ARS | Publication request: Assessing net ecosystem carbon exchange of U.S. terrestrial ecosystems by integrating eddy covariance flux measurements and satellite observations

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2009
Publication Date: November 1, 2010
Citation: Xiao, J., Zhuang, Q., Law, B.E., Baldocchi, D.D., Chen, J., Richardson, A.D., Melillo, J.M., Davis, K.J., Hollinger, D.Y., Wharton, S., Oren, R., Noormets, `., Fischer M., L., Verma, S.B., Cook, D.R., Sun, G., Mcnulty, S., Wofsy, S.C., Bolstad, P.V., Burns, S.P., Curtis, P.S., Drake, B.G., Falk, M., Foster, D.R., Gu, L., Hadley, J.L., Katul, G.G., Litvak, M., Ma, S., Martin, T.A., Matamula, R., Meyers, T.P., Monson, R.K., Munger, J.W., Oechel, W.C., Tha Paw, U.K., Schmid, H.P., Scott, R.L., Starr, G., Suyker, A.E., Torn, M.S. 2010. Assessing net ecosystem carbon exchange of U.S. terrestrial ecosystems by integrating eddy covariance flux measurements and satellite observations. Agricultural and Forest Meteorology. 151: 60-69.

Interpretive Summary: More accurate projections of future carbon dioxide concentrations in the atmosphere and associated climate change as well as carbon accounting and climate policy-making depend on improved scientific understanding of the terrestrial carbon cycle. Despite the consensus that U.S. terrestrial ecosystems act as a carbon sink, the size, distribution, and interannual variability of the sink remain uncertain. Here we report total terrestrial carbon uptake in the conterminous U.S. at -0.68 Pg C yr-1 with the majority of the sink in regions dominated by evergreen and deciduous forests and savannas based on novel estimates of net ecosystem carbon exchange with high spatial (1km) and temporal (8-day) resolution derived from flux tower measurements and wall-to-wall satellite observations. We find that the U.S. terrestrial ecosystems could offset 40% of the fossil-fuel carbon emissions. The dominant sources of interannual variation of the carbon sink include extreme climate events and disturbances, with droughts in 2002 and 2006 reducing U.S. net ecosystem carbon uptake by ~20% relative to a normal year, and disturbances including wildfires and hurricanes resulting in reduced carbon uptake or carbon release into the atmosphere.

Technical Abstract: More accurate projections of future carbon dioxide concentrations in the atmosphere and associated climate change as well as carbon accounting and climate policy-making depend on improved scientific understanding of the terrestrial carbon cycle. Despite the consensus that U.S. terrestrial ecosystems act as a carbon sink, the size, distribution, and interannual variability of the sink remain uncertain. Here we report total terrestrial carbon uptake in the conterminous U.S. at -0.68 Pg C yr-1 with the majority of the sink in regions dominated by evergreen and deciduous forests and savannas based on novel estimates of net ecosystem carbon exchange with high spatial (1km) and temporal (8-day) resolution derived from eddy covariance flux tower measurements and wall-to-wall satellite observations. We find that the U.S. terrestrial ecosystems could offset 40% of the fossil-fuel carbon emissions. Our results show that the U.S. terrestrial carbon uptake varied between -0.51 and -0.70 Pg C yr-1 over the period 2001-2006. The dominant sources of interannual variation of the carbon sink include extreme climate events and disturbances, with droughts in 2002 and 2006 reducing U.S. net ecosystem carbon uptake by ~20% relative to a normal year, and disturbances including wildfires and hurricanes resulting in reduced carbon uptake or carbon release into the atmosphere. Our novel approach can be applied to other regions of the globe to refine the global terrestrial carbon budget.