Like greenhouse gases, landscape changes may alter climate

Greenhouse gases, the long-standing villains of climate change, may have a significant new partner in crime: wholesale changes to the world’s landscapes by humans.

It’s a safe bet to say that the future climate of the Amazon will be determined by deforestation and carbon dioxide concentrations. Jonathan Foley Climatologist UW–Madison

From the deforestation of the Amazon to the transformation of millions of acres of North American prairies to farmland, humans have remolded the surface of the Earth. Those changes, scientists now suspect, may have a significant influence on climate, changing regional weather patterns at least, and possibly contributing to global shifts in climate.

Addressing scientists Dec. 8 at the fall meeting of the American Geophysical Union, UW–Madison climatologist Jonathan Foley said changes to the landscape, coupled with global increases in atmospheric carbon dioxide concentrations, could pack a one-two punch capable of remaking regional climates by altering patterns of rainfall and temperature.

Such changes, he said, are reflected in a novel study of the Amazon Basin, the first to simulate the combined effects on regional climate of large-scale deforestation and increases in global concentrations of carbon dioxide in the atmosphere. The study was conducted by Marcos Heil Costa, now a professor at the University of Viçosa in Brazil and Foley, a UW–Madison professor of atmospheric and oceanic sciences.

“It’s a safe bet to say that the future climate of the Amazon will be determined by deforestation and carbon dioxide concentrations,” Foley said in an interview.

In the past, climate change scenarios have been linked primarily to the increasing concentrations in the world’s atmosphere of the so-called greenhouse gases, carbon dioxide and methane, for example. But some scientists are now beginning to examine the influence on climate of the surface of the Earth and the collection of landscapes, such as forests, farmland, and deserts that collectively make up what is known as the biosphere.

The biosphere, argues Foley, probably exerts an influence on climate that is equal to, and in some instances greater than, the accumulation of greenhouses gases.

When you alter landscapes on a massive scale, as has happened across the globe in the past 200 years as farms have replaced forests, basic patterns of climate are disrupted and climatic change is set in motion.

In his study of the Amazon basin, for example, when forest is cleared and replaced with pasture or cropland, the hydrologic cycle, where rain is recycled back to the atmosphere through soil, trees and forest plants by the process of evapotranspiration. That process, said Foley, is essential to a landscape’s ability to cool itself. Without it, as reflected in the model developed by Costa and Foley, it acts together with increased atmospheric carbon dioxide concentrations to significantly warm the regional climate of the Amazon basin.

“If you were to cut down a big chunk of the Amazon and replace it with grasslands and pasture, the ability of that land to cool itself off is diminished,” said Foley. “The dominating effect is cutting off evapotranspiration. Forest (landscapes) can cool themselves off by releasing water (to the atmosphere).”

Moreover, Foley noted that wholesale changes to the landscape can affect climate by altering the ability of the biosphere to store carbon dioxide. Forests and prairies, for example, have a far greater ability to store carbon dioxide than cornfields or pasture. With deforestation, these “sinks” are eliminated and the ability of the biosphere to absorb and store greenhouse gases like carbon dioxide is reduced.

The supercomputer-driven model used by Costa and Foley showed that a significant deforestation of the Amazon, combined with increased carbon dioxide levels, would reduce annual rainfall by as much as 20 percent. Such a reduction, in addition to environmental consequences, would have an impact on such things as Brazil’s hydroelectric capacity, a primary source of power there.

The scale of change factored into the model of a deforested Amazon is unlikely to occur, said Foley, because many parts of the Amazon basin flood annually and are unlikely to be converted to farmland. However, he said the results are important because it demonstrates how altered landscapes can feed climate change.