Allerød oscillation

Definition

The Alleroed oscillation was a temperature fluctuation that occurred near the end of the last glaciation period, about 13,000 years ago. For several centuries following the oscillation, Europe, the British Isles, and the northern Atlantic warmed to present-day levels. Other areas remained untouched. The Alleroed oscillation is just one of several climate swings affecting the North Atlantic region over a period from 17,700 years ago to 11,500 years ago. These oscillations, revealed in European terrestrial sediments, are known as the Oldest Dryas, Boelling, Older Dryas, Alleroed, and Younger Dryas oscillations. Some oscillations provided warming conditions to circumscribed areas, as during the Alleroed oscillation, while others—such as the Younger Dryas oscillation, the last major cold event—were probably global in scope.

Significance for Climate Change

The North Atlantic region's climate underwent a series of abrupt cold/warm oscillations when the Northern Hemisphere's ice sheets retreated. During the Alleroed oscillation, the warm temperatures allowed for a mix of flora and fauna that would look familiar today: Deer, horses, bear, and beaver found a congenial environment in Europe’s evergreen and deciduous forests.

Questions arise about the feedback effects of such warming swings. For example, a phenomenon called the thermohaline circulation, or the great ocean conveyor, involves large ocean currents that flow like rivers around the globe, affecting local climates in the process. These currents depend on the ocean’s salinity and temperature, the Alleroed oscillation probably altered their course or strength. However, modeling for the effect of climate warming on the thermohaline circulation is difficult, and definitive conclusions are therefore elusive.

A more likely climate impact from the Alleroed oscillation would have resulted from warmer Arctic temperatures stimulating plant growth. Plant cover darkens the landscape and causes more sunlight to be absorbed, rather than reflecting into the atmosphere. Thus, as plant growth spreads, a feedback effect promotes higher temperatures, which encourages further plant growth over an increasing terrestrial range.

Some climate simulations further support the idea that temperature swings in the North Atlantic Ocean may have wide-ranging climatic effects. These simulations indicate that North Atlantic Ocean cooling causes North Pacific Ocean cooling, which in turn results in a drier climate in western North America.

Finally, current observable increases in global temperature are threatening glacier systems in the Antarctic. This may lead to sea-level increases worldwide, with serious consequences as the sea encroaches on vulnerable landmasses. Whether this occurred during the Alleroed oscillation is subject to speculation, but research indicates that it probably did.

Fermamdes, M., et al. "Glacial Oscillations During the Bølling–Allerød Interstadial-Younger Dryas Transition in the Ruda Valley, Central Pyrenees." Journal of Quaternary Science, 11 Oct. 2021, doi:10.1002/jqs.3379. Accessed 28 Aug. 2025.

Kovanen, Dori J., and Easterbrook, Don J. "Timing and Extent of Allerød and Younger Dryas Age (ca. 12,500–10,000 14C yr B.P.) Oscillations of the Cordilleran Ice Sheet in the Fraser Lowland, Western North America." Quaternary Research, 2002, doi.org/10.1006/qres.2001.2307. Accessed 28 Aug. 2025.

Mercer, J. H. "The Allerød Oscillation: A European Climatic Anomaly?" Arctic and Alpine Research, May 2018, doi:10.1080/00040851.1969.12003550. Accessed 28 Aug. 2025.