A Wildlife Conservation Society researcher has leant some new meaning to the word “rainforest.” Turns out, says WCS’s Douglas Sheil, these moist landscapes may actually be generating most of their own rain, rather than merely collecting the heavy rainfall in the surrounding environment, as climatologists long assumed.
Climate experts have largely ignored a recent study on how rainforests influence regional hydrology—called the “biotic pump.” That study, by Russian researchers Anastassia Makarieva and Victor Gorshkov, explains how large rainforests such as the Amazon and Congo Forest Basin could drive the global water cycle. Sheil, who also directs the Institute of Tropical Forest Conservation, gives new support to the overlooked theory in his article “How Forests Attract Rain: An Examination of a New Hypothesis,” recently published in BioScience Magazine.
“If proven, this theory could explain why continental interiors with huge rainforests remain so moist, which will lead to more accurate predictive models of climate,” said Sheil. “It could also underline the dangers of widespread deforestation in the world’s major rainforests.”
Sheil’s work explains the physics of how forested regions pull in enormous quantities of water vapor from adjacent regions and water bodies. As water vapor condenses into rainfall, the atmospheric pressure drops, and draws in more water vapor from surrounding areas. More water condenses than evaporates, creating a positive feedback loop.
The hypothesis explains how in coastal areas without forests, water vapor is drawn out toward the ocean where condensation levels are much higher and atmospheric pressures lower. This leaves the continental interiors in such places as Patagonia, Australia, and West Africa—where the coastlines are not forested—much drier.
By contrast, forested coasts draw in vapor from the ocean, and the largest swaths of rainforest, such as the Amazon, transport enormous quantities of water inland. This highly important role of rainforests is drastically curtailed when forests are cut down. Makarieva and Gorshkov starkly predict that rainfall in interior regions will decrease by more than 95 percent when significant deforestation occurs, as opposed to the 20 to 30 percent drop predicted by conventional climate models.
Sheil points out that additional data from several fields are needed to further support the biotic pump theory. Specifically, he refers to how variations in landform and vegetation cover types influence air circulation patterns; the role of fire damage in forest degradation; the role of different vegetation types (i.e. coniferous and deciduous forests) in climate; and an examination of past climates, especially instances where human activities have altered regional rainfall and climate.
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