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For more information contact:

Rob Gutro/Mike Bettwy
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026)

Jana Goldman
NOAA Public Affairs
(Phone: 301/713-2483)

Steven Schultz
Princeton University,
Princeton, N.J.
Phone: 609/258-5729

Joe Blumberg
Rutgers University
(Phone: 732/932-7084 ext. 652)

David Sims
University of New Hampshire
(Phone: 603-862-5369)

Click here for more information about RAMS

Click here for more information about Ecosystem Demography (ED) model

For more information about GEWEX click here.

Click here for a related story, see “In Tropics, Forests are Cool, but Croplands are Hotter”


Viewable Images

Caption for Item 1: 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

Caption for Item 2: 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

Caption for Item 3: 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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March 24, 2004 - (date of web publication)



Item 1


While climate may be impacted by carbon dioxide emissions, aerosols and other factors, a new study offers further evidence that land surface changes may also play a significant role.

Recently, a study of the United States summer climate, using data and computer models from NASA and other organizations such as the National Oceanic and Atmospheric Administration (NOAA), reported that changes in land cover, particularly vegetation, over the past 300 years have impacted regional temperatures and precipitation.

“The largest human impacts on nature have occurred since the Industrial Revolution,” said Somnath Baidya Roy, a research scientist at Princeton University and lead author of the study that appeared in a recent issue of the Journal of Geophysical Research-Atmospheres. Co-authors include George Hurtt, University of New Hampshire, Christopher Weaver, Rutgers University, and Stephen Pacala of Princeton University.


Item 2


Unlike previous studies that simulated and compared past and present climates with potential and current vegetation respectively, this research used NASA’s Ecosystem Demography (ED) computer model to trace the evolution of vegetation distribution patterns over the United States for nearly 300 years. “The ED model is truly a technological breakthrough and enables scientists to study the potential impacts of land use and climate change across a wide range of scales, from individual plants to continental regions,” said Hurtt.

Baidya Roy and colleagues, using NASA’s ED model found that since 1700, land cover changes produced a significant cooling effect of more than 1 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.


Item 3


In addition, a warming effect was found along the Atlantic coast where croplands have 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 also found that land cover changes can impact local precipitation, but not as significantly as it affects temperature, because U.S. summer rainfall is not largely dependent on local land cover and evapotranspiration. However, the researchers say the relatively strong cooling over the central U.S. has probably weakened the temperature difference between land and the Gulf of Mexico, slowing the northward 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,” said Baidya Roy. The forest cover in the U.S. has actually increased in the last 100 years – mostly due to farm abandonment in the East and fire suppression in the West. Additionally large parts of the Great Plains have been converted into irrigated croplands. Both tend to produce cooling.

The research also carries additional implications. “It is important to understand the effects of changing land cover because depending on the nature of the land cover change, it can mitigate or exacerbate greenhouse warming,” said Baidya Roy. “In the United States, in the last 100 years, it seems to be offsetting the greenhouse warming, to some extent. The opposite is probably true in most other parts of the world. This finding has also been supported in previous research.”

The NASA-funded Ecosystem Demography model that incorporates data from the Global Energy and Water Cycle Experiment (GEWEX), initially conceived to take advantage of the development of the new series of environmental satellites, including NASA’s Terra, Aqua, the Tropical Rainfall Measuring Mission (TRMM), ADEOS I and II satellites. The study also used the Regional Atmospheric Modeling System (RAMS) for regional climate simulations.

NASA’s mission is to understand and protect our home planet by studying the primary causes of climate variability, including what changes are occurring in global land cover and land use, and their causes and impacts. This research was funded by NASA, Princeton University, and NOAA.

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