Abstract
The seasonal-cycle amplitude (SCA) of the atmosphere–ecosystem carbon dioxide (CO2) exchange rate is a useful metric of the responsiveness of the terrestrial biosphere to environmental variations. It is unclear, however, what underlying mechanisms are responsible for the observed increasing trend of SCA in atmospheric CO2 concentration. Using output data from the Multi-scale Terrestrial Model Intercomparison Project (MsTMIP), we investigated how well the SCA of atmosphere–ecosystem CO2 exchange was simulated with 15 contemporary terrestrial ecosystem models during the period 1901–2010. Also, we made attempt to evaluate the contributions of potential mechanisms such as atmospheric CO2, climate, land-use, and nitrogen deposition, through factorial experiments using different combinations of forcing data. Under contemporary conditions, the simulated global-scale SCA of the cumulative net ecosystem carbon flux of most models was comparable in magnitude with the SCA of atmospheric CO2 concentrations. Results from factorial simulation experiments showed that elevated atmospheric CO2 exerted a strong influence on the seasonality amplification. When the model considered not only climate change but also land-use and atmospheric CO2 changes, the majority of the models showed amplification trends of the SCAs of photosynthesis, respiration, and net ecosystem production (+0.19 % to +0.50 % yr−1). In the case of land-use change, it was difficult to separate the contribution of agricultural management to SCA because of inadequacies in both the data and models. The simulated amplification of SCA was approximately consistent with the observational evidence of the SCA in atmospheric CO2 concentrations. Large inter-model differences remained, however, in the simulated global tendencies and spatial patterns of CO2 exchanges. Further studies are required to identify a consistent explanation for the simulated and observed amplification trends, including their underlying mechanisms. Nevertheless, this study implied that monitoring of ecosystem seasonality would provide useful insights concerning ecosystem dynamics.
Highlights
The carbon budget of terrestrial ecosystems is one of the most important mechanisms, in addition to anthropogenic emissions and ocean fluxes, that regulate atmospheric CO2 concentrations (Le Quereet al., 2015; Schimel et al, 2015)
We used data from the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP), which was conducted as a part of the North American Carbon Program
This study focused on the seasonalcycle amplitude (SCA) of atmosphereÁecosystem CO2 exchange in relation to atmospheric CO2 concentration
Summary
The carbon budget of terrestrial ecosystems is one of the most important mechanisms, in addition to anthropogenic emissions and ocean fluxes, that regulate atmospheric CO2 concentrations (Le Quereet al., 2015; Schimel et al, 2015). On the basis of long-term observations, Graven et al (2013) have recently indicated that the SCA of the northern atmospheric CO2 concentration increased by 25Á50 % over the last 50 yr. This finding is consistent with previous results reported by researchers such as Bacastow et al (1985) and Keeling et al (1996) but is associated with a higher level of confidence because it is based on novel analyses using longterm ground and aircraft data and atmospheric transport models. We analysed global-scale, long-term trends of the SCA of atmosphereÁecosystem CO2 fluxes using data simulated by 15 contemporary terrestrial ecosystem models. We discuss the correspondence between the trends of terrestrial ecosystems and atmospheric CO2, the potential limitations of this study, the implications for observation and future directions
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