-- Campaign For Old Growth | Reference Articles --

WASHINGTON (AP) -- New estimates of the United States' contribution to global warming show that forest growth, crops and rivers absorb a quarter to a half of the nation's yearly 1.5 billion tons of carbon dioxide emitted from burning fossil fuels.

But that cushion against a buildup in atmospheric greenhouse gases will likely disappear over the next century as forests mature and absorb less carbon, said Stephen Pacala, a Princeton University researcher.

"That means the greenhouse problem is going to get worse, not better, because fossil fuel emissions are going up at the same time," said Pacala, lead author of one of two carbon absorption studies appearing Friday in the journal Science.

In the other study in Science, researchers estimate there is enhanced carbon storage in China as the result of timber management programs and reforestation efforts.

President Bush's opposition to the Kyoto climate treaty, a 1997 international plan to curb global warming, has brought attention to the issue of increasing man-made gases, principally carbon dioxide from the burning of oil, gas and coal.

European Union leaders and Bush this month promised to "agree to disagree." The treaty calls for a sharp reduction of carbon dioxide emissions, and Bush says compliance is too expensive and ill-timed with the nation's energy and economic problems.

Bush is pressing for more research instead, while the EU says it will continue backing the treaty, though none of its member nations has ratified it.

Pacala said his study shows the amount of carbon dioxide emissions absorbed through natural processes in the United States is higher than many estimates though less than a previous Princeton-led study. That 1998 study, saying the lower 48 states, southern Canada and Mexico absorb 1.5 billion tons of carbon per year, was criticized by many scientists as inflated.

The new study found that the United States absorbs 407 million to 781 million tons of carbon each year.

"Our own work would suggest that the lower end of that range is certainly reasonable," said Jerry Melillo, co-director of The Ecosystems Center of the Marine Biological Laboratory in Woods Hole, Mass.

Much of the absorption, or what some scientists call a natural "sink," occurs through photosynthesis as growing trees take in carbon dioxide. The nation has benefited from allowing cleared tracts to recover, letting farmland revert to forests and suppressing forest fires.

But as the forests mature, they take in less carbon dioxide. "What that all means is that a sink caused by land use or ecosystem recovery is generally expected to decline," said George Hurtt, a co-author of the study and researcher at the University of New Hampshire.

Of the amount absorbed naturally, some 77 million to 143 million tons are exported to other countries as agricultural products. That means the total impact of the United States' absorption is, from a global perspective, the removal from the atmosphere of 330 million to 638 million tons of carbon per year.

A team led by Jingyun Fang of Peking University found that a program of preserving and planting trees in China begun in the mid-1970s has resulted in storage in the forests of about 495 million tons of carbon.

China's forests now cover 330 million acres and are growing, the researchers say.

The study concludes that improved forest management could contribute significantly to carbon storage, thus helping to offset the release of carbon dioxide.