Sensitivity of Future Ocean Acidification to Carbon Climate Feedbacks 1 CSIRO Oceans and Atmosphere, Hobart, Tasmania, Australia 2 Antarctic Climate and Ecosystems CRC, Hobart, Tasmania, Australia Correspondence to: Richard Matear (richard.matear@csiro.au) Abstract. Carbon-climate feedbacks have the potential to significantly impact the future climate by altering atmospheric CO 2 concentrations (Zaehle et al., 2010). By modifying the future atmo- spheric CO 2 concentrations, the carbon-climate feedbacks will also influence the future trajectory for ocean acidification. Here, we use the CO 2 emissions scenarios from 4 Representative Concen- tration Pathways (RCPs) with an Earth System Model to project the future trajectories of ocean 5 acidification with the inclusion of carbon-climate feedbacks. We show that simulated carbon-climate feedbacks can significantly impact the onset of under-saturated aragonite conditions in the Southern and Arctic Oceans, the suitable habitat for tropical coral and the deepwater saturation states. Under higher emission scenarios (RCP8.5 and RCP6.0), the carbon-climate feedbacks advance the onset of under-saturation conditions and the reduction in suitable coral reef habitat by a decade or more. 10 The impact of the carbon-climate feedback is most significant for the medium (RCP4.5) and low emission (RCP2.6) scenarios. For RCP4.5 scenario by 2100, the carbon-climate feedbacks nearly double the area of surface water under-saturated respect to aragonite and reduce by 50% the surface water suitable for coral reefs. For RCP2.6 scenario by 2100, the carbon-climate feedbacks reduce the area suitable for coral reefs by 40% and increase the area of under-saturated surface water by 15 20%. The high sensitivity of the impact of ocean acidification to the carbon-climate feedbacks in the low to medium emissions scenarios is important because our recent commitments to reduce CO 2 emissions are trying to move us on to such an emissions scenario. The study highlights the need to better characterise the carbon-climate feedbacks to ensure we do not excessively stress the oceans by under-estimating the future impact of ocean acidification. 20 1 Richard J. Matear 1 and Andrew Lenton 1,2 Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-225 Manuscript under review for journal Biogeosciences Discussion started: 16 June 2017 c  Author(s) 2017. CC BY 3.0 License.