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The strange case of the plow and the thermometer

A pattern of rising air near the Equator effects global tempsAs computerized models of climate grow more sophisticated, they show more detail of how human actions can affect the climate. Twenty-five years ago, when global warming caused by greenhouse gases started to take center stage as a scientific question and political-environmental conundrum, climate models were only able to look at the atmosphere in isolation. Next they began factoring in the critical role of oceans.

Hadley Cell: A major Earth atmospheric circulation pattern that consists of rising air at or near the Equator and sinking air at or near 30 degress north or south of the Equator Graphic: NASA

Newer climate models have begun to show how land and vegetation affect climate. In a new projection, Johannes Feddema, an associate professor of geography at the University of Kansas, and colleagues at the National Center for Atmospheric Research projected climates through year 2100, based on two contrasting scenarios for greenhouse gas emissions.

Feddema says that in either scenario, the global impact of land-use changes was slight -- less that 0.1 degree Celsius -- because changes tend to offset each other. In some places, changes in land use effectively double the warming caused by rising greenhouse gases alone. In other places, land-use changes cooled the local climate, offsetting greenhouse warming.

Land use: Two projections from IPCC:
world maps with colors representing land cover types
A computer model projected the effect on climate of possible changes in land cover, based on two United Nations estimates of future change. The land-cover changes were largest in South America, Africa, and Southeast Asia. Courtesy Johannes Feddema, University of Kansas

Making change
When forests are replaced by farms, we can expect changes in the amount of heat reflected to space and the amount of moisture evaporated from plants. The effects can be complex, however. In middle latitudes in winter, for example, forests absorb more solar heat than farm fields because a sheet of snow reflects more solar energy back to space than a darker, snow-covered forest.

In humid areas, water evaporation plays a key role in removing energy from a surface, but it does not raise air temperature. "If lots of water is present, it's usually the primary way that energy is dissipated," Feddema says. "Places that are wet tend be relatively cool when hot. ... If a place is dry, its tends to heat up because it doesn't have the ability to move energy by evaporation." Because forests evaporate more water than farms, the net result of converting tropical forests to farms is to warm surface air temperatures, Feddema says.scenarios show world maps: more greenhouse gas with more deforestation

But the effects of changing land use can vary by time of day, time of year, and latitude, so the simulation is only a "first pass" at studying how land use affects regional climates, Feddema said.

Differing scenarios for changes in land use, coupled with different growth in greenhouse gases, produced regional warming in some parts of the globe. Courtesy Johannes Feddema, University of Kansas

Simple simulations
The simulations that Feddema oversaw were based on two scenarios from the United Nations's Intergovernmental Panel on Climate Change: A2, which forecasts rapid deforestation and a rapid rise in greenhouse gas emissions, and 2° Celsius of global warming, and B1, which forecasts slower gas emissions, less deforestation, and 1° Celsius of warming.

In both cases, Feddema said, land-use changes added to the warming impact in some regions, but reduced it in others. Deforestation doubled the warming effect of greenhouse warming alone in Amazonia, but not in Indonesia, the study found.

Equally interesting were the long-distance effects of land-use changes. Columns of water vapor rising from the humid tropics move heat up and away in a giant circulation pattern called the Hadley cell, which is a prime driver of global weather that delivers energy and rainfall to distant locations. But if evaporation slows over the Amazon basin, the upper atmosphere, and the nearby Pacific and Atlantic Oceans, may actually cool. This could slow the Hadley circulation, cooling the U.S. Southwest while bringing it more rain.Converting forests to farms affects the regional climate, but the effects vary by region, according to a new study

Local decisions, obviously, have effects. Scientists have long known that cutting trees and building farms in the Amazon reduces local rainfall while paradoxically increasing floods by removing vegetation that soaks up rain. It now seems that the same action may also turn up the heat in a region that is already quite hot.

"One interesting thing about land use is that it happens in the places where we live, and the effects of land cover are typically very localized, so clearly the impacts are where people are," Feddema said. "Compared to global warming, land use is a relatively small influence. However, there are regions where it's really important."

-- David Tenenbaum

- The Importance of Land-Cover Change in Simulating Future Climates, J. J. Feddema et al, Science, 9 Dec. 2005.
- Frost Followed the Plow: Impacts of Deforestation on the Climate of the United States, Bonan, Gordon B. 1999, Ecological Applications: Vol. 9, No. 4, pp. 1305-1315.
- A Comparison of a GCM Response to Historical Anthropogenic Land Cover Change and Model Sensitivity to Uncertainty in Present-Day Land Cover Representations, Johannes Feddema et al, Climate Dynamics, Volume 25, Issue 6, Nov 2005, Pages 581 - 609..

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