Man-made climate change gave half the world an extra month of extreme heat: Study

A new global study finds that human-caused climate change added over 30 extra days of extreme heat for nearly half the world’s population last year, raising urgent calls to cut fossil fuel emissions and improve heat safety.

Nearly half the world’s population experienced over a month of extra extreme heat last year because of human-driven climate change, according to a new international study.

The research, conducted by scientists from World Weather Attribution, Climate Central, and the Red Cross Red Crescent Climate Centre, found that around 4 billion people, or 49% of the global population, endured at least 30 more extremely hot days between May 2024 and May 2025 than they would have in a world without manmade global warming.

Measuring the heat: What the study shows

Researchers defined “extreme heat days” as those hotter than 90% of all temperatures recorded at a location between 1991 and 2020. They used peer-reviewed climate models to compare real-world data with a simulated world unaffected by human activity.

The result: 67 extreme heat events globally in the past year, all clearly linked to climate change.

The Caribbean island of Aruba was the most severely affected, suffering 187 extreme heat days, 45 more than would have occurred without global warming.

Global records and a warming planet

The findings follow a year of record-breaking temperatures:

• 2024 was officially the hottest year ever recorded, surpassing the previous record set in 2023.
• January 2025 became the hottest January in history.
• Over the past five years, global temperatures have risen by 1.3°C above pre-industrial levels.

In 2024 alone, the global temperature briefly exceeded the 1.5°C mark, the critical limit set by the Paris Agreement to prevent catastrophic climate impacts.

Heat’s silent toll and lack of data

The report warns that developing countries are especially vulnerable, not only to heat but also to a lack of data.

For instance, Europe recorded 61,000 heat-related deaths in 2022, but many countries in Africa, Asia, and Latin America lack the systems to track similar data. Heat-related deaths in these areas are often misreported as heart attacks or respiratory failures, making the true toll invisible.

What needs to change: Adaptation and prevention

The scientists are urging urgent global action, including:

• Early warning systems to alert people ahead of heatwaves
• Public education on the risks of heat exhaustion and stroke
• Heat action plans designed specifically for cities
• Building improvements, such as better ventilation and shading
• Behavior changes, like staying indoors during peak temperatures
• But the authors also emphasized that adaptation measures will not be enough.

“With every barrel of oil burned, every tonne of carbon dioxide released, and every fraction of a degree of warming, heatwaves will affect more people,” said Dr. Friederike Otto, a climate scientist at Imperial College London and one of the report’s co-authors.

The only way to stop the worsening pattern, the study concluded, is to rapidly cut fossil fuel use and switch to clean energy.

Heat Action Day: Raising awareness

The report was published ahead of Heat Action Day on June 2, an annual campaign to raise awareness of heat-related health risks, especially heat stroke and exhaustion.

This year’s theme highlights how climate change is turning heat into a global health crisis and why the world can no longer afford to ignore it.