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 (see diagram). As a result, giant winds transport moisture thousands of kilometres into the interior of a continent.