Foggy Amazon valleys offer refuge from climate change

Fog is particularly prevalent in lowland basins of the Amazon rainforest. This additional water vapor can prevent moisture-dependent plants from drying out. As global warming will increase the frequency of droughts, these fog-rich valleys thus offer a refuge for biodiversity in the Amazon. This also helps preserve the rainforest as a bulwark against climate change. These are the conclusions of research published in the journal Communications Earth & Environment. The study was carried out by a German-Belgian research consortium led by the University of Marburg and LMU using observation data from satellites.

The Amazon rainforest not only sequesters large amounts of greenhouse gases, but also hosts a huge variety of species, making it a global biodiversity hotspot. “But climate change and human activities are threatening this habitat,” observes University of Marburg environmental geographer Professor Jörg Bendix, who led the project. “A general difficulty is that little data is available about this rather inaccessible habitat. That’s why our study relies primarily on satellite data,” adds Lukas Lehnert, Professor for Physical Geography and Environmental Remote Sensing at LMU.

Periods of drought occurred in the region in 2005 and 2010. And during the severe 2015/16 drought caused by the El Niño phenomenon, there was a shortage of rainfall all over Amazonia. On top of this come the forest clearances, which are proceeding largely unchecked. Positive feedback loops between deforestation and rainfall reductions are exacerbating the situation, emphasizes Bendix: “As some of the rainfall is caused by evaporation from the forest itself, the combination of climate change and deforestation risks producing a self-reinforcing downward spiral.” This could lead to the dieback of large sections of the rainforest. Not only would this reduce biodiversity – but because the rainforest stores substantial amounts of the greenhouse gas CO2, the loss of this habitat would further drive climate change.

Fog in lowland forests provides stable conditions in a warming climate with reduced rainfall. Such moist refuges ensure the survival of species that would otherwise struggle with climate change. This applies in particular to lichens, mosses, and other epiphytes in canopies, which are typical for tropical rainforests. For these organisms, fog is an important source of water, as they quickly lose moisture when the atmosphere is dry. “Fog improves the supply of water to epiphytes by furnishing a dark, cool, and moist environment,” explains Bendix.

“However, it was uncertain, where this source of moisture occurs most frequently and whether it would prove to be resilient in the face of increasingly dry conditions due to climate change,” adds Lehnert. “We analyzed drought events from the past 18 years using satellite data,” explains co-author Marius Pohl from Bendix’s research group. “Our studies revealed that fog is particularly prevalent in the valleys of the Amazon basin.”

As the analysis shows, such lowland cloud forests are spread across the entire Amazon region, although they become scarcer during the dry season. Fog is best preserved in depressions in the landscape, where it proves to be resilient against droughts. “The reason for this resilience is that fog formation works particularly well when nocturnal flows of cold air accumulate in valleys, which means that condensation and fog formation can occur even under extensive dry conditions,” says Lehnert. “Based on our results we highly recommend protecting these moist refuges, particularly in vulnerable sectors,” write the authors.

Marius J. Pohl, Lukas W. Lehnert & al.: Valleys are a potential refuge for the Amazon lowland forest in the face of increased risk of drought, Communications Earth & Environment, 2023