Heatwaves were the deadliest climate-related disasters of 2025, surpassing floods, storms and wildfires in terms of fatalities, according to a new scientific assessment by World Weather Attribution (WWA). The analysis found that extreme heat had a disproportionate impact on poor and marginalised communities, where poverty, limited access to cooling and weak infrastructure sharply increased vulnerability.

The report comes after a year marked by widespread climate extremes across continents. Although 2025 did not exceed the global temperature record set in 2024, it ranked among the hottest years ever recorded. For the first time, the three-year global average temperature crossed the 1.5 degrees Celsius threshold above pre-industrial levels, a milestone scientists describe as a warning sign that societies are approaching dangerous limits to adaptation.

WWA’s annual review identified 157 extreme weather events in 2025 that met humanitarian impact criteria, meaning they caused significant loss of life, displacement or economic damage. Floods and heatwaves were the most frequent events, with 49 occurrences each, followed by storms, wildfires and droughts. Of the 22 events analysed in depth, 17 were found to have been made more intense or more likely by human-induced climate change.

Among all extreme events, heatwaves stood out as the most lethal. In Europe, a single prolonged summer heatwave between June and August was estimated to have caused more than 24,000 deaths across 854 cities, covering nearly 30 per cent of the continent’s population. In many regions of the Global South, however, comparable mortality data remains unavailable, meaning the true global toll of heat-related deaths is likely far higher than reported figures suggest.

The report showed that continued greenhouse gas emissions played a central role in turning what might otherwise have been a relatively cooler year into one characterised by extreme heat. Rising baseline temperatures fuelled prolonged heatwaves, droughts, storms and floods across multiple regions, reinforcing the cascading nature of climate impacts.

Since the adoption of the Paris Agreement in 2015, global temperatures have increased by around 0.3°C. According to WWA’s analysis, this warming has added an average of 11 additional extremely hot days per year worldwide. Revisiting major heatwaves from the past decade in regions including the Amazon basin, Burkina Faso and Mali, researchers found that some events are now up to ten times more likely than they were a decade ago.

In 2025 alone, climate change intensified heatwaves in diverse regions such as South Sudan, Burkina Faso, Norway, Mexico, Argentina and England. In South Sudan, for example, a February heatwave was made around 4°C hotter by human-induced climate change. What was once considered an exceptionally rare event has now become something that could occur every other year, dramatically increasing health risks for the population.

The impacts of extreme heat were closely intertwined with poverty and inequality. In South Sudan’s capital, Juba, dozens of children collapsed from heatstroke during the heatwave, prompting nationwide school closures for two weeks. Many residents were unable to follow public health advice to stay cool and hydrated, as housing often consists of iron-roofed structures that trap heat, while access to electricity, cooling and clean water remains limited. Around one-third of Juba’s population lacks reliable water access, and only about one per cent of the city provides green space or shade for people without cooling at home.

WWA’s assessment also highlighted how heat impacts fall unevenly across society. Women were found to face heightened risks because they are overrepresented in informal, outdoor and heat-exposed work such as agriculture and street vending, while also shouldering most unpaid care responsibilities, including fetching water and cooking in extreme temperatures. This sustained exposure increases the likelihood of long-term health problems, including cardiovascular and kidney disease.

While heatwaves were the deadliest events of the year, rising global temperatures also intensified storms and floods that killed thousands and displaced millions. In Asia, a series of powerful storms caused more than 1,700 deaths and billions of dollars in damage, with climate change found to have significantly increased rainfall intensity. In the Caribbean, Hurricane Melissa devastated Jamaica and neighbouring islands, with warming shown to have amplified rainfall associated with the storm.

Additional analysis by Climate Central indicated that all Atlantic hurricanes during the 2025 season were made at least 14 kilometres per hour stronger by climate change. Although this represents an average increase of around 10 per cent in wind intensity, research shows that even small increases can translate into much larger rises in damage.

The report emphasised that measures such as early warning systems, heat-resilient schools, urban cooling and access to shade can substantially reduce harm from extreme heat. Low-cost interventions, including passive cooling, green spaces and adjusted school and work schedules, were highlighted as especially effective for protecting children and vulnerable groups.

However, the findings also underscored that adaptation has clear limits. Even with preparedness measures in place, many low-income regions and small island states experienced severe losses in 2025 as climate extremes intensified. While global climate agreements have reduced projected warming compared to worst-case scenarios, current policy trajectories still point toward a dangerously hot future, with heat-related risks becoming increasingly frequent and severe.