Seeing the future hot spells

Satellite observations of soil moisture, combined with regional climate models, could help scientists better predict the size and scope of future heat waves, a new study suggests.

One of the most devastating heat waves in recent years struck Europe in 2003 (SN: 7/3/04, p. 10). Along with an estimated 70,000 heat-related deaths across the continent — many of them in France, the epicenter of the heat wave — droughts before and during the hot spell helped trigger forest fires and crop losses, Richard de Jeu, an environmental scientist at VU University Amsterdam in the Netherlands, and his colleagues report online March 4 in the Journal of Geophysical Research–Atmospheres. The combination of high temperature and drought trimmed the continent’s foliage growth, on average, by about 30 percent that year (SN: 10/15/05, p. 254).

Climate models can often predict that a heat wave is coming, but a number of factors, and how they interact, affect the strength and extent of a hot spell in sometimes unpredictable ways — as they did in Europe in 2003.

Trends observed by meteorologists and by Earth-orbiting satellites in the months before the continent’s heat wave struck in earnest could have provided clues that summer temperatures would be abnormally warm, the researchers contend. The analysis focused on the Upper Danube watershed — an area surrounding Munich that is slightly smaller than South Carolina — but the results can be readily applied to larger areas as well, the researchers report.

The region began to suffer a pronounced drought in February 2003. Climate models suggest that by March, the soil moisture in the area’s grasslands, croplands and forest dropped below normal and stayed there until May. Three different sets of satellite observations show the same trend: From early March to mid-May, the amount of moisture in the uppermost layer of soil declined an average of about 0.2 percent each day, the researchers report. Throughout 2004, 2005 and 2006, none of which had a heat wave, soil moisture was close to average.

An early flush of vegetation in Europe in 2003 — another trend readily assessed by satellites — slurped water from deeper layers of soil and sent it skyward, accentuating the loss of soil moisture. The resulting dearth of water available to soak up summer heat via evaporation sent air temperatures skyrocketing.

While data from the ground-based instruments that monitor soil moisture is often both sparse and untimely — sometimes available only after a several-week delay — satellite data covers broad areas and can be processed much more quickly, de Jeu and his colleagues contend. As a result, scientists can get a faster heads-up about conditions that might aggravate heat waves.

“This is a nice study,” says Erich Fischer, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. Case studies show that regional climate models provide better results if they include accurate data about soil moisture, he notes. Satellites are a good way to gather such information because soil moisture can vary considerably even across small distances, he adds.

A substantial and lengthy decline in soil moisture doesn’t cause heat waves, Fischer emphasizes, but it can certainly amplify them. In 2007, he and his colleagues estimated that if the soil moisture in Europe had been normal during 2003, the summer temperature anomalies — which in some places measured 5 degrees Celsius — probably would have been 40 percent smaller.