If our simulated changes in ecosystem C storage are extrapolated for the ≈43 Pg C contained in arctic tundras globally, the maximum net gain or loss (≈0.3% per yr) from tundra would be equivalent to 0.13 Pg C/yr. Although fluxes of this magnitude would have a relatively minor impact on current changes in atmospheric CO2, the long-term impact on tundra C stores could be significant. The synthesis and insights provided by the model should make it possible to extrapolate into the future with a better understanding of the processes governing long-term changes in tundra C storage.