The Amazon rainforest is home to strange weather. One peculiarity is that rains begin 2 to 3 months before seasonal winds start to bring in moist air from the ocean. Now, researchers say they have finally figured out where this early moisture comes from: the trees themselves.
The study provides concrete data for something scientists had theorized for a long time, says Michael Keller, a forest ecologist and research scientist for the U.S. Forest ServiceÂ based in Pasadena, California, who was not involved with the work. The evidence the team provides, he says, is â€œthe smoking gun.â€
Previous research showed early accumulation of moisture in the atmosphere over the Amazon, but scientists werenâ€™t sure why. â€œAll you can see is the water vapor, but you donâ€™t know where it comes from,â€ says Rong Fu, a climate scientist at the University of California, Los Angeles. Satellite data showed that the increase coincided with a â€œgreeningâ€ of the rainforest, or an increase in fresh leaves, leading researchers to suspect the moisture might be water vapor released during photosynthesis. In a process called transpiration, plants release water vapor from small pores on the underside of their leaves.
Fu thought it was possible that plants were releasing enough moisture to build low-level clouds over the Amazon. But she needed to explicitly connect the moisture to the tropical forest.
So Fu and her colleagues observed water vapor over the Amazon with NASAâ€™s Aura satellite, a spacecraft dedicated to studying the chemistry of Earthâ€™s atmosphere. Moisture that evaporates from the ocean tends to be lighter than water vapor released into the atmosphere by plants. Thatâ€™s because during evaporation, water molecules containing deuterium, a heavy isotope of hydrogen made of one proton and one neutron, get left behind in the ocean. By contrast, in transpiration, plants simply suck water out of the soil and push it into the air without changing its isotopic composition.
Aura found that the early moisture accumulating over the rainforest was high in deuteriumâ€”â€œtoo high to be explained by water vapor from the ocean,â€ Fu says. Whatâ€™s more, the deuterium content was highest at the end of the Amazonâ€™s dry season, during the â€œgreeningâ€ period when photosynthesis was strongest.
The tree-induced rain clouds could have other domino effects on the weather. As those clouds release rain, they warm the atmosphere, causing air to rise and triggering circulation. Fu and colleagues believe that this circulation is large enough that it triggers the shift in wind patterns that will bring in more moisture from the ocean, they report in the Proceedings of the National Academy of Sciences.