Response of the seasonal carbon cycle in high latitudes to climate anomalies

Many observational studies have demonstrated that the amplitude of the seasonal carbon cycle in high latitudes is increasing and advancing in phase. Several hypotheses related to natural and anthropogenic landscape disturbance and climate change have been proposed to explain these observations. This study employs a regional atmosphere-biosphere model (the Arctic Regional Climate System Model (ARCSyM)) to investigate the impacts of the climatic effects of elevated carbon dioxide on terrestrial carbon exchange over Alaska. A perturbation experiment, driven by a transient global climate scenario at the point at which atmospheric carbon dioxide has doubled, is compared to a validated present-day simulation. The focus of this paper is on the role of physical climate in seasonal carbon exchange between the atmosphere and the terrestrial biosphere at regional scales. In the present-day simulation the ARCSyM reproduces most observed features of climate and the surface energy budget and simulates a clearly defined growing season in the seasonal cycle of carbon dioxide exchange. In the perturbation experiment, the model atmosphere shows a generally warmer and wetter climate, with increased permafrost thaw. Two dominant biomes, tundra and boreal forest, are analyzed in detail for their responses to the perturbed climate. The amplitudes of the seasonal carbon dioxide cycle for Alaskan boreal forest and tundra increase 12 and 8%, respectively, with phase advances of similar to 7 and 2 days, respectively. The forcing of photosynthetic productivity by soil moisture changes is a dominant contributor to the simulated changes in net ecosystem exchange. Boreal forest ecosystem is more sensitive to moisture anomalies due to a drier soil, whereas in the tundra regions, which are often close to saturation, changes are forced primarily by temperature variations. Thus in the event of a northward migration of tree line, sensitivity of regional carbon exchange to climate perturbations could increase.