Increasing droughts in the future
The Amazon Basin is home to the world's largest contiguous tropical rainforest, which currently still stores about 200 gigatons of carbon over the long term. But this rainforest is in danger, as severe droughts have occurred repeatedly in more than half of the Amazon in recent years, damaging numerous trees, causing fire outbreaks, and temporarily causing net carbon emissions. And the intervals between these extreme droughts are getting shorter: in 2005 and 2010, for example, there were droughts that were each classified as "droughts of the century" when they occurred. This was followed by an even more severe drought in 2015 and 2016.
According to scientific calculations, such exceptionally dry years as 2005 and 2010 could occur in the Amazon as early as every two years from 2025 and become the new normal from 2050. Thus, according to forecasts from 2050 to 2060, a hundred-year drought is predicted in up to nine out of ten years. According to scientists at the Potsdam Institute for Climate Impact Research, this is likely to have serious consequences: Whole swaths of land could turn into savannah or even completely treeless. The consequences for biodiversity would be catastrophic - as would those for the local, regional and global climate.
Deforestation partly responsible for developments
The temperature in the Amazon has increased by an average of 0.5 degrees Celsius since 1980. Because climate change is bringing more frequent and severe droughts to the Amazon basin, the rainforest could lose some of its rainfall, making it even more vulnerable to drought. In a healthy ecosystem, for example, plants absorb most of the precipitation through their roots and release much of it back into the atmosphere through transpiration and evaporation via their leaves. The resulting water vapor creates new precipitation directly on site or in nearby areas, where the rainwater then evaporates in turn, leading to new precipitation.
The entire forest is interconnected in this circulation system. Consequently, damage in one region spreads to neighboring regions. According to current research results from the Potsdam Institute for Climate Impact Research, for every three trees that dry out in the Amazon rainforest, a fourth tree dies - even if it is not directly affected by drought. Thus, when droughts occur, only a small amount of precipitation is stored by the vegetation, evaporation as well as humidity is reduced, and less water vapor is transported further. The lack of rain during droughts also greatly reduces the water recycling volume, resulting in less precipitation in neighboring regions as well. Thus, more and more parts of the forest come under significant stress. Drought-triggered forest dieback in the northern part of the Amazon can thus lead to further dieback reactions further south or west. The problem is that the natural water cycle between the atmosphere and the rainforest accounts for up to 50 percent of the rainfall in the Amazon.
The situation is further exacerbated by human-induced deforestation and slash-and-burn clearing of the Amazon forest: the areas currently most at risk of drought and associated desertification are the southeastern and southwestern fringes of the Amazon rainforest, where human activities such as logging, deforestation and grazing have already been putting pressure on the forest ecosystem for years, exacerbating the problems. That's because the same is true of deforestation, according to the scientists: When one hectare of forest is cut down, 1.3 hectares is actually destroyed.
Diverse forests can stop downward spiral
In a study, a team from the Technical University of Munich (TUM), the Potsdam Institute for Climate Impact Research and the Stockholm Resilience Centre recommends protecting and restoring diversity in the Amazon at all costs because it maintains the ecosystem's resilience. Because each species responds differently to stresses, biodiversity acts as a buffer and can improve ecosystem resilience and limit a domino effect of forest dieback. This finding is also important for rainforest land management: Agricultural land in the form of monocultures is not diverse, not resilient, and cannot replace the functions of primary rainforest. Alternatively, agroforestry systems can be used to bring biodiversity to agricultural land and simultaneously reforest diversely.
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Source:
https://www.pnas.org/doi/full/10.1073/pnas.2120777119
https://www.tum.de/die-tum/aktuelles/pressemitteilungen/detail/33771