The Rainforest That Makes Its Own Rain

Picture the Amazon from above: a green canopy stretching to the horizon, rivers winding through it like ribbons. The basin covers at least six million square kilometres, nearly twice the size of India, and its rainforest is home to around one-fifth of the world's land species, many of them found nowhere else on Earth.

But the forest is not simply soaking up the rain that falls on it. In a very real sense, it helps to make it.

It begins underground. Rainforest trees draw water from the soil through their roots and carry it up through their trunks to millions of leaves. Tiny pores on those leaves open to take in carbon dioxide for photosynthesis, and as they do, water vapour escapes into the air. Scientists call this transpiration, and across a forest of this size it adds up to an extraordinary amount of moisture.

In the heat of the day, warm air rising from the forest carries that vapour upward. As the air climbs, it cools, and the vapour condenses into clouds. NASA satellites have photographed neat fields of small "popcorn" clouds forming over the Amazon on dry-season afternoons, likely built from water released by trees and other plants. Remarkably, the clouds tend not to form over the rivers, where cooler air sinks instead of rising.

Those clouds return the water as rain, replenishing the soil so the trees can draw it up again. NASA scientists describe rainforests as generating as much as 80 per cent of their own rain, especially during the dry season. Moisture rising from the Amazon also feeds vast airborne currents, sometimes called "flying rivers", and helps supply about half of the rain that falls back on the region.

That is why this cycle is so precious, and so fragile. A NASA study found that the air above parts of the Amazon has become drier over recent decades, largely because of human activity. Fewer healthy trees mean less moisture rising, fewer clouds forming and less rain returning.

Protecting forests, then, is not only about saving trees and wildlife. It is about protecting a living engine of the water cycle, one leaf, one cloud and one rainfall at a time.