Abstract.  We modeled the effects of increasing CO2 and climate on net carbon storage in terrestrial ecosystems of the conterminous U.S. for the period 1895-1993 using new, detailed historical climate information. For the period 1980-1993, results from an ensemble of three models agree within 25%, simulating a land carbon sink from CO2 and climate effects of 0.08 Gt C per year. The best estimates of the total sink from inventory data are approximately three times larger (~0.3 Gt C per year), suggesting that processes such as regrowth on abandoned agricultural land or in forests harvested before 1980 have effects as large or larger than the direct effects of CO2 and climate. The modeled sink varies by ~100% from year-to-year as a result of climate variability, suggesting that any future policy or management measures based on estimates of net carbon storage must take this high variability into account. Recent published estimates of a nearly 2 Gt sink in the U.S. are in conflict with ecological process models and forest inventory-based techniques.