Last year much was made by climate-change deniers of a poorly referenced section of one of the IPCC reports of 2007 that said “up to 40% of the Amazon rainforest could be sensitive to future changes in rainfall.” It turned out that the claim was based on solid science, despite the best efforts of those who just can’t bring themselves to trust professional climatologists. You can read the whole sordid tale here. I revisit the issue because of a new paper about to be published by the American Geophysical Union that bears on this question.
“Widespread Decline in Greenness of Amazonian Vegetation Due to the 2010 Drought” isn’t available to the public yet, although there’s a press release here. The upshot is it’s beginning to look even more worrisome in the Amazon than previously thought. The 2007 IPCC reports used research on the 2005 drought that affected the rainforest there in ways that were subject to some debate. There is now less uncertainty about whether the forest gained or lost vegetative cover in the wake of that drought. The authors of new paper write:
Undisturbed Amazon rainforests were reported to have greened-up during the 2005 drought based on analysis of a previous version of the Enhanced Vegetation Index (EVI) data [Saleska et al., 2007]. This has now been shown not to be the case…
Why does all this matter?
There is concern that in a warming climate the ensuing moisture stress could result in Amazonian rainforests being replaced by savannas [Cox et al., 2004; Salazar et al., 2007; Huntingford et al., 2008; Malhi et al., 2008], in which case the large reserves of carbon stored in these forests, about 100 billion tons [Malhi et al., 2006], could be released to the atmosphere, which in turn would accelerate global warming significantly.
It is understandable, to an extent, why some people would be upset that the IPCC report referenced a non-peer-reviewed World Wide Fund for Nature paper, rather than the genuine science from which the WWF drew its conclusions. The fate of the Amazon will have a huge effect on the global climate, and careless citation won’t help make the case for doing something about it. But those who argued, before bothering to do even cursory research into the matter (like talking with an actual scientist), were even sloppier. I draw the guilty parties’ attention to the the full abstract of the new paper.
During this decade, the Amazon region has suffered two severe droughts in the short span of five years – 2005 and 2010. Studies on the 2005 drought present a complex, and sometimes contradictory, picture of how these forests have responded to the drought. Now, on the heels of the 2005 drought, comes an even stronger drought in 2010, as indicated by record low river levels in the 109 years of bookkeeping. How has the vegetation in this region responded to this record-breaking drought? Here we report widespread, severe and persistent declines in vegetation greenness, a proxy for photosynthetic carbon fixation, in the Amazon region during the 2010 drought based on analysis of satellite measurements. The 2010 drought, as measured by rainfall deficit, affected an area 1.65 times larger than the 2005 drought – nearly 5 million km2 of vegetated area in Amazonia. The decline in greenness during the 2010 drought spanned an area that was four times greater (2.4 million km22 and more severe than in 2005. Notably, 51% of all drought-stricken forests showed greenness declines in 2010 (1.68 million km2) compared to only 14% in 2005 (0.32 million km2). These declines in 2010 persisted following the end of the dry season drought and return of rainfall to normal levels, unlike in 2005. Overall, the widespread loss of photosynthetic capacity of Amazonian vegetation due to the 2010 drought may represent a significant perturbation to the global carbon cycle.
In case anyone is wondering, “a significant perturbation to the global carbon cycle” is not a good thing.
And things aren’t any better in the Indonesia rainforests.