Oschlies, A. (2021). A committed fourfold increase in ocean oxygen loss. Nature Communications, 12(1), 2307. https://doi.org/10.1038/s41467-021-22584-4
Less than a quarter of ocean deoxygenation that will ultimately be caused by historical CO2 emissions is already realized, according to millennial-scale model simulations that assume zero CO2 emissions from year 2021 onwards. About 80% of the committed oxygen loss occurs below 2000 m depth, where a more sluggish overturning circulation will increase water residence times and accumulation of respiratory oxygen demand. According to the model results, the deep ocean will thereby lose more than 10% of its pre-industrial oxygen content even if CO2 emissions and thus global warming were stopped today. In the surface layer, however, the ongoing deoxygenation will largely stop once CO2 emissions are stopped. Accounting for the joint effects of committed oxygen loss and ocean warming, metabolic viability representative for marine animals declines by up to 25% over large regions of the deep ocean, posing an unavoidable escalation of anthropogenic pressure on deep-ocean ecosystems.
Policy relevant message:
In the surface layer, the ongoing deoxygenation will largely stop once CO2 emissions are stopped. The deep ocean, however, will lose more than 10% of its pre-industrial oxygen content even if CO2 emissions and thus global warming were stopped today, posing an unavoidable escalation of anthropogenic pressure on deep-ocean ecosystems.
Lauderdale, J. M., & Cael, B. B. (2021). Impact of Remineralization Profile Shape on the Air-Sea Carbon Balance. Geophysical Research Letters, 48 (7), e2020GL091746. https://doi.org/10.1029/2020GL091746
The ocean’s “biological pump” regulates atmospheric carbon dioxide levels and climate by transferring organic carbon produced at the surface by phytoplankton to the ocean interior via “marine snow,” where the organic carbon is consumed and respired by microbes. This surface to deep transport is usually described by a power-law relationship of sinking particle concentration with depth. Uncertainty in biological pump strength can be related to different variable values (“parametric” uncertainty) or the underlying equations (“structural” uncertainty) that describe organic matter export. This study evaluates structural uncertainty using an ocean biogeochemistry model by systematically substituting six alternative remineralization profiles fit to a reference power-law curve. Structural uncertainty makes a substantial contribution, about one-third in atmospheric pCO2 terms, to total uncertainty of the biological pump, highlighting the importance of improving biological pump characterization from observations and its mechanistic inclusion in climate models.
García-Ibáñez, M. I., Bates, N. R., Bakker, D. C. E., Fontela, M., & Velo, A. (2021). Cold-water corals in the Subpolar North Atlantic Ocean exposed to aragonite undersaturation if the 2 ° C global warming target is not met. Global and Planetary Change, 201, 103480. https://doi.org/10.1016/j.gloplacha.2021.103480
The net uptake of carbon dioxide (CO2) from the atmosphere is changing the ocean’s chemical state. Such changes, commonly known as ocean acidification, include a reduction in pH and the carbonate ion concentration ([CO32−]), which in turn lowers oceanic saturation states (Ω) for calcium carbonate (CaCO3) minerals. The Ω values for aragonite (Ωaragonite; one of the main CaCO3 minerals formed by marine calcifying organisms) influence the calcification rate and geographic distribution of cold-water corals (CWCs), important for biodiversity. In this study, high-quality measurements, collected on thirteen cruises along the same track during 1991–2018, are used to determine the long-term changes in Ωaragonite in the Irminger and Iceland Basins of the North Atlantic Ocean, providing the first trends of Ωaragonite in the deep waters of these basins. The entire water column of both basins showed significant negative Ωaragonite trends with the decrease in Ωaragonite in the intermediate waters, where nearly half of the CWC reefs of the study region are located, caused the Ωaragonite isolines to rapidly migrate upwards at a rate between 6 and 34 m per year. The main driver of the decline in Ωaragonite in the Irminger and Iceland Basins was the increase in anthropogenic CO2. However, this was partially offset by increases in salinity (in Subpolar Mode Water), enhanced ventilation (in upper Labrador Sea Water), and increases in alkalinity (in classical Labrador Sea Water, cLSW; and overflow waters). The authors also found that water mass aging reinforced the Ωaragonite decrease in cLSW. Based on these Ωaragonite trends over the last three decades, it is projected that the entire water column of the Irminger and Iceland Basins will likely be undersaturated for aragonite when in equilibrium with an atmospheric mole fraction of CO2 (xCO2) of ~880 ppmv, corresponding to climate model projections for the end of the century based on the highest CO2 emission scenarios. However, intermediate waters will likely be aragonite undersaturated when in equilibrium with an atmospheric xCO2 exceeding ~630 ppmv, an xCO2 level slightly above that corresponding to 2 °C global warming, thus exposing CWCs inhabiting the intermediate waters to undersaturation for aragonite.
Terhaar, J., Torres, O., Bourgeois, T., & Kwiatkowski, L. (2021). Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble. Biogeosciences, 18 (6), 2221–2240. https://doi.org/10.5194/bg-18-2221-2021
The uptake of carbon, emitted as a result of human activities, results in ocean acidification. The authors of this study analyse 21st-century projections of acidification in the Arctic Ocean, a region of particular vulnerability, using the latest generation of Earth system models. In this new generation of models there is a large decrease in the uncertainty associated with projections of Arctic Ocean acidification, with freshening playing a greater role in driving acidification than previously simulated.
Le Grix, N., Zscheischler, J., Laufkötter, C., Rousseaux, C. S., & Frölicher, T. L. (2021). Compound high-temperature and low-chlorophyll extremes in the ocean over the satellite period. Biogeosciences, 18 (6), 2119–2137. https://doi.org/10.5194/bg-18-2119-2021
Marine ecosystems could suffer severe damage from the co-occurrence of a marine heat wave with extremely low chlorophyll concentration. This study provides a first assessment of compound marine heat wave and low-chlorophyll events in the global ocean from 1998 to 2018. The authors reveal hotspots of these compound events in the equatorial Pacific and in the Arabian Sea and show that they mostly occur in summer at high latitudes and their frequency is modulated by large-scale modes of climate variability.
Pöppelmeier, F., Scheen, J., Jeltsch-Thömmes, A., & Stocker, T. F. (2021). Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum. Climate of the Past, 17(2), 615–632. https://doi.org/10.5194/cp-17-615-2021
The stability of the Atlantic Meridional Overturning Circulation (AMOC) critically depends on its mean state. The authors of this study simulate the response of the AMOC to North Atlantic freshwater perturbations under different glacial boundary conditions. They find that a closed Bering Strait greatly increases the AMOC’s sensitivity to freshwater hosing. Further, the shift from mono- to bistability strongly depends on the chosen boundary conditions, with weaker circulation states exhibiting more abrupt transitions.
Steinbach, J., Holmstrand, H., Shcherbakova, K., Kosmach, D., Brüchert, V., Shakhova, N., Salyuk, A., Sapart, C. J., Chernykh, D., Noormets, R., Semiletov, I., & Gustafsson, Ö. (2021). Source apportionment of methane escaping the subsea permafrost system in the outer Eurasian Arctic Shelf. Proceedings of the National Academy of Sciences, 118(10), e2019672118 https://doi.org/10.1073/pnas.2019672118
Extensive release of methane from sediments of the world’s largest continental shelf, the East Siberian Arctic Ocean (ESAO), is one of the few Earth system processes that can cause a net transfer of carbon from land/ocean to the atmosphere and thus amplify global warming on the timescale of this century. An important gap in our current knowledge concerns the contributions of different subsea pools to the observed methane releases. This knowledge is a prerequisite to robust predictions on how these releases will develop in the future. Triple-isotope–based fingerprinting of the origin of the highly elevated ESAO methane levels points to a limited contribution from shallow microbial sources and instead a dominating contribution from a deep thermogenic pool.
Gregor, L., & Gruber, N. (2021). OceanSODA-ETHZ: a global gridded data set of the surface ocean carbonate system for seasonal to decadal studies of ocean acidification. Earth System Science Data, 13(2), 777–808. https://doi.org/10.5194/essd-13-777-2021
Ocean acidification (OA) has altered the ocean’s carbonate chemistry, with consequences for marine life. Yet, no observation-based data set exists that permits to study changes in OA. This study fills this gap with a global data set of relevant surface ocean parameters over the period 1985–2018. This data set, OceanSODA-ETHZ, was created by using satellite and other data to extrapolate ship-based measurements of carbon dioxide and total alkalinity from which parameters for OA were computed.
Torres, O., Kwiatkowski, L., Sutton, A. J., Dorey, N., Orr, J. C. (2021) Characterising mean and extreme diurnal variability of ocean CO2 system variables across marine environments, Geophys. Res. Lett., e2020GL090228, accepted, https://doi.org/10.1029/2020GL090228
Our understanding of how ocean pH and related chemical variables vary during the day (known as diurnal variability) is not well established. In this study, a recent data set was used of such observations collected every 3 h during 8–140 months from 37 buoys located across the oceans to assess these diurnal variations and what drives them. In extreme cases, observed changes over 24 h were found to be greater than those observed between seasons. Diurnal variations in these chemical variables are particularly large in coastal waters and near coral reefs and are not negligible further offshore. Along with the more gradual, long‐term acidification of the ocean from atmospheric CO2 increases year after year, diurnal and seasonal variability of ocean chemistry is also expected to change dramatically. Understanding how this diurnal variability will change in the future is important because it modulates the levels of acidification experienced by marine organisms from long‐term yearly changes.
Fassbender, A. J., Orr, J. C., and Dickson, A. G. (2021). Technical note: Interpreting pH changes, Biogeosciences, 18, 1407–1415, https://doi.org/10.5194/bg-18-1407-2021
A decline in upper-ocean pH with time is typically ascribed to ocean acidification. A more quantitative interpretation is often confused by failing to recognize the implications of pH being a logarithmic transform of hydrogen ion concentration rather than an absolute measure. This can lead to an unwitting misinterpretation of pH data. This study provides three real-world examples illustrating this and recommends the reporting of both hydrogen ion concentration and pH in studies of ocean chemical change.