The Amazon Could Soon Transition to a Dry, Savanna-like Ecosystem
A new study shows that more than 75 percent of the rainforest is losing its ability to recover from droughts and fires—a finding with huge consequences for humans and wildlife alike.
For decades, deforestation, drought, fires, and APP changehave ravagedthe Amazon, worrying scientists that the sprawlingSouth American rainforest could eventuallyhit a critical tipping point and irreversibly transition into a drier, savanna-like ecosystem.More recently, three once-in-a-centurydroughtsduring a 10-year span in 2005, 2010, and 2015causedsome researchers, including the late Thomas Lovejoy, to speculate that the Amazon was nearing thisbreaking point. But most attempts to gauge thehealth of the rainforest or predict when apoint of no return wouldoccur have relied ontheoretical models, leaving much uncertainty.
Now, ausing real-world data and published in Nature Climate Change revealsthat the Amazon is losing its ability to recover from disturbancesand indeed approaching a pivotal moment, with hugeconsequences for humans and wildlife alike.The team of researchers, led by Chris Boulton at the University of Exeter, tapped into a trove of satellite data collected between 1991 and 2016 toexaminehow the Amazon has changed over time and better understandhow close this possible tipping point could be.
Strikingly, the researchers found that more than 75 percent of the Amazon forest has lost resilience, meaning it can’t recover as quickly after drought or fire. In chunks of forest closer to human development—croplands, roads, or urban areas—the forest lost resilience faster. Similarly, areas in the basin that receive less rainfall and whichalready have a drier APP are becoming more stressed and rapidly losing resilience.
As a result, “if that same drought [in 2005] were to happen today, it would take a lot longer to recover,” Boulton says. The researchers see this severe loss in resilience as a strong signal that the Amazon will soon hitits much-feared tipping point. “This study tells us something about the actual world we live in,” Boulton says, setting the team's findings apart from modeling studies that have attempted to forecast when the Amazon might cross this looming threshold.
To estimate the forest’s resilience, the researchers calculated how much the amount of biomass varied from month to month across the Amazon basin throughout the 25-year study window. This required analyzing satellite data detailing the “vertical optical depth” of an area, a measure of tree biomass that is closely related to water content. An increase invariation for this measurewould indicate a loss in forest resilience.
The Amazon has endured changes in rainfall over millennia, butlarge-scale deforestation, drought, and firesare threatening the very processes that makes the Amazon a rainforest. Thanks to geography, the Amazonproduces almostof its own rainfall.Whenhumid air from the Atlantic Ocean hitsthe Andes Mountains, it gets trapped over the Amazon basin and falls as rain. Thetrees suck waterupthrough their roots and then release moisture back into the atmosphere, a process called evapotranspiration. A single tree can emitinto the atmosphere each day, falling as rain elsewhere in the basin. This natural water cycle—and the processes that the forest uses to restore itself after an event like a fire or a drought—breaks down when the forest is less resilient. The worse resiliency gets, the longer it takes the forest to recover—until it can't recoverat all.
At a press briefingfor the study, Tim Lenton, a coauthor on the paper anddirector of the Global Systems Institute at the University of Exeter, emphasized that hitting the Amazon’s tipping point would not only be devastating forthe world’slargest rainforest but also have repercussions for the wholeAPP.The Amazonacts as a carbon sink for the planet, trapping greenhouse gasses like carbon dioxide—but that could change if the rainforestbecomes a dry savanna.Instead,the regioncould start emitting more carbon than it absorbs. “That’s a major change in one of earth’s most important systems,” says Miles Silman, an ecologist at Wake Forest University who studies forest dynamics in the Amazon.
Climate changefuelsthedeadly trioof drought, fire, and deforestationthat is pushing the Amazonbeyond its limits. Arecent report by the United Nation's Intergovernmental Panel on Climate Changeforecasts in the Amazon with worseningdroughts. The warmer and drier conditions lead to increased fire and more dieback of trees, reducing available moisture in the air andtriggering the cycle to repeat in a dangerous pattern.“Put forests in the equation with humans and fire, and things change really dramatically,” says Silman.
Along with the peoplewho still call the Amazon home, the results of this study have dire implications for itswildlife.A biodiversity hotspot, the Amazon contains the of freshwater fish species in the world—almost 3,000—plus species, including the world’s largest rodent, the capybara. An estimated depend on the Amazon rainforest to find food, raise young, and survive. The new study throws that current existence into question.
“If [the forest] didn’t recover, that would be a game-changer,” says Philip Stouffer, a biologist at Louisiana State University who wasn’t involved with the study. For nearly four decades, Stouffer has studied Amazonian birds—rainforest specialists that occur only in closed-canopy areas with —in a preserved patch of the Amazon. His previous research detected a decrease in the abundance of ground foragers, most likely caused by APP change and the same resilience issues the new paper highlights. How the birds fare after we hit the tipping point, says Stouffer, depends on what the transition looks like. If it happens slowly, theymay still have access to food and adapt to new resources. But a rapid shift to a savanna-like ecosystem? “There’s no doubt there would be catastrophic changes to the rainforest bird community,” he says. Vitek Jirinec, an ecologist at Integral Ecology Research Center who also was not involved with the study, agrees that a fast transition would be a disaster for the birds.“There’s no way these habitat specialists can adapt in that sort of time frame.”
Previous research byJirinecandStoufferhas shownthat Amazonianbirds have the ability toadapt to certain changes: Theyfound that speciesthere are with APP change, weighing less but havinglonger wingsto dissipate heat more easily.Although birds could theoretically fly to remnant forest patches as the rainforest shifts to a new ecosystem, Jirinec says that these rainforestbirds seldom move beyond the several hectares of forest that they occupy their entire lives. They, so an open pasture and even rivers present insurmountable barriers—to the point that you may see a closely related but unique species on the other side of the river.
Thenew analysisdidn’t try to predict exactlywhen the Amazon might reach itstipping point, but aprevious study suggested that this threshold could come as soon as. Thiscurrent trajectoryisn’tirreversible, Boulton says, but a transition to savanna would be extremely difficult to prevent, perhaps requiring the global APP to revert to pre-industrial carbon emissions. While his team’s findingsdonot bode well for the Amazon or any of the life that currently calls it home,Boulton believes there’s still hope. “It gives people a chance to do something about it potentially,” he says.