Greenland Largely Melted 400,000 Years Ago

Kelsey Winsor Oregon State University. The margin of Greenland's Ice Sheet was hundreds of kilometers further north 400,000 years ago

The melting of Greenland's ice sheet was responsible for at least a third, and possibly most, of the rise in global sea levels during an interglacial 400,000 years ago.

During what is known as Marine Isotope State (MIS) 11 the cycle of Earth's orbit raised its average temperature to roughly those anticipated for late this century and sea levels rose 6-13m above current levels. However, the source of this water remains a matter of debate. A small amount would have come from thermal expansion of the oceans and melting of alpine glaciers, but most must have come from the polar ice caps. Establishing the balance between Greenland and Antarctica remains a topic of considerable research.

Examination of sediment cores in the Eirik Drift off Greenland's south coast indicate that for a substantial period of MIS 11 the southern section of the island was ice-free. “The sediments are only deposited when there is significant ice to erode the terrain. The absence of terrestrial deposits in the sediment suggests the absence of ice,” says The University of Wisconsin, Madison's Dr Andres Carlson, one of the authors of a letter in Nature documenting the investigation. Sediments from particular areas have a unique chemical signal, allowing the authors to identify periods when certain regions were not eroded.

Although Carlson says some ice survived in higher regions of central Greenland, enough melted to be responsible for 4.5-6m of sea level rise. 

Alberto Reyes. By studying the sediments at the edge of the Greenland Ice Sheet, researchers have identified a period when southern Greenland was ice free.

On the other hand, during the last interglacial 125,000 years ago, much more ice was retained, with the Greenland loss estimated to be worth 2.5m of sea level rise. Despite this the total rise during the most recent warm period was almost as large; 5-10m. “Other studies have shown that Antarctica may have been unstable at the time and melting there may have made up the difference," says Carlson.

The reasons for the differences between the two eras are poorly understood, but the paper notes that, “MIS 11 was an exceptionally long interglaciation”.

Just as MIS 11 and the last interglacial had different effects on Antarctic and Greenland melting, neither serves as a perfect model for our near future. Human induced warming is occurring at a rate far in excess of the natural trends in the lead up to either of these periods. Nevertheless, a better understanding of the events of previous warm eras can help us refine our understanding of how fast seas are likely to rise this time

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