Africa Water, Sanitation & Hygiene Africa Water & Sanitation & Hygiene May -June 2017 | Page 34
Water Supply
Cool insights for a hot world: trees and forests
recycle water
By Daisy Ouya
Anyone who has walked outside on a sunny day knows that forests and trees matter for temperature, humidity and
wind speed. Planting trees speaks to concerns about climate change, but the directly important aspects of the tree-
climate relationships have so far been overlooked in climate policy where it relates to forest.
redistribute water, they simultaneously cool
planetary surfaces”, says Dr David Ellison,
lead author of the study.
Trees are giant air conditioners with no power bills.
“Some of the more refined details of
how forests affect rainfall are still being
discussed among scientists of different
disciplines and backgrounds. But the
direct relevance of trees and forests for
protecting and intensifying the hydrologic
cycle, associated cooling and the sharing
of atmospheric moisture with downwind
locations is beyond reasonable doubt.”
T
hat, at least, is the conclusion of a new review. The
authors suggest that the global conversation on
trees, forests and climate needs to be turned on
its head: the direct effects via rainfall and cooling may be
more important than the well-studied effects through the
global carbon balance.
Yet, current climate policy only recognizes the latter. While
farmers understand that trees cool their homes, livestock
and crops, they had to learn the complex and abstract
language of greenhouse gasses and carbon stocks if
they wanted to be part of climate mitigation efforts. Not
anymore, if the new perspectives become widely accepted.
Water should become the primary motivation for growing and
conserving trees.
In the review, published in the journal Global
Environmental Change, the 22 authors provide examples
for the planet-cooling benefits of trees. Scientists found
evidence for the widespread perception that trees and
forests also influence rainfall. As such, the review insists
that water, and not carbon, should become the primary
motivation for adding and preserving trees in landscapes.
“Carbon sequestration is a co-benefit of the precipitation-
recycling and cooling power of trees. As trees process and
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Africa Water, Sanitation & Hygiene • May - June 2017
Trees are giant air conditioners with no
power bills. They use solar energy to
convert water into vapour, thereby cooling
their surroundings. On a hot day the
surface temperature of a forest—in an example discussed
in the paper—is similar to that of a nearby lake, while a
dry patch of meadow or a tarmac road in the vicinity are
more than 20 °C hotter. The cooling power equivalent is
around 70 kWh for every 100 liter of water transpired,
similar to the output of two home air-conditioning units.
“There are important implications for practice, as we
can no longer simply focus on carbon sequestration to
mitigate or adapt to climate change”, says Dr Victoria
Gutierrez, Chief Science Officer of the WeForest NGO
that supports forest landscape restoration efforts in
tropical countries, and co-author of the study.
“For organizations and agencies working to restore forest
ecosystems for climate and people, it is crucial that we
pay greater attention to the sustainability of the water
processing and cooling aspects of the trees.”
Rainbow water
Planting trees has long been an expression of intent to
do something of substance in the climate change debate;
scientists have found a new rationale for this
As they cool the planet, trees may also promote rainfall.
Two ingredients for rainfall are: i) water vapour in the