Rainforests exist because it rains a lot and that makes the 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’s 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’s 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 oceans stay so moist. The info would help climate models. And highlight the potential dangers of deforesting large parts of the pump.

—Steve Mirsky