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87-607: The Flandrian began as the relatively short-lived Younger Dryas climate downturn came to an end. This was the last phase of the Devensian glaciation , the final stage of the Pleistocene epoch. The Flandrian is traditionally seen as the latest warm interglacial in a series that has been occurring throughout the Quaternary geological period . The first part of the Flandrian, known as

174-564: A few decades. Cooling in Greenland was particularly rapid, taking place over just 3 years or less. At the same time, the Southern Hemisphere experienced warming. This period ended as rapidly as it began, with dramatic warming over ~50 years, which transitioned the Earth from the glacial Pleistocene epoch into the current Holocene . The Younger Dryas onset was not fully synchronized; in

261-469: A global climate similar to that of the Last Glacial Maximum . Less orbital eccentricity might have the effect of moderating this temperature downturn. However, orbital cycles are not the only influence on global temperature; atmospheric greenhouse gases also affect the radiative forcing . While there is agreement that post- Industrial Revolution greenhouse gas emissions are substantially warming

348-502: A high latitude volcanic eruption could have accelerated North Atlantic sea ice growth, finally tipping the AMOC sufficiently to cause the Younger Dryas. Cave deposits and glacial ice cores both contain evidence of at least one major volcanic eruption taking place in the northern hemisphere at a time close to Younger Dryas onset, perhaps even completely matching the stalagmite-derived date for

435-501: A pathway along the Mackenzie River in present-day Canada, and sediment cores show that the strongest outburst had occurred right before the onset of Younger Dryas. Other factors are also likely to have played a major role in the Younger Dryas climate. For instance, some research suggests climate in Greenland was primarily affected by the melting of then-present Fennoscandian ice sheet , which could explain why Greenland experienced

522-468: A plant which only thrives in glacial conditions, began to dominate where forests were able to grow during the preceding B-A Interstadial. This makes the Younger Dryas a key example of how biota responded to abrupt climate change . For instance, in what is now New England , cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when

609-528: A southward displacement of Intertropical Convergence Zone . Changes in precipitation under high-emissions scenarios would be far larger. 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

696-613: Is known as overturning . In the Pacific Ocean, the rest of the cold and salty water from the Atlantic undergoes haline forcing, and becomes warmer and fresher more quickly. The out-flowing undersea of cold and salty water makes the sea level of the Atlantic slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher than the Pacific. This generates a large but slow flow of warmer and fresher upper ocean water from

783-771: Is a stub . You can help Misplaced Pages by expanding it . Younger Dryas The Younger Dryas (YD, Greenland Stadial GS-1) was a period in Earth's geologic history that occurred circa 12,900 to 11,700 years Before Present (BP). It is primarily known for the sudden or "abrupt" cooling in the Northern Hemisphere, when the North Atlantic Ocean cooled and annual air temperatures decreased by ~3 °C (5.4 °F) over North America , 2–6 °C (3.6–10.8 °F) in Europe and up to 10 °C (18 °F) in Greenland , in

870-651: Is denser than the NADW, and so flows beneath it. AABW formed in the Weddell Sea will mainly fill the Atlantic and Indian Basins, whereas the AABW formed in the Ross Sea will flow towards the Pacific Ocean. At the Indian Ocean, a vertical exchange of a lower layer of cold and salty water from the Atlantic and the warmer and fresher upper ocean water from the tropical Pacific occurs, in what

957-529: Is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons. The Younger Dryas is often linked to the Neolithic Revolution , with the adoption of agriculture in the Levant . The cold and dry Younger Dryas arguably lowered the carrying capacity of

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1044-444: Is left behind as the sea ice forms around it (pure water preferentially being frozen). Increasing salinity lowers the freezing point of seawater, so cold liquid brine is formed in inclusions within a honeycomb of ice. The brine progressively melts the ice just beneath it, eventually dripping out of the ice matrix and sinking. This process is known as brine rejection . The resulting Antarctic bottom water sinks and flows north and east. It

1131-581: Is outweighed by evaporation , in part due to high windiness. When water evaporates, it leaves salt behind, and so the surface waters of the North Atlantic are particularly salty. North Atlantic is also an already cool region, and evaporative cooling reduces water temperature even further. Thus, this water sinks downward in the Norwegian Sea , fills the Arctic Ocean Basin and spills southwards through

1218-465: Is reconstructed through proxy data such as traces of pollen , ice cores and layers of marine and lake sediments . Collectively, this evidence shows that significant cooling across the Northern Hemisphere began around 12,870 ± 30 years BP. It was particularly severe in Greenland , where temperatures declined by 4–10 °C (7.2–18.0 °F), in an abrupt fashion. Temperatures at the Greenland summit were up to 15 °C (27 °F) colder than at

1305-423: Is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt, coined by climate scientist Wallace Smith Broecker . It is also referred to as the meridional overturning circulation, or MOC . This name is used because not every circulation pattern caused by temperature and salinity gradients is necessarily part of a single global circulation. Further, it is difficult to separate

1392-648: The Nahanagan Stadial , and in Great Britain it has been called the Loch Lomond Stadial . In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a). As with the other geologic periods, paleoclimate during the Younger Dryas

1479-620: The Puerto Princesa cave complex in the Philippines shows that the onset of the Younger Dryas in East Asia was delayed by several hundred years relative to the North Atlantic. Further, the cooling was uniform throughout the year, but had a distinct seasonal pattern. In most places in the Northern Hemisphere, winters became much colder than before, but springs cooled by less, while there was either no temperature change or even slight warming during

1566-462: The Scandinavian ice sheet advanced. Notably, ice sheet advance in this area appears to have begun about 600 years before the global onset of the Younger Dryas. Underwater, the deposits of methane clathrate - methane frozen into ice - remained stable throughout the Younger Dryas, including during the rapid warming as it ended. As the Northern Hemisphere cooled and the Southern Hemisphere warmed,

1653-536: The Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range. Effects in the Rocky Mountain region were varied. Several sites show little to no changes in vegetation. In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places. That

1740-915: The Swiss Alps and the Dinaric Alps in the Balkans , northern ranges of North America's Rocky Mountains , Two Creeks Buried Forest in Wisconsin and western parts of the New York State , and in the Pacific Northwest, including the Cascade Range . The entire Laurentide ice sheet had advanced between west Lake Superior and southeast Quebec , leaving behind a layer of rock debris ( moraine ) dated to this period. Southeastern Alaska appears to have escaped glaciation; speleothem calcite deposition continued in

1827-481: The boreal forests shifted north. Along the southern margins of the Great Lakes, spruce dropped rapidly, while pine increased, and herbaceous prairie vegetation decreased in abundance, but increased west of the region. The central Appalachian Mountains remained forested during the Younger Dryas, but they were covered in spruce and tamarack boreal forests, switching to temperate broadleaf and mixed forests during

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1914-450: The convection of heat could drive deeper currents. In 1908, Johan Sandström performed a series of experiments at a Bornö Marine Research Station which proved that the currents driven by thermal energy transfer exist, but require that "heating occurs at a greater depth than cooling". Normally, the opposite occurs, because ocean water is heated from above by the Sun and becomes less dense, so

2001-528: The density of sea water . Wind-driven surface currents (such as the Gulf Stream ) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water ). This dense water then flows into the ocean basins . While the bulk of it upwells in the Southern Ocean , the oldest waters (with a transit time of about 1000 years) upwell in

2088-624: The thermal equator would have shifted to the south. Because trade winds from either hemisphere cancel each other out above the thermal equator in a calm, heavily clouded area known as the Intertropical Convergence Zone (ITCZ), a change in its position affects wind patterns elsewhere. For instance, in East Africa , the sediments of Lake Tanganyika were mixed less strongly during this period, indicating weaker wind systems in this area. Shifts in atmospheric patterns are believed to be

2175-465: The 21st century and that there was a "high confidence" changes to it would be reversible within centuries if warming was reversed. Unlike the Fifth Assessment Report, it had only "medium confidence" rather than "high confidence" in the AMOC avoiding a collapse before the end of the 21st century. This reduction in confidence was likely influenced by several review studies that draw attention to

2262-463: The 21st century. 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. Furthermore, the largest long-term role in the state of the circulation is played by Antarctic meltwater, and Antarctic ice loss had been the least-certain aspect of future sea level rise projections for

2349-485: The AMOC on timescales of decades or centuries. The Younger Dryas is the best known and best understood because it is the most recent, but it is fundamentally similar to the previous cold phases over the past 120,000 years. This similarity makes the impact hypothesis very unlikely, and it may also contradict the Lake Agassiz hypothesis. On the other hand, some research links volcanism with D–O events, potentially supporting

2436-471: The AMOC. Once the Younger Dryas began, lowered temperatures would have elevated snowfall across the Northern Hemisphere, increasing the ice-albedo feedback . Further, melting snow would be more likely to flood back into the North Atlantic than rainfall would, as less water would be absorbed into the frozen ground. Other modelling shows that sea ice in the Arctic Ocean could have been tens of meters thick by

2523-589: The Earth's radiation budget . Large influxes of low-density meltwater from Lake Agassiz and deglaciation in North America are thought to have led to a shifting of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas . In 2021, the IPCC Sixth Assessment Report again said the AMOC is "very likely" to decline within

2610-587: The Greenland-Scotland-Ridge – crevasses in the submarine sills that connect Greenland , Iceland and Great Britain. It cannot flow towards the Pacific Ocean due to the narrow shallows of the Bering Strait , but it does slowly flow into the deep abyssal plains of the south Atlantic. In the Southern Ocean , strong katabatic winds blowing from the Antarctic continent onto the ice shelves will blow

2697-481: The Holocene. Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene. An increase of pine pollen indicates cooler winters within the central Cascades. Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon 's Klamath Mountains yield evidence of climatic cooling contemporaneous to

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2784-522: The IPCC, the most-likely effects of future AMOC decline are reduced precipitation in mid-latitudes, changing patterns of strong precipitation in the tropics and Europe, and strengthening storms that follow the North Atlantic track. In 2020, research found a weakened AMOC would slow the decline in Arctic sea ice . and result in atmospheric trends similar to those that likely occurred during the Younger Dryas , such as

2871-507: The Marine Isotope Stage 6, ~130,000 years BP), III (the end of Marine Isotope Stage 8, ~243,000 years BP) and Termination IV (the end of Marine Isotope Stage 10, ~337,000 years BP. When combined with additional evidence from ice cores and paleobotanical data, some have argued that YD-like events inevitably occur during every deglaciation. The 2004 film, The Day After Tomorrow depicts catastrophic climatic effects following

2958-460: The North Atlantic, by the UK-US RAPID programme. It combines direct estimates of ocean transport using current meters and subsea cable measurements with estimates of the geostrophic current from temperature and salinity measurements to provide continuous, full-depth, basin-wide estimates of the meridional overturning circulation. However, it has only been operating since 2004, which is too short when

3045-558: The North Pacific, using as evidence the high values of silicon found in these waters. Other investigators have not found such clear evidence. Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean, associated with the strong winds in the open latitudes between South America and Antarctica. Direct estimates of the strength of the thermohaline circulation have also been made at 26.5°N in

3132-462: 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 (in the form of heat) and mass (dissolved solids and gases) around the globe. As such, the state of the circulation has a large impact on the climate of the Earth. The thermohaline circulation

3219-470: The Northern Hemisphere, the length of the growing season declined. Land ice cover experienced little net change, but sea ice extent had increased, contributing to ice–albedo feedback . This increase in albedo was the main reason for net global cooling of 0.6 °C (1.1 °F). During the preceding period, the Bølling–Allerød Interstadial , rapid warming in the Northern Hemisphere

3306-657: The Younger Atlantic, was a period of fairly rapid sea level rise , known as the Flandrian transgression . It is associated with the melting of the Fenno-Scandian , Scottish , Laurentide and Cordilleran glaciers. Fjords were formed during the Flandrian transgression when U-shaped glaciated valleys were inundated. Milankovitch theory alone would forecast that the present Flandrian climate, like that of other interstadials , should eventually decline in temperature, towards

3393-481: The Younger Dryas with a significant reduction or shutdown of the thermohaline circulation , which circulates warm tropical waters northward through the Atlantic meridional overturning circulation (AMOC). This is consistent with climate model simulations, as well as a range of proxy evidence, such as the decreased ventilation (exposure to oxygen from the surface) of the lowest layers of North Atlantic water. Cores from

3480-463: The Younger Dryas. On the Olympic Peninsula, a mid-elevation site recorded a decrease in fire, but forest persisted and erosion increased during the Younger Dryas, which suggests cool and wet conditions. Speleothem records indicate an increase in precipitation in southern Oregon, the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin. Pollen record from

3567-614: The amount of dust blown by wind had also increased. Other areas became wetter including northern China (possibly excepting the Shanxi region) The Younger Dryas was initially discovered around the start of the 20th century, through paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, particularly the Allerød clay pit in Denmark. The analysis of fossilized pollen had consistently shown how Dryas octopetala ,

Flandrian interglacial - Misplaced Pages Continue

3654-500: The area and forced the sedentary early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While relative consensus exists regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated. The scientific consensus links

3741-405: The circulation stability bias within general circulation models , and simplified ocean-modelling studies suggesting the AMOC may be more vulnerable to abrupt change than larger-scale models suggest. As of 2024 , there is no consensus on whether a consistent slowing of the AMOC circulation has occurred but there is little doubt it will occur in the event of continued climate change. According to

3828-408: The circulation, which was established in 1960 by Henry Stommel and Arnold B. Arons. They have chemical, temperature and isotopic ratio signatures (such as Pa / Th ratios) which can be traced, their flow rate calculated, and their age determined. NADW is formed because North Atlantic is a rare place in the ocean where precipitation , which adds fresh water to the ocean and so reduces its salinity,

3915-426: The climate system . The hemisphere which experiences the collapse of its circulation would experience less precipitation and become drier, while the other hemisphere would become wetter. Marine ecosystems are also likely to receive fewer nutrients and experience greater ocean deoxygenation . In the Northern Hemisphere, AMOC's collapse would also substantially lower the temperatures in many European countries, while

4002-557: The coastal waters. It was originally hypothesized that the massive outburst from paleohistorical Lake Agassiz had flooded the North Atlantic via the Saint Lawrence Seaway , but little geological evidence had been found. For instance, the salinity in the Saint Lawrence Seaway did not decline, as would have been expected from massive quantities of meltwater. More recent research instead shows that floodwaters followed

4089-736: The continental interior. The Southeastern United States became warmer and wetter than before. There was warming in and around the Caribbean Sea , and in West Africa . It was once believed that the Younger Dryas cooling started at around the same time across the Northern Hemisphere. However, varve (sedimentary rock) analysis carried out in 2015 suggested that the cooling proceeded in two stages: first along latitude 56–54°N, 12,900–13,100 years ago, and then further north, 12,600–12,750 years ago. Evidence from Lake Suigetsu cores in Japan and

4176-477: The cooling to the impact of a disintegrating comet or asteroid. Because there is no impact crater dating to the Younger Dryas period, the proponents usually suggest the impact had struck the Laurentide ice sheet , so that the crater would have disappeared when the ice sheet melted during the Holocene, or that it was an airburst, which would only leave micro- and nanoparticles behind as evidence. Most experts reject

4263-404: The date of the eruption back to 13,006 years BP, or over a century before the Younger Dryas began. This analysis was also challenged in 2023, with some researchers suggesting that the radiocarbon analysis was tainted by magmatic carbon dioxide. For now, the debate continues without a conclusive proof or rejection of the volcanic hypothesis. The Younger Dryas impact hypothesis (YDIH) attributes

4350-624: The disruption of the North Atlantic Ocean circulation that results in a series of extreme weather events that create an abrupt climate change that leads to a new ice age . Thermohaline 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

4437-411: The east coast of North America would experience accelerated sea level rise . The collapse of either circulation is generally believed to be more than a century away and may only occur under high warming, but there is a lot of uncertainty about these projections. It has long been known that wind can drive ocean currents, but only at the surface. In the 19th century, some oceanographers suggested that

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4524-626: The eastern and central areas. While the Pacific Northwest region cooled by 2–3 °C (3.6–5.4 °F), cooling in western North America was generally less intense. While the Orca Basin in the Gulf of Mexico still experienced a drop in sea surface temperature of 2.4 ± 0.6°C, land areas closer to it, such as Texas , the Grand Canyon area and New Mexico , ultimately did not cool as much as

4611-482: The exception was in tropical Atlantic areas such as Costa Rica , where temperature change was similar to Greenland's. The Holocene warming then proceeded across the globe, following an increase in carbon dioxide levels during the YD period (from ~210 ppm to ~275 ppm ). Younger Dryas cooling was often accompanied by glacier advance and lowering of the regional snow line , with evidence found in areas such as Scandinavia,

4698-474: The freezing point. That freezing point is also lower than for fresh water due to salinity, and can be below −2 °C, depending on salinity and pressure. These density differences caused by temperature and salinity ultimately separate ocean water into distinct water masses , such as the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). These two waters are the main drivers of

4785-479: The glaciers retreat, and it drops if glaciers grow. Altogether, there appears to have been little change in sea level throughout the Younger Dryas. This is in contrast to rapid increases before and after, such as the Meltwater Pulse 1A . On the coasts, glacier advance and retreat also affects relative sea level . Western Norway experienced a relative sea level rise of 10 m ( 32 + 2 ⁄ 3  ft) as

4872-530: The human population lives in the Northern Hemisphere , the AMOC has been far better studied, but both are very important for the global climate. Both of them also appear to be slowing down due to climate change , as the melting of the ice sheets dilutes salty flows such as the Antarctic bottom water . Either one could outright collapse to a much weaker state, which would be an example of tipping points in

4959-529: The hypothesis, and argue that all of the microparticles are adequately explained by the terrestrial processes. For instance, mineral inclusions from YD-period sediments in Hall's Cave, Texas, have been interpreted by YDIH proponents as extraterrestrial in origin, but a paper published in 2020 argues that they are more likely to be volcanic. Opponents argue that there is no evidence for massive wildfires which would have been caused by an airburst of sufficient size to affect

5046-417: The lack of sea level rise during this period, so other theories have also emerged. An extraterrestrial impact into the Laurentide ice sheet (where it would have left no impact crater) was proposed as an explanation, but this hypothesis has numerous issues and no support from mainstream science. A volcanic eruption as an initial trigger for cooling and sea ice growth has been proposed more recently, and

5133-410: The lack of sea level rise during the Younger Dryas onset by connecting it with a volcanic eruption. Eruptions often deposit large quantities of sulfur dioxide particles in the atmosphere, where they are known as aerosols , and can have a large cooling effect by reflecting sunlight. This phenomenon can also be caused by anthropogenic sulfur pollution, where it is known as global dimming . Cooling from

5220-489: The last and the strongest of these events. However, there is some debate over what caused the AMOC to become so weak in the first place. The hypothesis historically most supported by scientists was an interruption from an influx of fresh, cold water from North America's Lake Agassiz into the Atlantic Ocean. While there is evidence of meltwater travelling via the Mackenzie River , this hypothesis may not be consistent with

5307-546: The main reason why Northern Hemisphere summers generally did not cool during the Younger Dryas. Since winds carry moisture in the form of clouds, these changes also affect precipitation . Thus, evidence from the pollen record shows that some areas have become very arid, including Scotland, the North American Midwest , Anatolia and southern China . As North Africa, including the Sahara Desert , became drier,

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5394-455: The most abrupt climatic changes during the YD. Climate models also indicate that a single freshwater outburst, no matter how large, would not have been able to weaken the AMOC for over 1,000 years, as required by the Younger Dryas timeline, unless other factors were also involved. Some modelling explains this by showing that the melting of Laurentide Ice Sheet led to greater rainfall over the Atlantic Ocean, freshening it and so helping to weaken

5481-442: The newly formed sea ice away, opening polynyas in locations such as Weddell and Ross Seas , off the Adélie Coast and by Cape Darnley . The ocean, no longer protected by sea ice, suffers a brutal and strong cooling (see polynya ). Meanwhile, sea ice starts reforming, so the surface waters also get saltier, hence very dense. In fact, the formation of sea ice contributes to an increase in surface seawater salinity; saltier brine

5568-428: The ocean due to stronger westerlies , freshening the Southern Ocean further. 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

5655-460: The ocean floor, providing a continuous thermohaline circulation. As the deep waters sink into the ocean basins, they displace the older deep-water masses, which gradually become less dense due to continued ocean mixing. Thus, some water is rising, in what is known as upwelling . Its speeds are very slow even compared to the movement of the bottom water masses. It is therefore difficult to measure where upwelling occurs using current speeds, given all

5742-409: The onset of the Younger Dryas event. It has been suggested that this eruption would have been stronger than any during the Common Era , some of which have been able to cause several decades of cooling. According to 1990s research, the Laacher See eruption (present-day volcanic lake in Rhineland-Palatinate , Germany ) would have matched the criteria, but radiocarbon dating done in 2021 pushes

5829-408: The onset of the Younger Dryas, so that it would have been able to shed icebergs into the North Atlantic, which would have been able to weaken the circulation consistently. Notably, changes in sea ice cover would have had no impact on sea levels, which is consistent with the absence of significant sea level rise during the Younger Dryas, and particularly during its onset. Some scientists also explain

5916-463: The opposite happens when it is weak. The scientific consensus is that severe AMOC weakening explains the climatic effects of the Younger Dryas. It also explains why the Holocene warming had proceeded so rapidly once the AMOC change was no longer counteracting the increase in carbon dioxide levels. AMOC weakening causing polar seesaw effects is also consistent with the accepted explanation for Dansgaard–Oeschger events , with YD likely to have been

6003-408: The other wind-driven processes going on in the surface ocean. Deep waters have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth. A number of scientists have tried to use these tracers to infer where the upwelling occurs. Wallace Broecker , using box models, has asserted that the bulk of deep upwelling occurs in

6090-421: The parts of the circulation driven by temperature and salinity alone from those driven by other factors, such as the wind and tidal forces . This global circulation has two major limbs - Atlantic meridional overturning circulation ( AMOC ), centered in the north Atlantic Ocean, and Southern Ocean overturning circulation or Southern Ocean meridional circulation ( SMOC ), around Antarctica . Because 90% of

6177-445: The planet, there is debate over whether early agriculture , beginning thousands of years earlier, has had a much smaller warming effect (due to methane emissions from rice paddies , or deforestation, for instance). If this is the case, the climate of at least the later Holocene has long deviated from what would be expected with only orbital forcings, and the Flandrian has long been an atypical interglacial. This glaciology article

6264-481: The presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores and cave deposits. The Younger Dryas is named after the alpine – tundra wildflower Dryas octopetala , because its fossils are abundant in the European (particularly Scandinavian ) sediments dating to this timeframe. The two earlier geologic time intervals where this flower

6351-454: The region despite being retarded, indicating the absence of permafrost and glaciation. On the other hand, the warming of the Southern Hemisphere led to ice loss in Antarctica, South America and New Zealand. Moreover, while Greenland as a whole had cooled, glaciers had only grown in the north of the island, and they had retreated from the rest of Greenland's coasts. This was likely driven by

6438-462: The role of salinity in ocean layer formation. Salinity is important because like temperature, it affects water density . Water becomes less dense as its temperature increases and the distance between its molecules expands, but more dense as the salinity increases, since there is a larger mass of salts dissolved within that water. Further, while fresh water is at its most dense at 4 °C, seawater only gets denser as it cools, up until it reaches

6525-412: The start of the 21st century. Strong cooling of around 2–6 °C (3.6–10.8 °F) had also taken place in Europe. Icefields and glaciers formed in upland areas of Great Britain , while many lowland areas developed permafrost , implying a cooling of −5 °C (23 °F) and a mean annual temperature no higher than −1 °C (30 °F). North America also became colder, particularly in

6612-544: The strengthened Irminger Current . The Jabllanica mountain range in the Balkans also experienced ice loss and glacial retreat: this was likely caused by the drop in annual precipitation, which would have otherwise frozen and helped to maintain the glaciers. Unlike now, the glaciers were still present in northern Scotland , but they had thinned during the Younger Dryas. The amount of water contained within glaciers directly influences global sea levels - sea level rise occurs if

6699-498: The summer. An exception appears to have taken place in what is now Maine , where winter temperatures remained stable, yet summer temperatures decreased by up to 7.5 °C (13.5 °F). While the Northern Hemisphere cooled, considerable warming occurred in the Southern Hemisphere. Sea surface temperatures were warmer by 0.3–1.9 °C (0.54–3.42 °F), and Antarctica , South America (south of Venezuela ) and New Zealand all experienced warming. The net temperature change

6786-411: The surface layer floats on the surface above the cooler, denser layers, resulting in ocean stratification . However, wind and tides cause mixing between these water layers, with diapycnal mixing caused by tidal currents being one example. This mixing is what enables the convection between ocean layers, and thus, deep water currents. In the 1920s, Sandström's framework was expanded by accounting for

6873-408: The thermohaline circulation, mineralogical and geochemical evidence or for simultaneous human population declines and mass animal extinctions which would have been required by this hypothesis. Statistical analysis shows that the Younger Dryas is merely the last of 25 or 26 Dansgaard–Oeschger events (D–O events) over the past 120,000 years. These episodes are characterized by abrupt changes in

6960-414: The timescale of the circulation is measured in centuries. The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean. Changes in the thermohaline circulation are thought to have significant impacts on

7047-523: The tropical Pacific to the Indian Ocean through the Indonesian Archipelago to replace the cold and salty Antarctic Bottom Water . This is also known as 'haline forcing' (net high latitude freshwater gain and low latitude evaporation). This warmer, fresher water from the Pacific flows up through the South Atlantic to Greenland , where it cools off and undergoes evaporative cooling and sinks to

7134-451: The tropics, the cooling was spread out over several centuries, and the same was true of the early-Holocene warming. Even in the Northern Hemisphere, temperature change was highly seasonal, with much colder winters, cooler springs, yet no change or even slight warming during the summer. Substantial changes in precipitation also took place, with cooler areas experiencing substantially lower rainfall, while warmer areas received more of it. In

7221-534: The volcanic hypothesis. Events similar to the Younger Dryas appear to have occurred during the other terminations - a term used to describe a comparatively rapid transition from cold glacial conditions to warm interglacials. The analysis of lake and marine sediments can reconstruct past temperatures from the presence or absence of certain lipids and long chain alkenones , as these molecules are very sensitive to temperature. This analysis provides evidence for YD-like events during Termination II (the end of

7308-547: The western subtropical North Atlantic show that the "bottom water" lingered there for 1,000 years, twice the age of Late Holocene bottom waters from the same site around 1,500 BP. Further, the otherwise anomalous warming of the southeastern United States matches the hypothesis that as the AMOC weakened and transported less heat from the Caribbean towards Europe through the North Atlantic Gyre , more of it would stay trapped in

7395-603: Was a relatively modest cooling of 0.6 °C (1.1 °F). Temperature changes of the Younger Dryas lasted 1,150–1,300 years. According to the International Commission on Stratigraphy , the Younger Dryas ended around 11,700 years ago, although some research places it closer to 11,550 years ago. The end of Younger Dryas was also abrupt: in previously cooled areas, warming to previous levels took place over 50–60 years. The tropics experienced more gradual temperature recovery over several centuries;

7482-616: Was abundant in Europe are the Oldest Dryas (approx. 18,500-14,000 BP) and Older Dryas (~14,050–13,900 BP), respectively. On the contrary, Dryas octopetala was rare during the Bølling–Allerød Interstadial . Instead, European temperatures were warm enough to support trees in Scandinavia, as seen at the Bølling and Allerød sites in Denmark . In Ireland , the Younger Dryas has also been known as

7569-459: Was offset by the equivalent cooling in the Southern Hemisphere. This "polar seesaw" pattern is consistent with changes in thermohaline circulation (particularly the Atlantic meridional overturning circulation or AMOC), which greatly affects how much heat is able to go from the Southern Hemisphere to the North. The Southern Hemisphere cools and the Northern Hemisphere warms when the AMOC is strong, and

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