The U.S. Climate Change Science Program, a U.S. government science panel, published a report today, “The North American Carbon Budget and Implications for the Global Carbon Cycle,” that finds North America’s fossil fuel emissions 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. The report details how the growth of vegetation blanketing North America absorbs large amounts of carbon from the atmosphere.
But the report points out a greater than three-to-one imbalance between the fossil fuel sources and the ability of vegetation to absorb carbon. This results in a net release to the atmosphere (over one gigaton of carbon per year in 2003), but there is still some uncertainty in quantifying the North American sink compared to the carbon emission sources. The 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.
What’s not known is how fast the carbon storage “sink” will decline and whether it might potentially become a source since changes in climate and atmospheric carbon dioxide could affect forest growth differently in different regions. Further warming, for example, could exacerbate drought, increasing carbon release through vegetation dieback and increased fire and insect disturbances.
“The large difference between current sources and sinks and the expectation that the difference could become larger if the growth of fossil-fuel emissions continues and land sinks decline suggest that addressing imbalances in the North American carbon budget will likely require actions focused on reducing fossil-fuel emissions,” according to the report. “Options to enhance sinks (growing forests or sequestering carbon in agricultural soils) can contribute, but enhancing sinks alone is likely insufficient to deal with either the current or future imbalance.”
Total U.S. emissions have grown at close to the North American average rate of about one percent per year over the past 30 years, but United States per capita emissions have been roughly constant.
The carbon intensity of the U.S. economy, which is the amount of carbon emitted per dollar of inflation adjusted GDP, has decreased at a rate of about two percent per year. The decline in the carbon intensity of the economy was caused both by 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. Accordingly, carbon emissions will likely continue to grow more slowly than GDP.
The extraction of fossil-fuels and other primary energy sources and their conversion to energy products and services, including electricity generation, is the single largest contributor to the North American fossil-fuel source, accounting for approximately 42 percent of North American fossil emissions in 2003.
Electricity generation is responsible for the largest share of those emissions: approximately 94 percent in the U.S. 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 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.