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The physiological tolerances of marine species toward ambient temperature and oxygen can jointly be evaluated in a single metric: the metabolic index. Changes therein characterize a changing aerobic habitat tailored to species‐specific thermal and hypoxia sensitivity traits. If the geographical limits of marine species as indicated by critical thresholds of the metabolic index shift abruptly in response to ocean warming and deoxygenation, aerobic habitat could potentially be lost abruptly. Here, we assess the spatio‐temporal detectability of abrupt shifts in potential habitats for selected marine species within the Shared Socioeconomic Pathway 5–8.5 (SSP5‐8.5) scenario run with the fully coupled Norwegian Earth System Model version 2 (NorESM2‐LM). We use an environmental time series changepoint detection routine and analyze the number and timing of these abrupt changes over the past, present and future. We construct nine ecophysiotypes with low, medium, and high resting vulnerability to hypoxia and sensitivity of hypoxia vulnerability to temperature, respectively, with six different thresholds for minimal oxygen demand. For all ecophysiotypes with positive temperature sensitivity to hypoxia, the volume of non‐viable habitat in the upper ocean expands between 1850 and 2100. Changepoints in the metabolic index are detected in 49.0 ± 9.2% of the volume that eventually becomes non‐viable for all ecophysiotypes over the course of the 21st century. More than 75% of these abrupt shifts occur in response to warming close to the surface, while at depth, the abrupt shifts driven by changes in oxygen partial pressure become more important, with potentially severe consequences for marine species, populations, and ecosystems. Plain Language Summary: Future ocean warming and the accompanying deoxygenation will change the extent of habitats of all marine species. If the rate of future warming and deoxygenation is slow enough, marine species could adapt to the lower oxygen availability or migrate away to places that match their metabolic habitat requirements. However, if these rates are fast, and potentially exacerbated by abrupt shifts, the consequences for the future distribution of marine species will be profound. Here, we analyze abrupt shifts, or changepoints, in the metabolic index as a measure of metabolic habitat requirements for different marine species. Using projections of an Earth system model, we find that approximately half of the eventually lost habitat between 1850 and 2100 experiences abrupt shifts, and most of them occur between 1950 and 2040, mainly driven by the projected warming. Key Points: Marine habitats described by the metabolic index are lost for a range of marine species in a strong emission scenarioAbrupt changes in the metabolic index are detected in all ocean basins over a wide range of depth levelsTemperature changes drive abrupt metabolic index changes close to the surface, while changes in oxygen become important at depth [ABSTRACT FROM AUTHOR]

Abrupt changes in ocean biogeochemical variables occur as a result of human-induced climate forcing as well as those which are more gradual and occur over longer timescales. These abrupt changes have not yet been identified and quantified to the same extent as the more gradual ones. We review and synthesise abrupt changes in ocean biogeochemistry under human-induced climatic forcing. We specifically address the ocean carbon and oxygen cycles because the related processes of acidification and deoxygenation provide important ecosystem hazards. Since biogeochemical cycles depend also on the physical environment, we also describe the relevant changes in warming, circulation, and sea ice. We include an overview of the reversibility or irreversibility of abrupt marine biogeochemical changes. Important implications of abrupt biogeochemical changes for ecosystems are also discussed. We conclude that there is evidence for increasing occurrence and extent of abrupt changes in ocean biogeochemistry as a consequence of rising greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

The Arctic Ocean is generally undersaturated in CO 2 and acts as a net sink of atmospheric CO 2. This oceanic uptake is strongly modulated by sea ice, which can prevent air–sea gas exchange and has major impacts on stratification and primary production. Moreover, carbon is stored in sea ice with a ratio of alkalinity to dissolved inorganic carbon that is larger than in seawater. It has been suggested that this storage amplifies the seasonal cycle of seawater p CO 2 and leads to an increase in oceanic carbon uptake in seasonally ice-covered regions compared to those that are ice-free. Given the rapidly changing ice scape in the Arctic Ocean, a better understanding of the link between the seasonal cycle of sea ice and oceanic uptake of CO 2 is needed. Here, we investigate how the storage of carbon in sea ice affects the air–sea CO 2 flux and quantify its dependence on the ratio of alkalinity to inorganic carbon in ice. To this end, we present two independent approaches: a theoretical framework that provides an analytical expression of the amplification of carbon uptake in seasonally ice-covered oceans and a simple parameterization of carbon storage in sea ice implemented in a 1D physical–biogeochemical ocean model. Sensitivity simulations show a linear relation between ice melt and the amplification of seasonal carbon uptake. A 30 % increase in carbon uptake in the Arctic Ocean is estimated compared to ice melt without amplification. Applying this relationship to different future scenarios from an earth system model that does not account for the effect of carbon storage in sea ice suggests that Arctic Ocean carbon uptake is underestimated by 5 % to 15 % in these simulations. [ABSTRACT FROM AUTHOR]

The Arctic Ocean is generally undersaturated in CO2 and acts as a net sink of atmospheric CO2. This oceanic uptake is strongly modulated by sea ice, which can prevent air-sea gas exchange and has major impacts on stratification and primary production. Moreover, carbon is stored in sea ice with a ratio of alkalinity to dissolved inorganic carbon that is larger than in seawater. It has been suggested that this storage amplifies the seasonal cycle of seawater p CO2 and leads to an increase in oceanic carbon uptake in seasonally ice-covered regions compared to those that are ice-free. Given the rapidly changing ice-scape in the Arctic Ocean, a better understanding of the link between the seasonal cycle of sea ice and oceanic uptake of CO2 is needed. Here, we investigate how the storage of carbon in sea ice affects the air-sea CO2 flux and quantify its dependence on the ratio of alkalinity to inorganic carbon in ice. To this end, we present two independent approaches: a theoretical framework that provides an analytical expression of the amplification of carbon uptake in seasonally ice-covered oceans, and a simple parameterization of carbon storage in sea ice implemented in a 1D physical-biogeochemical ocean model. Sensitivity simulations show a linear relation between ice melt and the amplification of seasonal carbon uptake. A 30 % increase in carbon uptake in the Arctic Ocean is estimated compared to ice melt without amplification. Applying this relationship to different future scenarios from an Earth System Model that does not account for the effect of carbon storage in sea ice suggests that Arctic Ocean carbon uptake is underestimated by 5 to 15 % in these simulations. [ABSTRACT FROM AUTHOR]

The uptake of anthropogenic carbon (Cant) by the ocean leads to ocean acidification, causing the reduction of pH and the calcium carbonate saturation states of aragonite (Ωarag) and calcite (Ωcalc). 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 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), the inter-model uncertainty of projected end-of-century Arctic Ocean Ωarag/calc is reduced by 44-64 %. The strong reduction in projection uncertainties of Ωarag/calc can be attributed to compensation between Cant uptake and alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean Cant over the 21st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in Cant simulate a relatively strong freshening and concurrent alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in Cant remains the dominant driver. Even under the low-emissions shared socioeconomic pathway SSP1-2.6, basin-wide averaged arag 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 Cant inventory, is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ωarag/calc is not apparent in the new generation of models. This is due to both the reduction in Ωarag/calc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean Cant and arag/calc. In CMIP6, models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of Cant into the Arctic Ocean interior, with simulated historical increases in Cant in improved agreement with observational products. [ABSTRACT FROM AUTHOR]