Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model

Abstract

Abstract. Ocean acidification from the uptake of anthropogenic carbon is simulated for the industrial period and IPCC SRES emission scenarios A2 and B1 with a global coupled carbon cycle-climate model. Earlier studies identified seawater saturation state with respect to aragonite, a mineral phase of calcium carbonate, as a key variable governing impacts on corals and other shell-forming organisms. Globally in the A2 scenario, water saturated by more than 300%, considered suitable for coral growth, vanishes by 2070 AD (CO 2 ≈630 ppm), and the ocean volume fraction occupied by saturated water decreases from 42% to 25% over this century. The largest simulated pH changes worldwide occur in Arctic surface waters, where hydrogen ion concentration increases by up to 185% ( pH=−0.45). Projected climate change amplifies the decrease in Arctic surface mean saturation and pH by more than 20%, mainly due to freshening and increased carbon uptake in response to sea ice retreat. Modeled saturation compares well with observation-based estimates along an Arctic transect and simulated changes have been corrected for remaining model-data differences in this region. Aragonite undersaturation in Arctic surface waters is projected to occur locally within a decade and to become more widespread as atmospheric CO 2 continues to grow. The results imply that surface waters in the Arctic Ocean will become corrosive to aragonite, with potentially large implications for the marine ecosystem, if anthropogenic carbon emissions are not reduced and atmospheric CO 2 not kept below 450 ppm.Correspondence to: M. Steinacher

A safe operating space for humanity

“…Even though globally-averaged ! arag values in surface waters remain above unity for a doubling of atmospheric CO 2 , large parts of the Southern Ocean and the Arctic Ocean are projected to become undersaturated with respect to aragonite as early as 2030-2060 (Orr et al 2005;McNeil and Matear 2008;Steinacher et al, 2009). Aragonite undersaturation means that these waters will become corrosive to the aragonite and high-magnesium calcite shells secreted by a wide variety of marine organisms.…”

Real-time estimation of pH and aragonite saturation state from Argo profiling floats: Prospects for an autonomous carbon observing strategy

“…Such a strategy could be used to complement (not replace) direct observations of pH, TCO 2 , and TA. Predictions of pH e and W arag e could also be used as a tool to evaluate global coupled model simulations of ocean carbon chemistry [Orr et al, 2005;Steinacher et al, 2009], identify regional vulnerabilities to ocean carbon chemistry given understanding of critical life history stages (spawning, juvenile development), and hindcast conditions for recent biological or ecological datasets for which concurrent pH, TCO 2 , and TA observations are unavailable.…”

Calcium carbonate saturation states in the waters of the Canadian Arctic Archipelago and the Labrador Sea

“…According to their model, decreased sea ice extent leads to greater atmosphere-ocean exchange of CO 2 , which in turn drives an accelerated decline for both pH and saturation states. Steinacher et al [2009], in a separate model study, demonstrated that the largest pH changes will occur in Arctic Ocean surface waters, where aragonite undersaturation is projected within a decade due to freshening by precipitation and ice melt and by increased carbon uptake as the ice cover is reduced.…”

“…Rapidly changing conditions in the Arctic, such as altered hydrological cycles, rising seawater temperature and reduction of ice cover, impact biogeochemical cycles [Arrigo et al, 2008]. Anticipated changes in saturation states in the Arctic surface waters [Steinacher et al, 2009] will propagate downstream through the CAA southward from the Labrador Shelf to the Middle Atlantic Bight. We report the current calcite and aragonite saturation states of seawater and the depths of saturation horizons based on dissolved inorganic carbon and total alkalinity measurements from the CAA and the Labrador Sea in [2003][2004][2005].…”