The changing CO2 seasonal cycle: past and future

Abstract:

A long-standing puzzle in the global carbon cycle is the increase in the amplitude of the seasonal cycle of atmospheric CO2 and its relation to long-term carbon sink. This increase likely reflects enhanced biological activity in the Northern Hemisphere. It has been hypothesized that vegetation growth may have been stimulated by higher concentrations of CO2 as well as warming in recent decades, and more recently, the intensification of agriculture due to the use of high-productivity cultivars, fertilizer use and irrigation has been identified as a missing driver. However, the state-of-the-art carbon cycle models do not agree on the relative roles of such mechanisms. Furthermore, even the simulation of the mean seasonal cycle may be problematic. Yet most models predict large changes in the CO2 seasonal amplitude in the future, up to 75% increase in the CMIP5 ensemble. We analzye and review the evidence and mechanisms of such changes, based on CO2 and satellite vegetation observations and the TRENDY and CMIP5 models. We argue that detail spatial patterns and temporal variability, aspects often ignored and less well constrained, may be a key in delineating the mechanisms and therefore enhancing our confidence in future projection of both seasonal cycle and mean sink.