The Younger Dryas (YD, Greenland Stadial GS-1) was a time in Earth's history that happened about 12,900 to 11,700 years ago. It is best known for the sudden and quick cooling in the Northern Hemisphere. During this time, the North Atlantic Ocean cooled, and air temperatures dropped by about 3°C (5°F) in North America, 2–6°C (4–11°F) in Europe, and up to 10°C (18°F) in Greenland. This cooling in Greenland happened very fast, within just 3 years or less. At the same time, the Southern Hemisphere got warmer. The Younger Dryas ended quickly, with a rapid warming over about 50 years, marking the start of the current warm period called the Holocene.
The cooling during the Younger Dryas did not happen the same way everywhere. In the tropics, the cooling took place over several centuries, and the warming that followed also happened slowly. Even in the Northern Hemisphere, temperature changes were different in each season. Winters were much colder, and springs were cooler, but summers stayed the same or even got slightly warmer. Rainfall also changed: cooler areas had less rain, while warmer areas had more. In the Northern Hemisphere, the time when plants could grow each year became shorter. Land ice did not change much, but sea ice increased, which helped cool the planet by reflecting more sunlight. This increase in reflected sunlight was the main reason for a global cooling of 0.6°C (1.1°F).
Before the Younger Dryas, a warm period called the Bølling–Allerød Interstadial occurred. During this time, the Northern Hemisphere warmed quickly, while the Southern Hemisphere cooled. This pattern, called the "polar seesaw," is linked to ocean currents that move heat, especially the Atlantic meridional overturning circulation (AMOC). When AMOC is strong, the Northern Hemisphere warms, and the Southern Hemisphere cools. When AMOC is weak, the opposite happens. Scientists believe that a major weakening of AMOC explains the cooling during the Younger Dryas. This also explains why warming during the Holocene happened so quickly once AMOC changes no longer blocked the effects of rising carbon dioxide levels.
AMOC weakening and the polar seesaw pattern are also connected to other climate events called Dansgaard–Oeschger events. The Younger Dryas was likely the last and strongest of these events. However, scientists still debate what caused AMOC to weaken. One idea is that a large amount of cold, fresh water from North America’s Lake Agassiz flowed into the Atlantic Ocean. Evidence shows that meltwater may have traveled through the Mackenzie River, but this idea has been questioned because sea levels did not rise much during this time. Other theories, such as a volcanic eruption causing cooling and ice growth, have also been suggested. Studies of ice cores and cave deposits show unusually high volcanic activity just before the Younger Dryas began.
Etymology
The Younger Dryas is named after the alpine-tundra wildflower Dryas octopetala because its fossils are found in large numbers in European sediments from this time, especially in Scandinavia. Before this period, the flower was common in Europe during two earlier times: the Oldest Dryas (about 18,500–14,000 years before present) and the Older Dryas (about 14,050–13,900 years before present). In contrast, Dryas octopetala was uncommon during the Bølling–Allerød Interstadial. During this time, temperatures in Scandinavia were warm enough to support trees, as seen in sites in Denmark.
In Ireland, the Younger Dryas is also called the Nahanagan Stadial, and in Great Britain, it is called the Loch Lomond Stadial. In Greenland’s ice core records, the Younger Dryas matches Greenland Stadial 1 (GS-1). The warm Allerød period before the Younger Dryas is divided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a).
Climate
Scientists study the climate of the Younger Dryas period using evidence from sources like pollen, ice cores, and layers of sediment found in lakes and oceans. This evidence shows that the Northern Hemisphere began to cool significantly around 12,870 years ago. Greenland experienced the most extreme cooling, with temperatures dropping by 4 to 10 degrees Celsius (7.2 to 18.0 degrees Fahrenheit). At the top of Greenland, temperatures were up to 15 degrees Celsius (27 degrees Fahrenheit) colder than at the start of the 21st century.
In Europe, temperatures dropped by 2 to 6 degrees Celsius (4 to 11 degrees Fahrenheit). Icefields and glaciers formed in high areas of Great Britain, and many lowland regions had permafrost, meaning the ground stayed frozen year-round. This suggests temperatures were as low as -5 degrees Celsius (-9.0 degrees Fahrenheit), with average yearly temperatures no higher than -1 degree Celsius (30 degrees Fahrenheit). North America also cooled, especially in the east and central regions. The Pacific Northwest cooled by 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit), while western North America cooled less. In the Gulf of Mexico, sea surface temperatures dropped by about 2.4 degrees Celsius. However, areas like Texas, the Grand Canyon, and New Mexico did not cool as much as the center of the continent. The southeastern United States became warmer and wetter, and similar warming occurred near the Caribbean Sea and in West Africa.
Earlier studies suggested the Younger Dryas cooling began at the same time across the Northern Hemisphere. However, research from 2015 showed the cooling happened in two stages: first in areas near 56–54°N latitude between 12,900 and 13,100 years ago, then further north between 12,600 and 12,750 years ago. In East Asia, the cooling started later than in the North Atlantic. The cooling affected all seasons, but winters became much colder, while springs cooled less, and summers either stayed the same or warmed slightly. In what is now Maine, winters stayed stable, but summers cooled by up to 7.5 degrees Celsius (13.5 degrees Fahrenheit).
At the same time, the Southern Hemisphere warmed. Sea surface temperatures rose by 0.3 to 1.9 degrees Celsius (0.5 to 3.4 degrees Fahrenheit), and Antarctica, South America (south of Venezuela), and New Zealand experienced warming. The overall temperature change for the globe was a small cooling of 0.6 degrees Celsius (1.1 degrees Fahrenheit). The Younger Dryas lasted about 1,150 to 1,300 years and ended around 11,700 years ago, though some studies suggest it ended closer to 11,550 years ago.
The end of the Younger Dryas was sudden. In areas that had cooled, temperatures returned to previous levels within 50 to 60 years. In the tropics, warming happened more slowly over centuries, except in parts of the tropical Atlantic, like Costa Rica, where temperatures changed as much as in Greenland. After the Younger Dryas, global warming began as carbon dioxide levels rose from about 210 parts per million to 275 parts per million.
During the Younger Dryas, glaciers advanced, and snow lines (the height where snow stays year-round) dropped. Evidence of this was found in places like Scandinavia, the Swiss Alps, the Dinaric Alps, the Rocky Mountains, Wisconsin, New York, and the Pacific Northwest. The Laurentide Ice Sheet expanded from west of Lake Superior to southeast Quebec, leaving behind rock debris dated to this time. Southeastern Alaska avoided glaciers, as speleothems (cave formations) continued to grow there, showing no permafrost.
In the Southern Hemisphere, warming caused glaciers to shrink in Antarctica, South America, and New Zealand. In Greenland, glaciers only grew in the northern part of the island, while they retreated elsewhere. This was likely due to changes in ocean currents. In the Balkans, glaciers also retreated because of less rainfall, which would have otherwise helped glaciers form. Northern Scotland still had glaciers, but they became thinner.
Glaciers hold large amounts of water, so their growth or shrinkage affects sea levels. During the Younger Dryas, sea levels changed little, unlike before and after the period, when they rose rapidly. In western Norway, the Scandinavian Ice Sheet advancing caused a relative sea level rise of 10 meters (32 2/3 feet). Methane deposits under the ocean remained stable during the Younger Dryas, even during the rapid warming that ended it.
As the Northern Hemisphere cooled and the Southern Hemisphere warmed, the thermal equator (the area around Earth with the warmest temperatures) shifted south. This change affected wind patterns, such as weaker winds in East Africa, as seen in Lake Tanganyika’s sediments. These shifts explain why summers in the Northern Hemisphere did not cool as much.
Wind patterns also influence rainfall. Pollen evidence shows some areas became very dry, including Scotland, the North American Midwest, Anatolia, and southern China. In North Africa, including the Sahara Desert, drier conditions led to more dust being carried by the wind. Other regions, like northern China (except possibly Shanxi), became wetter.
Biosphere
The Younger Dryas period was first discovered at the beginning of the 20th century through studies of ancient plants and rock layers in bogs and lakes in Sweden and Denmark, especially at the Allerød clay pit in Denmark. Scientists found that pollen from Dryas octopetala, a plant that grows only in cold, icy conditions, became more common in areas where forests had previously grown during the warm B-A Interstadial. This discovery shows how living things reacted to sudden changes in climate.
In what is now New England, cool summers, cold winters, and little rain created a treeless tundra that lasted until the start of the Holocene, when forests moved north. Along the southern edges of the Great Lakes, spruce trees declined quickly, while pine trees increased, and grasses that grow in open areas became less common there but more common farther west. In the central Appalachian Mountains, forests remained during the Younger Dryas, but they were made up of spruce and tamarack trees typical of colder regions. These forests later changed to temperate forests with broad-leafed and mixed trees during the Holocene. However, near Lake Ontario, evidence from pollen and plant remains shows that cold, boreal forests lasted into the early Holocene.
More pine pollen found in the central Cascades suggests colder winters in that area. Cave formations in the Oregon Caves National Monument and Preserve in southern Oregon’s Klamath Mountains show signs of cooling that happened at the same time as the Younger Dryas. On the Olympic Peninsula, a mid-elevation site recorded fewer fires, but forests remained, and erosion increased during the Younger Dryas, which points to cooler and wetter conditions. Records from cave formations in southern Oregon show more rainfall during this time, which matches the growth of larger lakes in the northern Great Basin. Pollen records from the Siskiyou Mountains suggest the Younger Dryas started later there, possibly because warm ocean air from the Pacific delayed the cooling effect.
In the Rocky Mountain region, the effects varied. Some areas showed little change in plant life, while others saw more pine and fir trees, which suggests warmer conditions and a shift to open areas with scattered trees. Scientists think this happened because the jet stream moved north, summer sunlight increased, and winter snowpack was higher than today, with longer and wetter springs.
In northwestern Europe, the population dropped significantly during the first half of the Younger Dryas.
The Younger Dryas is often connected to the Neolithic Revolution, which began in the Levant with the start of farming. The cold and dry conditions of the Younger Dryas likely made it harder for people to live in the area, pushing the early Natufian people, who lived in settled communities, to move more often and rely on hunting and gathering. Later, worsening climate conditions may have led to the development of farming. While most scientists agree the Younger Dryas influenced changes in how people lived during the Natufian period, whether it directly started agriculture at the end of that time is still being studied.
Cause
The scientific agreement states that the Younger Dryas period was connected to a major slowdown or stop in the thermohaline circulation, which moves warm tropical water northward through the Atlantic meridional overturning circulation (AMOC). This is supported by climate model results and evidence from natural sources, such as less oxygen reaching the deepest layers of North Atlantic water. Studies of sediment layers in the western subtropical North Atlantic show that "bottom water" remained in that area for about 1,000 years, twice as long as similar water from the same location around 1,500 years before the present. Additionally, the unusual warming in the southeastern United States fits the idea that as the AMOC weakened, less heat was transported from the Caribbean to Europe through the North Atlantic Gyre, leaving more heat trapped in coastal waters.
Early theories suggested that a large flood from ancient Lake Agassiz might have poured into the North Atlantic through the Saint Lawrence Seaway, but little geological proof supports this. For example, the salinity of the Saint Lawrence Seaway did not decrease, as would have been expected if massive meltwater entered the ocean. More recent research shows that floodwaters instead traveled along the Mackenzie River in present-day Canada, and sediment layers confirm the strongest flood occurred just before the Younger Dryas began.
Other factors likely contributed to the Younger Dryas climate. Some studies suggest that melting of the Fennoscandian ice sheet may have caused Greenland’s sudden climate changes. Climate models also indicate that a single freshwater flood, no matter how large, would not have weakened the AMOC for over 1,000 years unless other factors were involved. Some models suggest that melting of the Laurentide Ice Sheet increased rainfall over the Atlantic, making the ocean less salty and weakening the AMOC. Lower temperatures during the Younger Dryas may have increased snowfall across the Northern Hemisphere, creating a feedback loop where more snow reflected more sunlight, cooling the area further. Melting snow was more likely to flow back into the North Atlantic than rainfall, as frozen ground absorbed less water. Other models suggest Arctic Ocean sea ice could have grown to tens of meters thick, allowing icebergs to enter the North Atlantic and weaken the circulation. These changes would not have affected sea levels, which matches the lack of significant sea level rise during the Younger Dryas.
Some scientists link the lack of sea level rise during the Younger Dryas to a volcanic eruption. Volcanic eruptions release sulfur dioxide into the atmosphere, where it forms aerosols that reflect sunlight and cool the planet. This cooling could have increased North Atlantic sea ice growth, weakening the AMOC enough to cause the Younger Dryas. Evidence from cave deposits and ice cores shows a major volcanic eruption occurred in the Northern Hemisphere near the start of the Younger Dryas, possibly matching the timing of the event. Some suggest this eruption was stronger than any in recorded history, which could have caused long-term cooling.
Research from the 1990s linked the Laacher See eruption in Germany to the Younger Dryas, but a 2021 study pushed its date back to 13,006 years before the present, over a century before the Younger Dryas began. This finding was challenged in 2023, as some researchers claimed the dating might have been affected by carbon dioxide from magma. The debate remains unresolved.
The Younger Dryas impact hypothesis (YDIH) suggests that a comet or asteroid impact caused the cooling. However, no impact crater from that time has been found, and supporters argue the impact may have hit the Laurentide ice sheet, which later melted, or caused an airburst that left only tiny particles as evidence. Most scientists disagree, stating that these particles can be explained by natural processes. For example, some minerals found in Texas sediments were thought by YDIH supporters to be from space, but a 2020 study suggests they are more likely volcanic. Opponents also note a lack of evidence for massive wildfires or the sudden extinction of species that would have followed a large impact.
Statistical analysis shows the Younger Dryas was one of 25 or 26 Dansgaard–Oeschger events (D–O events) in the past 120,000 years. These events involve sudden changes in the AMOC over decades or centuries. The Younger Dryas is the best known because it is the most recent, but it is similar to earlier cold periods. This similarity makes the impact hypothesis unlikely and may contradict the Lake Agassiz hypothesis. Some research links volcanism to D–O events, supporting the volcanic hypothesis.
Events like the Younger Dryas occurred during other "terminations," which are rapid shifts from cold glacial periods to warm interglacial periods. Scientists use molecules like lipids and long-chain alkenones in lake and marine sediments to estimate past temperatures, as these molecules are sensitive to heat. This method shows YD-like events happened during Termination II (~130,000 years ago), Termination III (~243,000 years ago), and Termination IV (~337,000 years ago). Combined with ice core and plant evidence, some researchers argue that YD-like events occur during every deglaciation.
In popular culture
The 2004 movie The Day After Tomorrow shows big problems with the weather that happen when the North Atlantic Ocean circulation stops working properly. This causes many very strong weather events, which lead to sudden changes in the climate. These changes result in a very cold period, similar to an ice age.