Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections

International audienceAnthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients, and changes to primary production , all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercompar-ison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). A total of 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the multi-model global mean change (2080-2099 mean values relative to 1870-1899) ± the inter-model SD in sea surface temperature, surface pH, subsurface Published by Copernicus Publications on behalf of the European Geosciences Union. 3440 L. Kwiatkowski et al.: Twenty-first century marine biogeochemistry in CMIP6 (100-600 m) oxygen concentration, euphotic (0-100 m) nitrate concentration, and depth-integrated primary production is +3.47 ± 0.78 • C, −0.44 ± 0.005, −13.27 ± 5.28, −1.06 ± 0.45 mmol m −3 and −2.99 ± 9.11 %, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are +1.42±0.32 • C, −0.16±0.002, −6.36±2.92, −0.52±0.23 mmol m −3 , and −0.56±4.12 %. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions , consistent with previous studies. The ESMs in CMIP6 generally project greater warming, acidification, deoxygena-tion, and nitrate reductions but lesser primary production declines than those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming increases upper-ocean stratification in CMIP6 projections, which contributes to greater reductions in upper-ocean nitrate and sub-surface oxygen ventilation. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO 2 concentrations than their RCP analogues for the same radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an increase in net primary production inter-model uncertainties in CMIP6, as compared to CMIP5