How does the Amazon make its own rain, and what are flying rivers?

The Amazon makes a large share of its own rain by recycling water through its trees. Roots take up water from the soil, and tiny pores on the leaves release unused water as vapor into the air. This is called evapotranspiration. An average forest tree releases about 1,000 litres of water vapor into the air each day, and a patch of forest evaporates about eight times as much water as the same area of sea, because the crowns have many layers of leaves.

Winds then carry that vapor across the continent, and when it condenses it falls as rain somewhere else. This stream of vapor is called a flying river. Air flows mostly southwest across the Amazon basin until it meets the Andes, which act as a natural barrier and redirect huge volumes of humid air towards Colombia, Ecuador, Peru, central and southern Brazil, northern Argentina, Paraguay and Uruguay, where rain falls. Estimates for how much of the Amazon's own rain comes from recycled forest water range from 24 to 56 percent.

The idea has a human story. In 2007 a Swiss-Brazilian pilot, Gérard Moss, joined scientists to start the Flying Rivers Project, to trace the source of the water vapor over Brazil and to test whether recent droughts were linked to clearing the Amazon. In 2009 the Brazilian climate scientist Antonio Nobre said that without the flying river much of southern Brazil, which produces roughly 70 percent of the country's GNP, would be arid desert. The droughts of southern Brazil in 2010 and 2014 to 2015 are believed to be linked to the drying of the flying rivers.

Not everything about it is settled. A related and bolder idea, the biotic pump, says that forests do not just make rain but create the winds that pull moist air inland from the ocean. Critics argue that differential heating is enough to explain the winds and that the biotic pump effect is relatively weak.