The Younger Dryas (YD, Greenland Stadial GS-1) was a time in Earth’s history that happened about 12,900 to 11,700 years Before Present (BP). It is mainly known for the sudden or "abrupt" cooling in the Northern Hemisphere. During this time, the North Atlantic Ocean cooled, and yearly air temperatures dropped by about 3 °C (5 °F) over North America, 2–6 °C (4–11 °F) in Europe, and up to 10 °C (18 °F) in Greenland, all within a few decades. Cooling in Greenland was especially fast, happening in just 3 years or less. At the same time, the Southern Hemisphere warmed. This period ended quickly, with a sharp warming over about 50 years, marking the shift from the glacial Pleistocene epoch to the current Holocene epoch.
The start of the Younger Dryas was not the same everywhere. In the tropics, cooling happened over several centuries, and the same was true for warming in the early Holocene. Even in the Northern Hemisphere, temperature changes were seasonal, with colder winters and cooler springs, but little or no change in summer. Rainfall also changed: cooler areas had less rain, while warmer areas had more. In the Northern Hemisphere, the growing season became shorter. Land ice cover did not change much, but sea ice expanded, which increased the Earth’s ability to reflect sunlight (called ice–albedo feedback). This increase in albedo was the main reason for a global cooling of 0.6 °C (1.1 °F).
Before the Younger Dryas, during the Bølling–Allerød Interstadial, rapid warming in the Northern Hemisphere was balanced by cooling in the Southern Hemisphere. This "polar seesaw" pattern matches changes in ocean currents, especially the Atlantic meridional overturning circulation (AMOC), which controls how much heat moves between the hemispheres. When the AMOC is strong, the Southern Hemisphere cools and the Northern Hemisphere warms. When it is weak, the opposite happens. Scientists agree that a major weakening of the AMOC explains the climate changes during the Younger Dryas. This weakening also explains why warming during the Holocene happened so quickly once the AMOC no longer counteracted rising carbon dioxide levels.
AMOC weakening and the polar seesaw pattern are also linked to Dansgaard–Oeschger events, with the Younger Dryas likely being the last and strongest of these events. However, scientists debate what caused the AMOC to weaken. One widely supported idea is that a large amount of fresh, cold water from North America’s Lake Agassiz flowed into the Atlantic Ocean. Evidence shows meltwater traveled through the Mackenzie River, but this idea may not fit with the lack of sea level rise during this time. Other theories have emerged, including a volcanic eruption as a possible trigger for cooling and sea ice growth. Recent studies confirm unusually high volcanic activity just before the Younger Dryas began, as seen in ice cores and cave deposits.
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. Earlier periods when this flower was common in Europe include 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 rare during the Bølling–Allerød Interstadial. During this time, temperatures in Europe were warm enough to support tree growth in Scandinavia, as shown by evidence from sites in Denmark.
In Ireland, the Younger Dryas is also called the Nahanagan Stadial, and in Great Britain, it is known as the Loch Lomond Stadial. In the Greenland Summit ice core record, the Younger Dryas matches Greenland Stadial 1 (GS-1). The warm Allerød period, which occurred 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 clues like pollen, ice cores, and layers in ocean and lake sediments. These clues show that the Northern Hemisphere began cooling around 12,870 years ago, with the most severe cooling in Greenland. Temperatures there dropped by 4–10°C (7.2–18.0°F), making the Greenland summit up to 15°C (27°F) colder than at the start of the 21st century.
Europe also cooled by 2–6°C (4–11°F). Ice formed in high areas of Great Britain, and frozen ground (permafrost) developed in lowlands, suggesting temperatures no higher than –1°C (30°F). North America cooled, especially in the east and central regions, though western areas saw less cooling. In the Gulf of Mexico, sea surface temperatures dropped by 2.4°C, but nearby areas like Texas and New Mexico cooled less. Meanwhile, the southeastern United States became warmer and wetter, as did the Caribbean Sea and West Africa.
Early studies suggested the Younger Dryas cooling happened at the same time across the Northern Hemisphere. However, research from 2015 showed the cooling occurred in two stages: first in areas near 56–54°N latitude around 12,900–13,100 years ago, then further north around 12,600–12,750 years ago. In East Asia, cooling began later than in the North Atlantic. Cooling was strongest in winter, with less change in spring and slight warming in summer in most places. However, in what is now Maine, summers cooled by up to 7.5°C (13.5°F) while winters stayed stable.
While the Northern Hemisphere cooled, the Southern Hemisphere warmed. Sea surface temperatures rose by 0.3–1.9°C (0.5–3.4°F), and areas like Antarctica, South America, and New Zealand became warmer. Overall, the global temperature change was small, about 0.6°C (1.1°F). The Younger Dryas lasted 1,150–1,300 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 previously cold areas, temperatures returned to previous levels within 50–60 years. Tropical regions warmed more slowly, except in parts of the tropical Atlantic, like Costa Rica, where cooling matched Greenland’s. After the Younger Dryas, global temperatures rose during the Holocene, following an increase in carbon dioxide levels from about 210 ppm to 275 ppm.
During the Younger Dryas, glaciers expanded, and snow lines (the boundary between snow and ice) dropped. Evidence of this includes glaciers in Scandinavia, the Swiss Alps, the Dinaric Alps, the Rocky Mountains, and areas like Wisconsin and New York. The Laurentide Ice Sheet advanced from western Lake Superior to southeast Quebec, leaving behind rock debris (moraine) from this time. Southeastern Alaska avoided glaciation, as speleothem deposits suggest no permafrost or glaciers formed there.
In the Southern Hemisphere, warming caused ice loss in Antarctica, South America, and New Zealand. While Greenland as a whole cooled, glaciers only grew in the north of the island, and retreated elsewhere, likely due to the Irminger Current. In the Balkans, glaciers shrank because of less precipitation. Northern Scotland still had glaciers, but they thinned during the Younger Dryas.
Glacier changes affect global sea levels. If glaciers retreat, sea levels rise; if they grow, sea levels fall. During the Younger Dryas, sea levels changed little, unlike the rapid changes before and after, such as the Meltwater Pulse 1A. In western Norway, the Scandinavian Ice Sheet’s advance caused a 10-meter (32 2/3-foot) rise in relative sea level. Methane clathrate deposits under the ocean remained stable throughout the Younger Dryas, even during the rapid warming at its end.
As the Northern Hemisphere cooled and the Southern Hemisphere warmed, the thermal equator (the area with the most heat) shifted south. This changed wind patterns, such as weaker winds in East Africa, as seen in Lake Tanganyika sediments. These changes likely explain why Northern Hemisphere summers did not cool as much.
Wind patterns also influence rainfall. Pollen records show some areas became very dry, like Scotland, the North American Midwest, Anatolia, and southern China. In North Africa, including the Sahara Desert, increased dust was blown by wind due to drier conditions. Other areas, like northern China, became wetter.
Biosphere
The Younger Dryas was first found at the beginning of the 20th century through studies of ancient plants and rock layers in bogs and lakes in Sweden and Denmark, especially the Allerød clay pit in Denmark. Analysis of pollen from fossils showed that Dryas octopetala, a plant that only grows in cold, glacial conditions, became common in areas where forests had previously grown during the earlier B-A Interstadial. This event is an important example of how living organisms reacted to sudden climate changes.
For example, in what is now New England, cool summers, cold winters, and low rainfall created a treeless tundra that lasted until the start of the Holocene, when boreal forests moved north. Along the southern edges of the Great Lakes, spruce trees declined quickly, while pine trees increased, and grassy prairie plants became less common but grew more in western areas. The central Appalachian Mountains remained covered in forests during the Younger Dryas, but these were boreal forests made of spruce and tamarack. These forests later changed to temperate forests with broadleaf and mixed tree species during the Holocene. However, evidence from near Lake Ontario shows that cool, boreal forests remained until the early Holocene.
An increase in pine pollen suggests colder winters in the central Cascades. Speleothems, or 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 suggests cool and wet conditions. Speleothem records also show more rainfall in southern Oregon, which matches the timing of larger pluvial lakes in the northern Great Basin. Pollen records from the Siskiyou Mountains suggest the Younger Dryas occurred later in that area, indicating the influence of warmer Pacific Ocean conditions.
Effects in the Rocky Mountain region varied. Some sites showed little to no changes in plant life. In the northern Rockies, a large increase in pine and fir trees suggests warmer conditions than before, leading to subalpine parkland in some areas. This is thought to result from a shift in the jet stream northward, along with more summer sunlight and higher winter snowpack that lasted longer and included wetter spring seasons.
Northwestern Europe experienced a major drop in population during the first half of the Younger Dryas.
The Younger Dryas is often connected to the Neolithic Revolution, which involved the start of agriculture in the Levant. The cold and dry conditions of the Younger Dryas likely reduced the ability of the region to support people, leading the early Natufian population to adopt a more mobile lifestyle. Further climate changes are believed to have encouraged the cultivation of grains. While there is general agreement about the Younger Dryas’ role in changing subsistence patterns during the Natufian period, its direct link to the start of agriculture at the end of the period is still being studied.
Cause
The scientific agreement connects the Younger Dryas 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 models and evidence from natural sources, such as less oxygen exposure in the deepest layers of North Atlantic water. Studies of sediment cores from the western subtropical North Atlantic show that "bottom water" remained there for 1,000 years, twice as long as similar water from the same area around 1,500 years before the present. Additionally, unusual warming in the southeastern United States fits the idea that a weaker AMOC transported less heat from the Caribbean toward Europe, leaving more heat trapped in coastal waters.
Early ideas suggested that a flood from ancient Lake Agassiz into the North Atlantic through the Saint Lawrence Seaway caused the Younger Dryas. However, geological evidence does not support this, as salinity in the Saint Lawrence Seaway did not decrease as expected. Recent research shows that floodwaters instead followed the Mackenzie River in present-day Canada, with sediment cores indicating the strongest flood occurred just before the Younger Dryas began.
Other factors likely contributed to the Younger Dryas. For example, melting of the Fennoscandian ice sheet may have caused sudden climate changes in Greenland. Climate models suggest that a single freshwater flood alone could not have weakened the AMOC for over 1,000 years, as needed for the Younger Dryas, unless other factors were involved. Some models show that melting of the Laurentide Ice Sheet increased rainfall over the Atlantic, freshening the water and weakening the AMOC. Lower temperatures during the Younger Dryas likely increased snowfall across the Northern Hemisphere, creating a feedback loop where more snow reflected sunlight and further cooled the climate. Melting snow was more likely to flow into the North Atlantic than rainfall, as frozen ground absorbed less water. Other models suggest Arctic Ocean sea ice could have grown thick enough to release icebergs into the North Atlantic, weakening the circulation. These changes did not affect sea levels, which aligns with no major sea level rise during the Younger Dryas.
Some scientists link the lack of sea level rise to a volcanic eruption. Volcanoes release sulfur dioxide, which forms aerosols in the atmosphere and cools the planet by reflecting sunlight. Evidence from cave deposits and ice cores suggests a major volcanic eruption occurred in the Northern Hemisphere near the start of the Younger Dryas. Some researchers believe this eruption was stronger than any in the Common Era, which has caused decades of cooling. However, dating of the Laacher See eruption in Germany, once thought to match the Younger Dryas timeline, was revised to 13,006 years before the present, a century earlier than the Younger Dryas began. This date is still debated, as some argue the analysis may have been affected by magmatic carbon dioxide.
The Younger Dryas impact hypothesis suggests a comet or asteroid impact caused the cooling. Proponents argue that no impact crater has been found because the impact may have hit the Laurentide ice sheet, which later melted, or was an airburst that left only tiny particles. Most scientists disagree, stating that these particles can be explained by natural processes. For example, mineral evidence from Texas sediments was once thought to support the impact theory, but a 2020 study suggests it is more likely volcanic in origin. Opponents also note a lack of evidence for wildfires or mass extinctions that would follow a large impact.
Statistical analysis shows the Younger Dryas is one of 25 or 26 Dansgaard–Oeschger events over the past 120,000 years. These events involve sudden changes in the AMOC over decades or centuries. The Younger Dryas is the best understood 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 theory. Some research links volcanoes to Dansgaard–Oeschger events, supporting the volcanic idea.
Events like the Younger Dryas occurred during other transitions from glacial to warm periods. Scientists study lake and marine sediments to reconstruct past temperatures using molecules like lipids and long chain alkenones, which are sensitive to temperature. This method shows YD-like events 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 data, some argue YD-like events happen during every deglaciation.
In popular culture
The 2004 movie The Day After Tomorrow shows major climate problems that occur when the North Atlantic Ocean circulation is disrupted. This disruption causes many severe weather events, which lead to sudden climate changes and the start of a new ice age.