UNH maps land use changes to better understand global warming

Analyzing available satellite information and historical records, scientists at UNH have developed detailed maps of global land usage for the past 300 years. The group published a paper of their findings in the July issue of  “Global Change Biology.”

“This is the most comprehensive study to date,” says study leader George Hurtt, assistant professor of natural resources at the University of New Hampshire’s Institute for the Study of Earth, Oceans and Space.

“Land use has been known among scientists to be a very important factor in climate change,” Hurtt says. “The tricky thing about it is, even if you’re interested in the future of climate change, it turns out you have to go back in history because these land use practices have a long legacy of effects.”

The study spans 300 years, from 1700 to 2000. Hurtt says that satellite imaging for the last 30 years was very helpful in developing models for recent land usage; beyond that, the study had to rely on any available documentation—usually government records at the national or local level—to deduce the types of land usage for each year.

The study classified land use in four areas: cropland; pastureland; primary land, which has not been used by humans in the past 300 years; and secondary land, which has been used for agriculture and then abandoned. From computerized analysis of this data, the group produced its maps.

“There’s often not a unique answer,” Hurtt says of the data used. “There’s many different historical scenarios that are consistent with the data.” To deal with this variability, the study developed 216 unique models that are all compatible with the data. “In this way, we deal with the uncertainty inherent in this process,” Hurtt says. The study took a year to complete.

Hurtt and the other scientists involved in the project hope that their study will help others who are studying climate change. Land usage is a major cause of increased carbon in the atmosphere. As people clear trees to develop land, the amount of carbon stored in the earth decreases. Trees use carbon dioxide to produce energy; as such, their presence has an impact on the global carbon cycle.

“Some good news we found is the amount of forested land that is re-growing,” says Hurtt. “Most of the forested land in the eastern U.S. is recovering from historical use as farmland.” Scientists refer to areas of developing forest as “carbon sinks.” A carbon sink takes in large amounts of carbon from the atmosphere to fuel growth, leading to a reduction in atmospheric carbon. Once the forest stops growing, it still uses carbon dioxide from the atmosphere for energy, but at a reduced rate.

As far as land use and emissions policy is concerned, Hurtt hopes the study will help governments make informed decisions.
“There’s a lot of people interested in storing carbon in ecosytems to offset fossil fuel emissions. If the carbon sinks we have are going to run out or fill up, we’ll have to reduce our emissions even more.”