It’s not your imagination. Not only are extremely hot temperatures occurring more frequently across the globe, but those heat waves are getting more severe.
Back in the 1950s, temperatures on any given summer day were just as likely to be near average as they were to be unseasonably high or low. Climatologist James Hansen of NASA’s Goddard Institute for Space Studies in New York City likens that scenario to rolling a die with two sides each corresponding to low, average and above-normal temperature.
Since the 1980s, that metaphorical die has increasingly become weighted toward delivering a warm day, Hansen and his coworkers report August 6 in the Proceedings of the National Academy of Sciences. In fact, Hansen says, since 2000 it’s as though on any roll almost 4.5 sides will draw hotter than average summer heat.
Hansen says the study also suggests that a new level of extreme heat is emerging “that almost never occurred 50 years ago.” Formerly striking about 0.2 percent of the Northern Hemisphere in any given summer, this degree of anomalous warmth now strikes about 10 percent of the land area. Within a decade, his data suggest, these hot spells could reach 16.7 percent of the hemisphere's summer weather.
“We’re not showing that this is a consequence of an increase in carbon dioxide and other greenhouse gases,” Hansen says. In fact, his team’s analysis makes no attempt to attribute the underlying source of warming to any particular cause. He does volunteer, however, that there is “a strong consensus within the scientific community that the warming we’re seeing is primarily a response to an increase in greenhouse gases.”
Markus Donat and Lisa Alexander of the University of New South Wales in Sydney reported similar conclusions online July 31 in Geophysical Research Letters. Their group integrated daily temperature changes at monitoring stations across the globe to identify a shift toward warmer temperatures globally and year-round — with more days of extreme heat.