News•February 23, 2016

Searing Heat Waves Could Become Annual Threat

The scorching, deadly heat waves that today strike only about once every 20 years could become an annual occurrence for more than half the world if nothing is done to curb emissions of heat-trapping greenhouse gases, a new study reported Tuesday.

The work, detailed in the journal Climatic Change, also points to the worst heat waves of the future being much more intense. The results jibe with other research looking at how heat waves might change as the world warms, as well as those that have found that global warming has already juiced the heat waves we see today.

People cool off from the heat at the Miroir d'Eau (Mirror of Water), a public art piece on the quay of the Garonne River, during a warm summer day in Bordeaux, southwestern France, June 30, 2015.
Credit: REUTERS/Regis Duvignau

But if action is taken to significantly curtail emissions, it could make a significant difference in just how bad future heat waves get.

“Mitigation is crucial,” study author Claudia Tebaldi, a senior research scientist at the National Center for Atmospheric Research (NCAR), said in an email. “We have a lot to gain from limiting greenhouse gas emissions, and [those] benefits will be felt fairly soon” if we do so.

Extreme heat is one of the defining features of global warming. Climate models have long suggested that heat waves would become more intense and more frequent as Earth’s average temperature continues to rise. It has already risen nearly 2°F (1°C) since the late 19th century and tipped the odds toward record hot temperatures over record cold ones.

Tebaldi and her co-author Michael Wehner of the Lawrence Berkeley National Laboratory wanted to see how curtailing the amount of carbon dioxide and other greenhouse gases might affect the severity of future heat waves. They used an NCAR climate model to look at how the odds of today’s 20-year events — or those that have about a 5 percent chance of occurring in any given year — would shift in the future under scenarios where emissions were and were not curbed.

They found that for more than half of the world’s land area, such heat waves would become an annual event by 2075 (possibly even occurring more than once a year). Some of the worst affected areas were the northern tiers of North America, Europe and Asia, as well as the central part of South America.

With emissions reductions aimed at keeping warming below 2°C (4°F) by the end of the century, the affected area falls to between 10 and 25 percent. (More than 100 countries have made pledges toward such a goal.)

“We were fairly surprised by the extent and the size of the benefits we quantify” even by just mid-century, said Tebaldi, who also works as an advisor to Climate Central.

Tebaldi and Wehner also looked at how much more extreme the 1-in-20-year events of the future will be compared to today. They found that such events in 2050 would be 5.4°F (3°C) hotter than today for 60 percent of the globe’s land area. For 10 percent, such events would be even worse, coming in at 9°F (5°C) hotter than today.

With emissions reductions, those temperatures would be a degree or two lower for large portions of the globe.

“This study does a good job of highlighting the large benefits of emissions reduction in terms of extreme temperature events,” Ethan Coffel, a PhD candidate at Columbia University who led another recent study of heat waves, said in an email.

Climate change has helped shift the odds of extreme heat.
Credit: WXshift

While that may not seem like much — temperatures, after all, can swing that much from week to week — when you get to the extremes, it can be the difference between life and death, especially for vulnerable populations like the very young, elderly, and the poor.

Heat waves like the one that hit Europe in 2003, killing thousands, as well as more recent events ranging from the U.S. to Australia to India, have raised awareness of the need for societies to be better poised to cope with extreme heat. Pilot programs in India, for example, have helped local governments issue warnings to citizens, as well as provide cooling centers and clean drinking water.

Several such heat waves have been shown to have been made more likely, more intense, or both by so-called extreme attribution analyses. That work buttresses the findings of this study, Tebaldi said.

Tebaldi and Wehner view their work as a first take on just how big a difference reducing emissions can make, even if it won’t fully eliminate the threat of amped-up future heat waves.

“Even under more dramatic mitigation scenarios . . . future heat wave frequency and intensity increase very dramatically,” Wehner said in an email. But “we do have a choice about how dangerous the future will be.”