Carbon sinksamong them plants and the soilbind the carbon from carbon dioxide (CO2) in our atmosphere. In doing so, they play a vital role in counterbalancing the effects of man-made CO2 emissions. But exactly how much carbon these sinks bindand so the true threat of global warmingis difficult to determine. Researchers can measure carbon sinks either by tracking carbon dioxide concentrations in the atmosphere or by taking inventory of trees in the forests. Unfortunately, these methods have produced dramatically different results in the past. A study published in today's Science, though, tries to close the gap by combining an extensive land-based survey of the continental U.S. with findings obtained from 27 different atmospheric studies.

The new analysis concludes that trees and other plant matter took up one third to two thirds of a billion tons of carbon each year between 1980 and 1989. This final number lies below earlier estimates from atmospheric tests and above land-based figures, which did not take into account such carbon sinks as woody debris, forest litter and soil.

The researchers point out that much of the carbon that soil and plants are absorbing now was released earlier in the century. "When we chopped down the forests, we released carbon trapped in the trees into the atmosphere," says lead author Stephen Pacala of Princeton University. "When we plowed up the prairies, we released carbon from the grasslands and soils into the atmosphere. Now the ecosystem is taking some of it back."

The sinks won't absorb the greenhouse gas forever, though. Pacala estimates that they will gradually stop binding carbon over the next 50 to 100 years. Even now, they cannot compensate for the 1.4 billion tons of carbon released into the atmosphere each year through the burning of fossil fuels in the U.S. "The carbon sinks are going to decrease at the same time as our fossil fuel emissions increase," Pacala says. "The greenhouse problem is going to get worse faster than we expected."