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March 24, 2004
Land Cover Changes Affect U.S. Summer
Climate
While climate may be impacted by carbon
dioxide emissions, aerosols and other factors, a
new study offers further evidence land surface
changes may also play a significant role.
The study of summer climate in the United
States reported changes in land cover,
particularly vegetation, have impacted regional
temperatures and precipitation. The study used
data and computer models from NASA and other
organizations such as the National Oceanic and
Atmospheric Administration (NOAA).
“The largest human impacts on nature
have occurred since the Industrial
Revolution,” said Somnath Baidya Roy, a
research scientist at Princeton University, N.J.
Roy is lead author of the study published in a
recent issue of the Journal of Geophysical
Research-Atmospheres. Co-authors included George
Hurtt, University of New Hampshire; Christopher
Weaver, Rutgers University; and Stephen Pacala
also from of Princeton.
Previous studies simulated and compared past
and present climates with current and potential
vegetation. This research used the NASA-funded
Ecosystem Demography computer model to trace the
evolution of vegetation distribution patterns
over the U.S. for nearly 300 years. “The
model is truly a technological breakthrough and
enables scientists to study the potential impact
of land use and climate change across a wide
range of scales, from individual plants to
continental regions,” Hurtt said.
The researchers found land cover changes
produced a significant cooling effect of more
than one degree Fahrenheit in parts of the Great
Plains and Midwest as agriculture expanded and
replaced grasslands. Farmlands tend to create
lower temperatures through increased evaporation.
A warming effect was found along the Atlantic
coast where croplands replaced forests.
Compared to forests, croplands are less
efficient in transpiration; a daytime process
where water evaporates from leaves during
photosynthesis and cools the air. A slight
warming effect was also observed across the
Southwest, where woodlands replaced some
deserts.
The study found land cover changes could
impact local precipitation, but not as
significantly as they affect temperature. The
relatively strong cooling over the central U.S.
has probably weakened the temperature difference
between land and the Gulf of Mexico, slowing the
northern movement of weather systems and
resulting in enhanced rainfall across Texas.
Consequently, the air masses reaching the Central
Lowlands region, including Illinois and Indiana,
are drier, causing rainfall reductions.
“Land cover change is not uniform. Most
people associate land cover change with
deforestation, but the changes in the U.S. are
more complex, creating a temperature signal that
is more difficult to study,” Roy said. The
forest cover in the U.S. has actually increased
in the last 100 years mainly due to farm
abandonment in the East, fire suppression in the
West, and large parts of the Great Plains have
been converted into irrigated croplands, which
tends to produce cooling.
The research also carries additional
implications. “It is important to
understand the effects of changing land cover,
because it can mitigate or exacerbate greenhouse
warming,” Roy said. “In the U.S. over
the past 100 years, it seems to be offsetting
greenhouse warming. The opposite is probably true
in most other parts of the world. This finding
has also been supported in previous
research,” Roy said.
Researchers relied on several computer models.
These included the Ecosystem Demography model,
which incorporates data from NASA’s
International Satellite Land Surface Climatology
Project. The model contains data from the Global
Energy and Water Cycle Experiment. The experiment
was conceived to take advantage of environmental
monitoring satellites including NASA’s
Terra, Aqua, Tropical Rainfall Measuring Mission,
and ADEOS I and II. The study also used the
Regional Atmospheric Modeling System for regional
climate simulations.
NASA’s Earth Science Enterprise is
dedicated to understanding the Earth as an
integrated system and applying Earth System
Science to improve prediction of climate,
weather, and natural hazards using the unique
vantage point of space. NASA, Princeton, and NOAA
funded this research. For information and images
about this research, visit:
http://www.gsfc.nasa.gov/topstory/2004/
0223landsummer.html
For more information about NASA’s Earth
Science Enterprise on the Internet, visit:
http://www.earth.nasa.gov
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Contacts:
Elvia H. Thompson
Headquarters, Washington
Phone: 202/358-1696
Rob Gutro/Mike Bettwy
Goddard Space Flight Center, Greenbelt, Md.
Phone: 301/286-3026
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Changes in Vegetation across the United
States
These images show the (a) dominant vegetation
type and (b) fractional areal coverage (%) of
each grid cell by the dominant vegetation for the
1700, 1910, and 1990 vegetation cases.
The maps labeled 1700 are estimates of potential
vegetation for the country under current climate
conditions (i.e., without land use). Maps labeled
1910 and 1990 are estimates of land cover for
those periods that include the effects of
land-use history up to and including those years
respectively.
These figures illustrate some of the large
changes to the land cover patterns that are
estimated to have occurred across the United
States as the result of land-use activities. It
can be seen that between 1700 and 1910, much of
the U.S. was converted from forests to croplands.
By 1910, forest cover over the United States was
at or near its lowest point in 300 years. Since
1910, agriculture has substantially intensified
in the central and western U.S., but decreased in
the east. By 1990, croplands covered about 24
percent of the total land area, with farms
occupying more than 90 percent of some regions in
Iowa and eastern Nebraska. Forest cover has
steadily increased, to near 40 percent of the
total land area in 1990. CREDIT: Somnath Baidya
Roy, Princeton University
Impact of Changing Vegetation Pattern
on July Temperatures
These images show the difference in the
simulated near-surface air temperature (K)
averaged for the entire month of July between the
three cases.
The change in land cover between the 1710 and
1910 cases produces significant warming in the
east and southeast and significant cooling in the
northern Great Plains region. During this period,
these regions experienced large-scale growth of
croplands at the expense of the natural
vegetation (mostly forests in the east and
grasslands in the Midwest and Great Plains).
Between 1910 and 1990, there is significant
cooling in parts of the Great Plains where
agriculture continued to expand. Slight warming
is also observed in the southwest, where
woodlands have replaced a few deserts.
The white line demarcates where temperature
changes are significant. CREDIT: Somnath Baidya
Roy, Princeton University
Impact of Changing Vegetation Pattern
on July Precipitation
These images show the difference in the
simulated total precipitation (mm) for the entire
month of July between the three cases.
Clearly, the impact of the land cover changes on
precipitation is much smaller than that of
precipitation.
The difference plot between the 1700 and the 1910
cases exhibits a significant reduction (more than
20 mm) of precipitation in the Central Lowlands
and increase (more than 30 mm) over western
Texas. A similar increase in western Texas can
also be found when the 1910 and 1990 cases are
compared, but otherwise the differences are
small.
The white line demarcates where precipitation
changes are significant. CREDIT: Somnath Baidya
Roy, Princeton University
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