Basic biology suggests that plants might grow faster in a world with more carbon dioxide, and field experiments bear that out: when you pump extra CO2 into a field or a forest, trees and other vegetation tend to get bigger.

There are plenty of caveats attached: without other nutrients, the size and health of CO2-enriched plants can be compromised, and in some cases noxious weeds like poison ivy do better than the greenery you might prefer. But perhaps the biggest question of all is how closely such artificial situations translate in the real world.

That question is a long way from being answered, but a study published recently in the Proceedings of the National Academy of Sciences takes a small step in that direction. A team of researchers used 22 years' worth of carefully accumulated measurements of hardwood forests in and near the Smithsonian Environmental Research Center, in Edgewater, Md., to show that their growth has accelerated significantly. On average, the stands were expanding at a rate of two extra tons of mass per acre per year, by the end of the study -- the equivalent of a single two-foot-diameter tree, if you could grow a tree that big in a year. "We don't know exactly when it started," says co-author Geoffrey Parker, a forest ecologist at the Smithsonian. But the scientists do have an idea of the reason -- or rather, three possible reasons, all of which are likely to be interrelated.

The first is a 12% increase in atmospheric carbon dioxide since the study began -- the same CO2 increase implicated in global warming. The second is the warming itself. And the third is an increase in the growing season, defined as the length of time between the last frost in spring and the first frost in fall, which is also a consequence of warming. "We couldn't narrow it down further than that," says Parker.