42 | New Scientist | 2 November 2019
along the coast has remained stable, there has
been a 20 per cent decline inland, leaving
reservoirs that supply Perth parched, says
Jorg Imberger, former director of the Centre
for Water Research at the University of
Western Australia.
Why does the loss of forest hold such sway
over rainfall? Hydrologically speaking, trees
are giant water fountains. A single tree
typically transpires hundreds of litres of water
a day. Transpiration is a process by which
growing trees take water from the ground and
release it into the atmosphere through their
leaves. What has only recently become clear is
that transpiration is a major source of water to
the atmosphere, and is responsible for around
half of all precipitation, up to 60,000 cubic
kilometres of water a year, says Scott Jasechko
at the University of California, Santa Barbara.
“Transpiration moves more water than all the
world’s rivers combined,” he says.
Indeed, some physicists say that the
condensation of moisture in clouds above
transpiring forests creates air pressure changes
that draw in air and strengthen the winds that
take the moisture inland. This idea, known
as the biotic pump, has its detractors, but
Deborah Lawrence at the University of Virginia
says it suggests another reason why even
small-scale deforestation, if it occurs in coastal
areas, could disrupt the movement of moisture
inland. Sheil says a weakened pump might
explain declining surface winds seen across
many land areas in recent times.
Clearly, in a world where forests are being
lost and fresh water is in ever shorter supply,
tracking atmospheric moisture matters. The
first global attempt at this was made by Ruud
van der Ent at Delft University of Technology in
the Netherlands. He combined meteorologicaldata with a computer model of atmospheric
moisture flow to figure out the main source
and sink regions for moisture and the routes
of the main flying rivers that transport it.
Key source regions include western North
America, eastern Africa, Europe, western Asia,
India and, above all, the Brazilian Amazon.
Flying rivers often take this water long
distances. Around 70 per cent of the water in
the River Plate basin, which stretches from
southern Brazil through Bolivia, Paraguay andUruguay to Buenos Aires in Argentina, comes
from transpiration in the Amazon. China gets
the moisture for over 80 per cent of its rain
from far to the west in the forests of Siberia
and Scandinavia, a journey involving several
stages of water recycling by trees and taking
six months or more.
“The China finding was among my first,
and it was a real eye-opener,” says van der Ent.
“We learn in high school that rainfall comes
from the oceans. China is next to an ocean,
yet most of its rainfall is moisture recycled
from the land far to the west.”
Transpiration from forests may be crucial
for relieving droughts and ending dry seasons,
says Wang-Erlandsson. When there is no rain,
evaporation from soils, and transpiration fromFlying rivers
Meteorologists have mapped vast atmospheric jets that transport water
across the world. These are some of the major flowsshallow-rooted grasses and crops, ceases. But
tree roots tap into water deeper underground.
So trees keep transpiring, providing moisture
to relieve drought downwind. No trees means
more drought.
This isn’t just speculation. “In the Amazon,
the dry seasons are getting drier,” says Jessica
Baker at the University of Leeds, UK. And
longer. Where forests have been replaced with
cattle pasture and soya fields, they last an extra
month. The worry is that beyond a tipping
point – some models suggest a 20 to 25 per cent
loss of forest could be critical for the Amazon –
tree loss could turn the climate into one where
only savannah grassland thrives.
The Amazon is unlikely to be alone. In
South-East Asia, deforestation has removed
half of Borneo’s forests in the past half a
century. That has coincided with declining
rainfall, with both trends accelerating since the
1970s. “Watersheds with more than 15 per cent
forest loss had a more than 15 per cent
reduction in rainfall” compared with those
with intact forests, says Clive McAlpine of
the University of Queensland, Australia.
The central African region, more dependent
on moisture recycling than the Amazon,
has seen a persistent decline in rainfall“ Transpiration
moves more
water than all
the world’s
rivers combined”