From: Fred Pearce, NewScientist
Published April 3, 2009 09:20 AM

Rainforests may pump winds worldwide

THE acres upon acres of lush tropical forest in the Amazon and tropical Africa are often referred to as the planet's lungs. But what if they are also its heart? This is exactly what a couple of meteorologists claim in a controversial new theory that questions our fundamental understanding of what drives the weather. They believe vast forests generate winds that help pump water around the planet.

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 idea makes it even more vital that we recognise the crucial role forests play in the well-being of the planet.

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. Ocean winds can spread the moisture to create more rain. But now 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 quickly condenses to form droplets and clouds. The Russians point out that 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. As a result, giant winds transport moisture thousands of kilometres into the interior of a continent.

The volumes of water involved in this process can be huge. More moisture typically evaporates from rainforests than from the ocean. The Amazon rainforest, for example, releases 20 trillion litres of moisture every day.

"In conventional meteorology the only driver of atmospheric motion is the differential heating of the atmosphere. That is, warm air rises," Makarieva and Gorshkov told New Scientist. But, they say, "Nobody has looked at the pressure drop caused by water vapour turning to water." The scientists, whose theory is based on the basic physics that governs air movement have dubbed this the "biotic pump" and claim it could be "the major driver of atmospheric circulation on Earth". This is a dramatic claim. The two Russians argue that their biotic pump underlies many pressure-driven features of the tropical climate system, such as trade winds, and helps create intense local features like hurricanes.

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".

The Russians' ideas have languished since they were published in a small journal in 2007. "We are facing enormous difficulties in overcoming the initial resistance of the scientific community," they say. Antoon Meesters of the Free University in Amsterdam, the Netherlands, recently described it as "an untenable result of confused principles". Meesters does not dispute the physics behind the Russians' theory but claims the effect is negligible.

This week, a leading British forest scientist based at the Institute of Tropical Forest Conservation in Kabale, Uganda, came to the Russians' aid. In a review of the work in the journal Bioscience (DOI: 10.1525/bio.2009.59.4.12), Doug Sheil and his co-author Daniel Murdiyarso underline the importance of the idea. "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."

Sheil explains that current theory doesn't explain clearly how the lowlands in continental interiors maintain wet climates. "There is a missing element," Sheil says. The biotic pump "may be the answer". He calls the Russians' findings "a most profound insight into the impact of forest loss on climate. They will transform how we view forest loss, climate change and hydrology."

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