The outpouring of greenhouse gases from North America far outstrips the ability of the continent's fields, forests and wetlands to absorb all the carbon in the atmosphere, and the United States alone remains the world's largest emitter of climate-warming carbon dioxide, scientists reported Wednesday.
All told, the burning of fossil fuels by the United States, Canada and Mexico releases nearly 2 billion tons of carbon each year into the atmosphere, and the United States accounts for 85 percent of that total, says the report by the Climate Change Science Program, a research effort by government and private scientists sponsored by the Bush administration.
Until now, many scientists had thought the continent holds enough vegetation to absorb most of the carbon dioxide emissions, but the new report refutes that assumption and warns that the disparity is increasing.
The entire continent accounts for 27 percent of all the carbon dioxide emissions in the world, says the report, but China, where more and more coal-burning power plants go online every year, is already forecast to soon become the world's worst emitter.
"This is the first systematic assessment of America's contribution to the carbon budget in the context of global climate change, and it tells us what we really need to know," said Christopher B. Field of the Carnegie Institution's Department of Global Ecology at Stanford.
Field is the lead author of a section of the report that deals with the carbon cycle - a kind of balance sheet calculating how much climate-changing gas is emitted by North American power plants, vehicles and industry and how much is absorbed by the forests, crops, soils and surrounding ocean waters that constitute what scientists call the carbon sink.
"By burning fossil fuel and clearing forests, human beings have significantly altered the global carbon cycle," Field said.
As a result, he and his colleagues who drew up the report calculated that the continent emits more than three times the amount of carbon dioxide than its varied sinks are capable of absorbing. All the rest stays in the atmosphere and creates the heat-trapping greenhouse effect that has been warming the planet for the past century.
"The conversion of fossil fuels to energy, such as electricity generation, is the single largest carbon contributor, with transportation second," the report said.
As for the lagging ability of forests, parks, soils and green croplands to absorb the carbon, the report is highly pessimistic about the future.
"Carbon absorption by vegetation, primarily in the form of forest growth, is expected to decline as maturing forests grow more slowly and take up less carbon dioxide from the atmosphere," the experts said.
Wildfires that strip vegetation from huge swaths of land also reduce the ability of the carbon sink to function, and Field noted that last month's forest fires in Southern California that blackened 740,000 acres in five counties will have a significant impact on the state's future ability to absorb its output of greenhouse gases - despite California's leadership in green technology and in curbing carbon dioxide emissions.
"That land may recover in 10 or 20 years, and plants may return" Field said, "but there are more wildfires every year, and we haven't prevented them."
As climate continues to warm and droughts increase all across North America, widespread vegetation would die off and leave the bare land useless for absorbing carbon, the report noted.
In Washington last January, House and Senate Democrats accused the Bush administration of censoring the findings of scientists working on an earlier report for the same climate change program, and some of the program's scientists said they had been asked to delete reference to global warming or climate change in their findings.
From his own experience and discussions with colleagues working on the new report, none of that happened this time, Field said. The words "warming" and "climate change" appear in the report's summary released by the National Oceanic and Atmospheric Administration.
The full report is available at:
© 2007 The San Francisco Chronicle