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Updated 15 November 2004

Consequences Vol. 4, No. 1, 1998









It has long been known that carbon dioxide in the air entraps the heat that the surface of the Earth has received from the Sun, and with the help of a few other "greenhouse gases," provides the planet with its principal thermostat. We also know that the carbon in CO2 is continually exchanged with that found in plants and animals and the soil and the ocean, in a give-and-take process called the Earth's "carbon cycle."

What has lifted the carbon cycle out of the classroom and into the street is of course its obvious relevance to very practical questions about global warming, and the contentious economic choices that are involved in cutting back on carbon emissions.

There is little question that in adding so much CO2 to the air, we have altered an apparently stable balance among natural carbon reservoirs. There is far more carbon in the air and less underground. But could it be that the carbon cycle is self-regulating, and that natural processes are now acting, or will in time, to compensate? Won't warmer oceans and soil take up more CO2? Or could trees adjust things for us, by simply increasing their rate of growth and photosynthesis? To answer these pressing questions we need to know not only how much CO2 is in the atmosphere, but where the rest of the carbon is, and how it gets there, and where it goes next, and how soon.

The part of the carbon cycle that is most often invoked in public debate involves the trees, because of their natural appetite for CO2 in photosynthesis. As it should be, reforestation has been seriously considered as a possible way to soften the effects of global warming. But as Jim Kasting reminds us in this issue, it would take a lot of trees to make a major difference, and moreover, they give back to the air--rather soon, in terms of other reservoirs--most of the carbon that their leaves and needles have taken in. Still, until the trees or what we make from them are burned or decompose, adding forests where now there are none might slow, for a short while, the global buildup of carbon dioxide.

As we all know, trees have not always enjoyed such status, for through much of our early history they were seen--in terms of national goals--as mostly in the way. Michael Williams, in his book on the American forests, tells of how in colonial America the settler's arduous toil of felling and uprooting trees for the plow was associated with God's will. As in remote legends and nursery stories, forests were places of darkness and danger, and the dwelling place of evil things. Each hard-won clearing, however small, opened a little more of the land to the protective view from heaven on high. And so it seemed, when the world was new and we were not so many.

John A. Eddy

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