SCIENTISTS say four billion people — about half the world’s population — experienced at least one extra month of extreme heat from May 2024 to May 2025 because of human-caused climate change.

The extreme heat caused illness, death, crop losses and strained energy and healthcare systems, according to the analysis from World Weather Attribution, Climate Central and the Red Cross.

“Although floods and cyclones often dominate headlines, heat is arguably the deadliest extreme event,” the report said. 

Many heat-related deaths are unreported or are mislabelled by other conditions like heart disease or kidney failure.

The study shows how much climate change boosted temperatures in an extreme heat event and calculated how much more likely its occurrence was because of climate change. 

Caribbean islands were among the hardest hit by additional extreme heat days. Puerto Rico, a territory of the United States, endured 161 days of extreme heat. 

Charlotte Gossett Navarro, chief director for Puerto Rico at Hispanic Federation, a non-profit focused on social and environmental issues in Latino communities, said: “Even something as simple as trying to have a day outdoors with family, we weren’t able to do it because the heat was too high.” 

Low-income communities and vulnerable populations, such as older adults and people with medical conditions, suffer the most from extreme heat.

The high temperatures recorded in the extreme heat events that occurred in central Asia in March, South Sudan in February and in the Mediterranean last July would have not been possible without climate change, according to the report. 

At least 21 people died in Morocco after temperatures hit 48°C (118°F) last July. 

Roop Singh, head of urban and attribution at the Red Cross Red Crescent Climate Centre, said: “We need to quickly scale our responses to heat through better early warning systems, heat action plans, and long-term planning for heat in urban areas to meet the rising challenge.”