Large‐scale variations in the vegetation growing season and annual cycle of atmospheric <scp><scp>CO₂</scp></scp> at high northern latitudes from 1950 to 2011

We combine satellite and ground observations during 1950–2011 to study the long‐term links between multiple climate (air temperature and cryospheric dynamics) and vegetation (greenness and atmospheric CO 2 concentrations) indicators of the growing season of northern ecosystems (>45°N) and their connection with the carbon cycle. During the last three decades, the thermal potential growing season has lengthened by about 10.5 days ( P < 0.01, 1982–2011), which is unprecedented in the context of the past 60 years. The overall lengthening has been stronger and more significant in E urasia (12.6 days, P < 0.01) than N orth America (6.2 days, P > 0.05). The photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn when factors such as light and moisture limitation may constrain photosynthesis. The autumnal extension of the photosynthetic growing season since 1982 appears to be about half that of the thermal potential growing season, yielding a smaller lengthening of the photosynthetic growing season (6.7 days at the circumpolar scale, P < 0.01). Nevertheless, when integrated over the growing season, photosynthetic activity has closely followed the interannual variations and warming trend in cumulative growing season temperatures. This lengthening and intensification of the photosynthetic growing season, manifested principally over E urasia rather than N orth America, is associated with a long‐term increase (22.2% since 1972, P < 0.01) in the amplitude of the CO 2 annual cycle at northern latitudes. The springtime extension of the photosynthetic and potential growing seasons has apparently stimulated earlier and stronger net CO 2 uptake by northern ecosystems, while the autumnal extension is associated with an earlier net release of CO 2 to the atmosphere. These contrasting responses may be critical in determining the impact of continued warming on northern terrestrial ecosystems and the carbon cycle.