SENSITIVITY ANALYSIS OF A MODEL OF CO2 EXCHANGE IN TUNDRA ECOSYSTEMS BY THE ADJOINT METHOD

A model of net primary production (NPP), decomposition, and nitrogen c ycling in tundra ecosystems has been developed. The adjoint technique is used to study the sensitivity of the computed annual net CO2 flux t o perturbations in initial conditions, climatic inputs, and model's ma in parameters describing current seasonal CO2 exchange in wet sedge tu ndra at Barrow, Alaska. The results show that net CO2 flux is most sen sitive to parameters characterizing litter chemical composition and mo re sensitive to decomposition parameters than to NPP parameters. This underlines the fact that in nutrient-limited ecosystems, decomposition drives net CO2 exchange by controlling mineralization of main nutrien ts. The results also indicate that the shortterm (1 year) response of wet sedge tundra to CO2-induced warming is a significant increase in C O2 emission, creating a positive feedback to atmospheric CO2 accumulat ion. However, a cloudiness increase during the same year can severely alter this response and lead to either a slight decrease or a strong i ncrease in emitted CO2, depending on its exact timing. These results d emonstrate that the adjoint method is well suited to study systems enc ountering regime changes, as a single run of the adjoint model provide s sensitivities of the net CO2 flux to perturbations in all parameters and variables at any time of the year. Moreover, it is shown that lar ge errors due to the presence of thresholds can be avoided by first de limiting the range of applicability of tile adjoint results.