Keep rainforests ? they drive the planet's winds

WHAT creates the wind? Anyone will tell you that temperature differences are key. Hot air rises and is replaced by cooler air surging in beneath. Except that maybe the explanation found in every textbook is too simple.

What if, instead, the winds that drive atmospheric circulation are mainly created by the condensation of moisture? Much of this occurs over rainforests as water evaporates or is transpired from the trees. The physicists and foresters behind this controversial idea say that if we chop down the forests, we will lose the winds - and the rains they bring with them.

The physical process itself is not in dispute. Whenever water vapour condenses to form droplets, its volume is reduced, lowering the pressure. Air moves in, creating wind.

Climate scientists have always regarded this as a trivial effect. Criticism has been heaped on the theory since it was first aired four years ago. "This is not a mysterious effect. It is small and included in some atmospheric models," says Isaac Held of the US National Oceanic and Atmospheric Administration in Princeton, New Jersey.

But physicist Anastassia Makarieva of St Petersburg University in Russia says the pressure gradients it would create "have never received a theoretical investigation". Her calculations suggest that the condensation of billions of litres of water above giant forests produces a giant effect (Atmospheric Chemistry and Physics, doi.org/kbx).

Co-author Douglas Sheil at Southern Cross University in Lismore, Australia, says critics have yet to explain why they think Makarieva is wrong. Until they do, he said, "this looks like a powerful mechanism that governs weather patterns round the world".

Judith Curry of the Georgia Institute of Technology, an author of the standard textbook Thermodynamics of Atmospheres and Oceans, is encouraging. "The process they describe is physically correct," she said. "The main question is its relative magnitude compared with other processes." She thinks it could explain why climate models do not get monsoons and hurricanes right.

Nobody doubts that forests recycle rain through evaporation and transpiration. But this is the first suggestion that this recycling process also whips up the winds that suck moist ocean air across continents.

The implications are huge. "In standard theories, if we lose forests the rainfall in the continental interiors generally declines by 10 to 30 per cent. In our theory, it is likely to decline by 90 per cent or more," says Sheil.

But it is not all bad news. If lost forests are replanted, the theory suggests, then the winds they generate could return rain to even the most arid lands. After all, the Sahara was lush swampland 6000 years ago.

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