We have used the Terrestrial Ecosystem Model (TEM 4.2) to quantify the relative contributions of climate variability, increasing atmospheric carbon dioxide and land-use change to the US terrestrial carbon sink in the 20th century. Our preliminary analyses have indicated that the combination of carbon dioxide, climate and land use caused an overall decrease of 2.2 Pg C in the terrestrial carbon storage in the US over the period from 1900 to 1992. However, since the 1960s, these terrestrial ecosystems have acted primarily as a sink of atmospheric carbon dioxide. For the 1980s, terrestrial ecosystems in US accumulated 0.136 Pg C per year because of the combined effects of increasing atmospheric carbon dioxide, climate variability and land-use change: 1) carbon dioxide fertilization alone increased carbon storage by 0.055 Pg C per year; 2) climate variability increased carbon storage by 0.044 Pg C per year; and 3) recovery from cropland abandonment increased carbon storage by 0.037 Pg C per year. We estimate that US forests accumulated 0.088 Pg C per year for the 1980s, which is in the low range of forest inventory-based estimates (0.079 - 0.28 Pg C per year). Two factors not considered in our analysis, forest recovery following logging and N deposition, would both function to increase the forest carbon sequestration rate.