A study from the St Petersburg Nuclear Physics Institute in Russia says vast forests generate winds through vapor pressure differences that help pump water around the planet.
If correct, the theory would explain how forested continents get as much rain inland as they do on the coast, and why Australia with its limited coastal forests has a vast, almost treeless outback.
According to the research, as reported by Mongabay.com, scientists have known for some time that forests recycle rain. Up to half the precipitation falling on a typical tropical rainforest evaporates or transpires from trees. This keeps the air above moist.
But Victor Gorshkov and Anastassia Makarieva of the St Petersburg Nuclear Physics Institute are now saying that not only do forests pump moisture into the atmosphere above, they also generate the winds that carry this moisture inland.
According to their theory, when water vapour from coastal forests and oceans condenses to form droplets and clouds, it takes up less atmospheric space, thus lowering the local air pressure. Furthermore, with evaporation being stronger over the forest than over the ocean, the resulting low air pressure sucks in the moist air over the ocean. This causes the winds that carry the moisture further inland.
Although published in 2007, Gorshkov and Makarieva's research is only now beginning to capture the attention of international scientists. Chief among these have been Dr. Doug Sheil, formerly of CIFOR and now with Uganda's Institute of Tropical Forest Conservation, and CIFOR's Dr. Daniel Muridyarso.
In a review of the research How Forests Attract Rain: an examination of a new hypothesis, published in the journal Bioscience, Sheil and Murdiyarso say that the controversial suggestion that forests act as wind pumps needs to be given serious consideration.
"Conventional models typically predict a 20 to 30 per cent decline in rainfall after deforestation," Sheil says. "Makarieva and Gorshkov suggest even localised clearing might ultimately switch entire continental climates from wet to arid, with rainfall declining by more than 95 per cent."
According to Sheil, science is still unable to fully and clearly explain how the lowlands in continental interiors maintain wet climates. Gorshkov and Makarieva's suggestion that forests serve as "biotic pumps" may provide science with the missing answer.
Sheil describes the Russians' findings as "a most profound insight into the impact of forest loss on climate. They will transform how we view forest loss, climate change and hydrology."
Co-author, Daniel Murdiyarso, adds, “This is a new way of looking at the co-lateral benefits provided by forests when they are left standing. It’s particularly relevant to the current global focus on promoting the carbon market through REDD” (Reducing Emissions from Deforestation and forest Degradation).
Murdiyarso says that the findings would trigger further debates on the way the traditional wisdoms and public policy-making are linked with the roles of remaining forests in the tropics.
References:
Sheil D., and Murdiyarso, D. (2009) How forests attract rain: an examination of a new hypothesis. Bioscience. 59: 341–347
Makarieva AM,GorshkovVG. 2007. Biotic pump of atmospheric moisture as driver of the hydrological cycle on land.Hydrology and Earth System
Sciences 11: 1013-1033 (in English)
http://news.mongabay.com/2009/0401-ance_revolutionarytheory.html
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