Update: 18 DEC 2001

Desperately seeking stability
The study of global warming originally focused on the atmosphere. Now it's broadening to the ocean and biosphere. That's only logical, since these systems are interdependent. Look at two key greenhouse gases: The air would have a heck of a lot more carbon dioxide if plants didn't metabolize the gas into carbon and oxygen. And air would have a lot less water if not for evaporation from the oceans.

The first move beyond the "atmosphere only" paradigm was adding data on ocean circulation into computer climate models. Now comes the second link -- examining how changing the land cover affects the atmosphere.

This data on global croplands from 1700 to 1992 reflects satellite data and historical cropland census data.Courtesy Navin Ramankutty and Jonathan Foley (see "Estimating Historical Changes..." in the bibliography) animation

We intuitively understand the impact of land changes on climate, says University of Wisconsin-Madison climatologist Jonathan Foley. "There are many instances where changes in land use affect climate, like the Dust Bowl. It's been in the collective wisdom and we have forgotten that until recently."

To put some numbers on the interaction, Foley and Marcos Heil Costa, who is now at University of Vicosa in Brazil, asked a computer to compare what would happen if every tree was cut in Amazonia to what would happen if, as expected, carbon dioxide doubles in less than 100 years.

The evidence was clear. "With wall-to-wall deforestation, replacing the trees with pasture or crops, we see an even larger change, a very large warming," Foley says.

Hot time in the old forest tonite
While doubled carbon dioxide alone was predicted to raise local temperatures by 1 to 2 degrees C, deforestation with doubled carbon dioxide would raise temperatures by 4 to 6 degrees C, Foley and Costa found. "In that sense, deforestation's significance on climate looks bigger than that of greenhouse gases."

Deforestation may also create what Foley called "a fairly large decrease in rainfall," because so much rain in rain forests originates in plant transpiration -- breathing. By depriving the forest of evaporative cooling, the reduced transpiration also caused the warming. (Evaporative cooling occurs because heat is removed during evaporation.)

Although the trees in the model were replaced by crops or grasses, the smaller plants move only a fraction as much water to the atmosphere. Thus with deforestation, Foley says, "You get a fundamentally drier surface, and that explains the rainfall reduction. And if you can't recycle water vapor, that turns evaporative cooling way down."

That doesn't count the damage that deforestation does to biodiversity.

More research into the relation between land and atmosphere will come from the Large-Scale Biosphere-Atmosphere Experiment in Amazonia, a Brazilian-lead effort to piece together the "climatological, ecological, biogeochemical, and hydrological functioning of Amazonia." (For more coverage of the interaction between plants and global warming, see "Warm, Warm on the Range " in the bibliography)

Feedback frenzy
Foley and Costa's data may also illuminate the larger swings of global temperatures. The atmosphere is not a reasonable explanation for long-term climatic fluctuations, Foley argues, because it changes too quickly. Vegetation, however, could explain those swings if it caused feedback. (Feedback takes place when changes in one quantity affect other quantities, which then affect the first quantity. Remember how Jimi Hendrix used sound from his amplifier to vibrate the guitar strings, which then changed the amplifier's output?)

Vegetation and climate could play similar roles, Foley says. "You could imagine scenarios of climate change accelerating or slowing down by changes in vegetative cover." A warming atmosphere, for example, could allow trees to grow closer to the poles, reducing snow cover and decreasing the reflection of sunlight to space, which would tend to increase global temperatures.

Only if climate interacts with slower-moving objects, like oceans, Earth's chemistry or vegetation, can these long-term changes be explained, Foley says. "You need something with a long memory, like the presence of trees in high latitudes, or the turnover of the oceans."

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