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 saturation states of aragonite (Omega(arag)) and calcite (Omega(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 total alkalinity in this region. Here, we analyse ocean acidification in the Arctic Ocean over the 21st century across 14 Earth system models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), 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. Moreover, in CMIP6 the inter-model uncertainty of projected changes over the 21st century in Arctic Ocean Omega(arag) and Omega(calc) averaged over the upper 1000m is reduced by 44-64 %. The strong reduction in projection uncertainties of Omega(arag) and Omega(calc) can be attributed to compensation between C-ant uptake and total alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean C-ant over the 21st 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 total alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in C-ant remains the dominant driver. Even under the low-emissions Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), basin-wide averaged Omega(arag) undersaturation in the upper 1000m 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 Omega(arag) and Omega(calc) is not apparent in the new generation of models. This is due to both the reduction in Omega(arag) and Omega(calc) projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean C-ant and changes in Omega(arag) and Omega(calc).