Effects of climate change on oceans

trapped by human-induced global warming is absorbed by the oceans, penetrating to its deeper layers. due to global warming (data from 2007). Presently (2020), atmospheric carbon dioxide (CO2) levels of more than 410 parts per million (ppm) are nearly 50% higher than preindustrial levels. These elevated levels and rapid growth rates are unprecedented in the geological record's 55 million years. The source for this excess is clearly established as human-driven, reflecting a mix of fossil fuel burning, industrial, and land-use/land-change emissions. Chart with data from NASA showing how land and sea surface air temperatures have changed vs a pre-industrial baseline. It is clear that the ocean is warming as a result of climate change, and this rate of warming is increasing. This is determined by the ocean heat content, which exceeded the previous 2021 maximum in 2022. The majority of ocean heat gain occurs in the Southern Ocean. For example, between the 1950s and the 1980s, the temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F), nearly twice the rate of the global ocean. The warming rate varies with depth. The upper ocean (above 700 m) is warming the fastest. At an ocean depth of a thousand metres the warming occurs at a rate of nearly 0.4 °C per century (data from 1981 to 2019). Marine heatwaves Marine heatwaves also take their toll on marine life: For example, due to fall-out from the 2019-2021 Pacific Northwest marine heatwave, Bering Sea snow crab populations declined 84% between 2018 and 2022, a loss of 9.8 billion crabs. Ocean heat content The ocean temperature varies from place to place. Temperatures are higher near the equator and lower at the poles. As a result, changes in total ocean heat content best illustrate ocean warming. When compared to 1969–1993, heat uptake has increased between 1993 and 2017. scenario is "low CO2 emissions". RCP8.5 scenario is "high CO2 emissions". Time scales Many ocean-related elements of the climate system respond slowly to warming. For instance, acidification of the deep ocean will continue for millennia, and the same is true for the increase in ocean heat content. Similarly, sea level rise will continue for centuries or even millennia even if greenhouse gas emissions are brought to zero, due to the slow response of ice sheets to warming and the continued uptake of heat by the oceans, which expand when warmed. == Effects on the physical environment ==

Sea level rise Many coastal cities will experience coastal flooding in the coming decades and beyond. Coastal flooding will threaten hundreds of millions of people by 2050, particularly in Southeast Asia. Wind patterns associated with these circulation cells drive surface currents which push the surface water to higher latitudes where the air is colder. 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 sea. When global warming is factored in, changes occur, particularly in areas where deep water is formed. As the oceans warm and glaciers and polar ice caps melt, more and more fresh water is released into the high latitude regions where deep water forms, lowering the density of the surface water. As a result, the water sinks more slowly than it would normally. Changes in ocean stratification are significant because they can influence productivity and oxygen levels. The separation of water into layers based on density is known as stratification. Stratification by layers occurs in all ocean basins. The stratified layers limit how much vertical water mixing takes place, reducing the exchange of heat, carbon, oxygen and particles between the upper ocean and the interior. Since 1970, there has been an increase in stratification in the upper ocean due to global warming and, in some areas, salinity changes. Reduced oxygen levels s at 300 metres (blue shaded regions). Climate change has an impact on ocean oxygen, both in coastal areas and in the open ocean. Overall ocean oxygen concentrations are estimated to have declined 2% over 50 years from the 1960s. These dead zones are expanding driven particularly by increasing nutrient inputs, but also compounded by increasing ocean stratification driven by climate change. There is evidence that this in turn may be leading to widespread ocean darkening where changes to the optical properties of the water is preventing light from penetrating to greater depth. 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. The result is a higher ocean heat content and higher sea surface temperatures. In turn, this "invigorates tropical cyclones to make them more intense, bigger, longer lasting and greatly increases their flooding rains". 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 has 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 becoming less saline, which indicates that precipitation is becoming more intensified. 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. == Impacts on biological processes ==

associated with climate change 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. Ocean productivity under a very high emission scenario (RCP8.5) is very likely to drop by 4-11% by 2100. 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 increase in HABs globally is simply due to more severe bloom impacts and better monitoring and not due to climate change. == Impacts on coral reefs and fisheries ==

Coral reefs 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 characterised by reef-building corals. Reefs are formed by colonies of coral polyps held together by calcium carbonate. Coral reefs are important centres of biodiversity and vital to millions of people who rely on them for coastal protection, 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. These pressures are expected to intensify. Marine heatwaves have caused coral reef mass mortality. A 1-2°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 that, they began to occur more frequently. Between 2016 and 2020 there were three 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. Effects on fisheries == Impacts on marine mammals ==

Regions and habitats particularly affected 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. and is now also a hotspot for 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. Therefore, the ocean warming will lead to the migration of increased species, 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 experience a reduced global range. Sea ice changes may also have indirect effects on animal health due to changes in the transmission of pathogens, impacts on animals' body condition due to shifts in the prey-based food web, and increased exposure to toxicants as a result of 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 mammal species rely on. Polar bears Seals mother nursing pup on sea ice Seals are another marine mammal that are susceptible to climate change. 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 Climate change has had a significant impact on various dolphin species. For example: In the Mediterranean, increased sea surface temperatures, salinity, upwelling intensity, and sea levels have led to a reduction in prey resources, causing a steep decline in the short-beaked common dolphin subpopulation in the Mediterranean, which was classified as endangered in 2003. At the Shark Bay World Heritage Area in Western Australia, the local population of the Indo-Pacific bottlenose dolphin had a significant decline following 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. North Atlantic right whales == Potential feedback effects ==

Methane release from methane clathrate Rising ocean temperatures also have the potential to impact methane clathrate reservoirs located under the ocean floor sediments. These trap large amounts of the greenhouse gas methane, which ocean warming has the potential to release. However, it is currently considered unlikely that gas clathrates (mostly methane) in subsea clathrates will lead to a "detectable departure from the emissions trajectory during this century". ==See also==