Emergent Multi-model Constraints Suggest Greater Arctic Ocean Anthropogenic Carbon Uptake and Coincident Acidification than Previously Projected

The ongoing uptake of anthropogenic carbon by the ocean leads to ocean acidification, a process that results in a reduction in pH and the saturation state of biogenic calcium carbonate minerals (Ω_calc/arag). As a consequence of naturally low Ω_calc/arag and limited buffer capacity, the Arctic Ocean is considered the open ocean region most susceptible to future acidification and associated ecosystem impacts. However, estimates of the Arctic Ocean anthropogenic carbon inventory are highly uncertain, and with the exception of surface waters, the magnitude of projected twenty-first century acidification differs strongly across current generation Earth System models. Utilizing an ensemble of 10 CMIP5 models, we identify an emergent multi-model relationship between the simulated present-day density of Arctic Ocean surface waters, used as a proxy for deep-water formation, and projections of the anthropogenic carbon inventory and coincident acidification. By applying observations of sea surface density, we show that the end of twenty-first century Arctic Ocean anthropogenic carbon inventory can be constrained to 10­.0 ± 1.6 Pg C and basin averaged Ω_arag and Ω_calc to 0.74 ± 0.06 and 1.16 ± 0.09 respectively, under a business-as-usual emissions scenario (RCP8.5). Our results indicate greater regional anthropogenic carbon storage and ocean acidification than previously projected, and show that under high emissions scenarios such as RCP8.5, large parts of the mesopelagic Arctic Ocean will be undersaturated with respect to calcite by the end of the century. The inferred increase in the projected rate of Arctic Ocean acidification, combined with rapidly changing physical and biogeochemical Arctic conditions, is likely to exacerbate the impact of climate change on vulnerable Arctic marine ecosystems.