Sensitivity analysis of a model of CO 2 exchange in tundra ecosystems by the adjoint method

A model of net primary production (NPP), decomposition, and nitrogen cycling in tundra ecosystems has been developed. The adjoint technique is used to study the sensitivity of the computed annual net CO 2 flux to perturbations in initial conditions, climatic inputs, and model's main parameters describing current seasonal CO 2 exchange in wet sedge tundra at Barrow, Alaska. The results show that net CO 2 flux is most sensitive to parameters characterizing litter chemical composition and more sensitive to decomposition parameters than to NPP parameters. This underlines the fact that in nutrient‐limited ecosystems, decomposition drives net CO 2 exchange by controlling mineralization of main nutrients. The results also indicate that the short‐term (1 year) response of wet sedge tundra to CO 2 ‐induced warming is a significant increase in CO 2 emission, creating a positive feedback to atmospheric CO 2 accumulation. However, a cloudiness increase during the same year can severely alter this response and lead to either a slight decrease or a strong increase in emitted CO 2 , depending on its exact timing. These results demonstrate that the adjoint method is well suited to study systems encountering regime changes, as a single run of the adjoint model provides sensitivities of the net CO 2 flux to perturbations in all parameters and variables at any time of the year. Moreover, it is shown that large errors due to the presence of thresholds can be avoided by first delimiting the range of applicability of the adjoint results.