The Changing Carbon Cycle of the Arctic

The Arctic plays an important role in the global dynamics of both carbon dioxide and methane, and the responses of the carbon cycle in this region to global change have the potential to cause rapid changes in the earth system. This study used several model-based tools to analyze the contemporary dynamics of the Arctic Basin between 1997 and 2006 as a linked system of land–ocean–atmosphere C exchange. The analysis estimates that during this time period the terrestrial areas of the Arctic Basin lost 62.9 Tg C yr-1 and that the Arctic Ocean gained 94.1 Tg C yr-1. Arctic lands and oceans were a net sink of 108.9 Tg C yr-1 as CO₂ between 1997 and 2006, which is within the range of uncertainty in estimates from the atmospheric inversions. While both the lands and oceans of the Arctic were estimated to be sinks for atmospheric CO₂ between 1997 and 2006, our analysis shows that the land sink is diminishing in strength because of increased fire disturbance in this decade compared to previous decades and that the ocean sink is increasing in strength as a result of increased biological pump activity associated with the reduction in sea ice cover. Terrestrial areas of the Arctic were estimated to be a net source of 41.5 Tg CH₄ yr-1 that increased by 0.5 Tg CH₄ yr-1 during the decade of analysis, a magnitude that is comparable with an atmospheric inversion of CH₄. This magnitude of CH₄ emissions is equivalent to 954.5 Tg CO₂ yr-1 calculated on a 100-year time horizon, which is over 550 Tg CO₂ greater than the estimated net CO₂ sink of 399.3 Tg CO₂ yr-1 (108.9 Tg C-CO₂ yr-1). Thus, our results suggest that the Arctic System as a whole is a substantial net source of greenhouse gas forcing to the climate system. Key research issues are to evaluate (1) whether the influence of continued warming on terrestrial processes affecting CO₂ exchange with the atmosphere will more than overwhelm the effects of reduced sea ice in enhancing CO₂ uptake by the Arctic Ocean; and (2) whether the greenhouse gas forcing of CH₄ emissions will be enhanced or compensated by the response of CO₂ exchange to warming. The results of this study emphasize the importance of analyzing the dynamics of the C balance in the Arctic as a linked system across the land–ocean–atmosphere domain in order to evaluate these questions.