A vast expanse of unique biological diversity hangs in the balance as the "lungs of the world" approach a tipping point from which there is no recovery. The Amazon Rainforest is losing its ability to regenerate, reported a peer-reviewed study, Monday, in Nature.
For 10% of all known species on Earth, that means the destruction of their only habitat with a profoundly amplifying effect on climate change. Over two-thirds of the size of the United States, the Amazon provides a critical "carbon sink," sequestering about 123 billion tons of carbon dioxide. As climate change and deforestation progress, the Amazon verges on outright collapse.
If it does, only savanna-like grasslands will remain, many experts believe. However, what that transition actually looks like depends on how fast climate change progresses, Dr. Chris Boulton, lead author of the study, told Salon.
"If it was slower then there's the chance that drought-resilient trees could come in and establish themselves," he suggested. "Then you might see a seasonally dry forest."
Through analysis of satellite data, using a measurement called Vegetation Optical Depth — a measure of the total biomass of trees and other plants in a given area — researchers found that three-quarters of the Amazon have been losing resilience against logging, fires, and other biomass loss since 2000. This loss was "consistent with the approach to a critical transition," they write.
Dr. Niklas Boers, another collaborator from Potsdam Institute for Climate Impact Research and Technical University of Munich, called the findings "alarming," particularly given the projections from a United Nations climate committee that predicted drying of the Amazon Basin under a likely climate scenario.
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Boulton, a researcher at the University of Exeter, compares such shifts to suddenly stumbling over a cliff. On the surface, the Amazon may not appear to change much over time, with the forest roughly maintaining its biomass. Meanwhile, underlying stability continues to erode. Actual die backs of vegetation, he said, will be sudden.
"When it will be observable, it would likely be too late to stop it," Boers said in a statement.
What is unusual about tropical rainforests is that despite lush growth, they are considered "wet deserts." They are so efficient at absorbing nutrients that almost all carbon is contained within living organisms, meaning when the forest dies back, nearly all of that carbon returns to the atmosphere. On an ecological time scale, a sudden transition could take more than 20 years, but the loss of the world's largest rainforest would significantly hinder global efforts to reduce carbon emissions.
"The Amazon rainforest is a home to a unique host of biodiversity, strongly influences rainfall all over South America by way of its enormous evapotranspiration, and stores huge amounts of carbon that could be released as greenhouse gasses in the case of even partial dieback, in turn contributing to further global warming," Boers asserted. "This is why the rainforest is of global relevance."
However, great uncertainty remains about when that threshold for "Savannahfication" might be met.
"Models that are predicting the future tend to disagree with each other quite a lot," Boulton explained. "There's lots of models that say the Amazon will die back and there's others that say it might be quite healthy with future climate change. People argue about why. There's all these future climate scenarios and the models are set up to determine how the forest is affected by those things."
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Three major droughts of severity only seen, on average, once a century dealt resounding blows to the Amazon. Amazingly, it continues to rebound, but more slowly with each successive blow.
"The reason we look at these indicators over time is because it's quite difficult to pinpoint specific events," Boulton added. Looking at forest density actually reveals very little about how soon the ecosystem might unravel. "Weather's always happening. There might have been a drought in 2005, but then there's a fire a couple months later. It's really hard to untangle all those things," Boulton continued.
Far more significant is the robustness of its response. Resilience has not consistently declined as a result of climate change. In fact, resilience increased from 1991 to 2000. Some recent studies project widespread diebacks by the end of the century, but these have generally relied on modeling to do so.
"There's all this uncertainty, and looking at the real world data gives you an idea of the trajectory that the forest is on because it is heading more towards those models that say the system is going to tip," Boulton continued.
Researchers found that the Amazon has yet to cross that threshold, but something has to give. Regions closer to human activity and those that experience less rainfall had the most pronounced impacts.
"The river of the sky as people poetically call it — where this water is recycled back into the heart of the Amazon about six or seven times — is losing forest on the edge, degrading that system," Boulton pointed out. "You're getting gradually less rain that comes back through."
This causes the whole system to suffer in turn, but also offers hope. Either climate action or regional protections may afford partial reversal of the trend, according to Boulton.
"If you start to prevent that from happening, keeping the trees safe on the edge, you're going to bring back that atmospheric river," Boulton concluded. "That's going to go a long way to helping the forest restore itself. Then that's also helping itself when it comes to the global climate crisis because your lungs are working properly. By solving one of those problems, you can certainly help it and even try and combat the second one at the same time."
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