Features

SAN FRANCISCO — Scientists are uncertain how much of the carbon dioxide given off naturally each year within the North American ecosystem is reabsorbed by that system, complicating calculations of the net effect of human activities on emissions of the greenhouse gas. 

The calculation is important because it establishes a baseline to gauge incremental sources of carbon dioxide — namely that produced by the burning of fossil fuels, scientists said Thursday at the fall meeting of the American Geophysical Union. 

Stating outright whether North America is a source or sink is currently “problematic,” said Pieter Tans, a National Oceanic and Atmospheric Administration scientist. 

“The evidence is not strong enough,” Tans said. 

Should scientists determine that the United States absorbs more carbon dioxide than is naturally emitted within its borders, it could subtract that from the total amount that escapes to the atmosphere from its smokestacks and tailpipes, said Christopher Potter, a scientist with the National Aeronautics and Space Administration’s Ames Research Center. 

“It’s very important we try to pin this down and know its variability,” Potter said. 

Carbon dioxide is the main culprit behind the rise in global temperatures that is widely accepted by scientists, Establishing how much individual nations emit is a thorny issue. 

Last month, negotiators from 165 countries agreed on rules for implementing the 1997 Kyoto Protocol, which calls on about 40 industrialized nations to limit carbon dioxide emissions or cut them to below 1990 levels. The United States has rejected the accord. 

Together, the U.S. and Canada emit about 1.7 billion tons of carbon each year, mainly as carbon dioxide. The amount is huge but still a fraction of the estimated 140 billion tons of carbon that cycles through the atmosphere, land and oceans during the same period. 

Of the total, scientists are unsure how much is taken up within the two nations’ forests, farms and wetlands. Year-to-year variations in temperature and rainfall can skew the numbers significantly. 

Estimates derived from NASA satellite measurement of plant growth across the United States and Canada suggest that the region absorbs anywhere from one-fourth to one-third more carbon than it emits. In short, that means the amount taken up by plant growth exceeds that rereleased to the atmosphere through rot and fire. 

Jing Chen of the University of Toronto said warmer and wetter weather — possibly due to global warming — has extended the growing season by as much as a week over the last century. That increased growth could translate into more carbon dioxide being removed from the atmosphere, he said. 

“It is difficult to have high confidence in these calculations,” Chen said.