Land use is a significant player among the tangled web of climate change actors, according to a new study published in the Journal of Geophysical Research-Atmospheres that compares land use patterns during the past 300 years with temperature trends.

The study adds to a growing body of research that shows land use changes such as deforestation and shifts in agricultural production can alter air temperature as well as the water vapor content of the atmosphere. For example, croplands and forests absorb different amounts of solar radiation and transpire different quantities of water. Irrigated croplands cool the air because a portion of the sun's energy goes into evaporating moisture rather than heating the surface.

The study, led by Somnath Baidya Roy of Princeton University, focused on two periods: From 1700 to 1910, when pristine vegetation gave way to maximum deforestation as agriculture blossomed, and from 1910 to 1990, when Eastern forests became re-established and croplands became concentrated in the Midwest.

The researchers used a variety of computer models and surface temperature records, including ecological models and a regional atmospheric simulation to produce what they say is a unique dataset. Roy said most land use research to date has focused on the Amazon and southeast Asia. According to the researchers, in 1910 forest cover in the United States reached its lowest level in the 300 year period of the study, making up about 21 percent of total land area. In contrast, forest cover amounted to almost 40 percent of the total area in 1990.

These changes corresponded to significant warming in the eastern and southeastern United States between 1700 and 1910. From then until 1990, the Great Plains cooled by more than 1 degree Fahrenheit as agriculture expanded, which Roy said is statistically significant. "This tells you that the magnitude of climate change from land cover changes is quite large," he said.

The maps on the left show the dominant vegetation, with red representing croplands and green representing forests, in 1700, 1910 and 1990. The second series of maps show changes in temperature, with blue representing cooler temperatures and red representing warmer temperatures, from 1700 to 1910, 1700 to 1990 and 1910 to 1990. Maps courtesy of the National Aeronautic and Space Administration/Somnath Baidya Roy.

The authors did not find a strong connection between land use and precipitation, although on a local level areas with increased evaporation can experience higher amounts of precipitation. The study found that cooling over the central United States has weakened the air pressure gradient between the land and the Gulf of Mexico, a major source of moisture for the expansive area east of the Rocky Mountains. Differences in atmospheric pressure are essential in drawing that moisture inland.

The study found increased precipitation in Texas and less in Indiana and Illinois. "The slowing down of the moist southerly flow increases the residence time of moisture over Texas, resulting in enhanced precipitation," the study states.

Roy said land use-induced cooling could be offsetting temperature increases caused by rising concentrations of greenhouse gases, but scientists do not know by how much. Computer models have consistently predicted more rapid temperature increases than have been observed. Some researchers have attributed the discrepancy to the cooling effects of sulfate aerosols, but it is becoming clear that land use is also a factor.

"One could conclude that the warming due to greenhouse gases [is] being to some extent mitigated by the land cover change signals," Roy said, referring to the midwestern cooling. "To what extent is hard to tell."

Ben Preston of the Pew Center on Climate Change said more studies are needed to determine the magnitude of the land use influence, and whether it causes net warming or cooling across the globe. But, he said, "there's consistency in the literature that human changes to the land surface, whether it be urbanization or deforestation for the purpose of agriculture ... all have an effect on the climate system."

Teleconnections

Land use decisions may be made on the local level, but experts believe they can have global implications.

"It appears that land use change can affect the energy and water budget regionally and that effect can teleconnect globally so it affects the entire atmospheric circulation," said Roger Pielke Sr. of Colorado State University. Pielke argues that computer models that rely mainly on carbon dioxide concentrations to predict climate fail to capture the intricacies of the climate system and should not be relied upon as "predictive tools."

"[Land use] makes climate a much more complicated issue than just trying to infer what happens due to influxes of carbon dioxide alone," he said.

As an example, Pielke said deforestation in southeast Asia has shifted the favored location of thunderstorms, and that has rippled downstream, with similar patterns occurring elsewhere. "The net result is the entire weather systems around the globe are altered from what they would be in the natural landscape," he said. "If you change land in Asia it could affect a national park in the United States."

Policy implications

Although land use factors into the climate change equation, it is unlikely to translate into policy decisions because its contribution to climate change is unknown and land use decisions are dictated by other, more powerful needs such as growing food and building homes.

"I don't think we want to get into a game of having concerns about climate dictate all of our land use patterns, such as we're going to cut down all of our forests and turn them into croplands in order to produce a small amount of cooling. ... Then you're just trading one problem for another," the Pew Center's Preston said.

Roy said warming from greenhouse gases is likely to be much more significant than variations from land use changes, making land use manipulation to stabilize the climate an unattractive option. "By being smart about our land use probably we can to some extent mitigate the extent of global warming, but it can only go so far," he said. "[With] the rate at which carbon dioxide is increasing in the atmosphere, I don't think you can totally solve this problem."