Le roast. Europe was hit by the worst heat wave in its history in the summer of 2003, and researchers worry it could happen more often due to global warming.

No Upside to Global Warming

John is a Science contributing correspondent.

Global warming may not be all bad, according to optimists who say that rising Earth temperatures could be a boon for plant growth. This could help farmers in currently chilly areas, such as Canada and northern Europe, they say, and actually slow climate change as the extra plants sequester carbon dioxide from the atmosphere. But a study of the impact that a recent European heat wave had on plant growth and carbon emissions shows that such claims may be no more than hot air.

In the summer of 2003, Europe roasted during a catastrophic heat wave. The temperatures in August 2003 were the hottest on record, killing an estimated 35,000 people. During the heat wave, drought parched cropland and fires devastated forests.

To get a handle on how the European heat wave affected plant growth and carbon dioxide levels, the European Union commissioned a study called CARBOEUROPE in 2004. Teams of researchers from 17 countries pooled data collected in 2003 from measurements of the emission of the gas at sites across Europe that covered the range of different European ecosystems, while satellite images and crop yields were used to tally changes in plant biomass. Europe lost 30% of its plant life in the summer of 2003, and the net effect on the carbon budget--a combination of a decrease in biomass and an increase in the amount of carbon dioxide to the atmosphere through respiration--erased the previous 4 years of carbon sequestration, the team reports 22 September in Nature. The picture could even be worse, the authors warn, because some effects have not yet been accounted for, such as the release of carbon from forest fires.

The researchers also integrated their data into a computer simulation that indicated the impact of extreme heat waves and droughts may be severe enough to erase the benefits of global warming in temperate zones.

"This is a very good analysis of the [heat wave's] impact," says Steve Running, a climate scientist at the University of Montana in Missoula. To gauge the study's significance for climate change, he says, scientists will have to determine how regular and intense such droughts and heat waves may become.

Related sites
CARBOEUROPE
Running's site

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