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LANDCOVER
CHANGES MAY RIVAL GREENHOUSE GASES AS CAUSE OF CLIMATE CHANGE
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While
many scientists and policy makers have focused only on how
heat-trapping gases like carbon dioxide are altering our global
climate, a new NASA-funded study points to the importance
of also including human-caused land-use changes as a major
factor contributing to climate change.
Land
surface changes, like urban sprawl, deforestation and reforestation,
and agricultural and irrigation practices strongly affect
regional surface temperatures, precipitation and larger-scale
atmospheric circulation. The study argues that human-caused
land surface changes in places like North America, Europe,
and southeast Asia, redistribute heat regionally and globally
within the atmosphere and may actually have a greater impact
on climate than that due to anthropogenic greenhouse gases
combined.
The
study also proposes a new method for comparing different human-influenced
agents of climate change in terms of the redistribution of
heat over land and in the atmosphere. Using a single unit
of measurement may open the door to future work that more
accurately represents human-caused climate change.
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"Our
work suggests that the impacts of human-caused landcover changes
on climate are at least as important, and quite possibly more
important than those of carbon dioxide," said Roger Pielke,
Sr., an atmospheric scientist at Colorado State University,
Fort Collins, Colo., and lead author of the study. "Through
landcover changes over the last 300 years, we may have already
altered the climate more than would occur associated with
the radiative effect of a doubling of carbon dioxide."
If carbon dioxide (CO2) emissions continue at current rates,
atmospheric CO2 concentrations are expected to double by 2050.
Land surface changes will also continue to occur.
Types
of land surface strongly influence how the Sun's energy is
distributed back to the atmosphere. For example, if a rainforest
is removed and replaced with crops, there is less transpiration,
or evaporation of water from leaves. Less transpiration leads
to warmer temperatures in that area. On the other hand, if
farmland is irrigated, more water is transpired and also evaporated
from moist soils, which cools and moistens the atmosphere,
and can affect precipitation and cloudiness.
Similarly,
forests may influence the climate in more complicated ways
than previously thought. For example, in regions with heavy
snowfall, reforestation or afforestation would cause the land
to reflect less sunlight, and more heat would be absorbed,
resulting in a net warming effect despite the removal of CO2
from the atmosphere through photosynthesis during the growing
season. Further, reforestation could increase transpiration
in an area, putting more water vapor in the air. Water vapor
in the troposphere is the biggest contributor to greenhouse
gas warming.
Local
land surface changes can also influence the atmosphere in
far-reaching ways, much like regional warming of tropical
eastern and central Pacific Ocean waters known as El Niño.
El Niño events create moist rising air, thunderstorms
and cumulus clouds, which in turn alter atmospheric circulations
that export heat, moisture, and energy to higher latitudes.
Tropical land surface changes should be expected to play a
greater role on global climate than El Niño, given
that thunderstorms prefer to form over land, and the fact
that the large area of tropical land-use changes far exceeds
the relatively small area of water responsible for El Niño.
Impacts of land use changes are harder to detect because they
are permanent, as opposed to El Niño, which comes and
goes.
Pielke
Sr., and colleagues propose a new method for measuring the
impacts of both greenhouse gases and landcover changes by
using a formula that quantifies all the various anthropogenic
climate change factors in terms of the amount of heat that
is redistributed from one area to another. This heat redistribution
is stated in terms of watts per meter squared, or the amount
of heat associated with a square meter area. For example,
if a flashlight generated heat of one watt that covers a square
meter, then the heat energy emitted would be one watt per
meter squared.
By
using a measure based on the spatial redistribution of heat
to quantify the different human influences on climate, including
landcover changes and greenhouse gases, the researchers hope
to achieve a more accurate portrayal of all of the anthropogenic
influences on climate change in future research.
The
paper was published in a recent issue of the Philosophical
Transactions of the Royal Society of London. The research
was funded by grants from NASA and the National Science Foundation.
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