In the ongoing onslaught on the science underlying global warming, one recent criticism has focused on the finding that global warming could cause large-scale dieback of Amazon rainforest as cited in the Intergovernmental Panel on Climate Change (IPCC) report (a point spun by the opponents of action on global warming as this post highlights). Leading scientists with years of experience studying the Amazon rainforest have just released a letter that puts into perspective this recent controversy. The gist of their conclusion is that we should still be concerned.
In the letter (available here), 19 highly respected scientists who conduct research on Amazon forests, climate, and/or fire refute claims that there is no link between drought (one of the possible impacts of global warming in this region) and Amazon dieback. This “controversy” arose because the IPCC cited a study by the World Wildlife Fund (WWF) instead of a peer reviewed journal as is common practice by the IPCC. And a recent study from researchers at Boston University claims to “debunk myths about vulnerability of Amazon rainforests to drought” (as you can see here by the coverage).
But as it turns out the WWF study was merely a collection of a large body of peer reviewed articles and as these leading scientists conclude:
In response to the Boston University study that supposedly “debunked” the “myth” of Amazon dieback due to drought their response was:
"First, there is no myth. Rather, there are multiple, consistent lines of evidence from ground-based studies published in the peer-reviewed literature that Amazon forests are, indeed, very susceptible to drought stress. Second, nothing is debunked by the new study."
So why do we care about this potential dieback of the Amazon rainforest? The Amazon Basin’s trees hold carbon stocks equivalent to more than a decade of global fossil fuel emissions. The forest also releases enough water to the atmosphere via evapotranspiration and to the ocean via river outflow to influence world climate and ocean circulation systems. So if you are trying to reduce global warming pollution and minimize the impacts from global warming, the last thing you want to do is create conditions that lead to a huge increase in emissions (massive Amazon dieback) and that could exacerbate one of the possible impacts of global warming (changes in ocean circulation).
So what is the connection between this huge carbon stock, rainfall, fires, and global warming? As this post on RealClimate from a forest scientist highlights: “The evidence for the possibility of a major die-back of the Amazon rainforest is due to two factors:
- That climate change induced decreases in rainfall in the dry season occur, and
- The trees cannot tolerate these reductions in rainfall.
As the 19 scientists point the link can be as follows:
“Reductions in rainfall can affect Amazon forests by increasing tree mortality, but also by increasing their susceptibility to fire. The initial fire kills trees, increasing the likelihood of subsequent fires for years afterwards in a vicious positive feedback loop.”
As the RealClimate post points out there is some uncertainty about whether rainfall will be reduced in the Amazon due to global warming, but several peer-reviewed studies have shown that this is a real possibility.
So the Amazon rainforests aren’t totally “out of the woods” or guaranteed to “go up in smoke”, but they are still very susceptible to dieback due to global warming as these respected scientists highlighted.
I for one would rather not risk losing an area of rainforest larger than the continental U.S. because some ill-cited peer reviewed studies or new study raised some uncertainty. As my colleague has pointed out (here and here) there are plenty of facts on-the-ground that point us clearly towards the need to reduce our global warming pollution. The Amazon and the planet aren’t worth such a risky bet.