Acclimation of ecosystem CO2 exchange in the Alaskan Arctic in response todecadal climate warming

Long-term sequestration of carbon in Alaskan Arctic tundra ecosystems was r eversed by warming and drying of the climate in the early 1980s, resulting in substantial losses of terrestrial carbon(1,2). But recent measurements s uggest that continued warming and drying has resulted in diminished CO2 eff lux, and in some cases, summer CO2 sink activity(3,4). Here we compile summ er CO2 flux data for two Arctic ecosystems from 1960 to the end of 1998. Th e results show that a return to summer sink activity has come during the wa rmest and driest period observed over the past four decades, and indicates a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term (decadal or longer) changes in climate. The mechanisms involv ed are likely to include changes in nutrient cycling, physiological acclima tion, and population and community reorganization. Nevertheless, despite th e observed acclimation, the Arctic ecosystems studied are still annual net sources of CO2 to the atmosphere of at least 40 g C m(-2) yr(-1), due to wi nter release of CO2, implying that further climate change may still exacerb ate CO2 emissions from Arctic ecosystems.