Carbon-concentration and carbon-climate feedbacks in CMIP6 models and their comparison to CMIP5 models

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 degrees C-1) is about 3 times larger than over ocean (-17.2 +/- 5.0 PgC degrees 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.