Effects of climate change on oceans: Difference between revisions

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small change to the section on deoxygenation
→‎Impacts on marine life: added new para on timescales. I did use two quotes from IPCC report. Will come back to it later to paraphrase but I am finding it not very easy to do it in the same go. Easier to do it a few days later after it has sunk in. Or maybe someone else can also help.
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The surface wind pattern may also change with climate change, particularly its intensity and this may lead to locally variable climate change effects.<ref name="SROCC_SPM_20190925" /> These changes in ocean currents also affect the ability of the ocean to take up carbon dioxide (which depends on water temperature) and also ocean productivity because the currents transport nutrients (see [[#Impacts on phytoplankton and net primary production|Impacts on phytoplankton and net primary production]]). The AMOC deep ocean circulation is slow (hundreds to thousands of years to circulate around the whole ocean) and so it is slow to respond to climate change. The impacts of climate change will be slow to start but equally take a long time to play out.<ref name=":15">Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V.  Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, N.P. Gillett, L. Goldfarb, I. Gorodetskaya, J.M. Gutierrez, R. Hamdi, E. Hawkins, H.T. Hewitt, P. Hope, A.S. Islam, C. Jones, D.S. Kaufman, R.E. Kopp, Y. Kosaka, J.  Kossin, S.  Krakovska, J.-Y. Lee, et al., 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 33−144. doi:10.1017/9781009157896.002.</ref>{{rp|42}}
The surface wind pattern may also change with climate change, particularly its intensity and this may lead to locally variable climate change effects.<ref name="SROCC_SPM_20190925" /> These changes in ocean currents also affect the ability of the ocean to take up carbon dioxide (which depends on water temperature) and also ocean productivity because the currents transport nutrients (see [[#Impacts on phytoplankton and net primary production|Impacts on phytoplankton and net primary production]]). The AMOC deep ocean circulation is slow (hundreds to thousands of years to circulate around the whole ocean) and so it is slow to respond to climate change.


=== Increasing stratification ===
=== Increasing stratification ===
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[[File:Global areas of hypoxia.jpg|thumb| {{center|Global map of low and declining oxygen levels in the open ocean and coastal waters. The map indicates coastal sites where anthropogenic nutrients have resulted in oxygen declines to less than 2 mg L<sup>–1</sup> (red dots), as well as ocean [[oxygen minimum zone]]s at 300 metres (blue shaded regions).<ref>{{cite journal |doi = 10.1126/science.aam7240|title = Declining oxygen in the global ocean and coastal waters|year = 2018|last1 = Breitburg|first1 = Denise|last2 = Levin|first2 = Lisa A.|last3 = Oschlies|first3 = Andreas|last4 = Grégoire|first4 = Marilaure|last5 = Chavez|first5 = Francisco P.|last6 = Conley|first6 = Daniel J.|last7 = Garçon|first7 = Véronique|last8 = Gilbert|first8 = Denis|last9 = Gutiérrez|first9 = Dimitri|last10 = Isensee|first10 = Kirsten|last11 = Jacinto|first11 = Gil S.|last12 = Limburg|first12 = Karin E.|last13 = Montes|first13 = Ivonne|last14 = Naqvi|first14 = S. W. A.|last15 = Pitcher|first15 = Grant C.|last16 = Rabalais|first16 = Nancy N.|last17 = Roman|first17 = Michael R.|last18 = Rose|first18 = Kenneth A.|last19 = Seibel|first19 = Brad A.|last20 = Telszewski|first20 = Maciej|last21 = Yasuhara|first21 = Moriaki|last22 = Zhang|first22 = Jing|journal = Science|volume = 359|issue = 6371|pages = eaam7240|pmid = 29301986|bibcode = 2018Sci...359M7240B|s2cid = 206657115|doi-access = free}}</ref>}}]]
[[File:Global areas of hypoxia.jpg|thumb| {{center|Global map of low and declining oxygen levels in the open ocean and coastal waters. The map indicates coastal sites where anthropogenic nutrients have resulted in oxygen declines to less than 2 mg L<sup>–1</sup> (red dots), as well as ocean [[oxygen minimum zone]]s at 300 metres (blue shaded regions).<ref>{{cite journal |doi = 10.1126/science.aam7240|title = Declining oxygen in the global ocean and coastal waters|year = 2018|last1 = Breitburg|first1 = Denise|last2 = Levin|first2 = Lisa A.|last3 = Oschlies|first3 = Andreas|last4 = Grégoire|first4 = Marilaure|last5 = Chavez|first5 = Francisco P.|last6 = Conley|first6 = Daniel J.|last7 = Garçon|first7 = Véronique|last8 = Gilbert|first8 = Denis|last9 = Gutiérrez|first9 = Dimitri|last10 = Isensee|first10 = Kirsten|last11 = Jacinto|first11 = Gil S.|last12 = Limburg|first12 = Karin E.|last13 = Montes|first13 = Ivonne|last14 = Naqvi|first14 = S. W. A.|last15 = Pitcher|first15 = Grant C.|last16 = Rabalais|first16 = Nancy N.|last17 = Roman|first17 = Michael R.|last18 = Rose|first18 = Kenneth A.|last19 = Seibel|first19 = Brad A.|last20 = Telszewski|first20 = Maciej|last21 = Yasuhara|first21 = Moriaki|last22 = Zhang|first22 = Jing|journal = Science|volume = 359|issue = 6371|pages = eaam7240|pmid = 29301986|bibcode = 2018Sci...359M7240B|s2cid = 206657115|doi-access = free}}</ref>}}]]


The oxygen content of the ocean is vital for the survival of most larger animals and plants and also serves a long term role in controlling atmospheric oxygen upon which terrestrial life depends.<ref name=":1">{{Cite journal |last1=Oschlies |first1=Andreas |last2=Brandt |first2=Peter |last3=Stramma |first3=Lothar |last4=Schmidtko |first4=Sunke |date=2018 |title=Drivers and mechanisms of ocean deoxygenation |url=http://www.nature.com/articles/s41561-018-0152-2 |journal=Nature Geoscience |language=en |volume=11 |issue=7 |pages=467–473 |doi=10.1038/s41561-018-0152-2 |s2cid=135112478 |issn=1752-0894}}</ref> Climate change affects ocean oxygen. There are two areas of concern in terms of ocean oxygen levels: The open ocean mid depth waters and the coastal waters. <ref name=":1" />
The oxygen content of the ocean is vital for the survival of most larger animals and plants and also serves a long term role in controlling atmospheric oxygen upon which terrestrial life depends.<ref name=":1">{{Cite journal |last1=Oschlies |first1=Andreas |last2=Brandt |first2=Peter |last3=Stramma |first3=Lothar |last4=Schmidtko |first4=Sunke |date=2018 |title=Drivers and mechanisms of ocean deoxygenation |url=http://www.nature.com/articles/s41561-018-0152-2 |journal=Nature Geoscience |language=en |volume=11 |issue=7 |pages=467–473 |doi=10.1038/s41561-018-0152-2 |s2cid=135112478 |issn=1752-0894}}</ref> Climate change affects ocean oxygen. There are two areas of concern in terms of ocean oxygen levels: The open ocean mid depth waters and the coastal waters.<ref name=":1" />


The first area of concern relates to the [[Pelagic zone|open ocean]] mid depth waters which are naturally low in oxygen ([[Oxygen minimum zone|oxygen minimum zones]]) because of sluggish ocean circulation isolating these waters from the atmosphere (and hence oxygen) for decades, while sinking organic matter from surface waters is broken down consuming oxygen. These low oxygen ocean areas are expanding as a result of ocean warming which both reduces water circulation and also reduces the oxygen content of that water. This is because the solubility of oxygen declines as temperature rises.
The first area of concern relates to the [[Pelagic zone|open ocean]] mid depth waters which are naturally low in oxygen ([[Oxygen minimum zone|oxygen minimum zones]]) because of sluggish ocean circulation isolating these waters from the atmosphere (and hence oxygen) for decades, while sinking organic matter from surface waters is broken down consuming oxygen. These low oxygen ocean areas are expanding as a result of ocean warming which both reduces water circulation and also reduces the oxygen content of that water. This is because the solubility of oxygen declines as temperature rises.
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{{excerpt|Arctic sea ice decline|paragraphs=1|file=no}}
{{excerpt|Arctic sea ice decline|paragraphs=1|file=no}}
{{excerpt|Antarctic sea ice#Recent trends and climate change|paragraphs=1}}
{{excerpt|Antarctic sea ice#Recent trends and climate change|paragraphs=1}}

== Timescales ==
Many of the ocean-related processes are "slow-responding elements of the [[climate system]]", namely the loss of ice (sea ice or glaciers), increase in ocean heat content, sea level rise and deep ocean acidification.<ref name=":15">Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V.  Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, N.P. Gillett, L. Goldfarb, I. Gorodetskaya, J.M. Gutierrez, R. Hamdi, E. Hawkins, H.T. Hewitt, P. Hope, A.S. Islam, C. Jones, D.S. Kaufman, R.E. Kopp, Y. Kosaka, J.  Kossin, S.  Krakovska, J.-Y. Lee, et al., 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 33−144. doi:10.1017/9781009157896.002.</ref>{{rp|43}} They therefore represent a "millennial-scale commitment" (committed changes that are associated with past [[greenhouse gas emissions]]. The [[IPCC Sixth Assessment Report]] found that "The response of these variables depends on the time it takes to reach the global warming level, differs if the warming is reached in a transient warming state or after a temporary overshoot of the warming level, and will continue to evolve, over centuries to millennia, even after global warming has stabilized."<ref name=":15" />{{rp|55}} Different [[Climate change scenario|global warming levels]] are for example 1.5°C or 2°C above the 1850–1900 period.

This means that the impacts of climate change on oceans will be slow to start but equally take a long time (centuries to millennia) to play out. For example, the "global mean sea level will continue to rise for thousands of years, even if future CO2 emissions are reduced to net zero and global warming halted".<ref name=":15" />{{rp|39}} This is because excess energy due to past emissions will continue to extend into the deep ocean, and glaciers and ice sheets will continue to melt.


== Impacts on marine life ==
== Impacts on marine life ==

Revision as of 21:26, 24 October 2022

Overview of climatic changes and their effects on the ocean. Regional effects are displayed in italics.[1]

There are many significant effects of climate change on oceans including: an increase in sea surface temperature as well as ocean temperatures at greater depths, more marine heatwaves, a reduction in pH value, a rise in sea level from ocean warming and ice sheet melting, sea ice decline in the Arctic, increased upper ocean stratification, reductions in oxygen levels, increased contrasts in salinity (salty areas becoming saltier and fresher areas becoming less salty),[2] changes to ocean currents including a weakening of the Atlantic meridional overturning circulation, stronger tropical cyclones and monsoons, and changing wind patterns.[3] All these changes have knock-on effects which disturb marine ecosystems. The root cause of these observed changes is the Earth warming due to anthropogenic emissions of greenhouse gases, such as for example carbon dioxide and methane. This leads inevitably to ocean warming, because the ocean is taking up most of the additional heat in the climate system.[4] Some of the additional carbon dioxide in the atmosphere is taken up by the ocean (via carbon sequestration), which leads to ocean acidification of the ocean water.[5] It is estimated that the ocean takes up roughly a quarter of total anthropogenic CO2 emissions.[5]

Warming of the ocean surface due to higher air temperatures leads to increased ocean temperature stratification.[6]: 471  The decline in mixing of the ocean layers piles up warm water near the surface while reducing cold, deep water circulation. The reduced up and down mixing reduces the ability of the ocean to absorb heat, directing a larger fraction of future warming toward the atmosphere and land. Energy available for tropical cyclones and other storms is expected to increase, nutrients for fish in the upper ocean layers are set to decrease, as is the capacity of the oceans to store carbon.[7]

Warmer water cannot contain as much oxygen as cold water. As a result, the gas exchange equilibrium changes to reduce ocean oxygen levels and increase oxygen in the atmosphere. Increased thermal stratification may lead to reduced supply of oxygen from the surface waters to deeper waters, and therefore further decrease the water's oxygen content. The ocean has already lost oxygen throughout the water column, and oxygen minimum zones are expanding worldwide.[6]: 471 

These changes disturb marine ecosystems, which can cause both extinctions and population explosions, change the distribution of species,[3] and impact coastal fishing and tourism. Increase of water temperature will also have a devastating effect on various oceanic ecosystems, such as coral reefs. The direct effect is the coral bleaching of these reefs, which live within a narrow temperature margin, so a small increase in temperature would have a drastic effect in these environments. Ocean acidification and temperature rise will also affect the productivity and distribution of species within the ocean, threatening fisheries and disrupting marine ecosystems. Loss of sea ice habitats due to warming will severely impact the many polar species which depend on this sea ice. Many of these climate change pressures interact, compounding the pressures on the climate system and on ocean ecosystems.[3]

Changes due to rising greenhouse gas levels

Ocean heat content changes since 1955 (annual estimates for the first 2,000 meters of ocean depth, the shaded blue region indicates the 95% margin of uncertainty.)[8]
Energy (heat) added to various parts of the climate system due to global warming (data from 2007).

Present-day (2020) atmospheric carbon dioxide (CO2) levels of more than 410 ppm are nearly 50% higher than preindustrial concentrations, and the current elevated levels and rapid growth rates are unprecedented in the past 55 million years of the geological record.[5] The source for this excess CO2 is clearly established as human-driven, reflecting a mix of anthropogenic fossil fuel, industrial, and land-use/land-change emissions.[5] The concept that the ocean acts as a major sink for anthropogenic CO2 has been present in the scientific literature since at least the late 1950s.[5] Multiple lines of evidence support the finding that the ocean takes up roughly a quarter of total anthropogenic CO2 emissions.[5]

The latest key findings about the observed changes and impacts from 2019 include:

It is virtually certain that the global ocean has warmed unabated since 1970 and has taken up more than 90% of the excess heat in the climate system [...]. Since 1993, the rate of ocean warming has more than doubled [...]. Marine heatwaves have very likely doubled in frequency since 1982 and are increasing in intensity [...]. By absorbing more CO2, the ocean has undergone increasing surface acidification [...]. A loss of oxygen has occurred from the surface to 1000 m [...].

Rising ocean temperature

Land surface temperatures have increased faster than ocean temperatures as the ocean absorbs about 92% of excess heat generated by climate change.[9] Chart with data from NASA[10] showing how land and sea surface air temperatures have changed vs a pre-industrial baseline.

It is clear that the oceans are warming as a result of climate change and this rate of warming is increasing.[3]: 9  The upper ocean (above 700 m) is warming fastest, but the warming trend extends throughout the ocean. Most of the ocean heat gain is taking place in the Southern Ocean. For example, the temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F) between the 1950s and the 1980s, nearly twice the rate for the world's oceans as a whole.[11]

Ocean temperatures vary from place to place. They are warmer near the equator and cooler at the poles. Therefore, ocean warming is best illustrated by the changes in total ocean heat content.

The heat uptake has accelerated in the 1993–2017 period compared to 1969–1993.[6]: 457  The warming rate varies with depth: at a depth of a thousand metres the warming occurs at a rate of almost 0.4 °C per century (data from 1981 to 2019), whereas the warming rate at two kilometres depth is only half.[6]: 463 

The illustration of temperature changes from 1960 to 2019 across each ocean starting at the Southern Ocean around Antarctica.[12]

From 1960 to through 2019, the average temperature for the upper 2000 meters of the oceans has increased by 0.12 degree Celsius, whereas the ocean surface temperature has warmed up to 1.2 degree Celsius from the pre-industrial era.[12]

Ocean heat content

Ocean heat content (OHC) or ocean heat uptake (OHU) is the energy absorbed and stored by oceans. To calculate the ocean heat content, it is necessary to measure ocean temperature at many different locations and depths. Integrating the areal density of a change in enthalpic energy over an ocean basin or entire ocean gives the total ocean heat uptake.[13] Between 1971 and 2018, the rise in ocean heat content accounted for over 90% of Earth's excess energy from global heating.[14][15] The main driver of this increase was anthropogenic forcing via rising greenhouse gas emissions.[16]: 1228  By 2020, about one third of the added energy had propagated to depths below 700 meters.[17][18]

Reducing ocean pH value

Ocean acidification: mean seawater pH. Mean seawater pH is shown based on in-situ measurements of pH from the Aloha station.[19]
Change in pH since the beginning of the industrial revolution. RCP 2.6 scenario is "low CO2 emissions" . RCP 8.5 scenario is "high CO2 emissions", the path we are currently on.[20]
Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.[21] Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO2) levels exceeding 410 ppm (in 2020). CO2 from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H2CO3) which dissociates into a bicarbonate ion (HCO3) and a hydrogen ion (H+). The presence of free hydrogen ions (H+) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.[22]

Example effects of ocean acidification

The increase of ocean acidity decelerates the rate of calcification in salt water, leading to smaller and slower growing coral reefs which supports approximately 25% of marine life.[23][24] Impacts are far-reaching from fisheries and coastal environments down to the deepest depths of the ocean.[25] The increase in ocean acidity in not only killing the coral, but also the wildly diverse population of marine inhabitants which coral reefs support.[26]

At depths of 1000s of meters in the ocean, calcium carbonate shells begin to dissolve as increasing pressure and decreasing temperature shift the chemical equilibria controlling calcium carbonate precipitation.[27] The depth at which this occurs is known as the carbonate compensation depth. Ocean acidification will increase such dissolution and shallow the carbonate compensation depth on timescales of tens to hundreds of years.[27] Zones of downwelling are being affected first.[28]

Observed effects on the physical environment

This animation helps to convey the importance of Earth's oceanic processes as one component of Earth's interrelated systems (source: NASA).

Sea level rise

Waves on an ocean coast

Since about 1900, the sea level has risen worldwide at an average rate of 1–2 mm/yr (the global average sea level was about 15–25 cm higher in 2018 compared to 1900).[29]: 1318  The pace of sea level rise is now increasing: The sea level rose by about 4 mm per year from 2006 to 2018.[29]: 1318 

This will threaten many coastal cities with coastal flooding over coming decades and longer.[29]: 1318  Coastal flooding can be exacerbated further by local subsidence which may be natural but can be increased by human activity.[30] By 2050 hundreds of millions of people are at risk from coastal flooding, particularly in Southeast Asia.[30]

Between 1901 and 2018, average global sea level rose by 15–25 cm (6–10 in), an average of 1–2 mm (0.039–0.079 in) per year.[31] This rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022.[32] Climate change due to human activities is the main cause.[33]: 5, 8  Between 1993 and 2018, thermal expansion of water accounted for 42% of sea level rise. Melting temperate glaciers accounted for 21%, while polar glaciers in Greenland accounted for 15% and those in Antarctica for 8%.[34]: 1576 

Changing ocean currents

Ocean currents are caused by varying temperatures associated with sunlight and air temperatures at different latitudes, as well as by prevailing winds and the different densities of saline and fresh water.

Air tends to be warmed and thus rise near the equator, then cool and thus sink slightly further poleward. Near the poles, cool air sinks, but is warmed and rises as it travels along the surface equatorward. This creates large-scale wind patterns known as Hadley cells, with similar effects driving a mid-latitude cell in each hemisphere.[35] Wind patterns associated with these circulation cells drive surface currents which push the surface water to the higher latitudes where the air is colder.[35] This cools the water down, causing it to become very dense in relation to lower latitude waters, which in turn causes it to sink to the bottom of the ocean, forming what is known as North Atlantic Deep Water (NADW) in the north and Antarctic Bottom Water (AABW) in the south.[36]

Driven by this sinking and the upwelling that occurs in lower latitudes, as well as the driving force of the winds on surface water, the ocean currents act to circulate water throughout the entire sea. When global warming is added into the equation, changes occur, especially in the regions where deep water is formed. With the warming of the oceans and subsequent melting of glaciers and the polar ice caps, more and more fresh water is released into the high latitude regions where deep water is formed, reducing the density of the surface water. Consequently, the water sinks more slowly than it normally would.[37]

Modern observations, climate simulations and paleoclimate reconstructions suggest that the Atlantic Meridional Overturning Circulation (AMOC) has weakened since the preindustrial era (the AMOC is part of a global thermohaline circulation) The latest climate change projections in 2021 suggest that the AMOC is likely to weaken further over the 21st century.[3]: 19  Such a weakening could cause large changes to global climate, with the North Atlantic particularly vulnerable.[3]: 19  This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.[38]

The surface wind pattern may also change with climate change, particularly its intensity and this may lead to locally variable climate change effects.[3] These changes in ocean currents also affect the ability of the ocean to take up carbon dioxide (which depends on water temperature) and also ocean productivity because the currents transport nutrients (see Impacts on phytoplankton and net primary production). The AMOC deep ocean circulation is slow (hundreds to thousands of years to circulate around the whole ocean) and so it is slow to respond to climate change.

Increasing stratification

Drivers of hypoxia and ocean acidification intensification in upwelling shelf systems. Equatorward winds drive the upwelling of low dissolved oxygen (DO), high nutrient, and high dissolved inorganic carbon (DIC) water from above the oxygen minimum zone. Cross-shelf gradients in productivity and bottom water residence times drive the strength of DO (DIC) decrease (increase) as water transits across a productive continental shelf.[39][40]

Changes in stratification within the ocean are important because they can drive changes in productivity and oxygen level. Stratification is defined as the separation of water in layers based on a specific quantity. Layered stratification occurs in all of the ocean basins. The stratified layers act as a barrier to the mixing of water, which can impact the exchange of heat, carbon, oxygen and other nutrients.[41] There has been an increase in stratification in the upper ocean since 1970 due to global warming and also in some regions changes in salinity.[29] The salinity changes are due to evaporation in tropical waters increasing salinity and density and at high latitudes where ice melt can reduce salinity.[29]

The density of water depends on its temperature and salinity. Hence the water column in the vast ocean basins is stratified with less dense water at the surface and denser water at depth.[42] This stratification is not only important in creating the Atlantic Meridional Overturning Circulation with its impacts on global weather and climate. It is also important because stratification also controls the transport of nutrients up from deep water to the surface. This helps fuel ocean productivity, and is connected to the compensatory downward flow of water that carries oxygen from the air and surface waters into the deep ocean.[43]

The mid depth waters of the ocean mix only slowly with surface waters and the decay of sinking organic matter from primary production in these waters naturally leads to low oxygen.[43] However, the effect of warming is to reduce the amount of oxygen which dissolves in surface waters. Furthermore, increasing stratification acts to isolate these mid depth waters even more, both factors leading to lower oxygen (see also oxygen depletion section). There is now clear evidence that the open ocean is losing oxygen and this trend is expected to continue as a result of climate change with overall oxygen declining by several percent. This will have ecological effects in regions where the oxygen concentrations fall to low levels, although the overall biological impacts are rather uncertain.[29]

Reduced oxygen levels

Global map of low and declining oxygen levels in the open ocean and coastal waters. The map indicates coastal sites where anthropogenic nutrients have resulted in oxygen declines to less than 2 mg L–1 (red dots), as well as ocean oxygen minimum zones at 300 metres (blue shaded regions).[44]

The oxygen content of the ocean is vital for the survival of most larger animals and plants and also serves a long term role in controlling atmospheric oxygen upon which terrestrial life depends.[45] Climate change affects ocean oxygen. There are two areas of concern in terms of ocean oxygen levels: The open ocean mid depth waters and the coastal waters.[45]

The first area of concern relates to the open ocean mid depth waters which are naturally low in oxygen (oxygen minimum zones) because of sluggish ocean circulation isolating these waters from the atmosphere (and hence oxygen) for decades, while sinking organic matter from surface waters is broken down consuming oxygen. These low oxygen ocean areas are expanding as a result of ocean warming which both reduces water circulation and also reduces the oxygen content of that water. This is because the solubility of oxygen declines as temperature rises.

Overall ocean oxygen concentrations are estimated to have declined 2% over 50 years from the 1960s.[3] The nature of the ocean circulation means that in general these low oxygen regions are more pronounced in the Pacific Ocean. Low oxygen represents a stress for almost all marine animals. Very low oxygen levels create regions with much reduced fauna. It is predicted that these low oxygen zones will expand in future due to climate change, and this represents a serious threat to marine life in these oxygen minimum zones.[3]  

The second area of concern relates to coastal waters where increasing nutrient supply from rivers to coastal areas leads to increasing production and sinking organic matter which in some coastal regions leads to extreme oxygen depletion sometimes referred to as “dead zones”.[46] These dead zones are expanding driven particularly by increasing nutrient inputs, but also compounded by increasing ocean stratification driven by climate change.[3]  

Changes to Earth's weather system and wind patterns

Climate change and the associated warming of the ocean will lead to widespread changes to the Earth’s climate and weather system including increased tropical cyclone and monsoon intensities and extremes with some areas becoming wetter and others drier challenging current systems of agriculture.[29] Changing wind patterns are predicted to increase wave heights.[29]: 1310  This can pose risks to mariners and also to marine structures.[29]: 1310 

Intensifying tropical cyclones

Human-induced climate change continues to warm the oceans which provide the memory of past accumulated effects.[47] The resulting environment, including higher ocean heat content and sea surface temperatures, invigorates tropical cyclones to make them more intense, bigger, and longer lasting and greatly increases their flooding rains. The main example here is Hurricane Harvey in August 2017. Accordingly, record high ocean heat values not only increased the fuel available to sustain and intensify Harvey but also increased its flooding rains on land. Harvey could not have produced so much rain without human-induced climate change.[47]

North Atlantic tropical cyclone activity according to the Power Dissipation Index, 1949–2015. Sea surface temperature has been plotted alongside the PDI to show how they compare. The lines have been smoothed using a five-year weighted average, plotted at the middle year.

Climate change can affect tropical cyclones in a variety of ways: an intensification of rainfall and wind speed, a decrease in overall frequency, an increase in the frequency of very intense storms and a poleward extension of where the cyclones reach maximum intensity are among the possible consequences of human-induced climate change.[48] Tropical cyclones use warm, moist air as their source of energy or fuel. As climate change is warming ocean temperatures, there is potentially more of this fuel available.[49]

Between 1979 and 2017, there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir–Simpson scale. The trend was most clear in the North Atlantic and in the Southern Indian Ocean. In the North Pacific, tropical cyclones have been moving poleward into colder waters and there was no increase in intensity over this period.[50] With 2 °C (3.6 °F) warming, a greater percentage (+13%) of tropical cyclones are expected to reach Category 4 and 5 strength.[48] A 2019 study indicates that climate change has been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin. Rapidly intensifying cyclones are hard to forecast and therefore pose additional risk to coastal communities.[51]

Salinity changes

Due to global warming and increased glacier melt, thermohaline circulation patterns may be altered by increasing amounts of freshwater released into oceans and, therefore, changing ocean salinity. Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as upwelling.[52]

Seawater consists of fresh water and salt, and the concentration of salt in seawater is called salinity. Salt does not evaporate, thus the precipitation and evaporation of freshwater influences salinity strongly. Changes in the water cycle are therefore strongly visible in surface salinity measurements, which is already acknowledged since the 1930s.[2][53]

The long term observation records show a clear trend: the global salinity patterns are amplifying in this period.[54][55] This means that the high saline regions have become more saline, and regions of low salinity have become less saline. The regions of high salinity are dominated by evaporation, and the increase in salinity shows that evaporation is increasing even more. The same goes for regions of low salinity that are become less saline, which indicates that precipitation is intensifying only more.[56][6]

Sea ice decline and changes

Decline in arctic sea ice extent (area) from 1979 to 2022

Sea ice decline occurs more in the Arctic than in Antarctica, where it is more a matter of changing sea ice conditions.

Sea ice in the Arctic region has declined in recent decades in area and volume due to climate change. It has been melting more in summer than it refreezes in winter. Global warming, caused by greenhouse gas forcing is responsible for the decline in Arctic sea ice. The decline of sea ice in the Arctic has been accelerating during the early twenty‐first century, with a decline rate of 4.7% per decade (it has declined over 50% since the first satellite records).[57][58][59] It is also thought that summertime sea ice will cease to exist sometime during the 21st century.[60]

Sea ice extent in Antarctica varies a lot year by year. This makes it difficult determine a trend, and record highs and record lows have been observed between 2013 and 2023. The general trend since 1979, the start of the satellite measurements, has been roughly flat. Between 2015 and 2023, there has been a decline in sea ice, but due to the high variability, this does not correspond to a significant trend.[61] The flat trend is in contrast with Arctic sea ice, which has seen a declining trend.[61][62]

Reporting reducing Antarctic sea ice extent in mid 2023, researchers concluded that a "regime shift" may be taking place "in which previously important relationships no longer dominate sea ice variability".[63]
The (then-record) 2012 Antarctic sea ice extent; compare with the yellow outline, which shows the median September extent from 1979 to 2000.
Antarctic sea ice cover shrinks to its minimum extent each year in February or March; the ice cover then grows until reaching its maximum extent in September or October.
An animation of the Antarctic sea ice growing from its seasonal minimum to seasonal maximum extent during southern hemisphere autumn and winter (between March 21 and September 19, 2014; note labels on animation). Spring melting in not shown.

Timescales

Many of the ocean-related processes are "slow-responding elements of the climate system", namely the loss of ice (sea ice or glaciers), increase in ocean heat content, sea level rise and deep ocean acidification.[64]: 43  They therefore represent a "millennial-scale commitment" (committed changes that are associated with past greenhouse gas emissions. The IPCC Sixth Assessment Report found that "The response of these variables depends on the time it takes to reach the global warming level, differs if the warming is reached in a transient warming state or after a temporary overshoot of the warming level, and will continue to evolve, over centuries to millennia, even after global warming has stabilized."[64]: 55  Different global warming levels are for example 1.5°C or 2°C above the 1850–1900 period.

This means that the impacts of climate change on oceans will be slow to start but equally take a long time (centuries to millennia) to play out. For example, the "global mean sea level will continue to rise for thousands of years, even if future CO2 emissions are reduced to net zero and global warming halted".[64]: 39  This is because excess energy due to past emissions will continue to extend into the deep ocean, and glaciers and ice sheets will continue to melt.

Impacts on marine life

Examples of projected impacts and vulnerabilities for fisheries associated with climate change

Phytoplankton and net primary production

Research indicates that increasing ocean temperatures are taking a toll on the marine ecosystem. A study on phytoplankton changes in the Indian Ocean indicates a decline of up to 20% in marine phytoplankton during the past six decades.[65] During the summer, the western Indian Ocean is home to one of the largest concentrations of marine phytoplankton blooms in the world when compared to other oceans in the tropics. Increased warming in the Indian Ocean enhances ocean stratification, which prevents nutrient mixing in the euphotic zone where there is ample light available for photosynthesis. Thus, primary production is constrained and the region's entire food web is disrupted. If rapid warming continues, experts predict that the Indian Ocean will transform into an ecological desert and will no longer be productive.[65] The same study also addresses the abrupt decline of tuna catch rates in the Indian Ocean during the past half century. This decrease is mostly due to increased industrial fisheries, with ocean warming adding further stress to the fish species. These rates show a 50-90% decrease over 5 decades.[65]

A study that describes climate-driven trends in contemporary ocean productivity looked at global-ocean net primary production (NPP) changes detected from satellite measurements of ocean color from 1997 to 2006.[66] These measurements can be used to quantify ocean productivity on a global scale and relate changes to environmental factors. They found an initial increase in NPP from 1997 to 1999 followed by a continuous decrease in productivity after 1999. These trends are propelled by the expansive stratified low-latitude oceans and are closely linked to climate variability. This relationship between the physical environment and ocean biology effects the availability of nutrients for phytoplankton growth since these factors influence variations in upper-ocean temperature and stratification.[66] The downward trends of ocean productivity after 1999 observed in this study can give insight into how climate change can affect marine life in the future.

Satellite measurement and chlorophyll observations indicate a decline in the number of phytoplankton, microorganisms that produce half of the earth's oxygen, absorb half of the world carbon dioxide and serve foundation of the entire marine food chain.[67] Phytoplankton are vital to Earth systems and critical for global ecosystem functioning and services, and vary with environmental parameters such as, temperature, water column mixing, nutrients, sunlight, and consumption by grazers.[68][69] Climate change results in fluctuations and modification of these parameters, which in turn may impact phytoplankton community composition, structure, and annual and seasonal dynamics.[69] Recent research and models have predicted a decline in phytoplankton productivity in response to warming ocean waters resulting in increased stratification where there is less vertical mixing in the water column to cycle nutrients from the deep ocean to surface waters.[70][71] Studies over the past decade confirm this prediction with data showing a slight decline in global phytoplankton productivity, particularly due to the expansion of "ocean deserts," such as subtropical ocean gyres with low-nutrient availability, as a result of rising seawater temperatures.[72]

Phytoplankton are critical to the carbon cycle as they consume CO2 via photosynthesis on similar scale to forests and terrestrial plants. As phytoplankton die and sink, carbon is then transported to deeper layers of the ocean where it is then eaten by consumers, and this cycle continues. The biological carbon pump is responsible for approximately 10 gigatonnes of carbon from the atmosphere to the deep ocean every year.[73] Fluctuations in phytoplankton in growth, abundance, or composition would greatly affect this system, as well as global climate.[73]

Changes in temperatures will impact the location of areas with high primary productivity. Primary producers, such as plankton,[74][75][76][77] are the main food source for marine mammals such as some whales. Species migration will therefore be directly affected by locations of high primary productivity. Water temperature changes also affect ocean turbulence, which has a major impact on the dispersion of plankton and other primary producers.[78]

Ocean warming can also result in a reduction of the solubility of CO2 in seawater,[79] resulting in discharge of CO2 from the ocean to the atmosphere. In addition to temperature, alkalinity and primary productivity modulate the CO2 flux between the ocean and the atmosphere.[80] In basins with very low primary productivity and rapid warming, such as the Eastern Mediterranean sea, a shift from CO2 sink to source has already been observed.[81]

Coral reefs and other shelf-sea ecosystems

Bleached Staghorn coral in the Great Barrier Reef.

While some mobile marine species can migrate in response to climate change, others such as corals find this much more difficult. A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate.[82] Coral reefs are important centres of biodiversity and vital to many millions of people who rely on them for coastal protection and food and for sustaining tourism in many regions.[83]

Ocean acidification reduces coralline algal biodiversity, according to a 2021 study. [84]

Warm water corals are clearly in decline, with losses of 50% over the last 30-50 years due to multiple threats from ocean warming, ocean acidification, pollution and physical damage from activities such as fishing, and these pressures are expected to intensify.[83]

The warming ocean surface waters can lead to bleaching of the corals which can cause serious damage and/or coral death. The IPCC Sixth Assessment Report in 2022 found that: "Since the early 1980s, the frequency and severity of mass coral bleaching events have increased sharply worldwide".[85]: 416  Coral reefs, as well as other shelf-sea ecosystems, such as rocky shores, kelp forests, seagrasses and mangroves have recently undergone mass mortalities from marine heatwaves.[85]: 381  It is expected that many coral reefs will "undergo irreversible phase shifts due to marine heatwaves with global warming levels >1.5°C".[85]: 382 

Coral bleaching occurs when thermal stress from a warming ocean results in the expulsion of the symbiotic algae that resides within coral tissues and is the reason for the bright, vibrant colors of coral reefs.[26] A 1-2 degree C sustained increase in seawater temperatures is sufficient for bleaching to occur, which turns corals white.[86] If a coral is bleached for a prolonged period of time, death may result. In the Great Barrier Reef, before 1998 there were no such events. The first event happened in 1998 and after it they begun to occur more and more frequently so in the years 2016 - 2020 there were 3 of them.[87]

Harmful algal blooms

Although the drivers of harmful algal blooms are poorly understood they do appear to have increased in range expansion and frequency in coastal areas since the 1980s.[3]: 16  The is the result of human induced factors such as increased nutrient inputs (nutrient pollution) and climate change (in particular the warming of water temperatures).[3]: 16  The parameters that affect the formation of HABs are ocean warming, marine heatwaves, oxygen loss, eutrophication and water pollution.[88]: 582  These increases in HABs are of concern because of the impact of their occurrence on local food security, tourism and the economy.[3]: 16 

It is however also possible that the perceived global increase in HABs is simply due to better monitoring and more detrimental bloom impacts and not due to a climate-linked mechanism.[85]: 463 

Spatially, algal species may experience range expansion, contraction, or latitudinal shifts.[89] Temporally, the seasonal windows of growth may expand or shorten.[89]

Impacts on marine mammals

Some effects on marine mammals, especially those in the Arctic, are very direct such as loss of habitat, temperature stress, and exposure to severe weather. Other effects are more indirect, such as changes in host pathogen associations, changes in body condition because of predator–prey interaction, changes in exposure to toxins and CO2 emissions, and increased human interactions.[90] Despite the large potential impacts of ocean warming on marine mammals, the global vulnerability of marine mammals to global warming is still poorly understood.[91]

Marine mammals have evolved to live in oceans, but climate change is affecting their natural habitat.[92][93][94][95] Some species may not adapt fast enough, which might lead to their extinction.[96]

It has been generally assumed that the Arctic marine mammals were the most vulnerable in the face of climate change given the substantial observed and projected decline in Arctic sea ice. However, research has shown that the North Pacific Ocean, the Greenland Sea and the Barents Sea host the species that are most vulnerable to global warming.[91] The North Pacific has already been identified as a hotspot for human threats for marine mammals[97] and now is also a hotspot of vulnerability to global warming. Marine mammals in this region will face double jeopardy from both human activities (e.g., marine traffic, pollution and offshore oil and gas development) and global warming, with potential additive or synergetic effects. As a result, these ecosystems face irreversible consequences for marine ecosystem functioning.[91]

Marine organisms usually tend to encounter relatively stable temperatures compared with terrestrial species and thus are likely to be more sensitive to temperature change than terrestrial organisms.[98] Therefore, the ocean warming will lead to increased species migration, as endangered species look for a more suitable habitat. If sea temperatures continue to rise, then some fauna may move to cooler water and some range-edge species may disappear from regional waters or experienced a reduced global range.[98] Change in the abundance of some species will alter the food resources available to marine mammals, which then results in marine mammals' biogeographic shifts. Additionally, if a species cannot successfully migrate to a suitable environment, unless it learns to adapt to rising ocean temperatures, it will face extinction.

Arctic sea ice decline leads to loss of the sea ice habitat, elevations of water and air temperature, and increased occurrence of severe weather. The loss of sea ice habitat will reduce the abundance of seal prey for marine mammals, particularly polar bears.[99] There also may be some indirect effect of sea ice changes on animal heath due to alterations in pathogen transmission, effect on animals on body condition caused by shift in the prey based/food web, changes in toxicant exposure associated with increased human habitation in the Arctic habitat.[100]

Sea level rise is also important when assessing the impacts of global warming on marine mammals, since it affects coastal environments that marine mammals species rely on.[101]

Example marine mammals

Polar bears

A polar bear waiting in the Fall for the sea ice to form.
Section 'Climate change' not found

Seals

Harp seal mother nursing pup on sea ice

Seals are another marine mammal that are susceptible to climate change. Much like polar bears, seals have evolved to rely on sea ice. They use the ice platforms for breeding and raising young seal pups. In 2010 and 2011, sea ice in the Northwest Atlantic was at or near an all-time low and harp seals as well as ringed seals that bred on thin ice saw increased death rates.[102][103] Antarctic fur seals in South Georgia saw extreme reductions over a 20-year study, during which scientists measured increased sea surface temperature anomalies.[104]

Dolphins

Dolphins are marine mammals with broad geographic extent, making them susceptible to climate change in various ways. The most common effect of climate change on dolphins is the increasing water temperatures across the globe.[105] This has caused a large variety of dolphin species to experience range shifts, in which the species move from their typical geographic region to warmer waters.[106][107] Another side effect of increasing water temperatures is the increase in toxic algae blooms, which has caused a mass die-off of bottlenose dolphins.[105]

Some examples for specific dolphin species include: In the Mediterranean, sea surface temperatures have increased, as well as salinity, upwelling intensity, and sea levels. Because of this, prey resources have been reduced causing a steep decline in the short-beaked common dolphin Mediterranean subpopulation, which was deemed endangered in 2003.[108] At the Shark Bay World Heritage Area in Western Australia, the local Indo-Pacific bottlenose dolphin population had a significant decline after a marine heatwave in 2011.[109] River dolphins are highly affected by climate change as high evaporation rates, increased water temperatures, decreased precipitation, and increased acidification occur.[106][110]

Economic effects

Effects on fisheries

Fisheries are affected by climate change in many ways: marine aquatic ecosystems are being affected by rising ocean temperatures,[111] ocean acidification[112] and ocean deoxygenation, while freshwater ecosystems are being impacted by changes in water temperature, water flow, and fish habitat loss.[113] These effects vary in the context of each fishery.[114] Climate change is modifying fish distributions[115] and the productivity of marine and freshwater species. Climate change is expected to lead to significant changes in the availability and trade of fish products.[116] The geopolitical and economic consequences will be significant, especially for the countries most dependent on the sector. The biggest decreases in maximum catch potential can be expected in the tropics, mostly in the South Pacific regions.[116]: iv 

The impacts of climate change on ocean systems has impacts on the sustainability of fisheries and aquaculture, on the livelihoods of the communities that depend on fisheries, and on the ability of the oceans to capture and store carbon (biological pump). The effect of sea level rise means that coastal fishing communities are significantly impacted by climate change, while changing rainfall patterns and water use impact on inland freshwater fisheries and aquaculture.[117] Increased risks of floods, diseases, parasites and harmful algal blooms are climate change impacts on aquaculture which can lead to losses of production and infrastructure.[116]

Potential feedback effects

Methane release from methane clathrate

Rising ocean temperatures can also have an effect on the methane clathrate reservoirs found under sediments on the ocean floors. These trap large amounts of the greenhouse gas methane, which ocean warming has the potential to release. However, it is currently regarded as very unlikely that gas clathrates (mostly methane) in subsea clathrates will lead to a "detectable departure from the emissions trajectory during this century".[64]: 107 

In 2004 the global inventory of ocean methane clathrates was estimated to occupy between one and five million cubic kilometres.[118]

Prevention

The methods to prevent or reduce further effects of climate change on oceans involves global-scale reduction in greenhouse gas emissions (climate change mitigation), as well as regional and local mitigation and management strategies moving forward.[119]

See also

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