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LAND COVER CHANGES AFFECT U.S. SUMMER CLIMATE
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Item 1
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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.
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Item 2
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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.
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Item 3
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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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