The Marine Biological Laboratory General Ecosystem Model was calibrated for an arctic tussock tundra system using data from long-term observations and experiments at Toolik Lake, Alaska. These experiments include the effects of changes in temperature, light, CO2, and nutrients, so the model could be applied to five regions comprising the entire Kuparuk River basin. Net primary production, averaged for the entire basin, was 92 g C m-2 yr-1. A 150 year simulation of carbon storage under a doubling of CO2 (slow ramp-up) and a temperature increase of 3.5 ¿C gave an estimate of +400 g C m-2 when soil moisture increased and +500 g C m-2 when soil moisture decreased. Drier soils stimulated decomposition producing an increase in nitrogen availability; the increased N led to increased net primary production. If this result is applicable to other arctic ecosystems, then it is unlikely that warming will enhance carbon loss to the atmosphere to further enhance warming. ¿ 1998 American Geophysical Union