英文摘要: | Ocean acidification may lead to seasonal aragonite undersaturation in surface waters of the Southern Ocean as early as 2030 (ref. 1). These conditions are harmful to key organisms such as pteropods2, which contribute significantly to the pelagic foodweb and carbon export fluxes in this region3. Although the severity of ocean acidification impacts is mainly determined by the duration, intensity and spatial extent of aragonite undersaturation events, little is known about the nature of these events, their evolving attributes and the timing of their onset in the Southern Ocean. Using an ensemble of ten Earth system models, we show that starting around 2030, aragonite undersaturation events will spread rapidly, affecting ~30% of Southern Ocean surface waters by 2060 and >70% by 2100, including the Patagonian Shelf. On their onset, the duration of these events will increase abruptly from 1 month to 6 months per year in less than 20 years in >75% of the area affected by end-of-century aragonite undersaturation. This is likely to decrease the ability of organisms to adapt to a quickly evolving environment4. The rapid equatorward progression of surface aragonite undersaturation can be explained by the uptake of anthropogenic CO2, whereas climate-driven physical or biological changes will play a minor role.
The Southern Ocean (south of 40° S) ecosystem plays a fundamental role in global biogeochemical cycling through its effect on nutrient distributions and the air–sea balance of CO2 (refs 5,6). Despite its remoteness, this region also hosts valuable krill and toothfish fisheries7. Recent trends in the Southern Ocean foodweb8, which can be linked partly to regional ocean warming and sea-ice retreat9, prompt the concern that further progression of anthropogenic stressors on sensitive marine organisms can have ripple effects far beyond the Southern Ocean. One key threat to Southern Ocean biota is the rapid progression of ocean acidification2, 10, 11, which is caused by the uptake of anthropogenic CO2. Along with a decreasing pH, the uptake of anthropogenic CO2 decreases the CO32− concentration and thereby the saturation state (Ω) of the CaCO3 minerals aragonite (arag), calcite (calc) and magnesian calcite. These minerals chemically dissolve once Ω decreases below the well-established thermodynamic threshold of Ω = 1. Many marine calcifiers are sensitive to a decreasing Ω in the ocean and develop species-dependent responses already well above this thermodynamical threshold12. For example, aragonite-forming organisms such as soft clams and pteropods exhibit a negative net calcification rate at Ωarag ~ 1.5 (ref. 12) and close to 1 (ref. 2), respectively. In the following, we will refer to these species-dependent thresholds as biological thresholds. Here, we use monthly output from ten Earth system models from the Coupled Model Intercomparison Project, Phase 5 (CMIP5, see Methods) to study the history and future development of Southern Ocean low-Ωarag and -Ωcalc events. These CMIP5 models are the most advanced, global, state-of-the-art climate–carbon-cycle models. By using a multi-model ensemble we expect to add robustness to our analysis as potential shortcomings of individual models are of less consequence for the overall results13. Our analysis focuses mostly on aragonite undersaturation events, as aragonite is the most soluble CaCO3 mineral and because Ωarag = 1 closely lines up with the biological threshold of pteropods. Aragonite undersaturation in sea water can occur sporadically and naturally as a result of background variability; superimposed on this natural variability is the long-term ocean acidification trend14, 15. The Southern Ocean is at particular risk of becoming undersaturated with respect to aragonite in the near future1, 11, as thermodynamics and upwelling of CO2-rich deep waters cause a naturally low Ωarag environment16. The duration of these aragonite undersaturation events is an important indicator for the survival chances of organisms sensitive to these conditions, such as pteropods, as it quantifies how long these organisms will be exposed to lower calcification and increased dissolution rates, higher energetic cost, and suppressed metabolism, eventually leading to reduced growth and reproduction17, 18. Furthermore, the rates at which the intensity and duration of aragonite undersaturation events change are crucial, as they may be faster than the evolutionary processes that could eventually lead to adaptation to low-Ωarag habitats4. Compared with previous studies that were based on annual mean values of Ωarag simulated by ocean-only models11 or the extrapolation of relatively sparse measurements of Southern Ocean near-surface carbon-cycle parameters1, monthly output from the ten CMIP5 coupled Earth system models employed in our study enables us to explore the spatial characteristics and temporal evolution of the habitat of pteropods and other sensitive organisms with unprecedented detail and robustness, and in the presence of natural climate variability and greenhouse warming. Under the high-emissions Representative Concentration Pathway 8.5 (RCP8.5; ref. 19, see Methods), the ensemble mean of ten CMIP5 models documents that the duration and spatial extent of aragonite undersaturation events in the Southern Ocean will change rapidly over the next 40 years (Figs 1 and 2). According to the modelling results, regions in the Bellingshausen and Ross seas already experience sporadic short surface aragonite undersaturation events under present-day conditions (Fig. 1a). Although such short events are masked out in multi-year data products such as the Global Ocean Data Analysis Project (Supplementary Fig. 5a and reference in figure caption) as well as in the comparable ten-year ensemble means (Supplementary Fig. 5b), observations and salinity-based estimates of surface Ωarag from these regions sometimes attain values near 1, thus supporting the model results20, 21. The absence of simulated surface aragonite undersaturation events before 1965 indicates that their present occurrence may already be a result of the uptake of anthropogenic CO2 (Supplementary Fig. 2a).
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