Carbon dioxide shakes off its pursuers.

Climate scientists last year reported that forests and fields in the United States are acting like sponges, sopping up most of the nation's carbon dioxide pollution. A new study, however, puts the squeeze on the idea of a U.S. carbon sponge.

The research is important, say scientists, because it plays into debates about how much the United States must reduce its emissions of carbon dioxide to combat global warming.

Atmospheric and oceanic scientists have spent the past decade searching the Northern Hemisphere for a hiding place that calculations suggest is absorbing more than a billion tons of carbon dioxide out of the air each year. In late 1998, a team led by Princeton researchers announced that it had located such a carbon "sink" in the United States and southern Canada (SN: 11/21/98, p. 332). This area, they said, takes up between 1.2 and 2.2 billion tons of carbon annually. For comparison, the United States emits 1.6 billion tons a year.

"If they were right, it would mean that the U.S. could be pretty relaxed in terms of reducing emissions," says Richard A. Houghton of the Woods Hole (Mass.) Research Center.

Houghton and his colleagues decided to test the Princeton group's findings. Working like ecological bookkeepers, they tallied changes in forest and agricultural acreage between 1700 and 1990 and then determined how these alterations affected carbon supplies.

Early on, for instance, clearing of forests added carbon dioxide to the atmosphere. This process reversed after 1945, as forests started to regrow in formerly cleared fields. Suppression of forest fires has contributed to the sink by allowing carbon to build up in thickening stands of trees. Also, modern agricultural practices, with their higher crop yields, have stored extra carbon in soil.

Adding these factors and several others together, Houghton and colleagues pull the plug on the idea of a large U.S. carbon sink. They conclude that land in the United States during the 1980s absorbed between 0.15 and 0.35 billion tons of carbon dioxide each year--offsetting only 10 to 30 percent of the country's emissions during that period. They report their conclusions in the July 23 SCIENCE.

The new study corroborates what many researchers had suspected. "The sink is not as large as the Princeton group implies it to be. It cannot be," comments Inez Y. Fung, an atmospheric scientist at the University of California, Berkeley.

The biggest problem with the previous study, she says, is an unrealistic inequality: North America appeared to absorb billions of tons of carbon dioxide, but Europe and Asia, next to nothing. Such a disparity, she says, makes little ecological sense. The actual Northern Hemisphere sink is probably distributed across the continents and perhaps the Atlantic Ocean, says Fung.

Song-Miao Fan of Princeton says that the new study may not refute what his group reported last year. Its analysis of carbon dioxide measurements covered the period 1988 through 1992--a time when sinks were quite large, judging from other evidence. Houghton's study, however, included a period when sinks absorbed less carbon dioxide.

All the scientists agree that resolution to this debate will require better global gas measurements.