Executive summary
Extreme heat affected billions of people worldwide between May 2024 and May 2025, making heat one of the most widespread and damaging climate hazards today. World Weather Attribution’s Heat Action Day Report shows that human-caused climate change dramatically increased the frequency, intensity and duration of extreme heat across all inhabited continents. The findings emphasise that extreme heat is no longer an episodic risk but a persistent and escalating global threat with serious implications for health, productivity and infrastructure.
What happened
Over the 12 months leading up to Heat Action Day 2025, large regions of the world experienced prolonged and repeated periods of extreme heat. Heatwaves affected parts of Africa, South and Southeast Asia, the Middle East, Europe, the Americas and Australia, often breaking local temperature records and occurring outside traditional peak summer periods.
These heat events led to excess mortality, increased hospital admissions, reduced labour productivity, crop losses and strain on electricity systems. In many locations, heat impacts accumulated over time, with limited recovery between events, increasing stress on people and systems.
What the attribution analysis found
World Weather Attribution finds that climate change was the primary driver of the observed escalation in extreme heat. Rising global temperatures shifted the entire temperature distribution, making extreme heat events far more likely and more intense than in a pre-industrial climate.
The analysis shows that many of the heat events observed during the reporting period would have been extremely unlikely, or virtually impossible, without human-induced warming. Even relatively small increases in global average temperature have resulted in large increases in the number of dangerously hot days experienced each year.
Because temperature responds directly to greenhouse gas emissions, confidence in the attribution findings for extreme heat is very high.
Unequal impacts and compounding vulnerability
The report highlights that heat impacts are not evenly distributed. Low-income communities, outdoor workers, older people and those with limited access to cooling face disproportionately high risks. In many regions, social and economic constraints limited the ability to adapt, turning heat into a major public health emergency.
Urban heat island effects, inadequate housing and unreliable electricity further amplified impacts, particularly in rapidly growing cities.
How climate attribution fits into heat risk reporting
Climate attribution provides a direct link between emissions, warming and heat impacts, making extreme heat one of the clearest examples of present-day climate risk. Attribution evidence supports a shift away from viewing heat as an exceptional hazard towards treating it as a baseline planning condition.
This has important implications for health systems, labour regulations, energy planning and urban design.
Why this matters for organisations
For employers, governments, insurers and infrastructure operators, extreme heat is already a material operational and financial risk. Heat affects workforce safety, service delivery, asset performance and demand for energy and water, with impacts occurring across supply chains and regions simultaneously.
How to use this in your own risk work
Organisations should integrate climate-adjusted heat metrics into risk assessments, update heat-health and occupational safety plans, and stress-test operations against prolonged and repeated heat events. Investing in cooling, urban greening, resilient energy systems and social protection measures will be essential to managing escalating heat risk.
Source
World Weather Attribution (2025). Heat Action Day Report: Climate change and the escalation of global extreme heat.
https://www.worldweatherattribution.org/heat-action-day-report-climate-change-and-the-escalation-of-global-extreme-heat/