Humidity is the hidden factor that often makes heat dangerous and it has increased sharply in the recent decades, says a new global report.
Since 2000, extreme heat has claimed more than a quarter-million lives globally. Dangerous hot and humid heat days have more than doubled globally since the 1970s, principally due to climate change, says a new report from the global environment watch organisation Climate Central.
The Global South is most affected, with a map prepared as part of the report showing the majority of Indian territory affected by the condition.
Wet-bulb test
The report, Global Analysis: Dangerous Humid Heat Rising Due to Climate Change, explores the increasing risks across regions of the world. “Map layer data comes from the Global Administrative Unit Layers (2024) from the Food and Agriculture Organization of the United Nations,” Climate Central states.
“Dangerous humid heat days have a daily maximum wet-bulb temperature of 25° Celsius or higher,” says a Climate Central release. The wet-bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water alone. Such humidity and heat make many people vulnerable to heat-related illness.
South Asia hotspot
South Asia is a hotspot. Almost the entire region is affected by hot and humid days.
The Climate Central report segregates climate change-induced temperatures from temperatures affected by other factors. Much of north India and central India are vulnerable to climate-change triggered humid heat. In eastern India, including West Bengal, the north-east states, Bihar and Uttar Pradesh, dangerous humid heat would have occurred anyway, without the impact of climate change.
On June 26, according to the map, in Kolkata, the capital of West Bengal, the forecast is of dangerous humid heat. The daily high wet-bulb temperature is forecast to be 27.4° Celsius.
The humid heat in Bangladesh and Pakistan are not seen to be impacted so much by climate change.
Global high points
This report analysts 254 countries, territories, and dependencies, 50 US states and selected areas of Australia, Brazil, China and India. “Parts of tropical South America, coastal West Africa, and Southeast Asia now experience at least six months’ worth of dangerous humid heat days on average each year,” says Climate Central.
Some of the drier parts of the planet, including the Arabian Peninsula, central Australia and the southwestern US areas now experience dangerous humid heat.
“Dangerous humid heat days have more than doubled globally, from an average of 10 days per year in the 1970s to 23 days per year today,” says Climate Central. “The largest increases have occurred in tropical humid regions, where wet-bulb temperatures are typically higher and sit closer to the dangerous threshold,” it adds.
Doubled figures
According to the report, dangerous humid heat days have more than doubled globally, from 10 days per year in the 1970s to 23 days per year during the period 2016-2025. Climate change is responsible for almost two-thirds (64%) of global dangerous humid heat days since 1970.
“In 2025, the global average was 23 dangerous humid heat days — 19 (83%) of which were added by climate change,” says the report.
“Climate change is causing more dangerous humid heat days in 65% (161) of 247 US cities analysed — an average of 19 more days per year during the past decade,” the report says. The FIFA World Cup is being held in several venues in the US; the heat is expected to affect players’ performance.
Sweat mechanism collapse
When the air becomes more humid, it affects the primary mechanism the body uses to protect itself against high heat: sweating. As sweat evaporates into the air, it relieves the body of heat. But if the air is humidity-saturated, sweat does not evaporate, and leaves dangerous levels of heat trapped inside the body.
“As heat builds up in the body, the heart works harder and the risk of dehydration and heat-related illness (including heat exhaustion and heat stroke) increases,” points out the Climate Central analysis. “Heat stroke is a medical emergency that can lead to organ damage or even death without rapid treatment,” it says.