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:
