Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble

The uptake of anthropogenic carbon (C_ant) by the ocean leads to ocean acidification, causing the reduction of pH and the calcium carbonate saturation states of aragonite (Ω_arag) and calcite (Ω_calc). The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally low pH and saturation states and due to ongoing freshening and the concurrent reduction in alkalinity in this region. Here, we present projections of  C_ant and ocean acidification in the Arctic Ocean over the 21^st century across Earth System Models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), the inter-model uncertainty of projected end-of-century Arctic Ocean Ω_arag/calc is reduced by 44–64 %. The strong reduction in projection uncertainties of Ω_arag/calc can be attributed to compensation between C_ant uptake and alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean C_ant over the 21^st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in C_ant simulate a relatively strong freshening and concurrent alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21^st century, the increase in C_ant remains the dominant driver. Even under the low-emissions shared socioeconomic pathway SSP1-2.6, basin-wide averaged aragonite undersaturation occurs before the end of the century. While under the high-emissions pathway SSP5-8.5, the Arctic Ocean mesopelagic is projected to even become undersaturated with respect to calcite. An emergent constraint, identified in CMIP5, which relates present-day maximum sea surface densities in the Arctic Ocean to the projected end-of-century Arctic Ocean C_ant inventory, is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ω_arag/calc is not apparent in the new generation of models. This is due to both the reduction in Ω_arag/calc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean C_ant and Ω_arag/calc. In CMIP6, models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of C_ant into the Arctic Ocean interior, with simulated historical increases in C_ant in improved agreement with observational products.