By Matthew Long
HUMAN-DRIVEN climate warming is causing the ocean to lose oxygen on a global scale. This is an underappreciated consequence of climate change, but it is a deeply troubling one.
The history of life on Earth is punctuated by five major extinction events. In some of these, warming-driven ocean deoxygenation played an important role. For instance, during the Permian-Triassic extinction (about 252 million years ago), volcanism on a massive scale drove up carbon dioxide levels in the atmosphere, warming the planet, causing widespread deoxygenation. Roughly 90 percent of marine species perished.
The change in ocean oxygen now underway is following a similar pattern. Carbon dioxide emissions, primarily from fossil fuel combustion, are warming the planet. As the ocean absorbs heat, it loses oxygen. The question is, how far will we go? Can we cut emissions in time to avert ecological disaster in the ocean?
Oxygen is a sparingly soluble gas; it dissolves in seawater in amounts dependent on temperature. The atmosphere contains most of the planet’s oxygen — the air we breathe is about 21 percent oxygen. The ocean, by contrast, holds less than 1 percent of Earth's oxygen. This comparatively small inventory, however, is critical for marine life. Animals require oxygen to live and, except for those that breathe air directly, such as marine mammals, animals in the sea acquire oxygen from the supply dissolved in seawater.
Life cannot be sustained where dissolved oxygen levels are too low.
Ocean oxygen persists in a delicate balance with climate. Oxygen in surface waters remains high because it is produced there by photosynthesis and is free to exchange with the atmosphere. Oxygen declines with depth because it is continuously consumed by respiration, and must be replenished through circulation of waters from the surface. Climate warming drives ocean oxygen loss by a direct mechanism: seawater holds less oxygen as temperatures rise. However, warming also reduces circulation, thus shutting down the oxygen supply to the ocean depths. It is the compounding influence of direct warming effects amplified by changes in circulation that make oxygen so sensitive to climate.
Marine "dead zones" have received increased media attention in recent years. These can occur where nutrient pollution from agriculture and sewage causes oxygen depletion in coastal regions. While this is an important concern, and likely to be exacerbated by climate change, deoxygenation of the open ocean is a distinct issue, happening on a much larger scale.
Observations suggest that, globally, the ocean has lost substantial oxygen over the last several decades in amounts tightly related to ocean warming. Earth system models indicate that the rate of oxygen loss will dramatically accelerate over the next few decades. Oxygen loss will not abate without mitigation of climate warming.
The impacts of ocean deoxygenation are difficult to predict in detail. The geologic record indicates that consequences for marine ecosystems will be catastrophic if action is not taken to mitigate climate change. It may require more than a century under current carbon dioxide emissions for the magnitude of oxygen loss to equal that of the Permian-Triassic; however, shorter-term impacts on marine ecosystems are also likely to be profound.
Animals are constrained to live where there is sufficient oxygen. Perversely, however, an animal's demand for oxygen increases with temperature; thus, even current oxygen levels may be insufficient as the ocean warms. As climate change continues, species will be forced to migrate poleward to seek cooler, better oxygenated waters—or they will face extinction.
Ocean food chains are complex, involving multiple interdependencies between predators and prey. Rising temperatures and deoxygenation will affect species differently, pulling food chains apart and threatening their ability to sustain the marine ecosystem. Species important to fisheries may be directly impacted by oxygen declines—or subject to disruption as food chains collapse. These changes will impact human livelihoods, particularly for communities reliant on fisheries and other marine resources.
Our capacity to contend with deoxygenation will depend on proactive management. We must invest in social, political and scientific resources to adapt. The technology exists, for instance, to provide widespread real-time measurements of oxygen over large regions of the ocean. Synthesizing these observations into improved numerical models can provide a predictive capacity that could be important to managing impacts of deoxygenation.
Centuries to recover
The ocean will continue to lose oxygen unless we limit carbon dioxide emissions and mitigate climate change. Moreover, ocean oxygen loss is not reversible on timescales of human generations; the ocean will require centuries to recover, even from changes caused by current carbon dioxide emissions.
It is not too late to avert the most catastrophic outcomes, but the earlier we act to limit carbon dioxide emissions and reduce warming, the greater benefit these actions will have.
The ocean occupies more than two-thirds of Earth's surface. Through deoxygenation, humans are leaving an indelible imprint on this vast ecosystem. We are changing the most basic of properties: the concentration of oxygen, which is fundamental to life. As land-dwelling creatures, this change is difficult to comprehend. Unlike a clear-cut forest, such impacts are seemingly invisible. Nevertheless, observations, models and the planet’s history paint a consistent picture of potential devastation. The need for action is urgent.
Matthew Long is a scientist at the National Center for Atmospheric Research in Boulder, Colorado. He received his PhD in oceanography from Stanford University. Before beginning a career in Earth system science, he worked as a civil engineer.
Studies are not hard to find. Google is your friend. Here are a few to get you started. Enjoy. 1. Keeling, R. F., Körtzinger, A. & Gruber, N. Ocean deoxygenation in a warming world. Annu. Rev. Mar. Sci. 2, 199–229 (2010) Article 2. Long, M. C., Deutsch, C. A. & Ito, T. Finding forced trends in oceanic oxygen. Glob. Biogeochem. Cycles 30, 381–397 (2016) CASArticle 3. Diaz, R. J. & Rosenberg, R. Spreading dead zones and consequences for marine ecosystems. Science 321, 926–929 (2008) CASPubMedArticle 4. Stramma, L. et al. Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes. Nat. Clim. Change 2, 33–37 (2011) CASArticle 5. Vaquer-Sunyer, R. & Duarte, C. M. Thresholds of hypoxia for marine biodiversity. Proc. Natl Acad. Sci. USA 105, 15452–15457 (2008) PubMedArticle 6. Worm, B. Global patterns of predator diversity in the open oceans. Science 309, 1365–1369 (2005) CASPubMedArticle 7. Stramma, L., Johnson, G. C., Sprintall, J. & Mohrholz, V. Expanding oxygen-minimum zones in the tropical oceans. Science 320, 655–658 (2008) CASPubMedArticle 8. Whitney, F. A., Freeland, H. J. & Robert, M. Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific. Prog. Oceanogr. 75, 179–199 (2007) Article 9. Bograd, S. J. et al. Oxygen declines and the shoaling of the hypoxic boundary in the California Current. Geophys. Res. Lett. 35, L12607 (2008) CASArticle 10. Helm, K. P., Bindoff, N. L. & Church, J. A. Observed decreases in oxygen content of the global ocean. Geophys. Res. Lett. 38, L23602 (2011) CASArticle 11. Broecker, W. S., Sutherland, S. & Peng, T.-H. A possible 20th-century slowdown of Southern Ocean Deep Water formation. Science 286, 1132–1135 (1999) CASPubMedArticle 12. Keller, D. P., Kriest, I., Koeve, W. & Oschlies, A. Southern Ocean biological impacts on global ocean oxygen. Geophys. Res. Lett. 43, 6469–6477 (2016) CASArticle 13. Bopp, L., Le Quéré, C., Heimann, M., Manning, A. C. & Monfray, P. Climate-induced oceanic oxygen fluxes: implications for the contemporary carbon budget. Glob. Biogeochem. Cycles 16, http://dx.doi.org/10.1029/2001GB001445 (2002) 14. Deutsch, C., Brix, H., Ito, T., Frenzel, H. & Thompson, L. Climate-forced variability of ocean hypoxia. Science 333, 336–339 (2011) CASPubMedArticle 15. Keeling, R. F. & García, H. E. The change in oceanic O2 inventory associated with recent global warming. Proc. Natl Acad. Sci. USA 99, 7848–7853 (2002) CASPubMedArticle 16. Stendardo, I. & Gruber, N. Oxygen trends over five decades in the North Atlantic. J. Geophys. Res. 117, C11004 (2012) CASArticle 17. Deutsch, C. A., Emerson, S. & Thompson, L. Fingerprints of climate change in North Pacific oxygen. Geophys. Res. Lett. 32, L16604, http://dx.doi.org/10.1029/2005GL023190 (2005) CASArticle 18. Schmidtko, S., Johnson, G. C. & Lyman, J. M. MIMOC: A global monthly isopycnal upper-ocean climatology with mixed layers. J. Geophys. Res. Oceans 118, 1658–1672 (2013) Article 19. Schmidtko, S., Heywood, K. J., Thompson, A. F. & Aoki, S. Multidecadal warming of Antarctic waters. Science 346, 1227–1231 (2014) CASPubMedArticle 20. Frölicher, T. L., Joos, F., Plattner, G. K., Steinacher, M. & Doney, S. C. Natural variability and anthropogenic trends in oceanic oxygen in a coupled carbon cycle-climate model ensemble. Glob. Biogeochem. Cycles 23, GB1003, http://dx.doi.org/10.1029/2008GB003316 (2009) CASArticle 21. Bianchi, D., Galbraith, E. D., Carozza, D. A., Mislan, K. A. S. & Stock, C. A. Intensification of open-ocean oxygen depletion by vertically migrating animals. Nat. Geosci. 6, 545–548 (2013) CASArticle 22. Wyrtki, K. The thermohaline circulation in relation to the general circulation in the oceans. Deep-Sea Res. 8, 39–64 (1961) Article 23. Cheung, W. W. L. et al. Shrinking of fishes exacerbates impacts of global ocean changes on marine ecosystems. Nat. Clim. Change 3, 254–258 (2012) Article 24. Codispoti, L. A. Interesting times for marine N2O. Science 327, 1339–1340 (2010) CASPubMedArticle 25. Santoro, A. E., Buchwald, C., McIlvin, M. R. & Casciotti, K. L. Isotopic signature of N2O produced by marine ammonia-oxidizing Archaea. Science 333, 1282–1285 (2011) CASPubMedArticle 26. Kwon, E. Y., Deutsch, C. A., Xie, S.-P., Schmidtko, S. & Cho, Y.-K. The North Pacific Oxygen uptake rates over the past half century. J. Clim. 29, 61–76 (2016) Article 27. Watanabe, Y. W. et al. Probability of a reduction in the formation rate of the subsurface water in the North Pacific during the 1980s and 1990s. Geophys. Res. Lett. 28, 3289–3292 (2001) Article 28. Purkey, S. G. & Johnson, G. C. Global contraction of Antarctic bottom water between the 1980s and 2000s. J. Clim. 25, 5830–5844 (2012) Article 29. Kwok, R. & Rothrock, D. A. Decline in Arctic sea ice thickness from submarine and ICESat records: 1958–2008. Geophys. Res. Lett. 36, L15501, http://dx.doi.org/10.1029/2009GL039035 (2009) Article 30. Morison, J. et al. Changing Arctic Ocean freshwater pathways. Nature 481, 66–70 (2012) CASPubMedArticle 31. Biastoch, A., Böning, C. W., Getzlaff, J., Molines, J.-M. & Madec, G. Causes of interannual–decadal variability in the meridional overturning circulation of the midlatitude North Atlantic Ocean. J. Clim. 21, 6599–6615 (2008) Article 32. Cocco, V. et al. Oxygen and indicators of stress for marine life in multi-model global warming projections. Biogeosciences 10, 1849–1868 (2013) CASArticle 33. Carpenter, J. H. The accuracy of the Winkler method for dissolved oxygen analysis. Limnol. Oceanogr. 10, 135–140 (1965) CASArticle 34. Wilcock, R. J., Stevenson, C. D. & Roberts, C. A. An interlaboratory study of dissolved oxygen in water. Water Res. 15, 321–325 (1981) CASArticle 35. Knapp, G. P., Stalcup, M. C. & Stanley, R. J. Iodine losses during Winkler titrations. Deep-Sea Res. A 38, 121–128 (1991) CASArticle
A couple of years of weather swings has nothing to do with climate.