Extreme temperatures make forests emit carbon dioxide.
Nature 437, 529-533 (22 September 2005). The heatwave that parched Europe in 2003 caused the continent's grasslands and forests to release huge amounts of carbon dioxide into the atmosphere.
Heatwaves are predicted to become more common as a result of climate change, so the discovery raises fears that forests in temperate regions will become significant emitters of this greenhouse gas.
The stifling summer of 2003, which featured temperatures some 6 ºC above average and claimed thousands of lives, stunted plant growth through a combination of drought and extreme heat. Usually, Europe's crops and forests are thought to have the net effect of storing carbon as plants grow, rather than releasing it.
The carbon dioxide released as a result of the heatwave was equivalent to the amount of carbon stored over the previous four years of normal growth, reports a team led by Philippe Ciais of the Laboratory for
Climate Sciences and the Environment in Gif-sur-Yvette, France.
They found that at the height of the summer of 2003, carbon was baking out of Europe's vegetation at a rate of half a billion tonnes per year. By comparison, UK industrial emissions totalled 150 million tonnes of carbon equivalents in 2003.
Over the previous four years, carbon had been moving at roughly 125 million tonnes per year in the opposite direction, as it was taken up and used by plants to grow.
The 2003 event shows that forests, even in temperate areas such as Europe, cannot be guaranteed to suck up carbon dioxide from the atmosphere, says Chris Jones, who studies the carbon cycle at the Hadley Centre for Climate Prediction and Research in Exeter, UK.
One possibility is that rising temperatures could dent plant growth to the extent that many areas become overall emitters of carbon dioxide, Jones says. Warmer climes would accelerate the decomposition of old
plant matter, releasing more of the gas.
This effect on growth was borne out by the disappointing crop yields of 2003. France, hit hardest by the heatwave, lost 20% of its usual harvest. Yields usually rise year on year because of improvements in
management.
This work is part of the CARBOEUROPE-IP research program funded by the European Union.
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