The Amazon rainforest is considered the “green lung” of the planet and an important biodiversity hotspot. But as it turns out, the Amazon could be nearing its tipping point. Evaluations of 30 years of satellite data suggest that the resilience of this rainforest has already measurably decreased. This means that the forest takes longer and longer to get back to its original state, for example after a dry period. According to the research team, this could be because stabilizing feedback mechanisms have weakened – as is typical for systems close to the tipping point.
The Amazon rainforest is the largest tropical rainforest on earth and thus an important player in the earth’s climate system. Because its trees absorb large amounts of carbon dioxide from the atmosphere through their photosynthesis and thus act as a CO2 sink. Like a buffer, the rainforest thus offsets at least part of human CO2 emissions. At the same time, the Amazon region influences the climate of the entire region, because its evaporation enriches the air with moisture, which then falls elsewhere as rain. But in recent years, fires, deforestation and other human intervention as well as global warming have affected the rainforest: it is losing surface area, periods of drought are becoming more frequent and the rainforest has already lost part of its CO2 buffering effect.
Fading resilience as a sign of the tipping point
The Amazon rainforest has long been considered a tipping element in the climate system. Such a system can switch relatively abruptly into a completely new state of equilibrium after a long, gradual advance. In the case of the Amazon region, this would mean that the warm, humid rainforest would become a rather dry savannah landscape. “However, the Amazon rainforest is a highly complex system, so it is very difficult to predict when the tipping point might be reached,” explains lead author Chris Boulton from the University of Exeter. He and his team have therefore used satellite data to look for a possible sign of such a tipping point: a deterioration in resilience. As such, the researchers describe the ability of the rainforest to quickly recover from adverse circumstances such as drought.
“The rainforest can still look normal on the outside, but still lose resilience,” explains senior author Niklas Boers from the Potsdam Institute for Climate Impact Research. “When too much resilience has been lost, forest loss becomes irreversible. However, by the time this becomes visible, it will probably be too late to stop it.” As the team explains, one of the signs of dwindling resilience is that the rainforest is taking longer and longer to recover from a drought or a fire recover. They determined whether this is already the case using satellite data on biomass, leaf density and vegetation cover from the last 30 years. “Our study analyzed the reactions of the rainforest to the fluctuating weather conditions in detail and from month to month,” says Boers.
Resilience has already decreased
The evaluations showed that the resilience of the Amazon rainforest to adverse conditions has decreased measurably. “Resilience has already declined during the major droughts of 2005 and 2010,” reports Boulton. However, while the forest then recovered to some extent and south-eastern parts of the Amazon region were particularly affected, there has been a slow, steady decline in resilience since around the year 2000, which has continued to this day. Around three quarters of the Amazon rainforest are affected, as the team reports. “As a result, we expect the rainforest to recover from a dry spell at a slower rate today than it did 20 years ago,” Boulton said. As he and his colleagues explain, this is evidence that the Amazon rainforest is nearing its tipping point.
“Many scientists have predicted that the Amazon rainforest could reach a tipping point,” Boers said. “But our study now provides the decisive empirical evidence that we are actually approaching this threshold.” At the same time, the study also confirms that the gradual destabilization of an ecological system can already set in when there is no change in the overall condition. Even if the Amazon rainforest still seems intact in many places, its stability and resilience have already been measurably weakened. The parts of the rainforest that are already in drier areas and are therefore increasingly suffering from water shortages as a result of climate change are particularly badly affected, as the team explains. However, direct human interventions such as the construction of roads and settlements through the forests also seem to reduce the resilience of the rainforest.
“Our novel analysis of the empirical data provides further evidence that concerns about rainforest resilience are valid,” says co-author Timothy Lenton of the University of Exeter. “It confirms that there is an urgent need to limit global greenhouse gas emissions, but also to limit deforestation in order to preserve the Amazon rainforest.”
Source: Chris Boulton (University of Exeter) et al., Nature Climate Change, doi: 10.1038/s41558-022-01287-8