The effects of increasing carbon dioxide and climate on net carbon storage in terrestrial ecosystems of the conterminous United States were modeled using new, detailed historical climate information from the NOAA Historical Climate Network database. For the period 1980-1993, results from an ensemble of three models agree within 25 percent, simulating a land carbon sink from CO2 and climate effects of 0. 08 gigaton (1 gigaton=1 billion tonnes) of carbon per year. The best estimates of the total sink from inventory data are about three times larger, suggesting that processes such as regrowth on abandoned agricultural land or in forests harvested before 1980 have effects as large as or larger than the direct effects of CO2 and climate. The modeled sink varies by about 100 percent from year to year as a result of climate variability. Estimates from all three models were used for natural ecosystems; the Century model results were used for agricultural areas. These results were part of the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) aimed at understanding the contribution of ecosystem physiological mechanisms to terrestrial sinks in the conterminous United States.