Clearing Forests May Transform Local—and Global—Climate

http://www.scientificamerican.com/article.cfm?id=clearing-forests-may-transform-local-and-global-climate&WT.mc_id=SA_CAT_ENGYSUS_20130307
Clearing Forests May Transform Local�and Global�Climate
Researchers are finding that massive deforestation may have a
profound, and possibly catastrophic, impact on local weather

By Judith D. Schwartz


In the last 15 years 200,000 hectares of the Mau Forest in western
Kenya have been converted to agricultural land. Previously called a
�water tower� because it supplied water to the Rift Valley and Lake
Victoria, the forest region has dried up; in 2009 the rainy season�
from August to November�saw no rain, and since then precipitation has
been modest. Whereas hydropower used to provide the bulk of Kenya�s
power ongoing droughts have led investors to pull out of hydro
projects; power rationing and epic blackouts are common. In a
desperate move to halt environmental disaster by reducing population
pressure, the Kenyan government evicted tens of thousands of people
from the land.
Severe drought, temperature extremes, formerly productive land gone
barren: this is climate change. Yet, says botanist Jan Pokorny of
Charles University in Prague, these snippets from Kenya are not about
greenhouse gases, but rather the way that land-use changes�
specifically deforestation�affect climate; newly tree-free ground
�represents huge amounts of solar energy changed into sensible heat,
i.e. hot air.� Pokorny, who uses satellite technology to measure
changes in land-surface and temperatures, has done research in western
Kenya for 25 years, and watched the area grow hotter and drier. The
change from forest cover to bare ground leads to more heat and
drought, he says. More than half the country used to be forested; it's
now less than 2 percent.

Each year Earth loses 12 million to 15 million hectares of forest,
according to the World Wildlife Fund, the equivalent of 36 football
fields disappearing per minute. Although forests are ebbing throughout
the world, in Africa forest-climate dynamics are easily grasped:
according to the United Nations Environmental Programme, the continent
is losing forests at twice the global rate. Says Pokorny, the
conversion of forest to agricultural land, a change that took
centuries in Europe, �happened during one generation in western
Kenya.� Pokorny's work, coupled with a controversial new theory called
the �biotic pump,� suggests that transforming landscapes from forest
to field has at least as big an impact on regional climate as
greenhouse gas�induced global warming.

After all, de-treeing the landscape alters the way ecosystems function
and self-regulate. For Pokorny, the key is evapotranspiration, whereby
plants continuously absorb and emit water in the form of vapor.
Evaporation consumes heat and thus has a cooling effect. He calls this
"the perfect and only air-conditioning system on the planet." On a
moderately sunny day, a tree will transpire some 100 liters of water,
converting 70 kilowatt-hours of solar energy into the latent heat held
in water vapor. When soil is bare and dry�paved over or harvested�the
process comes to a halt. The sun hits and warms the ground directly.

This past November found Pokorny flying a small Cessna from Lake
Naivasha up the hills to the Mau Forest, where land surface
temperatures in woodlands measured 19 degrees C; agricultural land
that until recently had been forest hovered close to 50 degrees C. A
photo taken from the air shows the dark green of forests diminish
along the slope to the lowlands; the valley has clusters of deep
forest green among the broad, pale, geometric shapes of cultivated
land. His team measures surface temperature as opposed to the usual
air temperature metric, two meters above. The surface �is what you are
in contact with, and creates the dynamic movement of air,� he says�and
ground temperature �indicates the way solar radiation is transformed
at the Earth's surface.� His surveys from above combine three
measures: �a normal camera, Thermovision [thermal infrared sensors]
and our eyes. In putting the pictures together, we see the high
temperatures are where there is no vegetation,� which includes swaths
of land where forest has been cut.



The "biotic pump" theory argues that natural forests act as a �pump�
that draws moisture inland. According to this concept, first described
in a 2007 paper by Russian physicists Victor Gorshkov and Anastassia
Makarieva of the Saint Petersburg Nuclear Physics Institute in the
peer-reviewed Hydrology and Earth System Sciences, condensation,
rather than temperature differential, is a primary driver of weather.

Here's a snapshot of the concept: The concentration of trees in wooded
areas means a high rate of transpiration. This moist air cools as it
ascends and the water vapor condenses, producing a partial vacuum.
This creates an air pressure gradient, whereby the forest canopy sucks
in moist air from the ocean. According to Gorshkov and Makarieva,
forests don't merely grow in wet areas, they create and perpetuate the
conditions in which they grow. Without forest cover�specifically
mature, natural forest to ensure sufficient biomass and resilience�
moisture is no longer pulled in, the physicists say. Rain becomes
erratic and ultimately stalls.

The Russian scientists associate the unprecedented heat and drought in
their country over the last few years with rapid deforestation in
western Russia. The theory is controversial; indeed, it challenges the
viability of the climate models currently in use. The theory �explains
why in forested regions precipitation does not decrease with distance
from the ocean, even thousands of kilometers, while the interiors of
deforested parts of continents become dry already a few hundred
kilometers away from the oceanic coast,� they wrote in an e-mail.
"Condensation of water vapor over forests creates pressure gradients
that have been shown to be sufficient to drive winds that bring
moisture from ocean to land."

Should the biotic pump be confirmed by further research, it brings new
urgency to the need to protect forests. �Most climate models recognize
the role of "precipitation cycling" in forests, but not moisture
transport by forests,� Makarieva and Gorshkov say. The difference is
significant: if deforestation means simply reduced evaporation, the
decline of precipitation would be significant but not catastrophic,
around 15 percent; rains depend on imported moisture, however. If the
vehicle for transport�an intact forest�is impaired, that's a different
story. The physicists say: �In our theory, imported moisture will
decline if the forest is destroyed, especially in the inland portion
of the continent. If there is no imported moisture there is nothing to
be evaporated, so the water cycle will undergo a dramatic�not minor�
reduction of intensity.� In the Amazon, they add, this could be up to
90 percent.

Such ideas are not new. In his 1864 book Man and Nature (original
title: Man the Disturber of Nature's Harmonies), George Perkins Marsh
catalogued numerous examples of changing climate conditions on losing
forests and wrote, �When the forest is gone, the great reservoir of
moisture stored up in its vegetable mold [humus] is evaporated, and
returns only in deluges of rain to wash away the parched dust into
which that mold has been converted. The well-wooded and humid hills
are turned to ridges of dry rock[.]�

Back in Kenya Sarah Higgins, a conservationist who runs the Little Owl
Sanctuary for injured birds near Lake Naivasha, says she's seen
weather patterns change with the forest's fortunes. When she started
farming 30 years ago �we were almost guaranteed sufficient rainfall
for our crops.� Then came the destruction of the Mau Forest, and the
area above and on either side of the farm was �denuded of trees and
overgrazed, down to bare Earth. Our regular rainfall started to fail
and we were seeing dry years, poor yields and more droughts.�

Then there is the need to better understand the specific function of
forests and even individual tree species. For example, can planted
trees have the same effect on hydrology as intact natural forest? �A
practical question is whether we're able to mimic effects of forests,�
Pokorny says. �For we can't just have forests�we need agricultural
land. Can other land types, like savanna and high biomass grassland,
serve some functions of trees? Are there pioneer tree species that
create microclimates that help other trees grow?�

Deforestation has numerous untoward environmental consequences,
including the release of carbon: about one sixth of global emissions
are due to cleared or degraded forests. Such deforestation also
destroys the habitat for vast variety of species, and threatens the
welfare of the more than a billion people who rely on forests for
their livelihoods. It may also have unforeseen impacts on the water
cycle�which can in turn alter climate patterns. "If we don't pay
attention to this now, we could lose our forests," Makarieva told me
by phone. "This would make disaster come faster by destroying the
water cycle."
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