labs_title

Decadal trends in the seasonal-cycle amplitude of terrestrial CO2 exchange resulting from the ensemble of terrestrial biosphere models

A. Ito, M. Inatomi, D.N. Huntzinger, C. Schwalm, A.M. Michalak, R. Cook, A.W. King, J. Mao, Y. Wei, W. Mac Post, W. Wang, M. Altaf Arain, S. Huang, D.J. Hayes, D.M. Ricciuto, X. Shi, M. Huang, H. Lei, H. Tian, C. Lu, J. Yang, B. Tao, A. Jain, B. Poulter, S. Peng, P. Ciais, J.B. Fisher, N. Parazoo, K. Schaefer, C. Peng, N. Zeng and F. Zhao

The global biosphere currently absorbs about a quarter of fossil fuel emissions each year. It is imperative that we understand the mechanisms driving this process in order to anticipate how climate change will impact the future of this “carbon sink.” This paper uses an ensemble of terrestrial biosphere model simulations from the MsTMIP project to attribute a recent observed increase in the amplitude of the seasonal cycle in the exchange of carbon between the biosphere and the atmosphere. We found that, within the models, the increasing atmospheric concentrations of carbon dioxide exerted a strong influence on the seasonality amplification.


Figure: Interannual variability of the seasonal cycle amplitude (SCA) of global terrestrial CO2 fluxes. For each of the simulations (SG1, SG2, SG3 and BG1), model average anomalies of the models against the 1950s-mean (grey zone) are shown. (a) Monthly gross primary production (GPP), (b) cumulative (i.e. annual total) GPP, (c) monthly ecosystem respiration (RE), (d) cumulative RE, (e) monthly net ecosystem production (NEP) and (f) cumulative NEP.

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.

Ito, A., M. Inatomi, D.N. Huntzinger, C. Schwalm, A.M. Michalak, R. Cook, A.W. King, J. Mao, Y. Wei, W. Mac Post, W. Wang, M. Altaf Arain, S. Huang, D.J. Hayes, D.M. Ricciuto, X. Shi, M. Huang, H. Lei, H. Tian, C. Lu, J. Yang, B. Tao, A. Jain, B. Poulter, S. Peng, P. Ciais, J.B. Fisher, N. Parazoo, K. Schaefer, C. Peng, N. Zeng, F. Zhao (2016), "Decadal trends in the seasonal-cycle amplitude of terrestrial CO2 exchange resulting from the ensemble of terrestrial biosphere models", Tellus B, 68 (1), 28968, doi:10.3402/tellusb.v68.28968.