Abstract
Abstract. Earth system models (ESMs) that incorporate carbon–climate feedbacks represent the present state of the art in climate modelling. Here, we describe the Australian Community Climate and Earth System Simulator (ACCESS)-ESM1, which comprises atmosphere (UM7.3), land (CABLE), ocean (MOM4p1), and sea-ice (CICE4.1) components with OASIS-MCT coupling, to which ocean and land carbon modules have been added. The land carbon model (as part of CABLE) can optionally include both nitrogen and phosphorous limitation on the land carbon uptake. The ocean carbon model (WOMBAT, added to MOM) simulates the evolution of phosphate, oxygen, dissolved inorganic carbon, alkalinity and iron with one class of phytoplankton and zooplankton. We perform multi-centennial pre-industrial simulations with a fixed atmospheric CO2 concentration and different land carbon model configurations (prescribed or prognostic leaf area index). We evaluate the equilibration of the carbon cycle and present the spatial and temporal variability in key carbon exchanges. Simulating leaf area index results in a slight warming of the atmosphere relative to the prescribed leaf area index case. Seasonal and interannual variations in land carbon exchange are sensitive to whether leaf area index is simulated, with interannual variations driven by variability in precipitation and temperature. We find that the response of the ocean carbon cycle shows reasonable agreement with observations. While our model overestimates surface phosphate values, the global primary productivity agrees well with observations. Our analysis highlights some deficiencies inherent in the carbon models and where the carbon simulation is negatively impacted by known biases in the underlying physical model and consequent limits on the applicability of this model version. We conclude the study with a brief discussion of key developments required to further improve the realism of our model simulation.
Highlights
Over recent decades many climate models have evolved into Earth system models (ESMs), a term used to identify models that simulate biogeochemical cycles and their interaction with human and climate systems
We focus on the sensitivity to the ocean circulation simulation by comparing the ACCESS-ESM1 results with those from an ocean-only simulation (Sect. 4.4)
The two ACCESS-ESM1 simulations are very similar, and we only show the results from the simulation with prognostic leaf area index (LAI)
Summary
Over recent decades many climate models have evolved into Earth system models (ESMs), a term used to identify models that simulate biogeochemical cycles and their interaction with human and climate systems. While ESM simulations with the full carbon cycle and an interactive atmosphere (“emissions-driven simulations”) are essential for quantifying the carbon–climate feedbacks, simulations with a fixed atmospheric CO2 (“concentrationdriven”) are valuable These simulations are used to diagnose land and ocean carbon exchange with the atmosphere based on a prescribed atmospheric history of CO2 and any associated climate impacts of those atmospheric CO2 changes. The temporal evolution of carbon exchange can be evaluated for a range of future atmospheric CO2 trajectories This simpler concentration-driven mode for ESM simulations removes any direct impact of the carbon cycle on the climate simulation with indirect impacts only occurring through possible changes to the land surface characteristics such as the leaf area index. ACCESS-ESM1 simulations for future periods (2005–2100) will be presented elsewhere
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