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Ocean deoxygenation

Global map of low and declining oxygen levels in coastal waters (mainly due to eutrophication) and in the open ocean (due to climate change). The map indicates coastal sites where oxygen levels have declined to less than 2 mg/L (red dots), as well as expanding ocean oxygen minimum zones at 300 metres (blue shaded regions).[1]

Ocean deoxygenation is the reduction of the oxygen content in different parts of the ocean due to human activities.[2][3] There are two areas where this occurs. Firstly, it occurs in coastal zones where eutrophication has driven some quite rapid (in a few decades) declines in oxygen to very low levels.[2] This type of ocean deoxygenation is also called dead zones. Secondly, ocean deoxygenation occurs also in the open ocean. In that part of the ocean, there is nowadays an ongoing reduction in oxygen levels. As a result, the naturally occurring low oxygen areas (so called oxygen minimum zones (OMZs)) are now expanding slowly.[4] This expansion is happening as a consequence of human caused climate change.[5][6] The resulting decrease in oxygen content of the oceans poses a threat to marine life, as well as to people who depend on marine life for nutrition or livelihood.[7][8][9] A decrease in ocean oxygen levels affects how productive the ocean is, how nutrients and carbon move around, and how marine habitats function.[10][11]

As the oceans become warmer this increases the loss of oxygen in the oceans. This is because the warmer temperatures increase ocean stratification. The reason for this lies in the multiple connections between density and solubility effects that result from warming.[12][13] As a side effect, the availability of nutrients for marine life is reduced, therefore adding further stress to marine organisms.

The rising temperatures in the oceans also cause a reduced solubility of oxygen in the water, which can explain about 50% of oxygen loss in the upper level of the ocean (>1000 m). Warmer ocean water holds less oxygen and is more buoyant than cooler water. This leads to reduced mixing of oxygenated water near the surface with deeper water, which naturally contains less oxygen. Warmer water also raises oxygen demand from living organisms; as a result, less oxygen is available for marine life.[14]

Studies have shown that oceans have already lost 1-2% of their oxygen since the middle of the 20th century,[15][16] and model simulations predict a decline of up to 7% in the global ocean O2 content over the next hundred years. The decline of oxygen is projected to continue for a thousand years or more.[17]

  1. ^ Breitburg, Denise; Levin, Lisa A.; Oschlies, Andreas; Grégoire, Marilaure; Chavez, Francisco P.; Conley, Daniel J.; Garçon, Véronique; Gilbert, Denis; Gutiérrez, Dimitri; Isensee, Kirsten; Jacinto, Gil S.; Limburg, Karin E.; Montes, Ivonne; Naqvi, S. W. A.; Pitcher, Grant C.; Rabalais, Nancy N.; Roman, Michael R.; Rose, Kenneth A.; Seibel, Brad A.; Telszewski, Maciej; Yasuhara, Moriaki; Zhang, Jing (2018). "Declining oxygen in the global ocean and coastal waters". Science. 359 (6371): eaam7240. Bibcode:2018Sci...359M7240B. doi:10.1126/science.aam7240. PMID 29301986. S2CID 206657115.
  2. ^ a b Laffoley, D; Baxter, JM (2019). Ocean deoxygenation: everyone's problem. Switzerland: Gland. p. 562. ISBN 978-2-8317-2013-5.
  3. ^ Limburg, Karin E.; Breitburg, Denise; Swaney, Dennis P.; Jacinto, Gil (2020-01-24). "Ocean Deoxygenation: A Primer". One Earth. 2 (1): 24–29. Bibcode:2020OEart...2...24L. doi:10.1016/j.oneear.2020.01.001. ISSN 2590-3330. S2CID 214348057.
  4. ^ Oschlies, Andreas; Brandt, Peter; Stramma, Lothar; Schmidtko, Sunke (2018). "Drivers and mechanisms of ocean deoxygenation". Nature Geoscience. 11 (7): 467–473. Bibcode:2018NatGe..11..467O. doi:10.1038/s41561-018-0152-2. ISSN 1752-0894. S2CID 135112478.
  5. ^ Stramma, L; Johnson, GC; Printall, J; Mohrholz, V (2008). "Expanding Oxygen-Minimum Zones in the Tropical Oceans". Science. 320 (5876): 655–658. Bibcode:2008Sci...320..655S. doi:10.1126/science.1153847. PMID 18451300. S2CID 206510856.
  6. ^ Mora, C; et al. (2013). "Biotic and Human Vulnerability to Projected Changes in Ocean Biogeochemistry over the 21st Century". PLOS Biology. 11 (10): e1001682. doi:10.1371/journal.pbio.1001682. PMC 3797030. PMID 24143135.
  7. ^ Carrington (2018-01-04). "Environment. Oceans suffocating as huge dead zones quadruple since 1950, scientists warn". The Guardian. ISSN 0261-3077. Retrieved 2023-07-04.
  8. ^ Long, Matthew C.; Deutsch, Curtis; Ito, Taka (2016). "Finding forced trends in oceanic oxygen". Global Biogeochemical Cycles. 30 (2): 381–397. Bibcode:2016GBioC..30..381L. doi:10.1002/2015GB005310. ISSN 0886-6236. S2CID 130885459.
  9. ^ Pearce, Rosamund (2018-06-15). "Guest post: How global warming is causing ocean oxygen levels to fall". Carbon Brief. Retrieved 2023-07-04.
  10. ^ Harvey, Fiona (2019-12-07). "Oceans losing oxygen at unprecedented rate, experts warn". The Guardian. ISSN 0261-3077. Retrieved 2019-12-07.
  11. ^ Laffoley, D. & Baxter, J.M. (eds.) (2019). Ocean deoxygenation: Everyone's problem - Causes, impacts, consequences and solutions. IUCN, Switzerland.
  12. ^ Bednaršek, N., Harvey, C.J., Kaplan, I.C., Feely, R.A. and Možina, J. (2016) "Pteropods on the edge: Cumulative effects of ocean acidification, warming, and deoxygenation". Progress in Oceanography, 145: 1–24. doi:10.1016/j.pocean.2016.04.002
  13. ^ Keeling, Ralph F., and Hernan E. Garcia (2002) "The change in oceanic O2 inventory associated with recent global warming." Proceedings of the National Academy of Sciences, 99(12): 7848–7853. doi:10.1073/pnas.122154899
  14. ^ "Ocean deoxygenation". IUCN. 2019-12-06. Retrieved 2021-05-02.
  15. ^ Cite error: The named reference Bopp2013 was invoked but never defined (see the help page).
  16. ^ Schmidtko, S; Stramma, L; Visbeck, M (2017). "Decline in global oceanic oxygen content during the past five decades". Nature. 542 (7641): 335–339. Bibcode:2017Natur.542..335S. doi:10.1038/nature21399. PMID 28202958. S2CID 4404195.
  17. ^ Cite error: The named reference :0 was invoked but never defined (see the help page).

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