The impact of carbon/nitrogen dynamics in terrestrial ecosystems on the interaction between carbon cycle and climate is studied using an Earth system model of intermediate complexity, the MIT Integrated Global Systems Model (IGSM). Numerical simulations were carried out with two versions of the IGSM's Terrestrial Ecosystems Model, one with and one without carbon/nitrogen dynamics.

Our simulations show that consideration of carbon/nitrogen interactions not only limits the effect of CO₂ fertilization, but also changes the sign of the feedback between climate and terrestrial carbon cycle. In the absence of carbon/nitrogen interactions, surface warming significantly reduces carbon sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback). If plant carbon uptake, however, is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability stimulating plant growth. The resulting increase in carbon uptake by vegetation exceeds carbon loss from soil, leading to enhanced carbon sequestration (a negative feedback). Under very strong surface warming, however, terrestrial ecosystems become a carbon source whether or not carbon/nitrogen interactions are considered.

Overall, for small or moderate increases in surface temperatures, consideration of carbon/nitrogen interactions result in a larger increase in atmospheric CO₂ concentration in the simulations with prescribed carbon emissions. This suggests that models which ignore terrestrial carbon/nitrogen dynamics will underestimate reductions in carbon emissions required to achieve atmospheric CO₂ stabilization at a given level. At the same time, compensation between climate-related changes in the terrestrial and oceanic carbon uptakes significantly reduces uncertainty in projected CO₂ concentration.

© 2008 American Meteorological Society