperial College, London, UK 

Maja Vahlberg, Red Cross Red Crescent Climate Centre, The Hague, the Netherlands; Swedish Red Cross, Stockholm, Sweden (based in Ubmeje/Umeå, Sweden) 

Roop Singh, Red Cross Red Crescent Climate Centre, The Hague, the Netherlands (based in New Jersey, USA) 

Friederike Otto, Centre for Environmental Policy, Imperial College, London, UK 

Review authors

Izidine Pinto, Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands

Laura Scott, British Red Cross, London, UK

Henry Barnes, British Red Cross, London, UK

Julie Arrighi, Red Cross Red Crescent Climate Centre, The Hague, the Netherlands; Global Disaster Preparedness Center, American Red Cross, Washington D.C., USA; University of Twente, Enschede, the Netherlands (based in New York, USA)