PRINCETON, NJ — Forest growth absorbs a major portion of the United States’ carbon dioxide emissions — an effect called carbon sink — but that effect is going to gradually disappear this century as forest lands mature, according to a Princeton University-led research group. In a study published in the June 22 issue of Science, the Princeton group found that the U.S. is currently absorbing one-third to two-thirds of a billion metric tons of carbon per year. The main reason is that growing trees and shrubs are drawing considerable volumes of carbon dioxide from the air and using the carbon to build their trunks, branches, and foliage.

Despite the large U.S. carbon sink, the country still releases much more carbon into the atmosphere than can be absorbed. Burning of fossil fuels produces about 1.4 billion tons of carbon each year. Taking into account the sink, 800 million to 1.1 billion tons accumulate in the atmosphere each year. The new study is said to eliminate the possibility that the carbon sink is big enough to equal the nation’s fossil fuel release, as some had thought.

Stephen Pacala, lead author of the study and a Princeton professor of ecology and evolutionary biology, said that the carbon sink will disappear over the next 50 to 100 years. “The carbon sinks are going to decrease at the same time as our fossil fuel emissions increase,” Pacala said. “Thus, the greenhouse problem is going to get worse faster than we expected.”