North America's Carbon Budget Out of Balance

WASHINGTON, DC (ENS) - North America's carbon budget is increasingly overwhelmed by human-caused emissions, according to the first "State of the Carbon Cycle Report" for the continent, released online this week by the U.S. Climate Change Science Program.

The interagency government report finds North America's emissions of the greenhouse gas carbon dioxide are greater than 25 percent of global emissions. The conversion of fossil fuels to energy, such as electricity generation, is the single largest carbon contributor, with transportation second.

North American sources release nearly two billion tons of carbon into the atmosphere each year, mostly as carbon dioxide, according to the report, funded by four federal government agencies. This greenhouse gas blankets the Earth, raising the planetary temperature.

Carbon "sinks" such as growing forests may remove up to half this amount, but these current sinks may turn into new sources as climate changes.

"By burning fossil fuel and clearing forests, human beings have significantly altered the global carbon cycle," says one of the report's lead authors Chris Field of the Carnegie Institution for Science's Department of Global Ecology in Stanford, California.

A result has been the buildup of carbon dioxide in the atmosphere, but so far this has been partially offset by carbon uptake by the oceans and by plants and soils on land.

"In effect, we have been getting a huge subsidy from these unmanaged parts of the carbon cycle," says Field.

Overall, this subsidy has sequestered, or hidden from the atmosphere, approximately 200 billion tons of carbon. In North America much of it has come from the regrowth of forests on former farmland and the uptake of carbon by agricultural soils.

But these carbon sinks may be reaching their limit as forests mature and climate conditions change. Some may go up in smoke if wildfires become more frequent, as some climate simulations predict.

Planting forests and adopting carbon-conserving practices such as no-till agriculture may increase carbon sinks somewhat, but this would not come close to compensating for carbon emissions, which continue to accelerate.

"There are a lot of good reasons for replenishing our forests and encouraging better agricultural practices," says Ken Caldeira, another author of the report, also at Carnegie's Department of Global Ecology. "But if we want to mitigate our impact on the carbon cycle, there's no escaping the fact that we need to drastically reduce carbon dioxide emissions."

Tony King, report team leader and staff scientist at the Energy Department's Oak Ridge National Laboratory, said the first interagency State of the Carbon Cycle Report, "is a broadly conceived activity designed to provide accurate, unbiased, and policy-relevant scientific information concerning the carbon cycle to a broad range of stakeholders."

"It provides a baseline characterization of the North American carbon budget upon which future research and reports can build and refine," said King.

Electricity generation is responsible for the largest share of North America's carbon emissions - approximately 94 percent in the United Sates in 2004, 65 percent in Canada in 2003, and 67 percent in Mexico in 1998. These are the latest years for which data are available.

More than half of the electricity produced in North America is consumed in buildings, making that single use one of the largest factors in North American emissions. In the United States, 67 percent of all electricity generated is used in buildings.

In 2003, the carbon dioxide emissions resulting from energy consumed in U.S. buildings alone were greater than total carbon dioxide emissions of any country in the world except China.

Energy use in buildings in the United States and Canada, including the use of natural gas, wood, and other fuels as well as electricity, has increased by 30 percent since 1990, corresponding to an annual growth rate of 2.1 percent.

In the United States, the major drivers of energy consumption in the buildings sector are growth in commercial floor space and increase in the size of the average home. Carbon emissions from buildings are expected to grow with population and income.

The carbon intensity of the United States economy, which is the amount of carbon emitted per dollar of inflation adjusted Gross Domestic Product, GDP, has decreased at a rate of about two percent per year, the report states. The decrease is due to increased energy efficiency, particularly in the manufacturing sector, and structural changes in the economy with growing contributions from sectors such as services with lower energy consumption and carbon intensity.

The service sector is likely to continue to grow, the report states, and so carbon emissions will likely continue to grow more slowly than GDP.

The "State of the Carbon Cycle" is funded by the Department of Energy, the National Aeronautical and Space Administration, the National Oceanic and Atmospheric Administration, and the National Science Foundation.

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