Researchers from NASA’s Jet Propulsion Laboratory analyzed decades of weather station data, uncovering intolerable heat-humidity combinations appearing on Earth far sooner than anticipated—decades ahead of projections.
Humans have long adapted to extreme hot and cold environments, but physiological limits exist, particularly when temperature and humidity combine to overwhelm the body. A landmark study published in Science Advances on May 8, 2020, signals the resurgence of conditions unseen on Earth for millions of years due to climate change.
Led by Colin Raymond at NASA’s Jet Propulsion Laboratory, the research examined data from weather stations worldwide, zeroing in on wet-bulb temperature (TW)—a metric that integrates heat and humidity to gauge human survivability.
Findings show a doubling of extreme events between 27°C TW and 35°C TW from 1979 to 2017. Notably, thresholds above 35°C TW were breached briefly in Jacobabad, Pakistan, and Ras al-Khaimah, United Arab Emirates—episodes that raise grave alarms despite their brevity. For context, Europe’s deadly 2003 heatwave, which claimed 70,000 lives, peaked at just 28°C TW, but it was a dry event.
At 35°C TW, survival becomes nearly impossible. Human skin maintains a surface temperature around 35°C, so matching air conditions halt passive heat exchange. Sweating remains the sole cooling mechanism, but high humidity saturates the air, impairing evaporation and causing the body to overheat fatally.
Climate experts have scrutinized these humid heat extremes for just over a decade, yet the peril was clear from the start. Prior models, assuming high greenhouse gas emissions, forecasted initial 35°C TW exceedances around 2050. This study upends that timeline.
The authors highlight this rapid onset as a profound challenge for humanity. Vulnerable hotspots—subtropical regions near warm oceans prone to continental heatwaves—include swaths of the Middle East and South Asia. With Pakistan and the UAE already crossing the line, India and nearby areas may follow imminently.
