Rainforests are often described as the lungs of the Earth but they also perform another extraordinary function that receives far less attention.
How do trees make it rain?
Through their roots, trees draw vast quantities of water from the soil - acting like giant natural pumps - which travel up the trunk and into the leaves. As the tree breathes, tiny pores on the surface known as stomata then release it into the atmosphere as a vapour through a process called transpiration.
Across an entire forest, billions of trees collectively release enormous amounts of moisture into the air every day. But individual isolated trees also play a vital part - albeit on a much tinier scale.
As the vapour rises, it cools and condenses to form clouds before falling again as rain. In tropical rainforests, this recycling is so effective that around 40% of the Amazon's rainfall is generated by the forest itself. Scientists estimate that a single raindrop can fall, evaporate and be recycled through the forest five or six times as it journeys across the basin.
- It contains over half of Earth’s water and all of the continents could fit into its basin in one mega-puzzle
- They contain a quarter of all freshwater on Earth – but home to hundreds of ocean-like creatures
The rain released by trees does not simply fall back to Earth where it originated. Winds can carry clouds across continents in vast atmospheric currents known as aerial rivers - hundreds or even thousands of miles away - often carrying more water than the largest rivers.
The Amazon's flying rivers, for example, supply much of the rain that supports large parts of South America, while similar processes occur in the Congo Basin and Southeast Asia. Scientists have observed that air passing over dense tropical forests produces around twice as much rain as air travelling over sparsely vegetated land.
Some researchers describe forests as part of a "biotic pump" – the idea that forests not only recycle water but also help draw moist air inland from the oceans by creating areas of lower atmospheric pressure. It is so effective that downpours in some large areas, such as the Amazon Basin, are almost as heavy thousands of miles inland as they are on the coast.
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Peter Wohllben, author of The Hidden Life of Trees, explains that in order for this water transfer phenomenon to take place there needs to be forests, including coastal ones, to kick off the process. Without them, the chain reaction will collapse.
As forests are cleared, less water is returned to the atmosphere, reducing cloud formation and rainfall. This can lengthen dry seasons, increase drought risk and make remaining forests more vulnerable to fire, creating a damaging feedback loop.
