The Event 

Attribution studies continue to show that human induced climate change is making heatwaves hotter and deadlier. In many regions, the influence of human induced climate change is so large that temperatures recorded during heatwaves would not be possible without warming caused by the burning of fossil fuels. This includes parts of the US, the Sahel, West Africa, the Philippines and other countries in East Asia  as well as the Mediterranean. We assess how rare the extreme July 2024 heat in the Mediterranean and examine its impacts, focusing on observational data instead of conducting a detailed attribution study, which would likely produce similar results to previous studies. The results are contextualised with findings from our previous attribution studies in the region and global analyses.

Key Messages 

• Heatwaves are the deadliest type of extreme weather, with hundreds of thousands of people dying from heat-related causes each year. The July heatwave caused at least 21 deaths in Morocco after temperatures reached 48°C. However, it is likely there were dozens or hundreds of other heat-related deaths in the countries affected that have not been reported and the full impact of a heatwave is rarely known until months afterwards, once death certificates are collected, or scientists can analyse excess deaths. Many places lack good record-keeping of heat-related deaths, therefore global mortality figures are a significant underestimate.
• In line with past climate projections and IPCC reports, extreme heat events like July 2024 in the Mediterranean are no longer rare events. Similar heatwaves affecting Greece, Italy, Spain, Portugal and Morocco  are now  expected to occur on average about once every 10 years in today’s climate that has been warmed by 1.3°C due to human-induced climate change. 
• Based on the data set ERA5, the extreme temperatures reached in July would have been virtually impossible if humans had not warmed the planet by burning fossil fuels. In addition, the 1 in 10 year extreme July heat would have been 3°C [2.5 – 3.3°C] cooler in a world without climate change. 
• These results are based on observational data and do not include climate models. However, the results are very similar to the studies published  in 2023 that analysed heatwaves in the same region and included climate models. For both the April and e July heatwaves in 2023, we found the events would have been virtually impossible without climate change, but are not rare today. We also found the heatwaves were made 1.7-3.5 ºC hotter when compared to a pre-industrial world. 
• The results for the 2023 events synthesised observations and climate models. However, the models are known to systematically underestimate extreme heat in Europe (van Oldenborgh et al., 2022, Vautard et al., 2023, Schumacher et al., 2024)  meaning the real world changes due to climate change are probably closer to the changes  shown in observations. 
• Furthermore, the previous studies use slightly different temporal and spatial definitions of heat, but the numerical results remain very similar. Given the 2024 event is very similar to the observed changes found in studies published 2023, they are a good indicator of how climate change is affecting extreme heat in the Mediterranean.
• Unless the world rapidly stops burning fossil fuels, these events will become hotter, more frequent and longer-lasting. 
• Studies show that elite Olympic athletes who are exposed to high temperatures and are not acclimated to them may see impacts such as a decline in performance, and increase in heat-related illness, such as heat cramps and exhaustion, having implications for the Paris Olympics that are currently ongoing (de Korte et al., 2021, Griggs et al., 2019). Measures to reduce exposure, ensure adequate hydration and cooling, acclimatisation, and emergency plans can help keep athletes safe during periods of extreme heat.  
• Heat action plans that reduce heat-related deaths are increasingly being implemented across the region, which is encouraging. However, there remains an urgent need for an accelerated roll-out of heat action plans in light of increasing vulnerability driven by the intersecting trends of climate change, population ageing, and urbanisation. Cities are hot-spots for heat risk, so urban planning needs to focus on measures to  reduce the urban heat island effect, such as increasing cooling green and blue spaces. 

Extreme and persistent heat has been overwhelming south western parts of the US, Mexico, and the northern countries of Central America. The area has been lying under a large and lingering region of high pressure, known as a heat dome, whereby hot air is trapped close to the ground and further heated under blue skies and sunshine. Whilst heat domes have a well known mechanism for intensifying heatwaves, these past weeks have seen records broken in both daytime and nighttime temperatures in several countries, including Mexico, Guatemala, Honduras and in the south western US.

Heatwaves are among the deadliest types of extreme events. Even though often the death toll is typically underreported especially during or straight after the event, during this hot season Mexico has already reported 125 deaths (SwissInfo). The coinciding ongoing drought is enhancing impacts of the heat even more.

Scientists from Mexico, Panama, the Netherlands, the United Kingdom, the United States, and Sweden collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme heat in a region that included the US southwest, Mexico, Guatemala, Belize, Honduras and El Salvador. Using peer-reviewed methods, we analyse 5-day maximum daytime and nighttime temperatures in May and June over a large region (see Figure 1) encompassing the region where impacts associated with extreme temperature records were reported.

Main findings 

• The extreme heat in the north and central America has resulted in severe impacts, including more than 125 heat-related deaths in Mexico since March, thousands of cases of heat stroke, and power outages. We likely do not know the full picture of heat-related deaths, since they are usually only confirmed and reported months after the event, if at all. 
• Existing drought conditions have further aggravated the situation by preventing the dispersion of polluting particles, decreasing water availability, and reducing hydropower generation and electricity supply. 
• Observations show that 5-day maximum temperatures in May-June such as recorded this year are expected to occur about every 15 years in today’s climate that has been warmed by 1.2C. However, around the year 2000, when global temperatures were half a degree lower than now, such events were expected to occur only about once every 60 years.
• The night time temperatures over the same 5-day period were also high, but not extreme in today’s climate; there is now a 50% chance per year of similar temperatures occurring. At the turn of the millennium such events would only have been expected to occur with a 13% chance in any given year.
• These return times are estimated for the region as a whole. It is important to highlight that the heat was more rare in the southeastern part of the region, especially for the nighttime temperatures with return periods of up to 1000 years in individual locations. 
• To determine the role of climate change we combine observations with climate models and we conclude that human-induced warming from burning fossil fuels  made the 5-day maximum temperature event about 1.4 degrees hotter and about 35 times more likely. For nighttime temperatures this is about 1.6 degrees hotter and about 200 times more likely.
• These trends will continue with future warming and events like the one observed in 2024 will be very common in a 2C world. 
• Extreme heat warning systems and action plans can help fill important gaps in preparedness across Central America. Heat safety protection laws can be enacted and implemented to protect outdoor workers across all countries. 
• Strengthened grid resilience and water conservation strategies are critical to ensure reliable services during heat events. Improved urban planning, more green spaces, and enhanced infrastructure in informal settlements will also help protect the most vulnerable.

Over the 12-month period, 6.3 billion people (about 78% of the global population) experienced at least 31 days of extreme heat (hotter than 90% of temperatures observed in their local area over the 1991-2020 period) that was made at least two times more likely due to human-caused climate change.

Over the last 12 months, human-caused climate change added an average of 26 days of extreme heat (on average, across all places in the world) than there would have been without a warmed planet.

Heatwaves are arguably the deadliest type of extreme weather event and while the death toll is often underreported, hundreds of deaths have been reported already in most of the affected countries, including Palestine, Bangladesh, India, Thailand, Myanmar, Cambodia and the Philippines. The heat also had a large impact on agriculture, causing crop damage and reduced yields, as well as on education, with holidays having to be extended and schools closed in several countries, affecting millions of students. 

Extreme heat in South Asia during the pre-monsoon season is becoming more frequent. Two previous World Weather Attribution studies focused on extreme heat events in the region: the 2022 India and Pakistan heatwave and the 2023 humid heatwave that hit India, Bangladesh, Lao PDR and Thailand. Despite differences in the nature and impact of the events (drier heat in 2022 leading to widespread loss of harvest, and humid heat in 2023 with greater impacts on  people), both studies found that human-induced climate change influenced the events, making them around 30 times more likely and much hotter.

Scientists from Lebanon, Sweden, Malaysia, the Netherlands, the United States  and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme heat in three Asian regions: 1) West Asia, including Syria, Lebanon, Israel, Palestine and Jordan; 2) the Philippines in East Asia; and 3) South and Southeast Asia, including India, Bangladesh, Myanmar, Lao PDR, Vietnam, Thailand and Cambodia. 

Using published peer-reviewed methods, the scientists analysed how human-induced climate change altered the likelihood and intensity of the 3-day April heatwave in West Asia and a 15-day April heatwave in the Philippines. Figure 1 shows these two regions, outlined in blue, while figure 2 shows the South Asia region. For this region, the analysis focused on observed weather data, but not climate models, as the affected region largely overlaps with the study areas of the previous studies. The observational data for the whole month of April confirmed that the role of climate change is likely of similar magnitude to the heatwaves studied in 2022 and 2023, and the results of a full attribution analysis would not be significantly different. 

Main Findings

• • The heatwave exacerbated already precarious conditions faced by internally displaced people, migrants and those in refugee camps and conflict zones across West Asia. In Gaza, extreme heat worsened the living conditions of 1.7 million displaced people. The heatwave added pressure to the many challenges already faced by people in refugee camps and conflict zones, such as water shortages, difficulties to access medicines and poor living conditions for the large population that lives in makeshift tents that trap heat. With limited institutional support and options to adapt, the heat increases health risks and hardship. 
  • The extreme heat has forced thousands of schools to close down in South and Southeast Asia. These regions have previously also incurred school lockdowns during COVID-19, increasing the education gap faced by children from low-income families, enhancing the risk of dropouts, and negatively impacts the development of human capital.
  • Heat impacts certain groups like construction workers, transport drivers, farmers, fishermen etc. disproportionately. It both impacts their livelihoods and causes a reduction in income, and results in personal health risks.
  • In the current climate, warmed by 1.2°C since pre-industrial times due to human activities, this kind of extreme heat event is not very rare. In West Asia, the chance of it occurring in any given year is around 10% – or once every 10 years. In the Philippines, the chance of such an event happening in any given year is also around 10% – or once every 10 years under the current El Niño Southern Oscillation (ENSO) conditions, and a 1-in-20 year event, overall, without the influence of El Niño. In the larger South Asia region, an extremely warm April such as this one is a somewhat rarer event, with a 3% probability of happening in a given year – or once every 30 years.
  • To estimate the influence that human-caused climate change has had on extreme heat in West Asia and the Philippines, we combine climate models with observations. Observations and models both show a strong increase in likelihood and intensity. In the Philippines, the change in likelihood is so large that the event would have been impossible without human-caused climate change. In West Asia, climate change increased the probability of the event by about a factor of 5.
  • In terms of intensity, we estimate that a heatwave such as this one in West Asia is today about 1.7°C warmer than it would have been without the burning of fossil fuels. In the Philippines the intensity increase due to human-induced climate change is about 1.2°C.
  • We also look at the role of the ENSO. In the Philippines, we find that the current El Niño made the heatwave about 0.2°C hotter. In West Asia, on the other hand, we do not find any influence of ENSO in the event, which is consistent with previous research.  
  • If the world warms to 2°C above pre-industrial global mean temperatures, in both regions the likelihood of the extreme heat would increase further, by a factor of 2 in West Asia and 5 over the Philippines, while the temperatures will become another 1°C hotter in West Asia and 0.7°C hotter in the Philippines.
  • In South Asia, a region that we have studied twice in the last two years, our analysis was simpler and based only on observations. Similarly to what we found in previous studies, we observe a strong climate change signal in the 2024 April mean temperature. We find  that these extreme temperatures are now about 45 times more likely and 0.85ºC hotter. These results align with our previous studies, where we found that climate change made the extreme heat about 30 times more likely and 1ºC hotter. 
  • Existing heatwave action plans and strategies are challenged by rapidly growing cities, increase in informal settlements and exposed populations, reduction in green spaces and rise in energy demands. While many cities have been implementing solutions like cool roofs, nature based infrastructure design, and adherence to climate risk informed building codes, there is limited focus on retrofitting and upgrading of existing buildings and settlements, with infrastructure deficits (e.g. asbestos roofs), to make them more liveable.
  • Some countries such as India have comprehensive heat action plans in place, yet to protect some of the most vulnerable people, these must be expanded with mandatory regulations, such as workplace interventions for all workers to address heat stress, such as scheduled rest breaks, fixed work hours, and rest-shade-rehydrate programs (RSH) are necessary, but yet to become part of worker protection guidelines in the affected regions.
  • The recurrent heat events and associated impacts every year in these regions in the past few years have enabled heatwaves to be recognised as a serious hazard of concern in most countries, with proper guidelines and action plans in place. At the same time, cross-sectoral collaborative strategies that focus on providing immediate relief during the hot days are needed. 

Heatwaves are arguably the deadliest type of extreme weather event and while the death toll is often underreported and not known until months after the event, a surge in hospital admissions and deaths were reported from the Gabriel Touré hospital in Bamako, Mali between 1-4 April (Bahati, 2024).

The hospital recorded 102 deaths over the four-day period, which is significantly more than expected – in April 2023, the hospital recorded 130 deaths over the entire month (JolibaFM, 2024). While statistics for the cause of death have not been reported, around half were over the age of 60, and the hospital reports that heat likely played a role in many of the deaths. Furthermore, up to 44 bodies were buried in one cemetery in Bamako on Friday 5 April after the weekly service (DW 2024).

Scientists from Mali, Burkina Faso, Mozambique, the Netherlands, Sweden, the United States and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme heat across the Sahel. Our analysis focuses in detail on the countries that had the highest temperatures, especially also over night and the large reported death toll: The southern part of Mali and Burkina Faso (fig. 1a). Here we look at the 5-day maximum and minimum temperatures over the March to May season and in April respectively. In addition we analyse the 5-day maximum temperatures in March to May over a larger
region (fig. 1b; 10 to 17°N, 16W to 20°E) encompassing the most affected parts of Senegal, Guinea, Mali, Burkina Faso, Niger, Nigeria and Chad.

Main Findings

● While Burkina Faso and Mali are both countries with populations that are acclimated to high temperatures, the length and severity of this heatwave made it difficult for people to cope, as evidenced by the reported increased hospitalisations and deaths. Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-related causes each year
and many more suffering other severe health and livelihood consequences. Many places lack good record keeping of heat-related deaths, therefore currently available figures are likely an underestimate.
● This heatwave coincided with Ramadan (fasting) and power cuts, which compounded the risk for vulnerable groups and even those not traditionally considered vulnerable. Even minimum temperatures, overnight, remained relatively high, making it so that people did not get a break from the heat. The power cuts further made it difficult for those who did have access to
mechanical cooling to use it, thus reducing their coping capacity.
● Even in today’s climate, that has warmed by 1.2°C since pre-industrial times due to human activities, the extreme heat observed over the Mali/Burkina Faso region is still rare. While the daily temperatures were extreme, with a return time of about 100 years, the 5-day maximum temperatures were particularly unusual with a return time of 200 years. Minimum temperatures were less extreme but still rare with a return time of 20 years over Mali/Burkina Faso. For the Sahel region the return time of the 5-day maximum temperatures are 30 years.
● To estimate the influence that human-caused climate change has had on the extreme heat since the climate was 1.2°C cooler, we combine climate models with observations. Observations and models both show that heatwaves with the magnitude observed in March and April 2024 in the region would have been impossible to occur without the global warming of 1.2°C to date.
● Extreme 5-day maximum heat as rare as the observed event over Mali/Burkina Faso would have been 1.5 °C cooler and 1.4 °C cooler over the larger Sahel region if humans had not warmed the planet by burning fossil fuels. For minimum temperatures over Mali/Burkina Faso the change in intensity is even larger: in a 1.2°C cooler climate the nighttime temperatures would have been 2°C cooler.

● These trends will continue with future warming. Over Mali/Burkina Faso a heatwave like the observed event would be another 1°C hotter in a 0.8°C warmer world (2°C global warming since pre-industrial times). An event of the same magnitude as observed in 2024 would then not be very rare anymore but occur 10-times more frequent than in today’s climate.
● We also assessed whether the current El Niño event had an influence on the extreme temperatures and found that while there is some contribution it is small compared to human-induced climate change, explaining about 0.2°C of the observed 5-day heat event.
● Rapid urbanization and loss of green spaces in cities such as Bamako and Ouagadougou have increased the urban heat island effect. Coupled with high vulnerability, this highlights the need for sustainable urban planning that integrates green spaces, and building designs that account for high temperatures.
● Critical infrastructure such as electricity, water, and healthcare systems needs to be strengthened to adapt to the increasing frequency and intensity of extreme heat, requiring increased investment to ensure reliable access and service delivery.

Humid heatwaves are known to be particularly dangerous. While meteorological organisations in Ghana and Nigeria issued warnings, few heat-related impacts were reported by the media and government organisations across the Guinea zone. In February, West Africa was hit by an unusually intense early season heatwave, with temperatures not normally seen until March or April. The most severe heat occurred from February 11-15 with temperatures above 40°C. In Nigeria, doctors reported an increase in patients presenting for heat-related illness, people complained of poor sleep due to hot nights and the national meteorological agency issued several warnings about the heat. In Ghana, the national meteorological agency also warned people to prepare for dangerous temperatures. The heat occurred during the finals of the Africa Cup of Nations (AFCON) football tournament in Côte d’Ivoire. Due to the hot and humid conditions, additional ‘cooling breaks’ were taken during the matches so players could rehydrate.

Scientists from Nigeria, Burkina Faso, Switzerland, Sweden, South Africa, The Netherlands, Germany, the UK and the US collaborated to assess whether and to what extent human-induced climate change has modified the likelihood and intensity of this February humid heatwave. The team used published peer-reviewed methods to analyse the event. To account for the humidity, which increases the impacts of heat on the human body, the team analysed the Heat Index, which accounts for both daily maximum temperatures and relative humidity. The event was defined as the annual (Jul-Jun) maximum 5-day averaged Heat Index in a region near the southern coast of West Africa (see Figure 1).

● Very limited impact data are available across the studied area. It does not mean there were no impacts but suggests limited awareness about heat risks. To reduce heat-related morbidity and mortality in southern West Africa, there is an urgent need for improved monitoring and research on the impacts and risks associated with heat waves.
● Rapid, unplanned urbanization with about half of the urban residents on average living in informal housing renders a considerable portion of the region’s population highly exposed and vulnerable to extreme heat. Widespread energy deficiency and limited access to water, sanitation, and hygiene (WaSH) and healthcare services further aggravate heat-related health risks as individuals are left with very limited options for individual coping strategies, such as air conditioning.
● In January and February, the heat affected the African Cup of Nations football games in Côte d’Ivoire. Two-minute cooling breaks were proactively introduced at the 30th and 75th minutes of many matches, with provisions for additional breaks during the sessions so players could rehydrate. The study did not identify evidence of mandatory cooling breaks for outdoor workers across the region who are highly vulnerable to heatwaves
● In southern West Africa, the 5-day humid heat maximum usually occurs in March or April. The humid heat measured using the heat index was record high in February in terms of the annual average (usually occurring in March/April) but even more when considering it happened as early as in February.
● The datasets based on observations characterise the area’s average 5-day humid heat as a 1-in-10 year event in today’s climate.
● To estimate the influence of human-caused climate change on this excessive humid heat we use a combination of climate models and observations. We find that because of human-induced climate change, the area-averaged heat index is in today’s world about 4°C higher in today’s 1.2°C warmer climate. Also, such humid heat has become much more likely, it is at least 10 times more likely in today’s world.
● At global mean temperatures of 2°C above pre-industrial levels, humid heat such as observed this year is projected to be about another 1.2°C to 3.4°C warmer and about another factor of 3 to 10 times more likely, meaning similar events will occur about once every two years.
● Despite limited data and research, in recent years, there is an increase in awareness of heat-related risks by national weather services (e.g. Nigeria) and city authorities (e.g. Freetown, Sierra Leone). Further improvements and investments are needed, such as extending heat warnings to hot days outside of the typically hot season. This is especially urgent as the planet continues to heat, causing prolonged and hotter heat seasons.
● However, across the countries analysed, many do not appear to have carried out planning for heat extremes. Major investment is needed in Africa to build resilience to dangerous heat. The UN has estimated that the cost of adaptation for developing countries is between US$215-$387 billion per year this decade. However, rich countries haven’t yet met the promises they have made to help developing countries become more resilient to the growing risks of climate change. In addition, these commitments fall drastically short of the finance required – in 2021, the global community spent just US$21 billion to help developing countries adapt to climate
change

Scientists from Madagascar, South Africa, Denmark, the Netherlands and the UK collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of hot Octobers in Madagascar and the warmest 7-day maximum and minimum temperatures in Antananarivo during the month (fig 1), using published peer-reviewed methods. Drawing from studies in similar contexts and other regions in the world they also assessed what the impacts on humans and ecosystems in the region likely are.

Main findings

● Heatwaves in all regions of sub-saharan Africa are dramatically underreported leading to little awareness about the dangers of extreme heat. Heat-related mortality is estimated to increase by a factor of four by 2080, unless required investments to adaptation are made.
● South Madagascar is particularly vulnerable to impacts of heatwaves, as food and agricultural systems are likely to collapse under high temperatures and compounding drought conditions and frequent, highly destructive cyclones. Extremely dry air during heatwaves, including the nights, results in difficulty to breathe, and children are reported to be the worst affected with an overall high heat-related mortality.
● Using gridded observational products the very warm October is approximately a 1 in 100 year event in today’s climate, while the extreme warm 7-day maximum and minimum temperatures are less rare, estimated to be a 1 in 25 and 1 in 20 year event respectively.
● To estimate the influence of human-caused climate change on this extreme heat we use a combination of climate models and the observations. We find that because of human-induced climate change the event would have been approx. 1 to 2 °C cooler for all three event definitions had humans not warmed the planet by burning fossil fuels.
● Due to the strong trend the change in likelihood is very large: it has increased by at least 100 times for October mean and 7-day minimum temperatures and by at least 10 times for the 7-day maximum temperatures.
● Unless the world rapidly stops burning fossil fuels, these events will become more common in the future. In a world 2°C warmer than preindustrial, events like these would no longer be rare but occur up to 3 times per decade.
● In Madagascar, less than half of the population has access to electricity and clean water, making most common coping strategies in extreme heat inaccessible to a large part of the population. Linked to the lack of clean water is one of the lowest child survival rates in the world. With very young children being particularly vulnerable to extreme heat these young lives are even more endangered.
● A high degree of informal settlements and unplanned urbanisation have resulted in large parts of the population particularly vulnerable to heat exposure. Factors like urban poverty, large workforce in informal economies, and loss of productivity/income during hot days result in compounding vulnerabilities.
● There are no heat action plans, early actions protocols, or comprehensive early warning systems, indicating an overall limited preparedness for heat waves. Investments in extreme heat forecasting, warning, and response capabilities are the most urgent requirements for Madagascar to better adapt to a warming world.

Using published peer-reviewed methods, scientists from Brazil, Argentina, the Netherlands, United States of America and the United Kingdom, collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of 10-day maximum temperatures during the months August- September in the region most affected by the extreme heat (fig 1). 

Main findings

• While the full impacts of heatwaves remain unknown until months after the events, 4 casualties and many heat-related illnesses have been reported. Early spring extreme heat events often prove to be particularly impactful as local populations are not yet acclimatised to high temperatures. In addition, high population density, low vegetative cover and water-based spaces, high levels of air pollution, and inequality are additional risk drivers for mortality and morbidity within cities, rendering extreme heat particularly deadly for the urban poor. 
• Using gridded observational products and observations the heat event as defined above is approximately a 1 in 30 year event in today’s climate. 
• While there is a high-level of confidence in the gridded products used to carry out the analysis, these do not capture very local records, many of which were broken during this heat episode. To incorporate these in future studies and inform early warning systems high  quality and readily available weather station data is needed. 
• To estimate the influence of human-caused climate change on this extreme heat we combine climate models with the observations.We find that because of human-induced climate change the event would have been 1.4 to 4.3 °C cooler had humans not warmed the planet by burning fossil fuels. Due to the strong trend the change in likelihood is not well defined: it has increased by at least 100 times. 
• With future global warming, heat events like this will become even more common and hotter. At global mean temperatures of 2°C above pre-industrial levels, a heat event like this would be about another 5 times more likely and 1.1 to 1.6 °C hotter than today. 
• Although ENSO may have influenced the large-scale weather patterns, the direct contribution to the extreme heat is small, compared to the climate change signal. 
• While some losses will inevitably occur due to the extreme heat,  in particular with respect to ecosystems  it is misleading to assume that human impacts are inevitable. Adaptation to extreme heat can be effective at reducing morbidity and mortality. The authors were not able to identify any heat action plans that exist in the affected area, this leaves the potential for a window of opportunity to mitigate heat impacts on vulnerable people. Heat Action Plans that include early warning and early action, awareness raising and behaviour changing messaging, and supportive public services can reduce morbidity and mortality. 

Several heat deaths have been confirmed in the US, including migrants on the US Mexican border. In Mexico alone over 200 people died due to the heat. Spain, Italy, Greece, Cyprus, Algeria, and China also reported heat deaths, as well as a large increase in hospitalisation due to heat related illnesses. Large parts of the population in Italy and Spain and over 100 million people in Southern US are under heat alerts. In all three regions, demand for power spiked and negatively impacted a number of important crops, including olive oil in Spain and cotton in China. 

Scientists from the World Weather Attribution initiative collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme July heat in these three regions.

Using published peer-reviewed methods, we analysed how human-induced climate change altered the likelihood and intensity of 1) 18-day average maximum temperatures over the most affected regions in western US, Texas and northern Mexico (fig 1, top). 2) 7-day average maximum temperatures over land in the rectangular box(5W-25E, 36-45N) covering the most affected region (fig1, middle). 3) 14-day average maximum temperatures over the lowlands of China, again covering the most affected region (fig 1, bottom). 

Main findings

• Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-related causes each year. However, the full impact of a heatwave is rarely known until weeks or months afterwards, once death certificates are collected, or scientists can analyse excess deaths. Many places lack good record-keeping of heat-related deaths, therefore currently available global mortality figures are likely an underestimate.
• In line with what has been expected from past climate projections and IPCC reports these events are not rare anymore today. North America, Europe and China have experienced heatwaves increasingly frequently over the last years as a result of warming caused by human activities, hence the current heat waves are not rare in today’s climate with an event like the currently expected approximately once every 15 years in the US/Mexico region, once every 10 years in Southern Europe, and once in 5 years for China. 
• Without human induced climate change these heat events would however have been extremely rare. In China it would have been about a 1 in 250 year event while maximum heat like in July 2023 would have been virtually impossible to occur in the US/Mexico region and Southern Europe if humans had not warmed the planet by burning fossil fuels. 
• In all the regions a heatwave of the same likelihood as the one observed today would have been significantly cooler in a world without climate change. Similar to previous studies we found that the heatwaves defined above are 2.5°C warmer in Southern Europe, 2°C warmer in North America and about 1°C in China in today’s climate than they would have been if it was not for human-induced climate change. 
• Unless the world rapidly stops burning fossil fuels, these events will become even more common and the world will experience heatwaves that are even hotter and longer-lasting. A heatwave like the recent ones would occur every 2-5 years in a world that is 2°C warmer than the preindustrial climate.
• Heat action plans are increasingly being implemented across all three regions and there is evidence that they lead to reduced heat-related mortality. Furthermore, cities that have urban planning for extreme heat tend to be cooler and reduce the urban heat island effect. There is an urgent need for an accelerated roll-out of heat action plans in light of increasing vulnerability driven by the intersecting trends of climate change, population ageing, and urbanisation. 

These extreme temperatures, combined with humidity, caused a sudden increase in heat stroke cases, roads melting and a strong surge in electricity demand in all four countries. 13 casualties and about 50-60 hospitalisations due to heat stroke were reported in Navi Mumbai, Maharashtra on 16th of April alone, while other sources mention 650 hospitalisations. Casualties have also been reported in Thailand. The true cost to human lives will only be known months after the event. In India, in the states of West Bengal, Tripura and Odisha, schools closed three weeks earlier than planned due to the heat. In addition, a large number of forest fires occurred during the same time in India, Thailand and Lao PDR.

Scientists from India, Thailand, France, Australia, Denmark, Germany, Kenya, the Netherlands, the US and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the extreme heat in these four countries where temperature records were broken at several locations and harm to lives, livelihoods, and well-being were reported, also due to the accompanying humid conditions. 

Using published peer-reviewed methods and available datasets, scientists analysed how human-induced climate change altered the likelihood and intensity of the 4-day April heatwave event measured as a heat index that integrates temperature and humidity. Humidity is an important factor in how high temperatures affect the human body, as sweating, the way for humans to cool themselves, becomes less effective at high humidity. Thus, heat is more dangerous in humid conditions. The heat index (reported in °C) takes both temperature and humidity into account. Due to the high humidity conditions during the heatwave, heat index values are higher than the actual temperatures (reported in the first paragraph), as shown in Figure 1. Due to the heterogeneity in climate and landforms over this large area, the team separated the analysis into two smaller homogeneous regions as follows: (1) South and central parts of India and the whole of Bangladesh, excluding the dry, semi-arid region that runs parallel to the Western Ghats where humidity is low in the pre-monsoon season. (2) Thailand and Lao PDR together.

Main findings

• Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-related causes each year and many more suffering other severe health and livelihood consequences. However, the full impact of a heatwave is often not known until weeks or months later, once death certificates are collected, or scientists can analyse excess deaths. Many places lack good record keeping of heat-related deaths, therefore currently available global mortality figures are likely an underestimate.
• While people in the affected regions are used to hot and humid temperatures, those who are more physiologically susceptible to heat (e.g. due to pre-existing conditions, age, disability etc.) and/or are more exposed due to their occupation (e.g. outdoor workers, farmers) are at highest risk of heat-related health impacts. Such exposure and vulnerability are intensified by societal disadvantage based on factors such as socio-economic status, religion, caste, gender, migration, and living conditions. On top of this, factors such as air pollution, the urban heat island effect, and wildfires further compound health impacts, particularly among the most vulnerable populations. 
• In the current climate, which has warmed by 1.2°C since pre-industrial times due to human activities, the humid heat event (defined using the heat index) is not very unusual over India and Bangladesh, but is estimated to be rare in Thailand and Lao PDR. 
• The estimated heat index values exceeded the threshold considered as “dangerous” (41°C) over the large parts of the South Asian regions studied. In a few areas,  it neared the range of “extremely dangerous” values (above 54°C) under which the body temperature is difficult to be maintained.
• There is only one data set available for the entire region to calculate the heat index (ERA5). This dataset is known to underestimate extreme temperature trends over India, so the estimates for the rarity and severity of the event are more uncertain than when several datasets can be compared. 
• ​​To estimate the influence that human-caused climate change has had on extreme heat since the climate was 1.2°C cooler, we combine climate models with observations. Observations and models both show a strong increase in likelihood and intensity of April humid heat events similar to that of 2023. 
• The combined results give an increase in the likelihood of such an event to occur of at least a factor of 30 over India and Bangladesh due to human-induced climate change. At the same time, a heatwave with a chance of occurrence of 20% (1 in 5 years) in any given year over India and Bangladesh is now about 2°C hotter in heat index than it would be in a climate not warmed by human activities.
• Over Thailand and Lao PDR, a humid heatwave with a 0.5% chance of occuring in any given year (1 in 200 years) is now 2.3°C hotter in heat index. An event of the same magnitude as the observed heatwave would have been extremely rare in a 1.2°C cooler climate and hence it would have been virtually impossible to have occurred without climate change. 
• These trends will continue with further warming. They are stronger for the rarer event over Thailand and Lao PDR where a heatwave like the recent event would be about 10 times more likely in a 0.8°C warmer world (2°C global warming since pre-industrial times). In India and Bangladesh, the likelihood of this April’s event reoccuring would increase by about a factor of 3 between today and reaching 2°C global warming, meaning that this humid heat event could be expected every 1-2 years. 
• There are a range of solutions to heat-related harms from the individual to the regional level. They are currently implemented as a patchwork, to various degrees, across the countries studied, with India having the most advanced heatwave planning. Solutions, such as self-protective action, early warning systems for heat, passive and active cooling, urban planning, and Heat Action Plans can be effective at reducing fatalities and other negative impacts. In fact, heat-related fatalities have decreased in regions where heat action plans have been in place, e.g. in the city of Ahmedabad and the region of Odisha in India. However, these solutions are often out of reach for the most vulnerable people, highlighting the need to improve vulnerability assessments and design interventions that account for group-specific needs. 
• High social vulnerability among various segments of society, in combination with the strong increase in extreme heat in the region, is likely to exacerbate the impacts of extreme heat events on those already experiencing substantial disadvantages in their daily lives and hence requires comprehensive adaptation and development interventions.