Upper Dryas
   HOME

TheInfoList



OR:

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 Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...
(BP). It is primarily known for the sudden or "abrupt" cooling in the Northern Hemisphere, when the North
Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...
cooled and annual air temperatures decreased by ~ over
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
, in Europe and up to in
Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
, in 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, the transition from the glacial
Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...
epoch into the current
Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
. The Younger Dryas onset was not fully synchronized; in 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 In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
also took place, with cooler areas experiencing substantially lower rainfall, while warmer areas received more of it. In the Northern Hemisphere, the length of the
growing season A season is a division of the year marked by changes in weather, ecology, and the amount of daylight. The growing season is that portion of the year in which local conditions (i.e. rainfall, temperature, daylight) permit normal plant growth. Whi ...
declined. Land ice cover experienced little net change, but
sea ice Sea ice arises as seawater freezes. Because ice is less density, dense than water, it floats on the ocean's surface (as does fresh water ice). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world' ...
extent had increased, contributing to
ice–albedo feedback Ice–albedo feedback is a climate change feedback, where a change in the area of ice caps, glaciers, and sea ice alters the albedo and surface temperature of a planet. Because ice is very reflective, it reflects far more solar energy back to spac ...
. This increase in albedo was the main reason for net global cooling of . During the preceding period, the
Bølling–Allerød Interstadial The Bølling–Allerød Interstadial (), also called the Late Glacial Interstadial (LGI), was an interstadial period which occurred from 14,690 to years Before Present, during the final stages of the Last Glacial Period. It was defined by abrupt ...
, rapid warming in the Northern Hemisphere was offset by the equivalent cooling in the Southern Hemisphere. This "polar seesaw" pattern is consistent with changes in
thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes. The name ''thermohaline'' is derived from ''wikt:thermo-, thermo-'', r ...
(particularly the
Atlantic meridional overturning circulation The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean.IPCC, 2021Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Sem ...
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 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 Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
levels. AMOC weakening causing polar seesaw effects is also consistent with the accepted explanation for
Dansgaard–Oeschger event A Dansgaard–Oeschger event (often abbreviated D–O event), is a rapid climate fluctuation; such events occurred 25 times during the last glacial period. Some scientists say that the events occur quasi-periodically with a recurrence time being ...
s, with YD likely to have been 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 Lake Agassiz ( ) was a large proglacial lake that existed in central North America during the late Pleistocene, fed by meltwater from the retreating Laurentide Ice Sheet at the end of the last glacial period. At its peak, the lake's area wa ...
into the Atlantic Ocean. While there is evidence of meltwater travelling via the
Mackenzie River The Mackenzie River (French: ; Slavey language, Slavey: ' èh tʃʰò literally ''big river''; Inuvialuktun: ' uːkpɑk literally ''great river'') is a river in the Canadian Canadian boreal forest, boreal forest and tundra. It forms, ...
, this hypothesis may not be consistent with the lack of
sea level rise The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
during this period, so other theories have also emerged. Another proposed explanation is an extraterrestrial impact, but this is rejected by most experts. A volcanic eruption as an initial trigger for cooling and sea ice growth has been proposed more recently, and 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.


Etymology

The Younger Dryas is named after the alpine
tundra In physical geography, a tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. There are three regions and associated types of tundra: #Arctic, Arctic, Alpine tundra, Alpine, and #Antarctic ...
wildflower ''
Dryas octopetala ''Dryas octopetala'', the mountain avens, eightpetal mountain-avens, white dryas or white dryad, is an Arctic–alpine flowering plant in the family Rosaceae. It is a small prostrate evergreen subshrub forming large colonies. The specific epithe ...
'', because its
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
s are abundant in the European (particularly
Scandinavia Scandinavia is a subregion#Europe, subregion of northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. ''Scandinavia'' most commonly refers to Denmark, Norway, and Sweden. It can sometimes also ...
n) sediments dating to this timeframe. The two earlier geologic time intervals where this flower 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 The Bølling–Allerød Interstadial (), also called the Late Glacial Interstadial (LGI), was an interstadial period which occurred from 14,690 to years Before Present, during the final stages of the Last Glacial Period. It was defined by abrupt ...
. Instead, European temperatures were warm enough to support trees in Scandinavia, as seen at the Bølling and Allerød sites in
Denmark Denmark is a Nordic countries, Nordic country in Northern Europe. It is the metropole and most populous constituent of the Kingdom of Denmark,, . also known as the Danish Realm, a constitutionally unitary state that includes the Autonomous a ...
. In
Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...
, the Younger Dryas has also been known as the Nahanagan Stadial, and in Great Britain it has been called the Loch Lomond Stadial. In the
Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
Summit
ice core An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier ...
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).


Climate

As with the other geologic periods,
paleoclimate Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...
during the Younger Dryas is reconstructed through proxy data such as traces of
pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
,
ice core An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier ...
s and layers of marine and lake
sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
s. Collectively, this evidence shows that significant cooling across the Northern Hemisphere began around 12,870 ± 30 years BP. It was particularly severe in
Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
, where temperatures declined by , in an abrupt fashion. Temperatures at the Greenland summit were up to colder than at the start of the 21st century. Strong cooling of around had also taken place in Europe. Icefields and
glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...
s formed in upland areas of
Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...
, while many lowland areas developed
permafrost Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...
, implying a cooling of and a mean annual temperature no higher than .
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
also became colder, particularly in the eastern and central areas. While the
Pacific Northwest The Pacific Northwest (PNW; ) is a geographic region in Western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though no official boundary exists, the most common ...
region cooled by , cooling in western North America was generally less intense. While the Orca Basin in the
Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...
still experienced a drop in
sea surface temperature Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...
of 2.4 ± 0.6°C, land areas closer to it, such as
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
, the
Grand Canyon The Grand Canyon is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a mile (). The canyon and adjacent rim are contained within Grand Canyon Nati ...
area and
New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
, ultimately did not cool as much as the continental interior. The
Southeastern United States The Southeastern United States, also known as the American Southeast or simply the Southeast, is a geographical List of regions in the United States, region of the United States located in the eastern portion of the Southern United States and t ...
became warmer and wetter than before. There was warming in and around the
Caribbean Sea The Caribbean Sea is a sea of the Atlantic Ocean, North Atlantic Ocean in the tropics of the Western Hemisphere, located south of the Gulf of Mexico and southwest of the Sargasso Sea. It is bounded by the Greater Antilles to the north from Cuba ...
, and in
West Africa West Africa, also known as Western Africa, is the westernmost region of Africa. The United Nations geoscheme for Africa#Western Africa, United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Gha ...
. It was once believed that the Younger Dryas cooling started at around the same time across the Northern Hemisphere. However,
varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. Of the many rhythmites in the geological record ...
(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 Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
and the Puerto Princesa cave complex in the
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
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 the summer. An exception appears to have taken place in what is now
Maine Maine ( ) is a U.S. state, state in the New England region of the United States, and the northeasternmost state in the Contiguous United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Provinces and ...
, where winter temperatures remained stable, yet summer temperatures decreased by up to . While the Northern Hemisphere cooled, considerable warming occurred in the Southern Hemisphere. Sea surface temperatures were warmer by , and
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
,
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
(south of
Venezuela Venezuela, officially the Bolivarian Republic of Venezuela, is a country on the northern coast of South America, consisting of a continental landmass and many Federal Dependencies of Venezuela, islands and islets in the Caribbean Sea. It com ...
) and
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
all experienced warming. The net temperature change was a relatively modest cooling of . Temperature changes of the Younger Dryas lasted 1,150–1,300 years. According to the
International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes unofficially referred to as the International Stratigraphic Commission, is a daughter or major subcommittee grade scientific organization that concerns itself with stratigraphy, strati ...
, 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; the exception was in tropical Atlantic areas such as
Costa Rica Costa Rica, officially the Republic of Costa Rica, is a country in Central America. It borders Nicaragua to the north, the Caribbean Sea to the northeast, Panama to the southeast, and the Pacific Ocean to the southwest, as well as Maritime bo ...
, where temperature change was similar to Greenland's. The
Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
warming then proceeded across the globe, following an increase in
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
levels during the YD period (from ~210 ppm to ~275 ppm).


Ice cover

Younger Dryas cooling was often accompanied by
glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...
advance and lowering of the regional
snow line The climatic snow line is the boundary between a snow-covered and snow-free surface. The actual snow line may adjust seasonally, and be either significantly higher in elevation, or lower. The permanent snow line is the level above which snow wil ...
, with evidence found in areas such as Scandinavia, Björck, S. (2007) ''Younger Dryas oscillation, global evidence.'' In S. A. Elias, (Ed.): ''Encyclopedia of Quaternary Science,'' Volume 3, pp. 1987–1994. Elsevier B.V., Oxford. the
Swiss Alps The Alps, Alpine region of Switzerland, conventionally referred to as the Swiss Alps, represents a major natural feature of the country and is, along with the Swiss Plateau and the Swiss portion of the Jura Mountains, one of its three main Physica ...
and the
Dinaric Alps The Dinaric Alps (), also Dinarides, are a mountain range in Southern Europe, Southern and Southcentral Europe, separating the continental Balkan Peninsula from the Adriatic Sea. They stretch from Italy in the northwest through Slovenia, Croatia ...
in the
Balkans The Balkans ( , ), corresponding partially with the Balkan Peninsula, is a geographical area in southeastern Europe with various geographical and historical definitions. The region takes its name from the Balkan Mountains that stretch throug ...
, northern ranges of North America's
Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...
, Two Creeks Buried Forest in
Wisconsin Wisconsin ( ) is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest of the United States. It borders Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michig ...
and western parts of the
New York State New York, also called New York State, is a state in the northeastern United States. Bordered by New England to the east, Canada to the north, and Pennsylvania and New Jersey to the south, its territory extends into both the Atlantic Ocean and ...
, and in the Pacific Northwest, including the
Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...
. The entire Laurentide ice sheet had advanced between west
Lake Superior Lake Superior is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. Lake Michigan–Huron has a larger combined surface area than Superior, but is normally considered tw ...
and southeast
Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...
, leaving behind a layer of rock debris (
moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a gla ...
) dated to this period. Southeastern Alaska appears to have escaped glaciation; speleothem calcite deposition continued in 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 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 Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
, but they had thinned during the Younger Dryas. The amount of water contained within glaciers directly influences global sea levels -
sea level rise The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
occurs if 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 Meltwater pulse 1A (MWP1a) is the name used by Quaternary geologists, paleoclimatologists, and oceanographers for a period of rapid post-glacial sea level rise, between 14,700 and 13,500 years ago, during which the global sea level rose betwee ...
. On the coasts, glacier advance and retreat also affects
relative sea level Relative sea level (RSL) is defined as the sea level that is observed with respect to a land-based reference frame. It is often contrasted with eustatic sea level, which is a measure of the total mass or volume of the oceans. Relative sea level can ...
. Western
Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...
experienced a relative sea level rise of as 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 clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large a ...
- methane frozen into ice - remained stable throughout the Younger Dryas, including during the rapid warming as it ended.


Weather systems

As the Northern Hemisphere cooled and the Southern Hemisphere warmed, the
thermal equator The thermal equator (also known as "the heat equator") is a belt encircling Earth, defined by the set of locations having the highest mean annual temperature at each longitude around the globe. Because local temperatures are sensitive to the geo ...
would have shifted to the south. Because
trade winds The trade winds or easterlies are permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere ...
from either hemisphere cancel each other out above the thermal equator in a calm, heavily clouded area known as the
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ , or ICZ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the t ...
(ITCZ), a change in its position affects wind patterns elsewhere. For instance, in
East Africa East Africa, also known as Eastern Africa or the East of Africa, is a region at the eastern edge of the Africa, African continent, distinguished by its unique geographical, historical, and cultural landscape. Defined in varying scopes, the regi ...
, the sediments of
Lake Tanganyika Lake Tanganyika ( ; ) is an African Great Lakes, African Great Lake. It is the world's List of lakes by volume, second-largest freshwater lake by volume and the List of lakes by depth, second deepest, in both cases after Lake Baikal in Siberia. ...
were mixed less strongly during this period, indicating weaker wind systems in this area. Shifts in atmospheric patterns are believed to be 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 In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
. Thus, evidence from the pollen record shows that some areas have become very arid, including Scotland, the North American
Midwest The Midwestern United States (also referred to as the Midwest, the Heartland or the American Midwest) is one of the four census regions defined by the United States Census Bureau. It occupies the northern central part of the United States. It ...
,
Anatolia Anatolia (), also known as Asia Minor, is a peninsula in West Asia that makes up the majority of the land area of Turkey. It is the westernmost protrusion of Asia and is geographically bounded by the Mediterranean Sea to the south, the Aegean ...
and southern
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
. As North Africa, including the
Sahara Desert The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...
, became drier, the amount of dust blown by wind had also increased. Other areas became wetter including northern China (possibly excepting the
Shanxi Shanxi; Chinese postal romanization, formerly romanised as Shansi is a Provinces of China, province in North China. Its capital and largest city of the province is Taiyuan, while its next most populated prefecture-level cities are Changzhi a ...
region)


Biosphere

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 Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
had consistently shown how ''
Dryas octopetala ''Dryas octopetala'', the mountain avens, eightpetal mountain-avens, white dryas or white dryad, is an Arctic–alpine flowering plant in the family Rosaceae. It is a small prostrate evergreen subshrub forming large colonies. The specific epithe ...
'', 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 Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
. For instance, in what is now
New England New England is a region consisting of six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York (state), New York to the west and by the ...
, cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the
boreal forests Taiga or tayga ( ; , ), also known as boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces, and larches. The taiga, or boreal forest, is the world's largest land biome. In North Ame ...
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 The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...
remained forested during the Younger Dryas, but they were covered in
spruce A spruce is a tree of the genus ''Picea'' ( ), a genus of about 40 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal ecosystem, boreal (taiga) regions of the Northern hemisphere. ''Picea'' ...
and
tamarack ''Larix laricina'', commonly known as the tamarack, hackmatack, eastern larch, black larch, red larch, or American larch, is a species of larch native to Canada, from eastern Yukon and Inuvik, Northwest Territories east to Newfoundland, and als ...
boreal forests, switching to
temperate broadleaf and mixed forest Temperate broadleaf and mixed forest is a temperate climate terrestrial habitat type defined by the World Wide Fund for Nature, with broadleaf tree ecoregions, and with conifer and broadleaf tree mixed coniferous forest ecoregions. Thes ...
s during the Holocene. Conversely, pollen and
macrofossil Macrofossils, also known as megafossils, are the preserved remnants of organic beings and their activities that are large enough to be visible without a microscope. The term ''macrofossil'' stands in opposition to the term microfossil. Microfoss ...
evidence from near
Lake Ontario Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north, west, and southwest by the Canadian province of Ontario, and on the south and east by the U.S. state of New York (state), New York. The Canada–United Sta ...
indicates that cool, boreal forests persisted into the early Holocene. An increase of pine pollen indicates cooler winters within the central Cascades.
Speleothems A speleothem (; ) is a geological formation made by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depend ...
from the Oregon Caves National Monument and Preserve in southern
Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
's
Klamath Mountains The Klamath Mountains are a rugged and lightly populated mountain range in northwestern California and southwestern Oregon in the western United States. As a mountain system within both the greater Pacific Coast Ranges and the California Coast R ...
yield evidence of climatic cooling contemporaneous to 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 the
Siskiyou Mountains The Siskiyou Mountains are a Coast Ranges, coastal subrange of the Klamath Mountains, and located in northwestern California and southwestern Oregon in the United States. They extend in an arc for approximately from east of Crescent City, Calif ...
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 The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of Western Canada, to New Mexico in ...
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 Montane ecosystems are found on the slopes of mountains. The alpine climate in these regions strongly affects the ecosystem because temperatures fall as elevation increases, causing the ecosystem to stratify. This stratification is a crucial f ...
parkland in places. That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer
insolation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...
as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons.


Human societies

Northwestern Europe faced a significant population reduction during the first half of the Younger Dryas. The Younger Dryas is often linked to the
Neolithic Revolution The Neolithic Revolution, also known as the First Agricultural Revolution, was the wide-scale transition of many human cultures during the Neolithic period in Afro-Eurasia from a lifestyle of hunter-gatherer, hunting and gathering to one of a ...
, with the adoption of agriculture in the
Levant The Levant ( ) is the subregion that borders the Eastern Mediterranean, Eastern Mediterranean sea to the west, and forms the core of West Asia and the political term, Middle East, ''Middle East''. In its narrowest sense, which is in use toda ...
. . The cold and dry Younger Dryas arguably lowered the
carrying capacity The carrying capacity of an ecosystem is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as the ...
of the area and forced the sedentary early
Natufian The Natufian culture ( ) is an archaeological culture of the late Epipalaeolithic Near East in West Asia from 15–11,500 Before Present. The culture was unusual in that it supported a sedentism, sedentary or semi-sedentary population even befor ...
population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about
cereal A cereal is a grass cultivated for its edible grain. Cereals are the world's largest crops, and are therefore staple foods. They include rice, wheat, rye, oats, barley, millet, and maize ( Corn). Edible grains from other plant families, ...
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.


Cause

The scientific consensus links the Younger Dryas with a significant reduction or shutdown of the
thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes. The name ''thermohaline'' is derived from ''wikt:thermo-, thermo-'', r ...
, which circulates warm tropical waters northward through the
Atlantic meridional overturning circulation The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean.IPCC, 2021Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Sem ...
(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 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 The North Atlantic Gyre of the Atlantic Ocean is one of five great oceanic gyres. It is a circular ocean current, with offshoot eddies and sub-gyres, across the North Atlantic from the Intertropical Convergence Zone (calms or doldrums) to the pa ...
, more of it would stay trapped in the coastal waters. It was originally hypothesized that the massive outburst from paleohistorical
Lake Agassiz Lake Agassiz ( ) was a large proglacial lake that existed in central North America during the late Pleistocene, fed by meltwater from the retreating Laurentide Ice Sheet at the end of the last glacial period. At its peak, the lake's area wa ...
had flooded the North Atlantic via the
Saint Lawrence Seaway The St. Lawrence Seaway () is a system of rivers, locks, canals and channels in Eastern Canada and Northern United States that permits oceangoing vessels to travel from the Atlantic Ocean to the Great Lakes of North America, as far inland ...
, but little geological evidence for this hypothesis has been found. For instance, the
salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
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 a pathway along the
Mackenzie River The Mackenzie River (French: ; Slavey language, Slavey: ' èh tʃʰò literally ''big river''; Inuvialuktun: ' uːkpɑk literally ''great river'') is a river in the Canadian Canadian boreal forest, boreal forest and tundra. It forms, ...
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 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 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 Sea ice arises as seawater freezes. Because ice is less density, dense than water, it floats on the ocean's surface (as does fresh water ice). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world' ...
in the Arctic Ocean could have been tens of meters thick by 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 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 Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
particles in the atmosphere, where they are known as
aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...
s, 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 Global dimming is a decline in the amount of sunlight reaching the Earth's surface. It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution. Global dimming was observed soon after t ...
. Cooling from 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 the onset of the Younger Dryas event. It has been suggested that this eruption would have been stronger than any during the
Common Era Common Era (CE) and Before the Common Era (BCE) are year notations for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Common Era and Before the Common Era are alternatives to the ...
, some of which have been able to cause several decades of cooling. According to 1990s research, the
Laacher See Laacher See (), also known as Lake Laach or Laach Lake, is a volcanic Volcanic crater lake, caldera lake with a diameter of in Rhineland-Palatinate, Germany, about northwest of Koblenz, south of Bonn, and west of Andernach. It is in the Eifel ...
eruption (present-day volcanic lake in
Rhineland-Palatinate Rhineland-Palatinate ( , ; ; ; ) is a western state of Germany. It covers and has about 4.05 million residents. It is the ninth largest and sixth most populous of the sixteen states. Mainz is the capital and largest city. Other cities are ...
,
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
) would have matched the criteria, but
radiocarbon Carbon-14, C-14, C or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic matter is the basis of the radiocarbon dating method pioneered by Willard Libby and coll ...
dating done in 2021 pushes 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.


Younger Dryas impact hypothesis

The
Younger Dryas impact hypothesis The Younger Dryas impact hypothesis (YDIH) proposes that the onset of the Younger Dryas (YD) cool period (stadial) at the end of the Last Glacial Period, around 12,900 years ago was the result of some kind of cosmic event with specific details var ...
(YDIH) attributes the cooling to the impact of a disintegrating comet or asteroid. Because there is no
impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
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 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, while a paper published in 2020 argues that they are more likely to be volcanic. Opponents argue that there is no evidence for the massive wildfires which they assert would have been caused by an airburst of sufficient size to affect the thermohaline circulation, nor any mineralogical or
geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...
evidence for the simultaneous human population declines and mass animal extinctions that would ensue from such an impact.


Similar events

Statistical analysis shows that the Younger Dryas is merely the last of 25 or 26
Dansgaard–Oeschger event A Dansgaard–Oeschger event (often abbreviated D–O event), is a rapid climate fluctuation; such events occurred 25 times during the last glacial period. Some scientists say that the events occur quasi-periodically with a recurrence time being ...
s (D–O events) over the past 120,000 years. These episodes are characterized by abrupt changes in 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 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. Eglinton, G., A.B. Stuart, A. Rosell, M. Sarnthein, U. Pflaumann, and R. Tiedeman (1992) ''Molecular record of secular sea surface temperature changes on 100-year timescales for glacial terminations I, II and IV.'' Nature. 356:423–426. The analysis of lake and marine sediments can reconstruct past temperatures from the presence or absence of certain
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
s 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 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.


In popular culture

The 2004 film, ''
The Day After Tomorrow ''The Day After Tomorrow'' is a 2004 American science fiction disaster film conceived, co-written, co-produced, and directed by Roland Emmerich, based on the 1999 book '' The Coming Global Superstorm'' by Art Bell and Whitley Strieber, and ...
'' depicts catastrophic climatic effects following the disruption of the North Atlantic Ocean circulation that results in a series of
extreme weather Extreme weather includes unexpected, unusual, severe weather, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Extreme events are based on a location's recorded weat ...
events that create an
abrupt 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 ...
that leads to a new
ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and g ...
.


See also

* * * * * * * * Preboreal oscillation – Cooling episode within the preboreal * *


References


External links

* * * * * * * {{Authority control Blytt–Sernander system Nordic Stone Age Palynology Last Glacial Maximum Stone Age Europe 10th millennium BC Historical eras 11th millennium BC Dryas octopetala