R.T. Thomas, I.C. Prentice, H. Graven, P. Ciais, J.B. Fisher, D.J. Hayes, M. Huang, D.N. Huntzinger, A. Ito, A. Jain, J. Mao, A.M. Michalak, S. Peng, B. Poulter, D.M. Ricciuto, X. Shi, C. Schwalm, H. Tian and N. Zeng
Atmospheric observations of carbon dioxide show that the seasonal cycle has amplified over the last 50 years. It is important to understand what drives this increased seasonality in order to project how climate change will impact the global carbon cycle in the future. This paper assesses the degree to which an ensemble of terrestrial biospheric models participating in the Multiscale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) captures the observed changes in seasonality, and uses these models to attribute the change to various factors including warming and increased atmospheric CO2 concentrations, among others.
|Figure: Contributions from climate, LUC, and CO2 N deposition to modeled changes in (a) ASC, (b) GS-fAPAR, (c) mean annual NPP and Rh, and (d) GS-LUE. ASC is for 45°N–90°N at 500 mb between 1958–1961 and 2009–2010 (Figure a). Fluxes are for 30°N–90°N between 1982–2010 (Figure b) and 1958–1961 to 2009–2010 (Figure and Figure d). Fluxes are positive into the atmosphere. In Figure a, observations and uncertainty (grey shading) are from [Graven et al., 2013]. In Figure b, the 95% confidence interval in the observed trend is shown (grey shading). LAI was not available for DLEM and TEM6, and LAI was assimilated from remote sensing for SiB3, so no output is shown in Figures b or d for these models.|
Observations show an increasing amplitude in the seasonal cycle of CO2 (ASC) north of 45°N of 56 ± 9.8% over the last 50 years and an increase in vegetation greenness of 7.5–15% in high northern latitudes since the 1980s. However, the causes of these changes remain uncertain. Historical simulations from terrestrial biosphere models in the Multiscale Synthesis and Terrestrial Model Intercomparison Project are compared to the ASC and greenness observations, using the TM3 atmospheric transport model to translate surface fluxes into CO2 concentrations. We find that the modeled change in ASC is too small but the mean greening trend is generally captured. Modeled increases in greenness are primarily driven by warming, whereas ASC changes are primarily driven by increasing CO2. We suggest that increases in ecosystem-scale light use efficiency (LUE) have contributed to the observed ASC increase but are underestimated by current models. We highlight potential mechanisms that could increase modeled LUE.
Thomas, R.T., I.C. Prentice, H. Graven, P. Ciais, J.B. Fisher, D.J. Hayes, M. Huang, D.N. Huntzinger, A. Ito, A. Jain, J. Mao, A.M. Michalak, S. Peng, B. Poulter, D.M. Ricciuto, X. Shi, C. Schwalm, H. Tian, N. Zeng (2016) "Increased light-use efficiency in northern terrestrial ecosystems indicated by CO2 and greening observations", Geophysical Research Letters, 43 (21), 11339-11349, doi:10.1002/2016GL070710.