Vegetation may be absorbing the entire amount of carbon dioxide (CO2) produced by human activity in North America, says a study in the October 16, 1998, issue of Science magazine, and that may explain why the earth has not warmed as predicted by computer models of global warming.

Climatologists have speculated that so-called "carbon sinks" take up excess carbon dioxide in the atmosphere. Over the last century, the atmospheric concentration of CO2 has about doubled, and numerous studies have shown that plants grow faster and bigger in response to increased concentrations of CO2. Through photosynthesis, plants incorporate the carbon from CO2 and exhale oxygen.

The authors of the paper estimated the total global uptake of carbon using observational data and a model of the atmosphere. They found that the soil and vegetation in North America, about 20 percent of the world's vegetated land, absorbs annually as much carbon as is released into the atmosphere by North American sources. They attribute the existence of the North American carbon sink to four factors:

• Regrowth of U.S. forests from previous logging and agriculture.

• An increased amount of nitrogen in the atmosphere due to industry and agriculture that eventually winds up fertilizing plants, thereby stimulating their growth.

• An increased rate of photosynthesis due to the higher concentration of CO2.

• A longer growing season at higher latitudes, due to a slight warming.

The scientists did not find a similar carbon sink for other continents; however, some scientists say there is evidence the tropical rainforests may also be carbon sinks. Observers suggest that if intensive agriculture, which uses less land than traditional methods, and reforestation are utilized in the rest of the world to the same extent, projections of the future concentration of CO2 in the atmosphere will be drastically reduced.

Observers say the study could have ramifications for the climate change treaty signed in Kyoto, Japan, last December. For instance, the United States and Canada could argue that their ability to soak up excess CO2 should offset their emissions.

Source "Will Our Greening Planet "Sink" Kyoto?" World Climate Report, October 26, 1998.