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Amazon rainforest is creeping closer to a tipping point

A new analysis of high-resolution satellite images of the Amazon rainforest’s vegetation indicates that the forest is losing its resilience as it becomes less able to recover from perturbations such as droughts and fires. Ultimately, the forest is in danger of dying back and being replaced by drier habitat such as savannah. 

This is the conclusion reached by Chris Boulton and Tim Lenton from the Exeter University and Niklas Boers of the Potsdam Institute for Climate Impact Research (PIK). The researchers analyzed 30 years of satellite data in terms of forest biomass and greenness (productivity), and observed that about three quarters of the forest has shown signs of resilience loss since the early 2000s. 

“Reduced resilience – the ability to recover from perturbations like droughts or fires – can mean an increased risk of dieback of the Amazon rainforest. That we see such a resilience loss in observations is worrying,” said Boers. 

“The Amazon rainforest is 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 gases in the case of even partial dieback, in turn contributing to further global warming. This is why the rainforest is of global relevance.”

The results, published today in the journal Nature Climate Change, also indicate that the loss of ability to return to a previous equilibrium state has brought the forest closer to a tipping point, where it could undergo a sudden transition to a completely different habitat. The Amazon has long been considered vulnerable to the kinds of ecosystem changes that would result in loss of function, and a number of previous studies have highlighted this vulnerability. 

“However, computer simulation studies of its future yield quite a range of results,” said Boers. “We’ve therefore been looking into specific observational data for signs of resilience changes during the last decades. We see continuously decreasing rainforest resilience since the early 2000s, but we cannot tell when a potential transition from rainforest to savanna might happen. When it will be observable, it would likely be too late to stop it.”

The research team used stability indicators that had previously been applied to monitor changes in the Greenland ice sheet and the Atlantic overturning circulation. These statistical indicators aim to predict the approach of a system towards a tipping point by identifying a critical slowing down of the system’s dynamics. The analysis of the two satellite data sets, representing biomass and the greenness of the Amazon forest, revealed the critical slowing down of the forest’s ability to recover after disturbances such as fire and drought. 

”Our study shows that the Amazon is approaching a tipping point, but also that it has likely not yet crossed it,” said Boers.

To try and determine the reasons for the forest’s loss of resilience, the researchers explored its relation to rainfall in a given area in the Amazon that had experienced three ‘once in a century’ drought events. They found that drier areas are more at risk of resilience loss than wetter ones. “This is alarming, as the IPCC models project an overall drying of the Amazon region in response to anthropogenic global warming,” said Boers. 

Another factor is the distance of an area to roads and settlements from where people can access the forest. More pronounced losses of forest biomass and productivity were observed within 200 kilometers of human land use, such as large farms and settlements. This illustrates the urgent need to minimize human land use in the Amazon region, as well as to limit greenhouse gas emissions globally. 

“Our novel analysis of empirical data brings additional evidence to the worries about the forest’s resilience, especially in the near future,” said Lenton, Director of the Global Systems Institute. “It confirms that strongly limiting the logging, but also limiting global greenhouse gas emissions, is necessary to safeguard the Amazon.” 

The research is part of the Tipping Points in the Earth System (TiPES) project, which is funded by European Union’s Horizon 2020 program.

By Alison Bosman, Staff Writer 

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