Effects of climate change on oceans

This NASA animation conveys Earth's oceanic processes as a driving force among Earth's interrelated systems.

There are many effects of climate change on oceans. One of the main ones is an increase in ocean temperatures. More frequent marine heatwaves are linked to this. The rising temperature contributes to a rise in sea levels due to melting ice sheets. Other effects on oceans include sea ice decline, reducing pH values and oxygen levels, as well as increased ocean stratification. All this can lead to changes of ocean currents, for example a weakening of the Atlantic meridional overturning circulation (AMOC).^[2] The main root cause of these changes are the emissions of greenhouse gases from human activities, mainly burning of fossil fuels. Carbon dioxide and methane are examples of greenhouse gases. The additional greenhouse effect leads to ocean warming because the ocean takes up most of the additional heat in the climate system.^[3] The ocean also absorbs some of the extra carbon dioxide that is in the atmosphere. This causes the pH value of the seawater to drop.^[4] Scientists estimate that the ocean absorbs about 25% of all human-caused CO₂ emissions.^[4]

The various layers of the oceans have different temperatures. For example, the water is colder towards the bottom of the ocean. This temperature stratification will increase as the ocean surface warms due to rising air temperatures.^[5]^: 471 Connected to this is a decline in mixing of the ocean layers, so that warm water stabilises near the surface. A reduction of cold, deep water circulation follows. The reduced vertical mixing makes it harder for the ocean to absorb heat. So a larger share of future warming goes into the atmosphere and land. One result is an increase in the amount of energy available for tropical cyclones and other storms. Another result is a decrease in nutrients for fish in the upper ocean layers. These changes also reduce the ocean's capacity to store carbon.^[6] At the same time, contrasts in salinity are increasing. Salty areas are becoming saltier and fresher areas less salty.^[7]

Warmer water cannot contain the same amount of oxygen as cold water. As a result, oxygen from the oceans moves to the atmosphere. Increased thermal stratification may reduce the supply of oxygen from surface waters to deeper waters. This lowers the water's oxygen content even more.^[8] The ocean has already lost oxygen throughout its water column. Oxygen minimum zones are increasing in size worldwide.^[5]^: 471

These changes harm marine ecosystems, and this can lead to biodiversity loss or changes in species distribution.^[2] This in turn can affect fishing and coastal tourism. For example, rising water temperatures are harming tropical coral reefs. The direct effect is coral bleaching on these reefs, because they are sensitive to even minor temperature changes. So a small increase in water temperature could have a significant impact in these environments. Another example is loss of sea ice habitats due to warming. This will have severe impacts on polar bears and other animals that rely on it. The effects of climate change on oceans put additional pressures on ocean ecosystems which are already under pressure by other impacts from human activities.^[2]

^ Käse, Laura; Geuer, Jana K. (2018). "Phytoplankton Responses to Marine Climate Change – an Introduction". YOUMARES 8 – Oceans Across Boundaries: Learning from each other. pp. 55–71. doi:10.1007/978-3-319-93284-2_5. ISBN 978-3-319-93283-5. S2CID 134263396.
^ ^a ^b ^c "Summary for Policymakers". The Ocean and Cryosphere in a Changing Climate (PDF). 2019. pp. 3–36. doi:10.1017/9781009157964.001. ISBN 978-1-00-915796-4. Archived (PDF) from the original on 2023-03-29. Retrieved 2023-03-26.
^ Cheng, Lijing; Abraham, John; Hausfather, Zeke; Trenberth, Kevin E. (11 January 2019). "How fast are the oceans warming?". Science. 363 (6423): 128–129. Bibcode:2019Sci...363..128C. doi:10.1126/science.aav7619. PMID 30630919. S2CID 57825894.
^ ^a ^b Cite error: The named reference :12 was invoked but never defined (see the help page).
^ ^a ^b Bindoff, N.L., W.W.L. Cheung, J.G. Kairo, J. Arístegui, V.A. Guinder, R. Hallberg, N. Hilmi, N. Jiao, M.S. Karim, L. Levin, S. O'Donoghue, S.R. Purca Cuicapusa, B. Rinkevich, T. Suga, A. Tagliabue, and P. Williamson, 2019: Chapter 5: Changing Ocean, Marine Ecosystems, and Dependent Communities Archived 2019-12-20 at the Wayback Machine. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate Archived 2021-07-12 at the Wayback Machine [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In press.
^ Freedman, Andrew (29 September 2020). "Mixing of the planet's ocean waters is slowing down, speeding up global warming, study finds". The Washington Post. Archived from the original on 15 October 2020. Retrieved 12 October 2020.
^ Cheng, Lijing; Trenberth, Kevin E.; Gruber, Nicolas; Abraham, John P.; Fasullo, John T.; Li, Guancheng; Mann, Michael E.; Zhao, Xuanming; Zhu, Jiang (2020). "Improved Estimates of Changes in Upper Ocean Salinity and the Hydrological Cycle". Journal of Climate. 33 (23): 10357–10381. Bibcode:2020JCli...3310357C. doi:10.1175/jcli-d-20-0366.1.
^ Chester, R.; Jickells, Tim (2012). "Chapter 9: Nutrients oxygen organic carbon and the carbon cycle in seawater". Marine geochemistry (3rd ed.). Chichester, West Sussex, UK: Wiley/Blackwell. pp. 182–183. ISBN 978-1-118-34909-0. OCLC 781078031. Archived from the original on 2022-02-18. Retrieved 2022-10-20.

[1] Käse, Laura; Geuer, Jana K. (2018). "Phytoplankton Responses to Marine Climate Change – an Introduction". YOUMARES 8 – Oceans Across Boundaries: Learning from each other. pp. 55–71. doi:10.1007/978-3-319-93284-2_5. ISBN 978-3-319-93283-5. S2CID 134263396.

[SROCC_SPM_20190925-2] "Summary for Policymakers". The Ocean and Cryosphere in a Changing Climate (PDF). 2019. pp. 3–36. doi:10.1017/9781009157964.001. ISBN 978-1-00-915796-4. Archived (PDF) from the original on 2023-03-29. Retrieved 2023-03-26.

[3] Cheng, Lijing; Abraham, John; Hausfather, Zeke; Trenberth, Kevin E. (11 January 2019). "How fast are the oceans warming?". Science. 363 (6423): 128–129. Bibcode:2019Sci...363..128C. doi:10.1126/science.aav7619. PMID 30630919. S2CID 57825894.

[:12-4] Cite error: The named reference :12 was invoked but never defined (see the help page).

[:10-5] Bindoff, N.L., W.W.L. Cheung, J.G. Kairo, J. Arístegui, V.A. Guinder, R. Hallberg, N. Hilmi, N. Jiao, M.S. Karim, L. Levin, S. O'Donoghue, S.R. Purca Cuicapusa, B. Rinkevich, T. Suga, A. Tagliabue, and P. Williamson, 2019: Chapter 5: Changing Ocean, Marine Ecosystems, and Dependent Communities Archived 2019-12-20 at the Wayback Machine. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate Archived 2021-07-12 at the Wayback Machine [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In press.

[6] Freedman, Andrew (29 September 2020). "Mixing of the planet's ocean waters is slowing down, speeding up global warming, study finds". The Washington Post. Archived from the original on 15 October 2020. Retrieved 12 October 2020.

[ChengSalinity-7] Cheng, Lijing; Trenberth, Kevin E.; Gruber, Nicolas; Abraham, John P.; Fasullo, John T.; Li, Guancheng; Mann, Michael E.; Zhao, Xuanming; Zhu, Jiang (2020). "Improved Estimates of Changes in Upper Ocean Salinity and the Hydrological Cycle". Journal of Climate. 33 (23): 10357–10381. Bibcode:2020JCli...3310357C. doi:10.1175/jcli-d-20-0366.1.

[8] Chester, R.; Jickells, Tim (2012). "Chapter 9: Nutrients oxygen organic carbon and the carbon cycle in seawater". Marine geochemistry (3rd ed.). Chichester, West Sussex, UK: Wiley/Blackwell. pp. 182–183. ISBN 978-1-118-34909-0. OCLC 781078031. Archived from the original on 2022-02-18. Retrieved 2022-10-20.

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