Robust increase of seasonal amplitude in simulated terrestrial CO2 exchange in last decades

Abstract:

Ongoing climatic change is expected to affect terrestrial carbon budget by altering growing period and photosynthetic and respiratory processes. Indeed, a recent atmospheric study (Graven et al. 2013) shows that seasonal amplitude of atmospheric CO₂ concentration has bee increased since 1960. Because of the lack of long-term observational data except phenology, it is difficult to confirm such enhancement of vegetation activity through a direct approach. Therefore, we examined the simulated terrestrial CO₂ exchange by a process-based model (VISIT) from 1901 to 2010, using the MsTMIP protocol. Several metrics were used to characterize the seasonal amplitude: maximum photosynthetic uptake and respiratory release and cumulative net biome production (NBP). To separate the effects of climatic change and elevated atmospheric increase, two simulation results (climate, climate+CO₂) were compared. The results showed that the seasonal amplitude defined as the maximum?minimum cumulative NBP in the Northern Hemisphere increased by 16% from the 1960s to the 2000s. Such increase in seasonal amplitude was more evident in North America and Europe, compared with Siberia and Asia. In the climate-only simulation, northern seasonal amplitude decreased from 1965 to 1980, a climatic cooling period, and then increased since then. These results indicate that the observed increase of atmospheric CO₂ concentration amplitude is largely attributable to enhanced terrestrial vegetation activity. We would compare these results with those by other terrestrial ecosystem models and possibly with some observational record.