Seven hundred dead in Chicago in 1995. Fifty thousand across Europe in 2003. Eleven thousand in Russia in 2010. Heat waves have become hotter, longer and more frequent in recent decades, and scientists predict that climate change and the growth of cities—which trap heat—will boost body counts.
But what if we could see the heat coming ahead of time? If forecasters could predict heat waves weeks or months in advance, would more people survive them? That’s the hope of scientists racing to close the gap between the “nowcast” on the TV news and the long view of the Old Farmer’s Almanac.
At the National Center for Atmospheric Research, climate scientist Haiyan Teng and her colleagues recently simulated 12,000 years of weather, identifying a telltale sequence of pressure systems that tango through the troposphere 15 to 20 days before the onset of many American heat waves. “If this particular circulation pattern appears,” Teng says, “the chance of having a heat wave doubles.” But its predictive power wasn’t perfect: Often when the pattern appeared, no heat wave followed.
Another set of tea leaves: dry soils in winter and spring. Swiss researchers recently found that major summer heat waves in Europe between 1976 and 2005 were preceded by at least four months of low rainfall, which left summer soils tinderbox-dry. Wet soils sap heat through evaporation; without moisture, the earth bakes.
Also on the watch list: sea surface temperatures, vagaries of the Asian monsoon and a mysterious patch of Equator-circling rain called the Madden-Julian Oscillation. “It’s a very exciting and interesting time,” says Gabriel Vecchi, a top climate prediction researcher at the National Oceanic and Atmospheric Administration.
The National Weather Service hopes to have within half a decade tools for predicting the odds of extreme weather two to four weeks out. The aim isn’t just foresight; it’s also awareness. Extreme heat kills more people than floods, hurricanes or tornadoes. But in the public imagination, it remains a B-list natural disaster, overlooked because it offers few made-for-TV images.
“With flood or cyclone, you see the houses flattened,” says Adrian Barnett, an expert on temperature and public health at Queensland University of Technology in Australia, where 2013 was the country’s hottest year on record. In a heat wave, “there’s no smoking gun.”