By Tim Radford
Within one human lifetime, Amazon collapse could have turned the rainforest into open savannah.
The combined devastation of human-induced global warming, rapidly increasing degradation or destruction of the forest, natural climate cycles and catastrophic wildfires could be enough to bring the world’s biggest, richest and most vital forest to a tipping point: towards a new kind of habitat.
“The risk that our generation will preside over the irreversible collapse of Amazonian and Andean biodiversity is huge, literally existential,” warns Mark Bush of the Florida Institute of Technology, in the latest Annals of the Missouri Botanical Garden.
Professor Bush bases his argument on the evidence of history: painstaking study of fossil pollen and charcoal in the sediments of Andean lakes confirms that the profligate biodiversity of the Amazon has been disturbed many times in the past, as global climate has varied with the retreat and advance of the glaciers.
It has, however, never reached a tipping point towards collapse, if only because it has never before had to face the hazard of fire on the present scale.
There is another factor: ever-greater human intrusion into, degradation of, or conversion of forest into plantation or ranch land heightens the hazard of a dramatic shift from moist tropical canopy to open and wooded grasslands.
And then, the argument goes, there are the ever-higher temperatures driven by ever-greater greenhouse gas emissions from human investment in fossil fuel energy, and ever more extensive destruction of the natural habitats that in the past have absorbed atmospheric carbon. And with higher temperatures, there arrives the risk of ever more catastrophic drought.
“From a human perspective, the forest has just become much easier to clear”
A river of moist air flows from east to west across Amazonia to the Andes. What falls as rain is absorbed by the vegetation or evaporated by the sun and transpired through the treetops to provide yet more water vapour to fall again, and again. Effectively, the western Amazon rainforest and the Andean forests are almost entirely dependent on recycled moisture.
This recycling falls away as the canopy goes: evapo-transpiration from the savannah is less than two-thirds of that from the forest. Cropland returns only a tenth of its moisture to the skies. So that makes the forest inland from the Atlantic increasingly vulnerable to change.
The region has recovered from climate turbulence many times before. But the regional temperature has warmed by 1°C to 1.5C in the past century, and researchers have repeatedly warned that a combination of severe deforestation and a warming of 3°C or more could turn the forest into savannah.
In the last 15 years, Amazonia has experienced three “droughts of the century”, in 2005, 2010 and 2015-16. The effects of these, Professor Bush warns, “may be protracted, and possibly irreversible.”
His warning may sound apocalyptic. In fact, he is only saying out loud what has been implicit in research and reporting from the region for years.
Drought and fire present a kind of double jeopardy to any forest. Drought and fire could, researchers have repeatedly warned, turn the Amazon from an absorber of carbon to a source of greenhouse gases, to make global heating even worse.
Tipping point possible
High temperatures change ecosystems: some plants simply cannot cope. The region is one of the richest and most important on the planet. Loss of the Amazon would represent a climate tipping point, and researchers have been warning for years that such possible slides toward irreversible change are imminent.
In a drought, more trees die. Standing deadwood becomes treefall, and so much tinder waiting to catch fire. As the canopy opens up, local temperatures soar by as much as 10°C, and in a deforested region humidity drops by 30%.
For humans looking for roads to clear, minerals to mine, ground to plant or cattle to run, opportunity beckons. “From a human perspective, the forest has just become much easier to clear,” says Professor Bush.
So the effects of the droughts accumulate, and encourage the invasion of yet more humans with chainsaws and fire. The western Amazon is already a potential tipping point: in 2016, Bolivia’s second largest lake – an important commercial fishery – dried up between January and November.
Given the rates of deforestation and the temperatures to come, the Amazon tipping point – the loss of a massive rainforest – could occur by mid-century. The slide to a new kind of ecosystem would be irreversible.
“The immense biodiversity of the rainforest is at risk from fire,” said Professor Bush. “Warming alone could induce the tipping point by mid-century, but if the present policies that turn a blind eye to forest destruction aren’t stopped, we could reach the tipping point much sooner.”
He warned: “Beyond the loss of wildlife, the cascading effects of losing Amazonian rainforest would alter rainfall across the hemisphere. This is not a remote problem, but one of global importance and critical significance to food security that should concern us all.” – Climate News Network
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