Trees In The Amazon Make Their Own Rain

The Amazon
rainforest is home to strange weather (very very 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 Service based in Pasadena, California, who was not involved with the
work. The evidence the team provides, he says, is “the smoking gun.”

Previous research
showed early accumulation of moisture in the atmosphere over the Amazon, but
scientists weren’t sure why. “All you can see is the water vapor, but you don’t
know where it comes from,” says Rong Fu, a climate scientist at the University
of California, Los Angeles. Satellite data showed that the increase coincided
with a “greening” 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 NASA’s Aura satellite,
a spacecraft dedicated to studying the chemistry of Earth’s atmosphere.
Moisture that evaporates from the ocean tends to be lighter than water vapor
released into the atmosphere by plants. That’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
deuterium—“too high to be explained by water vapor from the ocean,” Fu says.
What’s more, the deuterium content was highest at the end of the Amazon’s dry
season, during the “greening” period when photosynthesis was strongest.

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.

have studied the connection between trees and rain in the Amazon before. A 2012
study found that plants help “seed” the atmosphere for rain by releasing tiny
salt particles. But the new study strongly supports the idea that plants play
an important role in triggering the rainy season, says Scott Saleska, an
ecologist at the University of Arizona in Tucson, who was not involved with the
work. The deuterium provides a clear “fingerprint” for what plants contribute
to the process, he says.

The findings
also address a long-standing debate about the role plants play in weather, says
Saleska, suggesting that they are more than just “passive recipients,” and that
they instead can play an active role in regulating rainfall. If that’s true in
the Amazon, Saleska says, climate scientists will need to take into account
practices like deforestation when predicting regional changes in weather
patterns. And curbing deforestation will be an important step for people to
take in preventing drought.

Article was
originally published on Sciencemag. Read the original article here.