Haverd, V. ; Smith, B. ; Nieradzik, L. ; Briggs, P. R. ; Woodgate, W. ; Trudinger, C. M. ; Canadell, J. G. ; Cuntz, M.
Geoscientific Model Development , 2018, 11 : 2995-3026.
Article
Pièces jointes : gmd-11-2995-2018.pdf
The Community Atmosphere–Biosphere Land Exchange model (CABLE) is a land surface model (LSM) that can be applied stand-alone and provides the land surface–atmosphere exchange within the Australian Com- munity Climate and Earth System Simulator (ACCESS). We describe new developments that extend the applicability of CABLE for regional and global carbon–climate simulations, accounting for vegetation responses to biophysical and an- thropogenic forcings. A land use and land cover change mod- ule driven by gross land use transitions and wood harvest area was implemented, tailored to the needs of the Coupled Model Intercomparison Project 6 (CMIP6). Novel aspects include the treatment of secondary woody vegetation, which benefits from a tight coupling between the land use module and the Population Orders Physiology (POP) module for woody de- mography and disturbance-mediated landscape heterogene- ity. Land use transitions and harvest associated with sec- ondary forest tiles modify the annually resolved patch age distribution within secondary vegetated tiles, in turn affecting biomass accumulation and turnover rates and hence the mag- nitude of the secondary forest sink. Additionally, we imple- mented a novel approach to constrain modelled GPP consis- tent with the coordination hypothesis and predicted by evo- lutionary theory, which suggests that electron-transport- and Rubisco-limited rates adjust seasonally and across biomes to be co-limiting. We show that the default prior assumption – common to CABLE and other LSMs – of a fixed ratio of electron transport to carboxylation capacity at standard tem- perature (Jmax, 0 /Vcmax, 0 ) is at odds with this hypothesis; we implement an alternative algorithm for dynamic optimisation of this ratio such that coordination is achieved as an outcome of fitness maximisation. The results have significant implica- tions for the magnitude of the simulated CO2 fertilisation ef- fect on photosynthesis in comparison to alternative estimates and observational proxies. These new developments enhance CABLE’s capability for use within an Earth system model and in stand-alone ap- plications to attribute trends and variability in the terres- trial carbon cycle to regions, processes and drivers. Model evaluation shows that the new model version satisfies sev- eral key observational constraints: (i) trend and interannual variations in the global land carbon sink, including sensi- tivities of interannual variations to global precipitation and temperature anomalies; (ii) centennial trends in global GPP; (iii) coordination of Rubisco-limited and electron-transport- limited photosynthesis; (iv) spatial distributions of global ET, GPP, biomass and soil carbon; and (v) age-dependent rates of biomass accumulation in boreal, temperate and tropical sec- ondary forests. CABLE simulations agree with recent independent assess- ments of the global land–atmosphere flux partition that use a combination of atmospheric inversions and bottom-up con- straints. In particular, there is agreement that the strong CO2- driven sink in the tropics is largely cancelled by net defor- estation and forest degradation emissions, leaving the North- ern Hemisphere (NH) extratropics as the dominant contribu- tor to the net land sink.