Temperature records reveal inexorable warming and increasing episodes of extreme heat

By Janet Raloff

Web edition: August 5, 2012
Print edition: September 8, 2012; Vol.182 #5 (p. 10)

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.

The Sydney researchers found that increases in daily minimum temperatures (which tend to occur at night) rose most strongly. Compared with the period from 1951 to 1980, daily minimum temperatures rose in the three decades ending in 2010 by 0.8 degrees Celsius (1.4 degrees Fahrenheit); the daily maximum increased in that second period by only 0.6  degrees Celsius.

What’s more, the new analysis shows, the temperature threshold exceeded on the hottest 5 percent of days in the first 30-year period was crossed 7 percent of the time in the more recent 30-year period, an increase of 40 percent.

John M. Wallace of the University of Washington in Seattle says he is not surprised to see this shift toward more extremely warm days — and hotter extremes within those periods. “Just as a rising tide lifts all ships,” he says, there is good reason to believe that growing global warming should elevate warming extremes. Computer projections of Earth’s changing climate call for such a pattern. “If a trend in that direction is detectable already,” Wallace says, “that would constitute an important finding.” But to be convinced, he’d like to see a longer track of temperatures broken down by region and covering both land and water (not just land temperature, as here).

The focus by both papers on temperature changes in recent decades — as opposed to all types of weather extremes, including floods, storms, droughts and more — constitutes “picking a relatively ‘easy target,” says Thomas Knutson of the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. Still, he says, the new data are interesting and “underscore the substantial changes already underway in terms of surface temperatures and their extremes.”