Dansgaard–Oeschger events
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| Dansgaard–Oeschger events | |
|---|---|
| Name | Dansgaard–Oeschger events |
| Time period | Last glacial period |
| Location | North Atlantic, Greenland, Eurasia |
| Causes | Internal climate variability, ocean circulation changes |
| Notable | Rapid warming events in Greenland ice cores |
Dansgaard–Oeschger events are abrupt climate fluctuations during the last glacial period characterized by rapid warming followed by gradual cooling, documented primarily in Greenland ice cores. Researchers first identified these phenomena in paleoclimate records from the Greenland Ice Sheet Project, linking them to variability in the North Atlantic Ocean and broader atmospheric circulation. Studies by teams associated with institutions such as the University of Copenhagen, Alfred Wegener Institute, Lamont–Doherty Earth Observatory, and the University of Bergen have compared proxies from sites including NGRIP, GRIP, and DYE-3 to establish a recurring pattern of stadial and interstadial phases.
Overview
The events manifest as abrupt temperature rises in Greenland spanning decades, recorded in records produced by projects like GISP2, EPICA, and the Vostok Station program. Identification involved scientists such as Willie Dansgaard and Hans Oeschger and collaborations across centers like Palaeoclimatology Unit, University of Cambridge, University of Copenhagen, and AWI Bremerhaven. The phenomena correlate with shifts observed in marine cores from the Irminger Sea, Iceland Basin, Norwegian Sea, and the North Atlantic Current, implicating links to circulatory features such as the Atlantic Meridional Overturning Circulation and the Gulf Stream.
Evidence and proxies
High-resolution isotopic records from NGRIP, GRIP, and GISP2 provide primary evidence via oxygen isotope ratios, complemented by dust proxies from EPICA Dome C and greenhouse gas concentrations measured in EPICA Dome C and Vostok Station ice cores. Marine sediment cores from programs like Ocean Drilling Program and International Ocean Discovery Program yield foraminifera assemblages and alkenone SST estimates that match signals from sites such as North Atlantic Drift and Rockall Trough. Terrestrial proxies include speleothem records from Höhlen, lake sediments from Lake Suigetsu and Lake Baikal, and pollen sequences from Greenland, Scandinavia, and Siberia. Paleoceanographers use assemblages of Neogloboquadrina pachyderma, sortable silt analyses from cruises by RRS James Clark Ross, and tracers like neodymium isotopes from samples collected by teams at Woods Hole Oceanographic Institution.
Mechanisms and hypotheses
Proposed mechanisms involve interactions among the Atlantic Meridional Overturning Circulation, sea ice dynamics in the Labrador Sea and Norwegian Sea, and freshwater fluxes from the Laurentide Ice Sheet and meltwater routing through outlets like the St. Lawrence River. Hypotheses by groups at NCAR, Max Planck Institute for Meteorology, and GEOMAR include bistability of thermohaline circulation, sea ice–albedo feedbacks studied at MPI-BGC, and atmospheric teleconnections involving the Arctic Oscillation and North Atlantic Oscillation. Alternative proposals draw on solar forcing recorded in IntCal chronologies and volcanic forcing cataloged by Smithsonian Institution teams, as well as ice sheet dynamics modeled at University College London and ETH Zurich.
Regional and global impacts
Regional consequences affected marine ecosystems in the Barents Sea, productivity shifts in the North Sea, and sea surface temperatures in the Irminger Sea. Terrestrial impacts altered vegetation zones across Scandinavia, permafrost distribution in Siberia, and hydrology in basins like Fennoscandia and the Baltic Sea. Teleconnected signals reached the Mediterranean Sea, influenced monsoon systems documented in records from Indian Ocean cores and speleothems in Hulu Cave, and are discussed in relation to records from Cariaco Basin and Lake Ohrid. Ice-rafted debris events in the Labrador Sea correspond with climate shifts recorded by teams from WHOI and Scripps Institution of Oceanography.
Chronology and pacing
Events recur quasi-periodically with spacing often between ~1,500 years during the last glacial, a pacing highlighted by analyses from GICC05 timescales and Antarctic synchrony work using AICC2012 chronology. Chronological comparisons rely on synchronization efforts by groups at LSCE, NILU, and PAGES to align Greenland ice cores with Antarctic records like EPICA and Vostok Station. Marine stratigraphy correlations employ tephrochronology linked to eruptions cataloged by Tephra Studies groups and luminescence dating from laboratories at University of Oxford and Australian National University.
Modeling and predictability
Climate models ranging from Earth system models at IPSL and Hadley Centre to coupled ocean–atmosphere models at NCAR and MPI have simulated abrupt transitions resembling the events under freshwater forcing and sea ice parameterizations. High-resolution proxy-model comparison initiatives involve teams at PAGES, PMIP, and CMIP intercomparison projects, while data assimilation efforts use frameworks developed at Max Planck Institute for Meteorology and LAMONT. Predictability studies explore thresholds and early warning indicators using methods from Cornell University and Princeton University, employing indices related to circulation shifts comparable to those analyzed in Paleoclimatology syntheses by NOAA paleoclimate groups.
Debates and open questions
Key debates concern the primary trigger (freshwater forcing versus internal oscillation), the role of sea ice versus AMOC variability, and the extent of interhemispheric coupling between Greenland and Antarctic responses, debated by researchers at University of Bergen, University of Copenhagen, US Geological Survey, and Max Planck Institute for Meteorology. Open questions include the mechanisms controlling the ~1,500-year pacing, the influence of ice sheet geometry studied at University of Toronto and Utrecht University, and the applicability of last-glacial insights to Holocene or future climate change scenarios investigated by groups at IPCC and World Climate Research Programme.