Southern Ocean overturning circulation

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A schematic overview of the Southern Ocean overturning circulation. The arrows point in the direction of the water movement. The lower cell of the circulation is depicted by the upwelling arrows south of the Antarctic Circumpolar Current (ACC) and the formation of Antarctic Bottom Water beneath the sea ice of Antarctica due to buoyancy loss. The upper cell is depicted by the upwelling arrows north of the ACC and the formation of lighter Antarctic Intermediate water due to buoyancy gain north of the ACC. Southern Ocean overturning circulation.jpg
A schematic overview of the Southern Ocean overturning circulation. The arrows point in the direction of the water movement. The lower cell of the circulation is depicted by the upwelling arrows south of the Antarctic Circumpolar Current (ACC) and the formation of Antarctic Bottom Water beneath the sea ice of Antarctica due to buoyancy loss. The upper cell is depicted by the upwelling arrows north of the ACC and the formation of lighter Antarctic Intermediate water due to buoyancy gain north of the ACC.

Southern Ocean overturning circulation (sometimes referred to as the Southern Meridional overturning circulation (SMOC) [1] or Antarctic overturning circulation) is the southern half of a global thermohaline circulation, which connects different water basins across the global ocean. Its better-known northern counterpart is the Atlantic meridional overturning circulation (AMOC). This circulation operates when certain currents send warm, oxygenated, nutrient-poor water into the deep ocean (downwelling), while the cold, oxygen-limited, nutrient-rich water travels upwards (or upwells) at specific points. Thermohaline circulation transports not only massive volumes of warm and cold water across the planet, but also dissolved oxygen, dissolved organic carbon and other nutrients such as iron. [2] Thus, both halves of the circulation have a great effect on Earth's energy budget and oceanic carbon cycle, and so play an essential role in the Earth's climate system. [3] [4]

Contents

Southern ocean overturning circulation itself consists of two parts, the upper and the lower cell. The smaller upper cell is most strongly affected by winds due to its proximity to the surface, while the behaviour of the larger lower cell is defined by the temperature and salinity of Antarctic bottom water. [5] The strength of both halves had undergone substantial changes in the recent decades: the flow of the upper cell has increased by 50-60% since 1970s, while the lower cell has weakened by 10-20%. [6] [3] Some of this has been due to the natural cycle of Interdecadal Pacific Oscillation, [7] [8] but climate change has also played a substantial role in both trends, as it had altered the Southern Annular Mode weather pattern, [9] [7] while the massive growth of ocean heat content in the Southern Ocean [10] has increased the melting of the Antarctic ice sheets, and this fresh meltwater dilutes salty Antarctic bottom water. [11] [12]

As the formation of dense and cold waters weakens near the coast while the flow of warm waters towards the coast strengthens, the surface waters become less likely to sink downwards and mix with the lower layers. [13] Consequently, ocean stratification increases. [6] [3] One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario, [14] with greater losses occurring afterwards. [15] This slowdown would have important effects on the global climate due to the strength of the Southern Ocean as a global carbon sink and heat sink. For instance, global warming will reach 2 °C (3.6 °F) in all scenarios where greenhouse gas emissions have not been strongly lowered, but the exact year depends on the status of the circulation more than any factor other than the overall emissions. [16]

Paleoclimate evidence shows that the entire circulation had strongly weakened or outright collapsed before: some preliminary research suggests that such a collapse may become likely once global warming reaches levels between 1.7 °C (3.1 °F) and 3 °C (5.4 °F). However, there is far less certainty than with the estimates for most other tipping points in the climate system. [16] Even if initiated in the near future, the circulation's collapse is unlikely to be complete until close to 2300, [1] Similarly, impacts such as the reduction in precipitation in the Southern Hemisphere, with a corresponding increase in the North, or a decline of fisheries in the Southern Ocean with a potential collapse of certain marine ecosystems, are also expected to unfold over multiple centuries. [15]

Dynamics

3D representation of North Atlantic Deep Water upwelling in the Southern Ocean basin, which closes the connection between the Atlantic and Southern circulation, and takes place along the defined pathways with limited mixing. Tamsitt 2017 NADW upwelling.jpg
3D representation of North Atlantic Deep Water upwelling in the Southern Ocean basin, which closes the connection between the Atlantic and Southern circulation, and takes place along the defined pathways with limited mixing.

Southern Ocean overturning circulation consists of two cells in the Southern Ocean, which are driven by upwelling and downwelling. The upwelling in the upper cell is associated with mid-deep water that is brought to the surface, whereas the upwelling in the lower cell is linked to the fresh and abyssal waters around Antarctica. Around 27 ± 7 Sverdrup (Sv) of deep water wells up to the surface in the Southern Ocean. This upwelled water is partly transformed to lighter water and denser water, respectively 22 ± 4 Sv and 5 ± 5 Sv. The densities of these waters change due to heat and buoyancy fluxes which result in upwelling in the upper cell and downwelling in the lower cell. [5]

The Southern Ocean plays a key role in the closure of the Atlantic meridional overturning circulation by compensating for the North Atlantic downwelling by upwelling of North Atlantic Deep Water and connects the interior ocean to the surface. This upwelling is induced by the strong westerly winds that blow over the ACC. [4] [17] Observations suggest that approximately 80 percent of global deep water is upwelled in the Southern Ocean. [18] Circulation is a slow process - for instance, the upwelling of North Atlantic Deep Water from the depths of 1,000–3,500 m (3,281–11,483 ft) to the surface mixed layer takes 60–90 years for just half of the water mass, and some water travels to the surface for more than a century. [17]

Upper cell

The upper cell is driven by wind generated flow, a result of the Westerlies, that brings water from the Circumpolar Deep Water (CDW) to the surface. [19] Zonal wind stress induces upwelling near the pole and downwelling at the equator due to the zonal surface-wind maximum. This wind-driven circulation is also called the Deacon cell and acts to overturn water supporting the thermal wind current of the Antarctic Circumpolar Current (ACC) and creating a storage of potential energy. This upper cell process is also known as Ekman transport. [4]

The meridional overturning flow is from the north to the south in deep waters and from the south to the north at the ocean surface. At the surface deep waters are exposed to the atmosphere and surface buoyancy forces. There is a net gain of buoyancy in the upper cell as a result of the freshening of the water caused by precipitation and the melting of sea ice during summer (on the Southern Hemipshere). This buoyancy gain transforms the waters into lighter, less dense waters, such as Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). Around 22 ± 4 Sv of the total upwelled water in the overturning circulation is transformed into lighter waters in the upper cell. The overturning process of density surfaces is balanced through the baroclinic instability of the thermal wind currents. This instability flattens the density surfaces and the transport towards the poles, resulting in energetic, time-dependent eddying motions. The potential energy from the wind-driven circulation is then flattened out by eddies. [5]

Missing-mixing paradox

The missing-mixing paradox assumes that dense water is upwelled through the thermocline to close the circulation. To achieve this, vertical mixing is needed in the thermocline, which is not observed. [20] Instead, dense water from sinking regions returned to the surface in nearly adiabatic pathways along density isopycnals, which was already written by Harald Sverdrup (oceanographer). [21]

Lower cell

The role of seasonal meltwater from the Antarctic ice sheet in driving the lower-cell circulation. Pellichero 2018 Southern Ocean mixed layer.jpg
The role of seasonal meltwater from the Antarctic ice sheet in driving the lower-cell circulation.

The lower cell is driven by freshwater fluxes where sea-ice formation and melting play an important role. [5] The formation of sea-ice is accompanied by brine rejection, resulting in water with a higher salinity and density and therefore buoyancy loss. When ice melts there is a freshwater flow and exposure to the atmosphere. If water turns into ice, there is more salt in the water and less exposure to the atmosphere. Due to seasonal variations, there is a gain of buoyancy during summer and a loss of buoyancy in winter. This cold and dense water filled with salt is called Dense Shelf Water (DSW). DSW is then transformed into Antarctic Bottom Water (AABW), originating from the Ross Sea, Weddell Sea and along the eastern coast of Antarctica. Around 5 ± 5 Sv of AABW is formed in the lower cell of the Southern Ocean circulation, which is around a third of the total AABW formation. [22] [23] [24]

Global carbon cycle

In the 1990s and 2000s, the concentration of dissolved organic carbon at the surface had been decreasing, as more was pushed to the depths through the circulation. In the 2010s, however, the weakening circulation moved less carbon downwards, and its concentration started to increase across the surface. Zemskova 2022 Southern Ocean DIC.png
In the 1990s and 2000s, the concentration of dissolved organic carbon at the surface had been decreasing, as more was pushed to the depths through the circulation. In the 2010s, however, the weakening circulation moved less carbon downwards, and its concentration started to increase across the surface.

The ocean is in normally in equilibrium with the atmospheric carbon dioxide concentration. The increase in atmospheric CO2 since the Industrial Revolution had turned the oceans into a net carbon sink, and they absorb around 25% of human-caused emissions. [26] Out of all oceans, the Southern Ocean plays the greatest role in carbon uptake, and on its own, it is responsible for around 40%. [27] [28] [29] In 2000s, some research suggested that climate-driven changes to Southern Hemisphere winds were reducing the amount of carbon it absorbed, [30] but subsequent research found that this carbon sink had been even stronger than estimated earlier, by some 14% to 18%. [27] [28] Ocean circulation is very important for this process, as it brings deep water to the surface, which has not been there for centuries and so was not in contact with anthropogenic emissions before. Thus, deep water's dissolved carbon concentrations are much lower than of the modern surface waters, and it absorbs a lot more carbon before it's transported back to the depths through downwelling. [31] [25]

On the other hand, regions where deep warm circumpolar carbon rich waters are brought to the surface through upwelling, outgas CO2 through exposure to the atmosphere, partly compensating the carbon sink effect of the overturning circulation. [32] Additionally, ocean upwelling brings mineral nutrients such as iron from the depths to the surface, which are then consumed by phytoplankton and allow them to increase their numbers, enhancing ocean primary production and boosting the carbon sink due to greater photosynthesis. [2] At the same time, downwelling circulation moves much of dead phytoplankton and other organic matter to the depths before it could decompose at the surface and release CO2 back to the atmosphere. This so-called biological pump is so important that a completely abiotic Southern Ocean, where this pump would be absent, would also be a net source of CO2. [29]

Climate change impacts

Even under the most intense climate change scenario, which is currently considered unlikely, the Southern Ocean would continue to function as a strong sink in the 21st century, and take up an increasing amount of carbon dioxide (left) and heat (middle). However, it would take up a smaller fraction of heat per every additional degree of warming than it does now (right), as well as a smaller fraction of emissions. Bourgeois 2022 Southern Ocean uptake.png
Even under the most intense climate change scenario, which is currently considered unlikely, the Southern Ocean would continue to function as a strong sink in the 21st century, and take up an increasing amount of carbon dioxide (left) and heat (middle). However, it would take up a smaller fraction of heat per every additional degree of warming than it does now (right), as well as a smaller fraction of emissions.

As human-caused greenhouse gas emissions cause increased warming, one of the most notable effects of climate change on oceans is the increase in ocean heat content, which accounted for over 90% of the total global heating since 1971. [36] Since 2005, from 67% to 98% of this increase has occurred in the Southern Ocean. [9] In West Antarctica, the temperature in the upper layer of the ocean has warmed 1 °C (1.8 °F) since 1955, and the Antarctic Circumpolar Current (ACC) is also warming faster than the global average. [37] This warming directly affects the flow of warm and cold water masses which make up the overturning circulation, and it also has negative impacts on sea ice cover in Southern Hemisphere, (which is highly reflective and so elevates the albedo of Earth's surface), as well as mass balance of Antarctica's ice shelves and peripheral glaciers. [38] For these reasons, climate models consistently show that the year when global warming will reach 2 °C (3.6 °F) (inevitable in all climate change scenarios where greenhouse gas emissions have not been strongly lowered) depends on the status of the circulation more than any other factor besides the emissions themselves. [16]

Greater warming of this ocean water increases ice loss from Antarctica, and also generates more fresh meltwater, at a rate of 1100-1500 billion tons (GT) per year. [38] : 1240  This meltwater from the Antarctic ice sheet then mixes back into the Southern Ocean, making its water fresher. [39] This freshening of the Southern Ocean results in increased stratification and stabilization of its layers, [40] [38] : 1240  and this has the single largest impact on the long-term properties of Southern Ocean circulation. [14] These changes in the Southern Ocean cause the upper cell circulation to speed up, accelerating the flow of major currents, [41] while the lower cell circulation slows down, as it is dependent on the highly saline Antarctic bottom water, which already appears to have been observably weakened by the freshening, in spite of the limited recovery during 2010s. [11] [42] [43] [38] : 1240  Since the 1970s, the upper cell has strengthened by 3-4 sverdrup (Sv; represents a flow of 1 million cubic meters per second), or 50-60% of its flow, while the lower cell has weakened by a similar amount, but because of its larger volume, these changes represent a 10-20% weakening. [6] [3] However, they were not fully caused by climate change, as the natural cycle of Interdecadal Pacific Oscillation had also played an important role. [7] [8]

Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened. NOAA SMOC changes.png
Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened.

Additionally, the main controlling pattern of the extratropical Southern Hemisphere's climate is the Southern Annular Mode (SAM), which has been spending more and more years in its positive phase due to climate change (as well as the aftermath of ozone depletion), which means more warming and more precipitation over the ocean due to stronger westerlies, freshening the Southern Ocean further. [9] [38] : 1240  Climate models currently disagree on whether the Southern Ocean circulation would continue to respond to changes in SAM the way it does now, or if it will eventually adjust to them. As of early 2020s, their best, limited-confidence estimate is that the lower cell would continue to weaken, while the upper cell may strengthen by around 20% over the 21st century. [38] A key reason for the uncertainty is the poor and inconsistent representation of ocean stratification in even the CMIP6 models - the most advanced generation available as of early 2020s. [10] Further, the largest long-term role in the state of the circulation is played by Antarctic meltwater, [14] and Antarctic ice loss had been the least-certain aspect of future sea level rise projections for a long time. [44]

Evidence suggests that the Antarctic bottom water requires a temperature range close to current conditions to be at full strength. During the Last Glacial Maximum (a cold period), it was too weak to flow out of the Weddell Sea and the overturning circulation was much weaker than now. It was also weaker during the periods warmer than now. Huang 2020 AABW Eemian.png
Evidence suggests that the Antarctic bottom water requires a temperature range close to current conditions to be at full strength. During the Last Glacial Maximum (a cold period), it was too weak to flow out of the Weddell Sea and the overturning circulation was much weaker than now. It was also weaker during the periods warmer than now.

Similar processes are taking place with Atlantic meridional overturning circulation (AMOC), which is also affected by the ocean warming and by meltwater flows from the declining Greenland ice sheet. [46] It is possible that both circulations may not simply continue to weaken in response to increased warming and freshening, but eventually collapse to a much weaker state outright, in a way which would be difficult to reverse and constitute an example of tipping points in the climate system. [16] There is paleoclimate evidence for the overturning circulation being substantially weaker than now during past periods that were both warmer and colder than now. [45] However, Southern Hemisphere is only inhabited by 10% of the world's population, and the Southern Ocean overturning circulation has historically received much less attention than the AMOC. Consequently, while multiple studies have set out to estimate the exact level of global warming which could result in AMOC collapsing, the timeframe over which such collapse may occur, and the regional impacts it would cause, much less equivalent research exists for the Southern Ocean overturning circulation as of the early 2020s. There has been a suggestion that its collapse may occur between 1.7 °C (3.1 °F) and 3 °C (5.4 °F), but this estimate is much less certain than for many other tipping points. [16]

The impacts of Southern Ocean overturning circulation collapse have also been less closely studied, though scientists expect them to unfold over multiple centuries. A notable example is the loss of nutrients from Antarctic bottom water diminishing ocean productivity and ultimately the state of Southern Ocean fisheries, potentially leading to the extinction of some species of fish, and the collapse of some marine ecosystems. [15] Reduced marine productivity would also mean that the ocean absorbs less carbon (though not within the 21st century [10] ), which could increase the ultimate long-term warming in response to anthropogenic emissions (thus raising the overall climate sensitivity) and/or prolong the time warming persists before it starts declining on the geological timescales. [1] There is also expected to be a decline in precipitation in the Southern Hemisphere countries like Australia, with a corresponding increase in the Northern Hemisphere. However, the decline or an outright collapse of the AMOC would have similar but opposite impacts, and the two would counteract each other up to a point. Both impacts would also occur alongside the other effects of climate change on the water cycle and effects of climate change on fisheries. [15]

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<span class="mw-page-title-main">Antarctic Circumpolar Current</span> Ocean current that flows clockwise from west to east around Antarctica

Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

<span class="mw-page-title-main">Antarctic</span> Polar region around Earths South Pole

The Antarctic is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole.

<span class="mw-page-title-main">Downwelling</span> Process of accumulation and sinking of higher density material beneath lower density material

Downwelling is the downward movement of a fluid parcel and its properties within a larger fluid. It is closely related to upwelling, the upward movement of fluid.

<span class="mw-page-title-main">Ocean current</span> Directional mass flow of oceanic water

An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents move both horizontally, on scales that can span entire oceans, as well as vertically, with vertical currents playing an important role in the movement of nutrients and gases, such as carbon dioxide, between the surface and the deep ocean.

<span class="mw-page-title-main">Thermohaline circulation</span> Part of large-scale ocean circulation

Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy and mass around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

<span class="mw-page-title-main">Ocean gyre</span> Any large system of circulating ocean surface currents

In oceanography, a gyre is any large system of circulating ocean surface currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the circulatory patterns from the wind stress curl (torque).

<span class="mw-page-title-main">West Antarctic Ice Sheet</span> Segment of Antarctic ice sheet

The West Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. It is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

<span class="mw-page-title-main">Antarctic ice sheet</span> Earths southern polar ice cap

The Antarctic ice sheet is a continental glacier covering 98% of the Antarctic continent, with an area of 14 million square kilometres and an average thickness of over 2 kilometres (1.2 mi). It is the largest of Earth's two current ice sheets, containing 26.5 million cubic kilometres of ice, which is equivalent to 61% of all fresh water on Earth. Its surface is nearly continuous, and the only ice-free areas on the continent are the dry valleys, nunataks of the Antarctic mountain ranges, and sparse coastal bedrock. However, it is often subdivided into East Antarctic ice sheet (EAIS), West Antarctic ice sheet (WAIS), and Antarctic Peninsula (AP), due to the large differences in topography, ice flow, and glacier mass balance between the three regions.

<span class="mw-page-title-main">Abrupt climate change</span> Form of climate change

An abrupt climate change occurs when the climate system is forced to transition at a rate that is determined by the climate system energy-balance. The transition rate is more rapid than the rate of change of the external forcing, though it may include sudden forcing events such as meteorite impacts. Abrupt climate change therefore is a variation beyond the variability of a climate. Past events include the end of the Carboniferous Rainforest Collapse, Younger Dryas, Dansgaard–Oeschger events, Heinrich events and possibly also the Paleocene–Eocene Thermal Maximum. The term is also used within the context of climate change to describe sudden climate change that is detectable over the time-scale of a human lifetime. Such a sudden climate change can be the result of feedback loops within the climate system or tipping points in the climate system.

Paleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation and biological productivity. Paleoceanographic studies using environment models and different proxies enable the scientific community to assess the role of the oceanic processes in the global climate by the re-construction of past climate at various intervals. Paleoceanographic research is also intimately tied to paleoclimatology.

<span class="mw-page-title-main">Antarctic bottom water</span> Cold, dense, water mass originating in the Southern Ocean surrounding Antarctica

The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from −0.8 to 2 °C (35 °F) and absolute salinities from 34.6 to 35.0 g/kg. As the densest water mass of the oceans, AABW is found to occupy the depth range below 4000 m of all ocean basins that have a connection to the Southern Ocean at that level. AABW forms the lower branch of the large-scale movement in the world's oceans through thermohaline circulation.

<span class="mw-page-title-main">Atlantic meridional overturning circulation</span> System of surface and deep currents in the Atlantic Ocean

The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean. It is a component of Earth's ocean circulation system and plays an important role in the climate system. The AMOC includes Atlantic currents at the surface and at great depths that are driven by changes in weather, temperature and salinity. Those currents comprise half of the global thermohaline circulation that includes the flow of major ocean currents, the other half being the Southern Ocean overturning circulation.

<span class="mw-page-title-main">Bølling–Allerød Interstadial</span> Interglacial period about 14,000 years ago

The Bølling–Allerød Interstadial, also called the Late Glacial Interstadial (LGI), was an interstadial period which occurred from 14,690 to c. 12,890 years Before Present, during the final stages of the Last Glacial Period. It was defined by abrupt warming in the Northern Hemisphere, and a corresponding cooling in the Southern Hemisphere, as well as a period of major ice sheet collapse and corresponding sea level rise known as Meltwater pulse 1A. This period was named after two sites in Denmark where paleoclimate evidence for it was first found, in the form of vegetation fossils that could have only survived during a comparatively warm period in Northern Europe. It is also referred to as Interstadial 1 or Dansgaard–Oeschger event 1.

<span class="mw-page-title-main">Ocean heat content</span> Energy stored by oceans

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. Between 1971 and 2018, the rise in ocean heat content accounted for over 90% of Earth's excess energy from global heating. The main driver of this increase was caused by humans via their rising greenhouse gas emissions. By 2020, about one third of the added energy had propagated to depths below 700 meters.

<span class="mw-page-title-main">Tipping points in the climate system</span> Concept in climate science on critical thresholds

In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patterns in the ocean, in ecosystems, and the atmosphere. Examples of tipping points include thawing permafrost, which will release methane, a powerful greenhouse gas, or melting ice sheets and glaciers reducing Earth's albedo, which would warm the planet faster. Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.

<span class="mw-page-title-main">Southern Ocean</span> Ocean around Antarctica

The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of 20,327,000 km2 (7,848,000 sq mi), it is the second-smallest of the five principal oceanic divisions, smaller than the Pacific, Atlantic and Indian oceans, and larger than the Arctic Ocean.

Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum, the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.

<span class="mw-page-title-main">Climate change in Antarctica</span> Impacts of climate change on Antarctica

Climate change caused by greenhouse gas emissions from human activities occurs everywhere on Earth, and while Antarctica is less vulnerable to it than any other continent, climate change in Antarctica has been observed. Since 1959, there has been an average temperature increase of >0.05 °C/decade since 1957 across the continent, although it had been uneven. West Antarctica warmed by over 0.1 °C/decade from the 1950s to the 2000s, and the exposed Antarctic Peninsula has warmed by 3 °C (5.4 °F) since the mid-20th century. The colder, stabler East Antarctica had been experiencing cooling until the 2000s. Around Antarctica, the Southern Ocean has absorbed more oceanic heat than any other ocean, and has seen strong warming at depths below 2,000 m (6,600 ft). Around the West Antarctic, the ocean has warmed by 1 °C (1.8 °F) since 1955.

<span class="mw-page-title-main">Cold blob</span> Cold temperature anomaly North Atlantic surface waters

The cold blob in the North Atlantic describes a cold temperature anomaly of ocean surface waters, affecting the Atlantic Meridional Overturning Circulation (AMOC) which is part of the thermohaline circulation, possibly related to global warming-induced melting of the Greenland ice sheet.

Jean Lynch-Stieglitz is a paleoceanographer known for her research on reconstructing changes in ocean circulation over the last 100,000 years.

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