Consistent Land- and Atmosphere-Based U.S. Carbon Sink Estimates

For the period 1980Ð89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon 5 1015 grams of carbon). The net carbon ßux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net ßux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year)because of the inclusion of additional processes and revised estimates of some component ßuxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980Ð89 are similar to those for 1985Ð89 and 1990Ð94, indicating a relatively stable U.S. sink throughout the period