Estimates from CarboEurope, a European Union research team based in Jena, Germany, suggest that during July and August 2003, around 500 million tonnes of carbon escaped from western Europe's forests and fields as crops shrivelled, soils desiccated and trees burnt. The releases are equivalent to around twice the emissions from fossil-fuel burning in the region over the same period.
The study, headed by CarboEurope's Philippe Ciais of the Laboratory for Climate and Environmental Sciences in Paris, France, used data from a network of 100sites across the continent. These each analysed air samples for CO2 and then plotted exchange of the gas between ecosystems and the atmosphere. The 2003 carbon releases coincided with a worldwide build-up of CO2 in the atmosphere. US figures show that in August 2003, the atmosphere contained 374parts per million of CO2, which at 3parts per million above the level of the previous August makes it a record year-on-year rise.
Two years ago, before the 2003 drought, CarboEurope estimated that Europe's ecosystems were absorbing 7 to 12 per cent of the continent's man-made carbon emissions. But its researchers agree that this year the ecosystems themselves will probably be net releasers of CO2. Much of western Europe- including Spain, Portugal, Italy, France and parts of the UK- is suffering drought on a similar scale to 2003.
Spain, for example, has received less than half its normal rainfall so far this year. And drought is also sweeping much of the US. Corn crops are failing and cattle are dying of heat stress in the Midwest, where many areas have seen less than half their typical rainfall. In New York, extra use of air conditioners resulted in record power demand last week. Summer CO2 releases may be rising across the world.
This week, US researchers reported that since the early 1990s, hot dry summers across the northern hemisphere have reduced the ability of plants to absorb CO2 during their normal growing season (Proceedings of the National Academy of Sciences, vol 102, p 10823).
Alon Angert and colleagues at the University of California, Berkeley, say this finding dashes the widespread expectation of a "greening trend", in which warm summer temperatures would speed plant growth and moderate climate change by soaking up some of the industrial CO2 emissions.
"Excess heating drives the dieback of forest, accelerates soil carbon loss and transforms the land from a sink to a source of carbon for the atmosphere," says team member and atmospheric chemist Inez Fung, also at UC Berkeley. So hotter temperatures amplify human-induced climate change, she adds.