The USGS released the first phase today of a national assessment that estimates "the conterminous U.S. presently stores 73 billion metric tons of carbon in soils and 17 billion metric tons in forests" which is equal to "more than 50 years of America’s current CO2 emissions from burning fossil fuels."
They conclude that "U.S. forests and soils could remove additional quantities of carbon dioxide (CO2) from the atmosphere as a means to mitigate climate change. The lower 48 states in the U.S. hypothetically have the potential to store an additional 3-7 billion metric tons of carbon in forests, if agricultural lands were to be used for planting forests. This potential is equivalent to 2 to 4 years of America’s current CO2 emissions from burning fossil fuels."
[right, hypothetical historical forest biomass carbon sequestration capacity. Credit, USGS OFR 2009-1283. Okay, you're going to have to read the paper to figure out what that means. The other figure captions are equally obtuse.]
Ref: Sundquist, E.T., Ackerman, K.V., Bliss, N.B., Kellndorfer, J.M., Reeves, M.C., and Rollins, M.G., 2009, Rapid assessment of U.S. forest and soil organic carbon storage and forest biomass carbon sequestration capacity: U.S. Geological Survey Open-File Report 2009–1283, 15 p., available at http://pubs.usgs.gov/of/2009/1283.