Richard Black at the BBC:
A new study shows that during the 2003 heatwave, European plants produced more carbon dioxide than they absorbed from the atmosphere.
They produced nearly a tenth as much as fossil fuel burning globally.
The study shows that ecosystems which currently absorb CO2 from the atmosphere may in future produce it, adding to the greenhouse effect.
The 2003 European summer was abnormally hot; but other studies show that these temperatures could become commonplace.
In some parts of Europe, 2003 saw temperatures soaring six degrees Celsius above normal; hot enough that estimates of the deaths which it caused run into the tens of thousands.
It was also significantly drier than usual; and these two factors appear to have had a major impact on plant growth.