There is debate on the potential release of the tundra's immense carbon stocks into the atmosphere in response to global warming. We present here results obtained with a model of CO2 exchanges, coupled to a model of the soil thermal and hydrological regime in the tundra. We show that, because of the partial thawing of permafrost and subsequent increase in nutrient availability, the ecosystem's response to warming may be a long-lasting increase in C accumulation, following a temporary increase in CO2 emissions. Our study also provides a consistent picture of CO2 exchanges in tundra ecosystems, reconciling the short-term experimental response to warming, recent field measurements, and Holocene C accumulation estimates.¿ 1997 American Geophysical Union