Before the last ice age, during a warm era some 125,000 years ago that was comparable to modern times, scientists know that the oceans reached levels that were some 15 to 20 feet higher than they are today. What they don't know is, where did the extra water come from?
Many have been assuming that it came from the melting Greenland ice sheet, but a new study points in the opposite direction — to West Antarctica — a circumstance that one researcher describes as "quite scary."
In a warming climate, questions about how ice sheets melt and how fast and how high sea levels rise are hot research topics.
Geoscientists from the University of Wisconsin, Madison, and Oregon State University, Corvallis, tested the prevailing thinking by analyzing ocean sediments deposited by glaciers melting from different, chemically distinct, regions in southern Greenland.
If the ice sheet in the regions melted down to bare land — as would be required to raise the oceans so high — the researchers reasoned that there should be a place in the ocean core samples where the glacier-fed flow of sediments was interrupted.
Instead, they found that all of the regions continued to contribute sediment throughout the period — the last interglacial — signifying that "the southern Greenland Ice Sheet did not completely deglaciate" any single region.
Writing in the new issue of the journal Science, the researchers say that ice-sheet computer models that most closely resemble their results point to a contribution to sea level rise of 1.6 to 2.2 meters — about 5 to 7 feet — from the Greenland ice sheet.
This means that to reach the observed ocean height of 15-20 feet, more or less, during the last warm era, at least another meter's worth must be coming from Antarctica, namely West Antarctica. That Greenland's ice sheet is more stable, and West Antarctica is less stable, than most scientists have been thinking is an idea that study co-author Anders Carlson describes as "quite scary."
"West Antarctica is mainly resting below present-day sea level, and it thus could have the potential for an "abrupt collapse," where a small forcing leads to a large retreat if ice becomes ungrounded and much of the ice sheet essentially coasts into the ocean," Carlson, a UW geologist, told Discovery News.
Greenland's ice, in contrast, is above sea level, so its melting may be subject to longer-term processes.
"Evidence in Earth's relatively recent past of West Antarctica disappearing raises the possibility that such a collapse could occur in the future from continued global warming," said Carlson.
IMAGE: University of Wisconsin geologist Anders Carlson surveys an outlet glacier in southwestern Greenland. CREDIT: Robert Hatfield, Oregon State University