The Arctic Ocean has experienced rapid warming and sea ice loss in recent decades, becoming the first open-ocean basin to experience widespread aragonite undersaturation [saturation state of aragonite (Ω arag ) < 1]. However, its trend toward long-term ocean acidification and the underlying mechanisms remain undocumented. Here, we report rapid acidification there, with rates three to four times higher than in other ocean basins, and attribute it to changing sea ice coverage on a decadal time scale. Sea ice melt exposes seawater to the atmosphere and promotes rapid uptake of atmospheric carbon dioxide, lowering its alkalinity and buffer capacity and thus leading to sharp declines in pH and Ω arag . We predict a further decrease in pH, particularly at higher latitudes where sea ice retreat is active, whereas Arctic warming may counteract decreases in Ω arag in the future.

The Ross Sea continental shelf is considered as the most productive area of the Southern Ocean (Arrigo, vanDijken, & Bushinsky, 2008). In particular, the Ross Ice Shelf Polynya (RISP), occupying half of the shelf area (Figure 1a) (Smith et al., 2012;Wang et al., 2021), sustains an annual productivity (166 g C m −2 ) comparable to that of the entire shelf (Smith & Kaufman, 2018). The large increase in size of this persistent polynya in austral spring and summer facilitates the occurrence of a large phytoplankton bloom (Arrigo & van Dijken, 2004), which is driven by decreased ice coverage, increasing irradiance, and shoaling mixed layer depths (MLDs) (