Deforestation drys continents - new theory explains how

Whilst many people have been aware (but have mostly been ignored) that vegetation, especially forests, creates rain, and whilst desertification has been linked to deforestation historically many times, there is a new and robust theory to explain how this may happen.
See also How logging causes forest fires

Original source of article:Rainforests may pump winds worldwide by Fred Pierce, New Scientist, 1st April 2009, issue no.2702.]


Whilst many thoughtful people have worked out (but have mostly been ignored) that vegetation, especially forests, create rain, and whilst desertification has been linked to deforestation historically many times, there is a new and robust theory to explain how this may happen.

The theory argues that forests generate winds that help pump water around the planet. This article says that the theory would explain how it rains as much in the interiors of forested continents as on the coasts.

"If correct, the theory would explain how the deep interiors of forested continents get as much rain as the coast, and how most of Australia turned from forest to desert. It suggests that much of North America could become desert - even without global warming."

The New Scientist article reports that "up to half the precipitation falling on a typical tropical rainforest evaporates or transpires from trees. ... Victor Gorshkov and Anastassia Makarieva of the St Petersburg Nuclear Physics Institute in Russia say that forests also create winds that pump moisture across continents."

How can forests create wind?

[Water vapour from coastal forests and oceans condenses into droplets and clouds. The gas takes up less space as it turns to liquid, lowering local air pressure.] Because evaporation is stronger over the forest than over the ocean, the pressure is lower over coastal forests, which suck in moist air from the ocean. This generates wind that drives moisture further inland. The process repeats itself as the moisture is recycled in stages, moving towards the continent's heart (see diagram). As a result, giant winds transport moisture thousands of kilometres into the interior of a continent.

Coastal forests create giant winds that push water thousands of kilometres inland

Huge volumes can be involved.

More moisture typically evaporates from rainforests than from the ocean. The Amazon rainforest, for example, releases 20 trillion litres of moisture every day.

New Scientist reports that Makarieva and Gorshkov told them that, "In conventional meteorology the only driver of atmospheric motion is the differential heating of the atmosphere. That is, warm air rises." "Nobody has looked at the pressure drop caused by water vapour turning to water." The scientists, [whose theory relies on the physics of air movement] have dubbed this the "biotic pump" and claim it could be "the major driver of atmospheric circulation on Earth".

To back up their hypothesis they show how regions without coastal forests, such as west Africa, become exponentially drier inland. Likewise, in northern Australia, rainfall drops from 1600 millimetres a year on the coast to 200 mm some 1500 kilometres inland. In contrast, on continents with large forests from the coast to interior, rainfall is as strong inland as on the coast, suggesting the trees help shuttle moisture inland (Ecological Complexity, DOI: 10.1016/j.ecocom.2008.11.004, in press). In the Congo, for instance, around 2000 mm of rain falls each year at the coast and the same amount falls inland. The same is true in the Amazon, the Siberian Arctic and the Mackenzie river basin in northern Canada. But the US, largely forested until recently, now seems be headed for desert. Makarieva and Gorshkov told New Scientist that without rapid reforestation "the degrading temperate forests of North America are on their way to desertification".

Read the full article here Consider reading, as well, two extraordinary books about deforestation and history. One is David Montgomery, Dirt, the erosion of civilisationsUC Press, 2008 and the other is John Perlin, A Forest Journey: The Role of Wood in the Development of Civilization, Harvard Univ Press, 2007. Dirt is the best written, but both are packed with information which will advance your state of knowledge way ahead of most environmentalists' and situate your understanding in a very wide context.