The devastating heat wave of 2003, which claimed 35,000 lives across Europe, also contributed to global warming, according to French researchers.
Plants, which usually absorb carbon dioxide, started emitting the gas as temperatures soared 6 degrees Centigrade above average. "It stopped the CO2 uptake by plants and within a few weeks the plants started, in the middle of the summer, to emit CO2 into the atmosphere instead of their normal behaviour where they pump CO2 out of the atmosphere," said Dr Philippe Ciais, a carbon cycle scientist at the Laboratoire des Sciences du Climat et de l'Environnement in Gif sur Yvette.
Hotter temperatures are usually thought to slow the rate of climate change by increasing the amount of CO2 that vegetation, so-called carbon sinks, absorbs from the atmosphere.
The scientists, whose findings are reported in the journal Nature, warned that future droughts and rising average temperatures may mean that plants cannot absorb CO2 and may contribute to global warming. They used computer models and studied records of crop yields, satellite photographs of plant cover and observations of CO2 taken up by ecosystems
Dennis Baldocchi of the University of California at Berkeley commented in the journal: "The report shows how episodes of heat and drought will affect the ability of European countries to comply with the requirements of the Kyoto protocol to reduce carbon emissions by limiting fossil-fuel combustion or increasing terrestrial carbon sinks."