Extreme heat on the rise

The heatwave that paralysed Europe last summer was hailed as a harbinger of global warming by many, including climatologists who predicted wilder extremes in floods, droughts and storms thanks to climate change. Results from a climate model now add evidence to the idea that extreme temperature events are set to rise - for Europe at least1.

More than 20,000 people are thought to have died as a result of Europe's heatwave last year.During June and July, temperatures across much of the continent
topped 40 °C.

Christoph Schär of the Swiss Federal Institute of Technology, Zurich, and his colleagues calculate that, based on average temperatures since 1990, a European summer such as that of 2003 should come along only once every 46,000 years - even after taking global warming into account. "Statistically, this event should not have happened," Schär says.

The researchers therefore reasoned that greenhouse gases such as CO2 might boost weather variability as well as the overall temperature. Using predicted greenhouse-gas levels for the end of this century, they ran a regional computer climate simulation to estimate the spread of future temperatures in Europe.

The simulation showed that extreme temperatures will indeed be more common in the future. "I wouldn't bet on how much the variation will increase," says Schär, "but I'm confident that it will."

Changes in variability are more difficult to predict than average temperature trends, says Gabriele Hegerl, who studies climate change at Duke University in Durham, North Carolina. But she believes that Schär's idea may well be correct. "The summer of 2003 would be very hard to explain without increased variability," she admits.

Freak event

Jens Christensen, a climate expert at the Danish Meteorological Institute in Copenhagen, agrees that "we will see more summers like last year".

But others say that killer heatwaves may simply be mere freaks. John Christy, director of the Earth System Science Center at the University of Alabama, Huntsville, says he doesn't see any evidence of increasing temperature variability in his own American data. "I am not an alarmist - I deal with real data and the data don't show this kind of effect," he says. Schär says that the summer of 2003 is the only evidence of increased variability he has seen so far.

Schär thinks that a similar fate could befall any area in which a semi-arid climate borders a wetter region. During the European heatwave, much of temperate central Europe took on a drier,'Mediterranean' pattern as plants and soil lost their moisture.

This worsened the heatwave, Schär explains. With less water in the plants, less of the Sun's energy went towards evaporation, and more towards heating the air. Effects such as this are difficult to represent in climate predictions, however, he adds.

He nevertheless remains convinced that Europe has more wild weather in store. "The summer of 2003 is proof that this kind of thing can happen not too far from the current climate," he says.

References
01. Schär, C. et al. The role of increasing temperature variability in European summer heatwaves. Nature, published online, doi:10.1038/nature02300 (2004).