Younger Dryas
The Younger Dryas refers to a significant climatic event that occurred approximately 12,900 to 11,600 years ago, marking a period of abrupt cooling after the Pleistocene epoch. During this time, temperatures dropped, leading to the reformation of ice sheets in various regions, primarily in Europe and North America. The event is named after the arctic-alpine herb Dryas octopetala, whose macrofossils were found in deposits indicative of this cooler climate. Research indicates that the Younger Dryas was characterized by shifts in vegetation and fauna, evident through pollen studies and ice core samples that documented higher dust levels and changes in atmospheric chemistry.
This climatic event is notable for its implications on global climate patterns, revealing that large areas of the Earth can transition from glacial to interglacial states within a short time frame. It highlights the potential for rapid climate shifts, a concern that resonates in discussions on current anthropogenic climate change. Scientists suggest that disturbances in oceanic thermohaline circulation, potentially triggered by substantial freshwater influx from melting glaciers, played a crucial role in initiating the Younger Dryas. Understanding this event is vital for predicting future climate scenarios, particularly as ongoing global warming could lead to similar abrupt climate changes.
Subject Terms
Younger Dryas
Definition
After the Pleistocene epoch ended, Earth’s climate began a variable warming trend, punctuated by episodes of cooler temperatures and reglaciations. The Younger Dryas, occurring 12,900 to 11,600 years ago, was a well-defined climatic event during which cool temperatures returned and ice sheets reformed. The term “Dryas” denotes the arctic-alpine herb, Dryas octopetala. Early evidence for the Younger Dryas were Dryas octopetala macrofossils found in Scandinavian terrestrial deposits that overlie warmer climate deposits.
After the Scandinavian discovery, pollen studies of European lake and bog sediments were undertaken during the 1930s. This field research found that tundra pollen is characteristic of the Younger Dryas and tundra pollen always appears after the deposition of forest pollen (which indicates a warmer climate). The occurrence of the Younger Dryas is documented by vegetation changes; faunal changes; higher dust levels; and elevated atmospheric chemical concentrations, as identified in ice cores.
Since early Younger Dryas evidence was collected in Europe, Scandinavia, and Greenland, it was thought that the Younger Dryas was confined to these northern geographic areas. However, vegetation changes from mixed conifer-deciduous forest to cooler climate boreal trees in southern New England indicate Younger Dryas events occurred in North America as well. The replacement of shrub tundra with herb tundra in northern Nova Scotia along with shrub tundra replacing spruce forest in southern New Brunswick and central Nova Scotia also indicates Younger Dryas occurrence in North America. In addition, Younger Dryas evidence is found in mid-America, on the coast of the Pacific Northwest, British Columbia, and Alaska.
Prior to the onset of the Younger Dryas, glaciers had retreated from Great Britain; Scandinavian and Laurentide ice sheets were in regression; and, according to global sea-level records, ice covered less than 15 percent of Earth’s surface. After the inception of the Younger Dryas, glaciers reformed in Europe and North America—from Scotland to the western Rocky Mountains. At the close of the Younger Dryas, temperatures rose quickly, 5 °Celsius to 10 °Celsius within ten years. They continued to increase to 15 °Celsius warmer than they had been. This abrupt rise in temperatures, as revealed in Greenland ice cores, was surprising to climate scientists. The Greenland cores had revealed other rapid climate changes—the abrupt deglaciation at the end of the Pleistocene.
Significance for Climate Change
Results from ice cores, glaciers, and land and deep-sea sediments disclose that large areas of Earth have had almost synchronous changes from glacial to interglacial in short intervals of ten to thirty years. This means the entire climate regime (cold-warm) can reorganize in less than one lifetime. With the reality of rapid changes in global climate regimes, there is the threat that anthropogenic global climate change might initiate abrupt shifts in climate, leaving human populations unprepared.
Antarctic indicate a high ratio of oxygen 18 (O18) to oxygen 16 (O16) during interglacials, suggesting that ice formed when atmospheric temperatures were elevated; also, high concentrations of and have been reported. High levels of greenhouse gases (GHGs), trapped in ice core air bubbles, indicate a period of atmospheric warming. GHGs are reduced during glacial periods.
Today, as a result of and fossil fuel burning, air temperatures, CO2, and CH4 levels are all rising. The CO2 generated before the occurred naturally; rising atmospheric CO2 after 1830 was generated by humans. This can be demonstrated, because the CO2 isotope of has a characteristic signature that matches current CO2. Climate models of high CO2 demonstrate that nighttime low temperatures should increase and that daily temperature variation should be reduced. Climate data collected from the last century confirm this prediction—the effects of global warming.
Research undertaken to discover the cause of the Younger Dryas points to perturbation of the global ocean Thermohaline circulation (THC), which transports tropical heat to higher latitudes. North Atlantic Ocean cooling is triggered by the addition of large amounts of terrestrial freshwater. Cyclic changes in quantities of meltwater (such as glacial lake outbursts or icebergs carrying glacial water), can supply large volumes of freshwater to the northern Atlantic Ocean, causing the THC to slow or stop.
The Younger Dryas event has significance for global warming: At its close, abrupt change of climate from glacial to warm conditions occurred, demonstrating that climatic processes are not gradual but rapid and dramatic, with the potential to plunge human populations, in less than a decade, into cold or warm conditions and back again. The Younger Dryas was initiated by some imbalance in the THC, such as flood waters or lake-bearing icebergs. Global warming may supply copious amounts of melted glacial water, thus triggering a perturbation of the THC and initiating a “Younger Dryas-like” event in the Northern Hemisphere. The study of the Younger Dryas is integral to understanding future climate change, especially due to warming caused by greenhouse gas emissions which have experienced a rebound after falling during the lockdowns attributed to the 2020 global pandemic. The warming caused by this increase in greenhouse gas emissions could lead to additional ice melt and thermohaline shutdown, as occurred during the Younger Dryas.
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