Millennial-scale warming and cooling episodes called Dansgaard-Oeschger (D-O) cycles punctuated the last ice age. These cycles were first discovered in the Greenland ice cores, where they appear as large and abrupt climate swings (years to decades). The swings are unlike anything else recorded in the last 120 thousand years, and represent a poorly understood behavior of the climate system.
Sea ice has long been thought to play an important role in D-O cycles because of its strong influence on regional temperature and its ability to grow and melt rapidly in response to relatively weak forcings. In this study, Camille Li of the Bjerknes Centre and collaborators David S. Battisti and Cecilia M. Bitz at the University of Washington ask what the sea ice cover around Greenland looked like during the last ice age, and where variations in this sea ice cover must have occurred in order to create climate changes as large as the D-O cycles.
The authors use a combination of coupled and atmosphere-only model simulations to answer these questions. Coupled climate models produce two main masses of sea ice in the glacial North Atlantic: one in the Nordic Seas, where the ice cover is controlled primarily by local growth and melt, and one in the western North Atlantic, where the ice cover owes its existence to the import of ice formed further north in Baffin Bay and Davis Strait. Additional simulations are used to show that, of the two ice masses, it is variations in the Nordic Seas ice cover that have the most influence on temperature and accumulation signals recorded in the Greenland ice cores.
The results of this study show that the abrupt advance and retreat of Nordic Seas ice cover is an effective way to produce large, abrupt D-O-like climate shifts on Greenland. The results may also be relevant for understanding a different class of abrupt climate changes, known as Heinrich events, which also occurred during the last ice age. Heinrich events involve massive iceberg discharges from the North American ice sheet into the western North Atlantic, and occur during the cold phases of some D-O cycles. While Heinrich events are exceptional signals in other locations worldwide, they appear merely as “regular” cold phases in the Greenland ice cores. However, if Greenland is indeed most sensitive to Nordic Seas ice conditions, then it would not be surprising that the ice cores do not record the presence or absence of icebergs in the western North Atlantic.
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| Surface warming due to retreat of sea ice in the western Atlantic and Nordic Seas. The thick and thin black contours show the winter and summer sea ice lines, respectively. The thin grey contours show today's continents and the thick grey contours show the continents at the Last Glacial Maximum (21 thousand years ago), when global sea level was 125m lower. The circle marks the location of the Greenland Summit ice cores. |
Reference:
Li, Camille, David S. Battisti, Cecilia M. Bitz, 2010: Can North Atlantic sea ice anomalies account for Dansgaard-Oeschger climate signals? J. Climate, 23, 5457-5475.