Key role of forests "may be lost."  / 2009

"The International Union of Forest Research Organizations (IUFRO) says forests are under increasing degrees of stress as a result of climate change. 

Forests could release vast amounts of carbon if temperatures rise 2.5C (4.5F) above pre-industrial levels, it adds."

...

"While deforestation is responsible for about 20% of greenhouse gas emissions from human activities, forests currently absorb more carbon than they emit. 

But the problem is that the balance could shift as the planet warms, the report concludes, and the sequestration service provided by the forest biomes 'could be lost entirely if the Earth heats up by 2.5C or more'."

See opposing argument.