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August 08, 2006

Another Look At Drought In The Amazon Environment

The climate scientists at RealClimate urge caution in interpreting the Woods Hole experiment that suggested the Amazon rainforest would collapse if it experienced three years of major drought. It's not that simple — the rainforest is not that fragile — but the picture in the Amazon still is not good. Excerpt:

The not-so-good part comes when [the Woods Hole] experiment is linked too directly to the ongoing drought in the southern Amazon...This is incorrect for a number of reasons. Firstly, drought conditions are not the same as no rain at all — the rainfall deficit in the middle of the Amazon is significant, but not close to 100%! Secondly, the rainfall deficits are quite regionally variable, so a forest-wide response is highly unlikely. Also, the trees won't all die in just one more year and could recover, depending on yearly variation in climate.

...[T]here are, however, some issues that should provoke genuine concern. Worries about the effects of the prolonged drought (and other natural and human-related disturbances) in the Amazon are indeed widespread and are partly related to the idea that there may be a 'tipping point' for the rainforest (see this recent article for some background). This idea is exemplified in a study last year...which looked at the sharp transition between forest and savannah and related that to the coupling of drought incidence and wild fires with the forest ecosystem. Modelling work has suggested that the Amazon may have two vegetation/regional climate equilibria due to vegetation and climate tending to reinforce each other if one is pushed in a particular direction...The two alternative states could be one rainforested and wet like today, the other mainly savannah and dry in the Eastern Amazon. Thus there is a fear that too much drought or disturbance could flip parts of the forest into a more savannah-like state. However, there is a great deal of uncertainty in where these thresholds may lie and how likely they are to be crossed, and the rate at which change will occur. Models go from predicting severe and rapid change..., to relatively mild changes... Locally these responses can be dramatic, but of course, these changes also have big implications for total carbon cycle feedback and so have global consequences as well. [Emphasis added]

So the conclusion drawn in the Independent article — namely, that since trees started dying in the Woods Hole experiment after rain was blocked for three years it follows that three years of drought could induce the collapse of the Amazon — was a gross over-simplification that exaggerated the fragility of the system.

But, that said, it remains true that the Amazon is in trouble. Parts of it are drying, and it's not clear when or if a tipping point will be reached after which the feedback loops in play will push the system to a new and different equilibrium state, possibly replacing forest with savannah — which would have significant global consequences. I.e., the Amazon's in trouble, but it's not going to collapse overnight, drought or no drought.

Posted by Jonathan at August 8, 2006 02:48 PM  del.icio.us digg NewsVine Reddit YahooMyWeb