Abstract. Results from the fully and biogeochemically coupled simulations in which CO2 increases at a rate of 1 % yr−1 (1pctCO2) from its preindustrial value are analyzed to quantify the magnitude of carbon–concentration and carbon–climate feedback parameters which measure the response of ocean and terrestrial carbon pools to changes in atmospheric CO2 concentration and the resulting change in global climate, respectively. The results are based on 11 comprehensive Earth system models from the most recent (sixth) Coupled Model Intercomparison Project (CMIP6) and compared with eight models from the fifth CMIP (CMIP5). The strength of the carbon–concentration feedback is of comparable magnitudes over land (mean ± standard deviation = 0.97 ± 0.40 PgC ppm−1) and ocean (0.79 ± 0.07 PgC ppm−1), while the carbon–climate feedback over land (−45.1 ± 50.6 PgC ∘C−1) is about 3 times larger than over ocean (−17.2 ± 5.0 PgC ∘C−1). The strength of both feedbacks is an order of magnitude more uncertain over land than over ocean as has been seen in existing studies. These values and their spread from 11 CMIP6 models have not changed significantly compared to CMIP5 models. The absolute values of feedback parameters are lower for land with models that include a representation of nitrogen cycle. The transient climate response to cumulative emissions (TCRE) from the 11 CMIP6 models considered here is 1.77 ± 0.37 ∘C EgC−1 and is similar to that found in CMIP5 models (1.63 ± 0.48 ∘C EgC−1) but with somewhat reduced model spread. The expressions for feedback parameters based on the fully and biogeochemically coupled configurations of the 1pctCO2 simulation are simplified when the small temperature change in the biogeochemically coupled simulation is ignored. Decomposition of the terms of these simplified expressions for the feedback parameters is used to gain insight into the reasons for differing responses among ocean and land carbon cycle models.

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<p><strong>Abstract.</strong> 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>

<p>A fraction of the carbon fixed in the surface ocean by phytoplankton is isolated away from the atmosphere in the ocean interior with the respiration of sinking detritus (Particles of Organic Carbon: POC) - a process known as the "Biological Carbon Pump'' (BCP). The BCP sequesters ~1700 Pg of dissolved inorganic carbon (DIC) in the ocean beyond the concentration expected solely with physio-chemical drivers, effectively lowering the base-line atmospheric CO2 concentration by ~150-250 ppm. The components that make up the BCP (export production, sinking and remineralisation of POC, ocean ventilation timescales) are all expected to change in response to a changing climate but there is currently low confidence in how these changes will influence the magnitude and direction of the ocean carbon feedback.</p><p>Here we quantify the predicted historical and future changes in the Biological Carbon Pump in the latest CMIP6 projections as fully as possible. We find that all models consistently predict that the BCP will accumulate carbon in the ocean interior by 2100, i.e., acting as a sink for atmospheric CO2, albeit contributing only a small fraction (~10%) of the net carbon sink. The accumulation of carbon along with a concurrent decrease in globally integrated export production at 100m is associated with warming-driven stratification. In contrast there is significant disagreement in both the magnitude and direction of global mean trends and spatial patterns of transfer efficiency of POC at 1000m. This uncertainty arises because of the range of processes resolved across the biogeochemical models that influence the sinking and remineralisation rate of POC such as: temperature and oxygen-dependent remineralisation, ballasting, and dependence of sinking velocities on cell size. We demonstrate that these changes in transfer efficiency could likely determine the larger long-term impact of the BCP on atmospheric CO2 beyond 2100. Our results have wider implications for the biogeochemical cycling of nutrients and oxygen as well as implications for future impacts on twilight zone ecology.</p>