Apart from the effects on the harvest, the decline in the productivity of plants spells bad news for the environment as a whole. Greater plant growth means that plants are removing more carbon dioxide from the atmosphere.

Global warming could reduce harvests and plant growth, with a serious effect on farm productivity, scientists warn in an article in the journal Nature that is published today.

The heatwave that parched Europe in 2003 caused the continent's grasslands and forests to release huge amounts of carbon dioxide into the atmosphere, climate experts have found.

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.

Forest flux

The researchers measured carbon dioxide concentrations using towers in fourteen forest sites and one grassland site. From half-hourly measurements, they worked out the change in carbon entering or leaving the atmosphere (its flux) from the plants below.

"There's a danger in assuming that climate change is only going to be gradual." Andrew Friend Laboratory for Climate Sciences and the Environment, Gif-sur-Yvette

Using a computer model, they built a picture of the carbon flux from similar forests and grasslands over the entire continent during the heatwave, and compared this with the picture for 2002.

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 researchers report their results in this week's Nature.

Sinking feeling

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.

Abrupt changes

Such an extreme summer has never been recorded before. "This was basically unprecedented," says Andrew Friend, an author on Ciais's paper. "It shows there's a danger in assuming that climate change is only going to be gradual."

Stopping the problem will take drastic cuts in human-driven emissions of carbon dioxide. In the meantime, land managers may have to make some changes, Friend says.

"Farmers might adopt different crops or irrigation programmes," he says. "But forest managers can't change so quickly; they will really have to think carefully about what they're planting now."

"Over the next 50 years the whole world's carbon cycle is going to be altered," Jones predicts. "It's not just Europe. Climate change is a global phenomenon; it could happen anywhere."