- Also called:
- Younger Dryas stadial
- Related Topics:
- global cooling
Younger Dryas
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What is the Younger Dryas?
The Younger Dryas was a cool period that interrupted the warming trend at the end of the Pleistocene Epoch, roughly 12,900 to 11,600 years ago. This period saw a return to ice age conditions in parts of Europe and North America, with the onset of these cooler temperatures occurring in less than 100 years and lasting for about 1,300 years.
The Younger Dryas is named after Dryas octopetala, a wildflower typical of cold Arctic environments. Ice core samples suggest that central Greenland was nearly 14 °C (24.5 °F) colder during the Younger Dryas than it is today, and the warming that ended the period took about 40 to 50 years.
The cause of the Younger Dryas is still debated. One hypothesis suggests that a large influx of fresh water into the North Atlantic Ocean disrupted thermohaline circulation, initiating a return to glacial conditions.
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When did the Younger Dryas period occur?
The Younger Dryas period, a cool climatic interval, occurred roughly 12,900 to 11,600 years ago, disrupting the general warming trend at the end of the Pleistocene Epoch. This period saw a return to near-glacial conditions in parts of Europe and North America and lasted approximately 1,300 years.
The Younger Dryas ended with another abrupt warming event around 11,600 years ago, marking the start of the Holocene Epoch and modern climate. During the Younger Dryas, central Greenland experienced temperatures nearly 14 °C (24.5 °F) colder than today, with the subsequent warming occurring rapidly over 40 to 50 years.
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What major climate changes happened during the Younger Dryas?
The Younger Dryas was a cool period that interrupted a warming trend at the end of the Pleistocene Epoch, roughly 12,900 to 11,600 years ago. During this period, Europe and North America returned to ice age conditions with cooler average temperatures. Forests were replaced by tundra vegetation. Central Greenland was nearly 14 °C (24.5 °F) colder than today. The end of the Younger Dryas was marked by a second abrupt warming event that resulted in temperatures increasing by as much as 10 °C (18 °F) in just a few decades.
The leading hypothesis for the cause of the Younger Dryas is that large amounts of fresh water were discharged into the North Atlantic. This influx of fresh water may have stopped higher-density seawater in the North Atlantic from descending to lower depths, thereby interrupting thermohaline circulation and initiating a short-term return to glacial conditions.
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How did the Younger Dryas affect plants and animals?
The Younger Dryas, a cool period from roughly 12,900 to 11,600 years ago, interrupted a warming trend at the end of the Pleistocene Epoch in the Northern Hemisphere. Examination of terrestrial plants and pollen indicates that forests were replaced by tundra vegetation during this period. The cooling was primarily limited to the Northern Hemisphere, especially the North Atlantic region, including Europe and eastern North America. Many parts of the tropics and the Southern Hemisphere became warmer and wetter during the Younger Dryas.
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What are some possible reasons scientists think the Younger Dryas happened?
The Younger Dryas was a cool period that interrupted the warming trend at the end of the Pleistocene Epoch, approximately 12,900 to 11,600 years ago. While the exact cause isn't known, several hypotheses attempt to explain this climatic reversal.
The leading hypothesis suggests that a large influx of fresh water into the North Atlantic, possibly from the drainage of Lake Agassiz due to the retreat of the Laurentide Ice Sheet, disrupted thermohaline circulation. Other hypotheses point to volcanic eruptions, negative radiative forcing, a comet impact, changes in atmospheric circulation, or reduced sunspot activity as potential factors.
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How do scientists study and find evidence of the Younger Dryas today?
Scientists study the Younger Dryas by examining various records and data. Ice cores from places such as Greenland offer information regarding past climatic fluctuations through the analysis of oxygen isotopes and air bubbles, which reveal air temperature and atmospheric composition. Additionally, the dating of glacial moraines, analysis of stalactites and stalagmites, and pollen records globally help determine climate shifts during the Younger Dryas. These methods help scientists piece together the geographic extent and impact of this period.
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Younger Dryas, cool period between roughly 12,900 and 11,600 years ago that disrupted the prevailing warming trend occurring in the Northern Hemisphere at the end of the Pleistocene Epoch (which lasted from 2.6 million to 11,700 years ago). The Younger Dryas was characterized by cooler average temperatures that returned parts of Europe and North America to ice age conditions. The onset of the Younger Dryas took less than 100 years, and the period persisted for roughly 1,300 years. After the period ended, an interval of rapid warming followed, and average temperatures increased to near present-day levels. The Younger Dryas was named after Dryas octopetala, a pale yellow wildflower of the rose family, typical of cold open Arctic environments.
During the Pleistocene Epoch, extensive ice sheets and other glaciers formed repeatedly on large landmasses. The Younger Dryas, one of several very abrupt climatic changes that took place near the end of the late Pleistocene, was preceded by a sudden warming interval beginning approximately 14,700 years ago. This interval, the Bølling-Allerød interstadial, saw the rapid retreat of the immense Pleistocene ice sheets. A second abrupt climatic warming event, approximately 11,600 years ago, marked the end of the Younger Dryas and the beginning of the Holocene Epoch (11,700 years ago to the present) and Earth’s modern climate. There is evidence that this warming was quite rapid; Greenland ice-core samples suggest that local temperatures increased by up to 10 °C (18 °F) in just a few decades.
Cause
Such dramatic climatic reversals occurring in such a short time cannot be explained by Milankovitch cycles (that is, cyclical changes to the shape of Earth’s orbit, the tilt of Earth’s axis, and the wobblelike movement of Earth on its axis with respect to the Sun), which play out over tens of thousands of years. A number of hypotheses have been proposed to explain the Younger Dryas, but so far there is no consensus on its cause.
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The leading hypothesis, first proposed by Finnish scientist Claes Rooth in 1982 and later expanded by American climatologist Wallace Broecker and others, postulated that large amounts of fresh water were discharged into the North Atlantic about 12,800 years ago. More specifically, the retreat of the Laurentide Ice Sheet allowed Lake Agassiz, a glacial meltwater lake that covered a large part of north-central North America, to drain eastward into the Atlantic Ocean rather than southward into the Mississippi River. Broecker and American geologist George Denton proposed that this large influx of fresh water may have stopped higher-density seawater in the North Atlantic from descending to lower depths, thereby interrupting thermohaline circulation (a system of surface and deepwater currents that distributes large amounts of heat around the globe) and initiating a short-term return to glacial conditions.
Some studies have noted the occurrence of similar freshwater pulses entering the Atlantic from the Mackenzie River in northern Canada. Such pulses may have slowed thermohaline circulation. Other hypotheses suggest that volcanic eruptions, negative radiative forcing, a large comet impact, modified atmospheric circulation patterns from the changing shape of the ice cap, or an extended period of reduced sunspot activity may have played significant roles in bringing about the Younger Dryas.
Initial discovery and subsequent research
The first evidence of the Younger Dryas came from ice cores taken from European maritime environments dating to the late Pleistocene. The ice cores showed that the warming process produced abrupt wholesale melting of late Pleistocene glaciers. Subsequent examination of terrestrial plants and pollen in the cores indicated that forests were replaced by tundra vegetation during a cool period.
The timing of past climatic fluctuations has been determined by measuring the ratio of two oxygen isotopes, oxygen-18 and oxygen-16, present in air bubbles trapped in different layers of the ice. Isotope data suggests that central Greenland was nearly 14 °C (24.5 °F) colder during the Younger Dryas than it is today and that the sudden warming that ended the Younger Dryas took about 40 to 50 years.
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Geographic extent
Radiocarbon dating of glacial moraines, measurements of oxygen isotopes in ice cores and in stalactites and stalagmites in caves, and pollen records indicate that climate shifts of various kinds took place during the Younger Dryas throughout the globe. Data show that the cooling was limited primarily to the Northern Hemisphere, specifically the North Atlantic region, which included Europe and eastern North America. Evidence of advancing continental ice sheets coincident with the Younger Dryas is reported from the Scandinavian Ice Sheet, the Laurentide Ice Sheet in eastern North America, the Cordilleran Ice Sheet in western North America, and the Siberian Ice Sheet in Russia. Alpine glaciers in the Northern Hemisphere responded to the abrupt cooling by expanding. Evidence for this appears in many places in the Rocky Mountains of the United States and Canada, the Cascade Range of Washington state, and the European Alps. Abrupt shifts in the plant communities in California and other parts of the southwestern United States have been linked to the onset of prolonged cooler and drier conditions coinciding with the beginning of the interval.
Researchers estimate that sea surface temperatures in eastern Canada declined by 5–10 °C (9–18 °F) during the Younger Dryas. Away from the western part of the North Atlantic, sea surface temperature declines were not as great; temperatures in the Mediterranean Sea fell by only 1–3 °C (1.8–5.4 °F), and the waters in Venezuela’s Cariaco Basin and along the coast of northern California cooled by about 3 °C (5.4 °F).
Many parts of the tropics and the Southern Hemisphere became warmer and wetter during the Younger Dryas. Evidence of sea surface temperature increases occur in the Caribbean Sea, the eastern equatorial Atlantic Ocean, and the tropical Pacific Ocean. Studies of high-altitude regions of the Southern Hemisphere—such as the Bolivian, Peruvian, and Patagonian Andes and the Southern Alps of New Zealand—reveal evidence of warmer conditions and declining glacier coverage.