Published ahead of Heat Action Day on 2 June, the report by World Weather Attribution, Climate Central, and the Red Cross Climate Centre presents some of the clearest evidence yet of the human cost of global warming. Researchers say in 195 countries and territories, climate change at least doubled the number of days with extreme heat over the past 12 months.

"This study is another stark warning: climate change is here—and it kills," said Dr Friederike Otto of Imperial College London.

Once-Rare Heatwaves Now Routine

To quantify the impact, researchers compared observed temperatures between May 2024 and May 2025 with a climate model simulating a world without human-induced warming. The contrast was dramatic.

Take Aruba, for example. The island endured 187 extreme heat days over the past year. Without climate change, that number would have been just 45.

Across the globe, 67 heat events were identified as being unusually severe. Scientists closely examined four of them—in Central Asia, South Sudan, Mexico and Central America, and the Mediterranean—and in each case, found climate change made the event either more likely or more intense.

"These frequent, intense hot spells are linked to a wide range of impacts—from illness and death to lower crop yields, blackouts and pressure on healthcare," said Dr Mariam Zachariah, a researcher at Imperial College London.

Europe: A Grim Forecast

Europe, long viewed as relatively insulated from climate extremes, is already facing some of the most deadly consequences.

In 2024, heatwaves scorched southern Europe, wildfires spread uncontrollably, and more than 47,000 heat-related deaths were recorded. Greece faced early-summer hospital strain and mass evacuations, with the Acropolis closed during peak heat hours. Spain, France, and Italy saw transport chaos, early school closures and grid strain.

In southeastern Europe, July brought the longest heatwave ever recorded—lasting 13 days and affecting over half the region.

Without decisive climate action, projections show up to 23 million additional heat-related deaths in Europe by 2099.

A Deadly, Often Invisible Threat

Experts warn that extreme heat remains one of the most underestimated dangers of the climate crisis. It accelerates ageing, damages cells, and silently overwhelms health systems. While floods and wildfires command headlines, heat is often more deadly—but less visible.

In 2024 alone, the world saw 41 extra days of dangerous heat directly linked to climate change.

And for children, the future is even more stark. One study found that 83% of today's five-year-olds will experience far more intense and frequent heatwaves than any prior generation.

"There's no corner of the Earth untouched by climate change," said Dr Kristina Dahl of Climate Central. "And heat is its most deadly consequence."