Land-use impact discovered in global warming

12/11/2005 01:00:00 AM

By Katy Human
Denver Post Staff Writer

In the Amazon, chopping trees to plant crops can warm the weather almost as much as extra greenhouse gases do. In Colorado, turning woodlands to wheat may counteract global warming, according to a new study.

Land-use changes profoundly affect local and regional climates, scientists wrote in a paper in Friday's issue of Science.

"The bottom line is we've changed the climate system in more significant ways than we realized," said one co-author, Linda Mearns, with the National Center for Atmospheric Research in Boulder.

"Land use is a major change that will have important impacts on regional climates. They may not be as strong as greenhouse gases, but they're certainly nontrivial."

Colorado State University climatologist Roger Pielke Sr. said the new study should "finally" make international climate scientists recognize that the planet's climate system is far more complicated than they once thought.

"I feel vindicated. Very much so," said Pielke, who has argued for a decade that land use can affect climate. Greenhouse gases, such as carbon dioxide, emitted during fossil-fuel combustion, are only part of the climate story, Pielke said.

On Colorado's Front Range, for example, the expansion of irrigated crops has made summers cooler and wetter than they were a few decades ago, he said.

Pielke is not an author of the new study, but commented on it in Science.

"I think Roger (Pielke) thinks this is a bigger deal than I do," said Johannes Feddema, a University of Kansas geographer and the lead author of the new paper.

For the new analysis, Feddema's team plugged land-use changes into a computer model of Earth's climate and asked how the changes would affect climate. In the tropics, converting forest to agriculture warmed the land, accelerating warming caused by greenhouse gases.

In other regions - such as most of the United States - cutting trees for crops actually counteracted warming, Feddema said.

The effects were triggered by a complicated interplay of soil-moisture level, rainfall, snow and albedo - the tendency of the land's surface to absorb or reflect sunlight.

The authors conclude that to predict Earth's future climate, modelers must take land-use changes into account. "We have a long way to go," Feddema said.

Still, he and Mearns said, greenhouse gases generally influence climate much more powerfully than do land-use changes.

In Colorado, for example, a conservative computer model predicts that doubling the carbon-dioxide level in the air will lead to a warming of about 5.5 degrees by 2100, Mearns said. Converting forest into crops might counteract part of that, leading to 3 degrees of warming, she said.

Staff writer Katy Human can be reached at 303-820-1910 or khuman@denverpost.com

Informant: binstock


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