Abstract
<p>Results from the fully-, biogeochemically-, and radiatively-coupled simulations in which CO<sub>2</sub> increases at a rate of 1% per year (1pctCO2) from its pre-industrial value are analyzed to quantify the magnitude of two feedback parameters which characterize the coupled carbon-climate system. These feedback parameters quantify the response of ocean and terrestrial carbon pools to changes in atmospheric CO<sub>2</sub> concentration and the resulting change in global climate. The results are based on eight comprehensive Earth system models from the fifth Coupled Model Intercomparison Project (CMIP5) and eleven models from the sixth CMIP (CMIP6). The comparison of model results from two CMIP phases shows that, for both land and ocean, the model mean values of the feedback parameters and their multi-model spread has not changed significantly across the two CMIP phases. The absolute values of feedback parameters are lower for land with models that include a representation of nitrogen cycle. The sensitivity of feedback parameters to the three different ways in which they may be calculated is shown and, consistent with existing studies, the most relevant definition is that calculated using results from the fully- and biogeochemically-coupled configurations. Based on these two simulations simplified expressions for the feedback parameters are obtained when the small temperature change in the biogeochemically-coupled simulation is ignored. Decomposition of the terms of these simplified expressions for the feedback parameters allows identification of the reasons for differing responses among ocean and land carbon cycle models.</p>
Highlights
The Earth system responds to the perturbation of atmospheric CO2 concentration ([CO2]), caused by anthropogenic emissions of CO2 or any other forcing, via changes in its physical climate
The eight models in the A13 study are a subset of 11 models considered in this study they have been updated since Coupled Model Intercomparison Project Phase 5 (CMIP5)
Index (LAI), which decreases land surface albedo and increases absorbed solar radiation; and (3) increase in vegetation fraction in models that explicitly simulate competition between their plant functional types (PFTs) over land (NOAA-Geophysical Fluid Dynamics Laboratory (GFDL)-ESM4, MPI-ESM1.2-LR, and UKESM1-0LL), which leads to reduced land surface albedo
Summary
The Earth system responds to the perturbation of atmospheric CO2 concentration ([CO2]), caused by anthropogenic emissions of CO2 or any other forcing, via changes in its physical climate. The response of the Earth’s carbon cycle for both land and ocean components has been characterized in terms of carbon–concentration and carbon–climate feedback parameters which quantify their response to changes in [CO2] and the physical climate, respectively (Friedlingstein et al, 2006; Arora et al, 2013). Intermodel comparisons offer several benefits, including common standards and experiment protocol, coordination, and documentation that facilitate the distribution of model outputs and the characterization of the mean model response (Eyring et al, 2016), as has been shown for multiple model intercomparison projects (MIPs) They allow for the quantification of the contribution of the two feedback processes to allowable anthropogenic emissions for a given CO2 pathway. All participating modelling groups are expected to perform DECK experiments to help document basic characteristics of models across different phases of CMIP (Eyring et al, 2016)
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