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Heatwaves were the deadliest climate disasters of 2025, killing more people than floods, storms or wildfires, a new scientific report finds
Poor and marginalised communities were hit hardest, as extreme heat interacted with poverty, lack of cooling and weak infrastructure
For the first time, the three-year global temperature average crossed 1.5°C, signalling dangerous limits to adaptation, scientists warn
While climate change fuelled a year of devastating extreme weather across the world in 2025, heatwaves — often described as “silent killers” — were the deadliest climate-related disasters, disproportionately affecting poor and marginalised communities, according to a new annual report by World Weather Attribution (WWA).
Although 2025 did not surpass the global temperature record set in 2024, it will rank among the hottest years ever recorded, exposing how continued fossil fuel use is pushing societies closer to the limits of adaptation. Crucially, for the first time in history, the three-year global average temperature crossed the 1.5 degrees Celsius (°C) threshold.
Scientists said this milestone marked a dangerous shift, as even years influenced by cooler natural climate cycles are now delivering extreme heat with devastating consequences. “Being the second or third hottest year on record is not good news,” the report noted.
WWA identified 157 extreme weather events in 2025 that met its humanitarian impact criteria. Floods and heatwaves were the most frequent, with 49 events each, followed by storms, wildfires and droughts. Of the 22 events studied in depth, 17 were found to have been made more severe or more likely by climate change.
In addition, six major heatwaves from previous years were revisited to assess how risks have evolved since the Paris Agreement was adopted.
Among all extremes, heatwaves stood out as the most lethal. In Europe alone, one study estimated that 24,400 people died during a single summer heatwave between June and August, across 854 cities representing nearly 30 per cent of the continent’s population.
In many parts of the Global South, however, comparable mortality data does not exist, the report noted, masking the full scale of heat-related deaths.
Continued greenhouse gas emissions meant that what might otherwise have been a relatively cooler year instead became an extremely warm one, fuelling prolonged heatwaves, droughts, storms and floods across the world.
The analysis showed that human-induced climate change has made extreme heat both more intense and far more frequent since the Paris Agreement was signed in 2015. Global temperatures have risen by about 0.3°C over that period, adding an average of 11 additional extremely hot days each year worldwide.
Revisiting past heat events in regions such as the Amazon, Burkina Faso and Mali, researchers found some heatwaves were now almost ten times more likely than a decade ago.
In regions as varied as South Sudan, Burkina Faso, Norway, Mexico, Argentina and England, WWA’s analysis showed that climate change intensified heatwaves in 2025.
In South Sudan, for example, human-induced climate change made a February heatwave around 4°C hotter and transformed what was once an exceptionally rare event into one expected to occur every other year.
The heatwave also underscored how extreme heat interacts with poverty and inequality to magnify harm. After dozens of children collapsed from heatstroke in the capital, Juba, schools were shut nationwide for two weeks, and residents were advised to stay indoors and keep hydrated.
“Both present a huge challenge for many across the country, as houses are often built with iron roofs, and lack cooling, electricity and access to clean water,” the report said.
In Juba, around a third of the population does not have access to water, and only 1 per cent of the city offers green space and shade for people without cooling at home.
WWA’s analysis found that impacts fell disproportionately on women, who are overwhelmingly employed in informal, heat-exposed work such as agriculture and street vending, while also carrying the bulk of unpaid care work, including fetching water and cooking in extreme temperatures. This sustained exposure, combined with limited resources and low literacy levels, increases the risk of long-term health impacts ranging from heat exhaustion to cardiovascular and kidney damage.
Rising global temperatures also intensified storms and floods that resulted in thousands of deaths and displaced millions of people in 2025. WWA found that climate change increased both the likelihood and severity of extreme rainfall in many of the storms and floods analysed.
In Asia, a series of powerful storms killed more than 1,700 people and caused billions of dollars in damage, with scientists concluding that human-induced warming made the associated rainfall significantly heavier.
In the Caribbean, Hurricane Melissa devastated Jamaica and neighbouring islands, with WWA’s analysis showing that climate change intensified rainfall linked to the storm.
Separate analysis by Climate Central showed that climate change made all Atlantic hurricanes during the 2025 season at least 9 metres per hour (about 14 kilometres per hour) stronger. Although this represents roughly a 10 per cent increase in intensity, the report noted that such changes can translate into much larger increases in damage — as much as 44 per cent, according to research on Hurricane Helene in 2024.
The report stressed that reducing vulnerability through measures such as heat-resilient schools, early-warning systems and urban cooling can save lives. Low-cost interventions — including shade, passive cooling and adjusted school schedules — could significantly reduce heat-related harm, particularly for children.
However, 2025 also showed that adaptation alone cannot keep pace with escalating extremes. Climate change is already “pushing millions close to the limits of adaptation”, the report warned, especially in small island states and low-income regions, where even relatively high levels of preparedness cannot prevent severe losses as storms intensify in a warming world.
While the Paris Agreement has helped lower projected warming from around 4 degrees Celsius to about 2.6 degrees Celsius, if current policies were fully implemented, WWA cautioned that this would still result in a dangerously hot world.
“Each year, the risks of climate change become less hypothetical and more brutal reality,” said Friederike Otto, climate scientist at Imperial College London and co-founder of World Weather Attribution.