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Pinning Down Carbon

Modeling climate is all in the parameters



A FACE experiment

Duke Univ
The problem of climate change—how to predict it, how to detect it, how to cope with it—could be called the problem of carbon. After all, carbon dioxide (CO2) is the greenhouse gas that human civilization is adding to the atmosphere. The Kyoto Accords of 1992 commit many nations to start expensive efforts to reduce these carbon emissions. And the carbon cycle is a fundamental problem in the growing scientific field of biogeochemistry.

But I would argue that the problem of climate change is the problem of parameterization. What the heck am I talking about?

I'm talking about the elaborate computer models of world climate that all the greenhouse policies are based on. Even after 30 years they leave a lot to be desired. The best of them can't give us much detail—for instance, each unit of geography in worldwide models, each "pixel" if you will, is 5 degrees of latitude and longitude on a side. That's all of Utah, or all of Ireland with lots of sea around it. And beyond mere geographic resolution, we need to boil down the host of factors influencing the carbon cycle into a reasonable set of numbers.

Consider just one factor: land vegetation. How much CO2 does it remove from the atmosphere? More specifically, how much carbon does one type of vegetation—a spruce forest—contain in its biomass (trees, brush, soil organisms, and buried organic matter)? How much carbon does it take up in summer and lose in winter? How much is released as CO2 during a forest fire? How do we define "spruce forest" as opposed to "taiga"? What if the forest contains large bogs, grassland, or lakes too? How many different kinds of vegetation communities are there, anyway? How does temperature affect each one, and how will they change as CO2 rises in the atmosphere?

All of those questions are answered in the models with parameters—numbers that represent the average behavior of the entities we're considering. So the problem of modeling climate change, as I said, is really the problem of how to parameterize the climate system. That's an abstract notion, so let me put a face on it.

Big research programs have been going on for years to learn how plants interact with carbon dioxide in the air. People have been measuring leaves, digging up roots and observing fields and forests from airplanes and satellites. Finally we've started looking at how real plant communities respond to higher carbon dioxide levels. An ingenious set of outdoor experiments is under way around the world using FACE (Free Air CO2 Enhancement) techniques.

In FACE experiments, a small plot of land is surrounded by towers that send measured amounts of CO2 into the air. The air within the plot is tested every second, and the flow from the towers is adjusted to keep CO2 level constant, even in windy conditions. These are the best way we have to see how the world will respond to the higher CO2 levels of this coming century—and where the experiments are done in croplands, they may give farmers clues about how to cope in their fields.

The FACE apparatuses have an uncanny resemblance to Stonehenge, so I've collected a small gallery of pictures on the next page.

The variables measured in FACE experiments are only a tiny fraction of the parameters that need to be improved for climate models. For a glimpse at the full spectrum of carbon studies in the works, visit the U.S. government site for the Carbon Cycle Science Program, www.carboncyclescience.gov.

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