Emerging negative impact of warming on summer carbon uptake in northern ecosystems.

Most studies of the northern hemisphere carbon cycle based on atmospheric CO concentration have focused on spring and autumn, but the climate change impact on summer carbon cycle remains unclear. Here we used atmospheric CO record from Point Barrow (Alaska) to show that summer CO drawdown between July and August, a proxy of summer carbon uptake, is significantly negatively correlated with terrestrial temperature north of 50°N interannually during 1979-2012. However, a refined analysis at the decadal scale reveals strong differences between the earlier (1979-1995) and later (1996-2012) periods, with the significant negative correlation only in the later period. This emerging negative temperature response is due to the disappearance of the positive temperature response of summer vegetation activities that prevailed in the earlier period. Our finding, together with the reported weakening temperature control on spring carbon uptake, suggests a diminished positive effect of warming on high-latitude carbon uptake.