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Abstract

Understanding the variations in global land carbon uptake, and their driving mechanisms, is essential if we are to predict future carbon-cycle feedbacks on global environmental changes. Satellite observations of vegetation greenness have shown consistent greening across the globe over the past three decades. Such greening has driven the increasing land carbon sink, especially over the growing season in northern latitudes. On the other hand, interannual variations in land carbon uptake are strongly influenced by El Niño–Southern Oscillation (ENSO) climate variations. Marked reductions in land uptake and strong positive anomalies in the atmospheric CO2 growth rates occur during El Niño events. Here we use the year 2015 as a natural experiment to examine the possible response of land ecosystems to a combination of vegetation greening and an El Niño event. The year 2015 was the greenest year since 2000 according to satellite observations, but a record atmospheric CO2 growth rate also occurred due to a weaker than usual land carbon sink. Two atmospheric inversions indicate that the year 2015 had a higher than usual northern land carbon uptake in boreal spring and summer, consistent with the positive greening anomaly and strong warming. This strong uptake was, however, followed by a larger source of CO2 in the autumn. For the year 2015, enhanced autumn carbon release clearly offset the extra uptake associated with greening during the summer. This finding leads us to speculate that a long-term greening trend may foster more uptakes during the growing season, but no large increase in annual carbon sequestration. For the tropics and Southern Hemisphere, a strong transition towards a large carbon source for the last 3 months of 2015 is discovered, concomitant with El Niño development. This transition of terrestrial tropical CO2 fluxes between two consecutive seasons is the largest ever found in the inversion records. The strong transition to a carbon source in the tropics with the peak of El Niño is consistent with historical observations, but the detailed mechanisms underlying such an extreme transition remain to be elucidated.