Effects of climate change on oceans

Among the effects of climate change on oceans are: an increase in sea surface temperature as well as ocean temperatures at greater depths, more frequent 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), changes to ocean currents including a weakening of the Atlantic meridional overturning circulation, and stronger tropical cyclones and monsoons. 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. 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. It is estimated that the ocean takes up roughly a quarter of total anthropogenic emissions.

Warming of the ocean surface due to higher air temperatures leads to increased ocean temperature stratification.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
In
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
.-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. The decline in mixing of the ocean layers stabilises 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.

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.

These changes disturb marine ecosystems, which can accelerate species extinctions or create population explosions, thus changing the distribution of species, 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.

Changes due to rising greenhouse gas levels

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. Text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
/ref> The source for this excess is clearly established as human-driven, reflecting a mix of anthropogenic fossil fuel, industrial, and land-use/land-change emissions. The concept that the ocean acts as a major sink for anthropogenic has been present in the scientific literature since at least the late 1950s. Multiple lines of evidence support the finding that the ocean takes up roughly a quarter of total anthropogenic emissions.

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

Rising ocean temperature

It is clear that the oceans are warming as a result of climate change and this rate of warming is increasing. 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. 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. 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.

Ocean heat content

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.

Reducing ocean pH value

Observed effects on the physical environment

Sea level rise

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).Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021
Chapter 9: Ocean, Cryosphere and Sea Level Change
I
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. 1211–1362 The pace of sea level rise is now increasing: The sea level rose by about 4 mm per year from 2006 to 2018.

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

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 then 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. Wind patterns associated with these circulation cells drive surface currents which push the surface water to the higher latitudes where the air is colder. 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.

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.

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. Such a weakening could cause large changes to global climate, with the North Atlantic particularly vulnerable. This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.

Any 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

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. 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. The salinity changes are due to evaporation in tropical waters increasing salinity and density and at high latitudes where ice melt can reduce salinity.

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. 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.

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. 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.

Reduced oxygen levels

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. 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.

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. 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.  

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". These dead zones are expanding driven particularly by increasing nutrient inputs, but also compounded by increasing ocean stratification driven by climate change.

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 weather extremes with some areas becoming wetter and others drier challenging current systems of agriculture. Changing wind patterns are predicted to increase wave heights in some areas. This can pose risks to mariners and also to marine structures.

Intensifying tropical cyclones

Human-induced climate change continues to warm the oceans which provide the memory of past accumulated effects. Text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
/ref> 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 energy 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.

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.

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 been known since the 1930s.

The long term observation records show a clear trend: the global salinity patterns are amplifying in this period. 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.

Sea ice decline and changes

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

Time scales

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.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
Technical Summary
I
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. 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." 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". 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

Climate change will not only alter the overall productivity of the oceans but also alter ocean biomass community structure and in general lead to a poleward migration of species. Some species have already moved hundreds of kilometres since the 1950s. Phytoplankton bloom timings are also already altering moving earlier in the season particularly in polar waters. These trends are projected to continue with further climate change.

There are additional potentially important impacts of climate change on seabirds, fish and mammals in polar regions where populations with highly specialised survival strategies will need to adapt to major changes in habitat and food supply. In addition sea ice often plays a key role in their life cycle. In the Arctic for example providing haul-out sites for seals and walruses and for hunting routes for polar bears. In the Antarctic sea bird and penguin distributions are also believed to be very sensitive to climate change, although the impacts to date are different in different regions.

Impacts on oceanic calcifying organisms

Coral reefs and other shelf-sea ecosystems

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. 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.

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.

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".Cooley, S., D. Schoeman, L. Bopp, P. Boyd, S. Donner, D.Y. Ghebrehiwet, S.-I. Ito, W. Kiessling, P. Martinetto, E. Ojea, M.-F. Racault, B. Rost, and M. Skern-Mauritzen, 2022
Chapter 3: Oceans and Coastal Ecosystems and Their Services
In
Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.) Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 379–550 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. It is expected that many coral reefs will suffer irreversible changes and loss due to marine heatwaves with global temperatures increasing by more than 1.5 °C.

Coral bleaching occurs when thermal stress from a warming ocean results in the expulsion of the symbiotic algae that resides within coral tissues. These symbiotic algae are the reason for the bright, vibrant colors of coral reefs. A 1-2 degree C sustained increase in seawater temperatures is sufficient for bleaching to occur, which turns corals white. 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.

Apart from coral bleaching, the reducing pH value in oceans is also a problem for coral reefs because ocean acidification reduces coralline algal biodiversity. The physiology of coralline algal calcification determines how the algae will respond to ocean acidification.

Ocean productivity

The process of photosynthesis in the surface ocean releases oxygen and consumes carbon dioxide. This photosynthesis in the ocean is dominated by phytoplankton, microscopic free floating algae. After the plants grow, bacterial decomposition of the organic matter formed by photosynthesis in the ocean consumes oxygen and releases carbon dioxide. The sinking and bacterial decomposition of some organic matter in deep ocean water, at depths where the waters are out of contact with the atmosphere, leads to a reduction in oxygen concentrations and increase in carbon dioxide, carbonate and bicarbonate. This cycling of carbon dioxide in oceans is an important part of the global carbon cycle.  

The photosynthesis in surface waters consumes nutrients (e.g. nitrogen and phosphorus) and transfers these nutrients to deep water as the organic matter produced by photosynthesis sinks upon the death of the organisms. Productivity in surface waters therefore depends in part on the transfer of nutrients from deep water back to the surface by ocean mixing and currents. The increasing stratification of the oceans due to climate change therefore acts generally to reduce ocean productivity. However, in some areas, such as previously ice covered regions, productivity may increase. This trend is already observable and is projected to continue under current projected climate change. In the Indian Ocean for example productivity is estimated to have declined by over the past sixty years due to climate warming and is projected to continue.

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. 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 expansion of stratified low-latitude oceans and are closely linked to climate variability.

This declining trend in ocean productivity is expected to continue with productivity likely to decline by 4-11% by 2100 (for the high greenhouse gas emissions scenario of RCP 8.5). The decline will show regional variations. For example, the tropical ocean NPP will decline more: by 7–16% for the same emissions scenario. The flux of organic matter from the upper ocean into the ocean interior will decrease because of increased stratification and reduced nutrient supply. The reduction in ocean productivity is due to the "combined effects of warming, stratification, light, nutrients and predation".

Effects on fisheries

The total amount of fish that can be harvested sustainably from the ocean depends on ocean productivity. Hence the reductions in ocean productivity leads to reductions in the available potential maximum fish catch from countries' exclusive economic zones. This catch is projected to decline globally, with different models predicting declines between 5 and 25% by the end of the century. Within this average global decline, declines in some regions such as the South Pacific are projected to be larger, and threaten the food security of local populations. 

Harmful algal blooms

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

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.

Spatially, all  algal species (including those causing harmful algal blooms) may experience range expansion, contraction, or latitudinal shifts. Temporally, the seasonal windows of growth may expand or shorten.

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 emissions, and increased human interactions. Despite the large potential impacts of ocean warming on marine mammals, the global vulnerability of marine mammals to global warming is still poorly understood.

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

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. The North Pacific has already been identified as a hotspot for human threats for marine mammals 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.

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. 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. 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. 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, and changes in toxicant exposure associated with increased human habitation in the Arctic habitat.

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.

Example marine mammals

= Polar bears

=

= Seals

=

Seals are another marine mammal that are susceptible to climate change. Much like polar bears, some seal species 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. Antarctic fur seals in South Georgia in the South Atlantic Ocean saw extreme reductions over a 20-year study, during which scientists measured increased sea surface temperature anomalies.

= 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. This has caused a large variety of dolphin species to experience range shifts, in which the species move from their typical geographic region to cooler waters. Another side effect of increasing water temperatures is the increase in harmful algae blooms, which has caused a mass die-off of bottlenose dolphins.

Some examples for the impact of climate change on 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. 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. River dolphins are highly affected by climate change as high evaporation rates, increased water temperatures, decreased precipitation, and increased acidification occur.

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".

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

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.IPCC, 2022
Summary for Policymakers
In
Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.) Cambridge University Press, Cambridge, UK and New York, NY, USA.

See also

* Blue carbon
* Carbon sequestration
* Effects of climate change on island nations
* Effects of climate change on the water cycle
* Special Report on the Ocean and Cryosphere in a Changing Climate (2019)
* Sustainable fisheries
*
*

References

External links

IPCC Working Group I (WG I)
Intergovernmental Panel on Climate Change group which assesses the physical scientific aspects of the climate system and climate change.

Climate
from the World Meteorological Organization

Climate change
UN Department of Economic and Social Affairs Sustainable Development

Effects of climate change
from the Met Office

United Nations Environment Programme and climate change

FAO - Fisheries and Aquaculture
{{Climate change

Effects of climate change
Oceans