By A'ndrea Elyse Messer
One major threat of planetary warming is the melting of the great polar ice sheets, and the resulting rise in global sea level. Particularly worrisome to researchers is the fragility of the West Antarctic ice sheet (WAIS), whose bed lies well below sea-level, accelerating the natural flow between the grounded ice sheet itself and the floating ice shelves that make up its boundary.
When these floating shelves melt sufficiently, David Pollard said, they no longer buttress the grounded ice upstream, which then flows faster and rapidly drains the massive interior ice. The grounding line (the junction between the floating ice shelf and upstream ice resting on bedrock) retreats, converting more grounded ice to floating ice. Eventually, nearly all of the ice sheet on the Pacific side of Antarctica can disappear.
Indeed it has done so, as past climates have waxed and waned, but little was actually known about this history. Recently, however, Pollard, a senior research scientist at Penn State, and Robert M. DeConto, professor of climatology at the University of Massachusetts, created a computer model of WAIS’s last 5 million years. To specify past variations in snowfall, snowmelt and ocean melting they relied on records of deep-sea oxygen isotope ratios that indicate temperature changes in the oceans.
"We found that the West Antarctic ice sheet varied a lot, collapsed and re-grew multiple times over that period," said Pollard. These changes have been rapid, and strongly influenced by ocean temperatures near the continent. "The ocean's warming and melting the bottom of the floating ice shelves has been the dominant control on West Antarctic ice variations."
Pollard and DeConto have compared their model to the early results of the ANtarctic geological DRILLing project (ANDRILL), a multinational collaboration to drill through the ice to ocean floor sediment, in effect going back in time to recover a history of paleoenvironmental changes.
"The ice sheets in our model changed in ways that agree well with the data collected by the ANDRILL project," Pollard said. "Our modeling extends the reach of the drilling data to justify that the data represent the entire West Antarctic area and not just the spot where they drilled."
Along with the rapid appearance and disappearance of ice, the researchers note that both the ANDRILL record and their model show that, early in the 5-million year history, the periodicity of glaciation and melting was about 40,000 years, which matches the pattern in the Northern Hemisphere. According to Pollard, this pattern is probably attributable to the tilt of the Earth's axis, which varies with the same period. Nearer to the present, cycle time increases to about 100,000 years, in alignment with the Northern Hemisphere’s ice ages.
During past warm periods, the model also shows, major collapses take a few thousand years—the expected time scale of future collapse of the West Antarctic ice sheet if ocean temperatures warm sufficiently.
The researchers note that when atmospheric carbon dioxide levels have been at about 400 parts per million, as in the early part of the ANDRILL record, West Antarctic ice sheet collapses were much more frequent.
"We are a little below 400 parts per million now and heading higher," Pollard said. "One of the next steps is to determine if human activity will make it warm enough to start the collapse."
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David Pollard is senior scientist in Penn State's College of Earth and Mineral Sciences' Earth and Environmental Systems Institute. Contact him at pollard@essc.psu.edu. Pollard and Robert M. DeConto, professor of climatology at the University of Massachusetts, reported their findings in the March 19 issue of Nature. The National Science Foundation supported this work.
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