Study Points To U.S. As Generator Of Greenhouse Gases

The new study, which will be published Friday in the journal Science, may have important implications for the role of the United States in combating the greenhouse effect and global warming.

"Some have argued that the U.S. does not need to reduce greenhouse gas emissions because we're not part of the problem," said Ronald Neilson, a professor of botany at Oregon State University and bioclimatologist with the USDA Forest Service. "Based on this study, we can no longer make that claim."

Neilson was a co-author on this research with scientists from the Max-Planck-Institute for Biogeochemistry in Germany, the Ecosystems Center at Woods Hole, Mass., the National Center for Atmospheric Research in Boulder, Colo., and other universities and agencies.

This debate and controversy, Neilson said, is a complicated but important part of the challenge facing nations around the world as they try to decide what to do about global warming and what responsibilities various countries should have. It's also a detective story of researchers looking for the "missing sink" of carbon. More carbon, they say, is being injected into the atmosphere by industrialized nations than can be clearly accounted for in the Earth's atmosphere, land, vegetation and oceans.

"Some past studies suggested that a big part of the missing carbon sink was in the forests and changing land use practices of North America," Neilson said.

Increasing levels of atmospheric carbon dioxide can literally "fertilize" plants and trees, researchers say, causing them to grow faster. Also, the United States in particular is converting a large amount of former agricultural land back into forests, which also tends to sequester carbon.

"On a global basis, we've estimated the missing sink of carbon at about 1.8 gigatons per year," Neilson said. "One earlier study suggested that changes in the forests and vegetation of North America were sequestering an extra 1.7 gigatons of carbon. Some people pointed to that as evidence that the U.S. had already done its part in the fight against global warming, that we were not contributing much to the problem."

The new research refutes that conclusion.

In their Vegetation and Ecosystem Modeling and Analysis Project, or VEMAP, the group of scientists found that atmospheric fertilization and other phenomena would sequester only an additional .08 gigatons of carbon within the lower 48 states, and possibly double that for all of North America. Regrowth of forest vegetation would sequester no more than an extra one or two times that amount.

In simpler terms, the study suggests at least 70-90 percent of the carbon injected into the atmosphere by fossil fuel use in the U.S. is either staying there or being sequestered somewhere besides North America.

As one of the largest industrialized nations in the world, the U.S. uses huge amounts of coal and petroleum products and is responsible for a major portion of the global total of greenhouse gases. But so far this country has not ratified agreements reached in Kyoto, Japan, under the United Nations Framework Convention on Climate Change, which sought international cooperation on reducing greenhouse gas emissions.

"In the U.S., some people and policy makers remain unconvinced of the reality of global warming and have concerns about the economic impact on jobs or industry if the country were to commit to greenhouse gas reductions," Neilson said. "Other nations, especially in Europe, have taken a different stance and are more receptive to the Kyoto accords."

But as the debate continues about what each nation should do, Neilson said, direct evidence and various computerized climate models make it increasingly clear that global warming is a scientific reality. "The U.S. has warmed by about one-half to one degree in the past century, and models suggest it will warm from about five to nine degrees in the next century," Neilson said. "There are still many people who don't believe these models are accurate, but the balance of evidence suggests they are getting increasingly accurate."

The impact of these climate changes may be profound, researchers say, ranging from drought and massive fires to dramatic changes in vegetation, ecology, and the agricultural potential of land. As the climate warms, there may also be feedback mechanisms that would cause even more carbon to be injected into the atmosphere and compound the greenhouse effects.

The new study also found that a nation's contribution or sequestration of carbon may be quite variable, even from one year to the next. In general, warmer or drier conditions cause carbon release to the atmosphere. So a drought or higher temperatures may change land that absorbs carbon into land that is releasing it, making it very difficult on a short-term basis to create accurate carbon measurements and fair, functional international agreements.

"All of these results indicate that we need to continue to improve our technologies for measuring carbon, determine where it is going, find ways to work through the annual variations and determine what the long-term impacts will be," Neilson said. "But this study clearly contradicts the suggestion that carbon uptake in North America is balancing our carbon emissions from fossil fuels. We are still part of the problem."