Climate change continues to drive up Earth’s temperatures. A new report from Climate Central reveals dangerous humid heat days have more than doubled globally since the 1970s, increasing from an average of 10 days per year to 23.
The analysis defines wet-bulb temperatures of 25° C (77° F) or higher as “dangerous” humid heat conditions, under which many people face an elevated risk of heat-related illness. Researchers analyzed global conditions from 1970 through 2025 to determine how often dangerous humid heat occurred and the extent to which human-caused climate change contributed to those conditions.
The analysis shows that human-caused climate change is now the primary driver of dangerous humid heat, contributing to nearly two-thirds of all dangerous humid heat days worldwide and increasing the risk of heat-related illness and death for hundreds of millions of people.
The report also reveals that extreme heat has claimed more than a quarter-million lives globally since 2000, making it one of the deadliest weather-related hazards. While high temperatures pose risks to everyone, older adults, children, pregnant people, individuals with underlying health conditions and people without access to cooling face disproportionately greater dangers.
The report states that humidity makes even seemingly mild days far more dangerous than they appear. Climate change is causing more dangerous humid heat days in 69 percent (665) of the 961 global cities analyzed, adding an average of 46 more days per year during the past decade. In 2025, the global average reached 23 dangerous humid heat days, with 19 of those days (83 percent) attributed to climate change.
Climate change is causing more dangerous humid heat days in 65 percent (161) of the 247 U.S. cities analyzed, adding an average of 19 such days per year during the past decade. Humidity plays a critical role in determining how dangerous heat is. High humidity reduces the body’s ability to cool itself through sweating, causing heat to build up internally and increasing the risk of dehydration, cardiovascular and respiratory problems, heat exhaustion, heat stroke and other serious health impacts.
