Extreme Heat Is Arriving Outside the Season People Are Ready For

The stretch of the year that brings a region its worst heat has grown longer across about half of the world's land over the past 45 years, and in most places it has grown at one end rather than both, researchers at NASA's Goddard Institute for Space Studies report.

The authors write that heat arriving unusually early or unusually late can do outsized harm, because people are less used to intense heat at those times of year and may not be ready to cool down. They say the results point to a need for heat alert systems that cover a longer part of the year.

The analysis appeared Sept. 10 in AGU Advances. Its first author is Catherine C. Ivanovich, a NASA Postdoctoral Program fellow at the institute; the senior author is Sonali McDermid, who works at the institute and at New York University. They set each location's baseline heat season as the three months that carried the largest share of its extreme heat events between 1980 and 1989.

Measured against that baseline, the extreme dry heat season has expanded across 50% of global land area, and the humid heat season, which combines heat with high moisture, across 48%. The paper calls both expansions statistically significant.

In the western United States, eastern China, northern Africa and eastern Europe, the share of extreme heat events falling after the historical heat season rose more than the share falling before it. In western Europe, southern Africa and northwestern India, the bigger increase came ahead of the season instead.

The baseline heat season does not line up with the usual summer months in many regions, and it accounts for most observed extreme heat days even in the tropics, where temperatures vary little through the year. The changes at the season's edges are generally not explained by existing seasonal patterns simply growing stronger as annual average temperatures rise.

Ivanovich and her coauthors call for further study of places where lengthening heat seasons increasingly overlap with wildfire and hurricane seasons.