Amazon forest tipping from carbon sink to carbon source?

That’s the gist of a new paper in Science, reported in the Guardian yesterday:

Billions of trees died in the record drought that struck the Amazon in 2010, raising fears that the vast forest is on the verge of a tipping point, where it will stop absorbing greenhouse gas emissions and instead increase them.

The dense forests of the Amazon soak up more than one-quarter of the world’s atmospheric carbon, making it a critically important buffer against global warming. But if the Amazon switches from a carbon sink to a carbon source that prompts further droughts and mass tree deaths, such a feedback loop could cause runaway climate change, with disastrous consequences.

“Put starkly, current emissions pathways risk playing Russian roulette with the world’s largest forest,” said tropical forest expert Simon Lewis, at the University of Leeds, and who led the research published today in the journal Science. Lewis was careful to note that significant scientific uncertainties remain and that the 2010 and 2005 drought – thought then to be of once-a-century severity – might yet be explained by natural climate variation.

“We can’t just wait and see because there is no going back,” he said. “We won’t know we have passed the point where the Amazon turns from a sink to a source until afterwards, when it will be too late.”

If we are in positive feedback territory on climate change, then all bets really are off.