Source: AGU Advances
Heat waves are becoming commonplace, and so too is high humidity, which can strain the electrical grid, hurt the economy, and endanger human health. But the global prevalence of record-breaking humidity events, some of which approach the physiological limit of what humans can safely handle—and all of which go beyond local expectations and adaptations—has not been widely studied.
To remedy that oversight, Raymond et al. used data from the European Centre for Medium-Range Weather Forecasts Reanalysis 5 (ERA5) and several other sources to establish the most intense humid heat that has occurred in recent years across the globe. They then used several climate models to estimate where instances of even more severe humid heat are most likely to occur in the future.
Relative to the local climate, humid heat can be most extreme in the Middle East and North Africa, with tropical regions coming in a close second, the researchers found. In these locales, the wet-bulb temperature (a measure of humid heat) is capable of reaching 4–5 standard deviations above the average for the warm season. The Middle East and North Africa are also among the regions that experience the longest stretches of humid heat, sometimes lasting 20 or more days.
Estimates of overall humid heat likelihood are very sensitive to a few extremely hot, humid days, the researchers found. In many locations, removing a single outlier led statistical models to predict fivefold fewer hot, humid days in the future. The finding highlights the need for accurate observational data, the researchers write.
Humid heat is particularly dangerous when it comes in spurts, offering areas little relief for concentrated periods. In the tropics, three quarters of the days when the wet-bulb temperature was in the top 5% occurred in only a quarter of the years included in the study. This is likely largely because El Niño heightens both atmospheric temperature and moisture levels, so record-setting days in the tropics tend to cluster in years when this weather pattern is active.
The researchers note that 2023 was a banner year for humid heat, with 23 different regions setting records. That’s entirely because of climate change, the researchers’ work suggests: Otherwise, no records would have been broken. (AGU Advances, https://doi.org/10.1029/2025AV001963, 2025)
—Saima May Sidik (@saimamay.bsky.social), Science Writer
