Overview
- Climate Central released an analysis on Wednesday, June 24, 2026, showing that days with daily maximum wet-bulb temperatures at or above 25°C have roughly doubled globally since the 1970s from about 10 to 23 days per year.
- The Climate Central study used ERA5 reanalysis to cover weather from 1970 through 2025 and estimated that about 64% of these dangerous humid heat days are attributable to human-caused climate change.
- Controlled experiments at Penn State reported on the same day found that young, healthy adults begin to lose the ability to keep core body temperature stable at a wet-bulb near 31°C, lowering the previously cited 35°C threshold.
- Indian cities show especially large increases in dangerous humid heat days, with Delhi rising from 96 to 135 days, Mumbai from 136 to 206, Chennai from 205 to 257, and Tirunelveli from 119 to 273 between the 1970s and 2016–25.
- Higher wet-bulb means sweat cannot cool the body, which increases heat illness risk for the elderly, children, pregnant people and anyone without reliable cooling; planners should watch power and healthcare strain and prioritize heat shelters, resilient power and low-cost cooling options that work at higher humidity.