SSS 2009-05-07（在线收听）

Rainforests exist because it rains a lot and that makes forests grow, right? Well, not so fast. What if it’s not the rain that makes the forests? What if it’s the forests that actually generate the rain? That is the contention of a paper in BioScience magazine called “How forests attract rain”. The article discusses a mostly overlooked hypothesis that, if right, would explain how big rainforests--like the Amazon--actually drive the entire global water cycle.

Here is the idea. Forests pull in large amounts of water vapor from surrounding regions and from nearby bodies of water. As the vapor condenses into rain, the local atmospheric pressure drops, which sucks in more water vapor from outside the forest, which repeats the process, creating a positive feedback loop. The whole rainforest water vapor system is called a biotic pump because the living forest matter is what’s moving the water. If proven, the biotic pump hypothesis could explain how big rainforests far from the oceans stay so moist. The info would help climate models and highlight the potential dangers of deforesting large parts of the pump.

Thanks for the minute for Scientific American's 60-second Science. I'm Steve Mirsky.