Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost are expected to be among the largest feedbacks to climate from arctic ecosystems. However, the current carbon (C) balance of terrestrial arctic ecosystems is unknown, with recent studies suggesting that these ecosystems are approximately sources, sinks, or approximately in balance at present. This uncertainty arises because there are few long-term continuous measurements of arctic tundra CO2 fluxes over the full annual cycle. Here, we describe an increasing trend of CO2 loss from the longest continuous record of direct measurements of CO2 fluxes in the Alaskan Arctic, from two representative tundra ecosystems, wet sedge and heath tundra. The rate of CO2 loss from both ecosystems increased over the past seven years, coincident with increases in soil temperatures in the fall and winter. Wet sedge tundra lost the most CO2 during the anomalously warm autumn periods of September – December 2013 and 2014, with CH4 emissions contributing little to the overall C budget. Losses of C translated to approximately 3.6% of the total soil C stocks (0.5% per year) in the wet sedge and 1.9% of the total C stocks (0.3% per year) in the heath tundra from 2008 – 2014. The increases in temperature during fall and early winter may have triggered a new trajectory of CO2 release, which will be a significant feedback to further warming if it is representative of larger areas of the Arctic.
(This study is associated with the Arctic LTER and Toolik Field Station.)