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

Arora, Vivek K. ; Katavouta, Anna ; Williams, Richard G. ; Jones, Chris D. ; Brovkin, Victor ; Friedlingstein, Pierre ; Schwinger, Jörg ; Bopp, Laurent ; Boucher, Olivier ; Cadule, Patricia ; Chamberlain, Matthew A. ; Christian, James R. ; Delire, Christine ; Fisher, Rosie A. ; Hajima, Tomohiro ; Ilyina, Tatiana ; Joetzjer, Emilie ; Kawamiya, Michio ; Koven, Charles D. ; Krasting, John P. ; Law, Rachel M. ; Lawrence, David M. ; Lenton, Andrew ; Lindsay, Keith ; Pongratz, Julia ; Raddatz, Thomas ; Séférian, Roland ; Tachiiri, Kaoru ; Tjiputra, Jerry F. ; Wiltshire, Andy ; Wu, Tongwen ; Ziehn, Tilo

Results from the fully and biogeochemically coupled simulations in which <span class="inline-formula">CO₂</span> increases at a rate of 1?%?yr<span class="inline-formula">^-1</span> (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 <span class="inline-formula">CO₂</span> 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?<span class="inline-formula">±</span>?standard deviation?<span class="inline-formula">=</span>?0.97?<span class="inline-formula">±</span>?0.40?PgC?ppm<span class="inline-formula">^-1</span>) and ocean (0.79?<span class="inline-formula">±</span>?0.07?PgC?ppm<span class="inline-formula">^-1</span>), while the carbon-climate feedback over land (<span class="inline-formula">-45.1</span>?<span class="inline-formula">±</span>?50.6?PgC°<span class="inline-formula"></span>C<span class="inline-formula">^-1</span>) is about 3 times larger than over ocean (<span class="inline-formula">-17.2</span>?<span class="inline-formula">±</span>?5.0?PgC°<span class="inline-formula"></span>C<span class="inline-formula">^-1</span>). The strength of both feedbacks is an order of magnitude more<span id="page4174"/> 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?<span class="inline-formula">±</span>?0.37?<span class="inline-formula"></span>°C?EgC<span class="inline-formula">^-1</span> and is similar to that found in CMIP5 models (1.63?<span class="inline-formula">±</span>?0.48?<span class="inline-formula"></span>°C?EgC<span class="inline-formula">^-1</span>) 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.</p>

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