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Depositphotos.com | gustavofrazao

© Depositphotos.com | gustavofrazao

Less rain in the forest: Amazon even more vulnerable than previously thought

For every three trees dying from drought in the Amazon rainforest, a fourth tree – even though not directly affected – will die, too.

In simplified terms, that’s what researchers have now found using network analysis to understand the complex workings of one of Earth’s most valuable and biodiverse carbon sinks. The regions most at risk of turning into savannah are located on the forest’s Southern fringes, where continuous clearing for pasture or soy has already been weakening the forest’s resilience for years.

The rainforest in South America maybe losingrain and with it its moisture supply as climate change sends more frequent and more severe drought spells to the Amazon Basin. That lack of rain is life-threatening to the forest because it breathes water: Once rained down, the soil takes it up as much as the plants, and both release a great amount again through evaporation and transpiration. In this atmospheric moisture recycling, the forest creates much of its own weather, generating up to half of the rainfall over the Amazon Basin. And while it is highly efficient, the moisture recycling system ultimately relies on how much water is initially put into the system.

The scientists have now found that even if a dry spell affects only one specific region of the forest, its harm stretches beyond that region by a factor of 1.3: As lack of rain strongly decreases the water recycling volume, there will also be less rainfall in neighbouring regions, hence putting even more parts of the forest under significant stress. “More intensive droughts put parts of the Amazon rainforest at risk of drying off. Subsequently, due to the network effect, less forest cover leads to less water in the system overall, and hence disproportionally more harm. And while we’ve investigated the impact of drought, that rule also holds for deforestation. It means essentially, when you chop down one acre of forest, what you actually are destroying is 1.3 acres,” Nico Wunderling, author and researcher at the Potsdam Institute for Climate Impact Research, explains.

© Sketch of the Amazon rainforest supplied by moisture recycling (blue arrows) showing that the southern part of the forest is most endangered to undergo a savanna transition. Image courtesy Boris Sakschewski. (C) PNAS / pik-potsdam.de
The new normal is much drier

Climate science predicts that what used to be extraordinarily dry years in the Amazon region, like 2005 and 2010, may well become the new normal from 2050 onwards, with centennial droughts occurring in up to nine out of ten years by 2060. But droughts have different effects on forest systems within the Amazon. “In the Amazon, trees and forest systems are differently adapted to water availability, as some regions commonly exhibit a distinct dry season while others have rain all year round. We specifically acknowledge these local adaptations as they can be a blessing or curse under climate change,” Boris Sakschewski, co-author of the study and also at the Potsdam Institute, remarks. “So we find that even the dry season adapted parts of the Amazon forest won’t necessarily survive a new climate normal and the risk of tipping into savannah or no trees at all is high. The consequences for biodiversity would be disastrous, but the same goes for the local, regional and global climate.”

“Yet not all is lost,” says Ricarda Winkelmann, co-author of the study and leader of tipping elements research at the Potsdam Institute. “Our simulations do not show an endless downward slope. That is because a good part of the forest is still in relatively stable conditions. The network effects of dry spells are likely limited to certain areas in the forest’s southeast and southwest – which happen to be those where the forest has been suffering from the human hand already, in clearing forest for pasture or soy.  So there is still a lot we can do to try and stabilize the Amazon, as preserving it and its ecological services is of utmost importance for local, regional and global climate stability. And we know how we can do that: by protecting the rainforest from logging, and by rapidly reducing greenhouse gas emissions to limit further global warming.”


Potsdam-Institut für Klimafolgenforschung (PIK) 2022

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