Increases in early season ecosystem uptake explain recent changes in the seasonal cycle of atmospheric CO2 at high northern latitudes

We report changes in the seasonal cycle of atmospheric CO2 at high northern latitudes from 1980 to 1997 based on NOAA/CMDL observation stations. Using a combination of biogeochemical and atmospheric modeling approaches, we sh ow that increases in early season net ecosystem uptake explain the recent t rends in the seasonal cycle. A strong year-to-year correlation between spri ng temperatures and early season uptake further suggests that increased pho tosynthetic activity is the primary mechanism. At the end of the growing se ason, a strong correlation between fall temperatures and late season releas es provides evidence for a large active pool of decomposing soil carbon. Ta ken together, our results suggest that the seasonal timing of temperature a nomalies may have important consequences for the interannual carbon balance of northern ecosystems.