by Tom Parisi
New scientific evidence fills in an important missing piece of the puzzle as scientists wrestle to determine how global warming will affect the West Antarctic Ice Sheet (WAIS).
NIU’s Ross Powell is co-lead author on a paper published by a team of scientists in the March 19 issue of the prestigious journal Nature. The research, which gained worldwide media attention, shows that the giant WAIS has had a history of dynamic retreat and even collapse during past cycles of global warming and elevated atmospheric carbon-dioxide concentrations that occurred during the Pliocene Epoch, between 2 million and 5 million years ago.
Any substantial melting of the ice sheet would cause a rise in global sea levels.
Powell is a Distinguished Research Professor in the NIU Department of Geology and Environmental Geosciences. NIU geologist Reed Scherer also is among the authors of the Nature study.
The research is based on investigations conducted by a 56-member team of scientists on a 4,200-foot-long sedimentary rock core taken from beneath the sea floor under Antarctica’s Ross Ice Shelf during the first project of the ANDRILL (ANtarctic geological DRILLing) research program. The drilling program was among the largest scientific projects ever conducted in the Antarctic.
The cores allowed the scientists to peer back in time to the Pliocene, when the Antarctic was in a natural climate state that was warmer than today and atmospheric carbon dioxide levels were higher. Data from the cores indicate the WAIS advanced and retreated numerous times in response to forcing driven by these climate cycles.
“The sedimentary record from the ANDRILL project provides scientists with an important analogue that can be used to help predict how ice shelves and the massive WAIS will respond to future global warming over the next few centuries,” Powell said.
“The sedimentary record indicates that under global warming conditions that were similar to those projected to occur over the next century, protective ice shelves could shrink or even disappear and the WAIS would become vulnerable to melting,” Powell said. “If the current warm period persists, the ice sheet could diminish substantially or even disappear over time. This would result in a potentially significant rise in sea levels.”
“For years scientists have been trying to figure out how the ice sheet behaved during the warm periods of the Pliocene,” Scherer added. “But we had very little data. Now we know the West Antarctic Ice Sheet has a reliable record of advancing and retreating on a large scale.”
The new research refines previous findings about the relationship between atmospheric carbon-dioxide concentration, atmospheric and oceanic temperatures, sea-level rise and natural cycles in Earth’s orbit around the sun. It also shows that even a slight rise in atmospheric concentrations of carbon dioxide, one of the gases that drives global warming, affects the WAIS stability.
Tim Naish, director of Victoria University of Wellington’s Antarctic Research Centre, said the new information gleaned from the core shows that changes in the tilt of Earth’s rotational axis has played a major role in ocean warming that has driven repeated cycles of growth and retreat of the WAIS for the period in Earth’s history between 3 million and 5 million years ago.
“It also appears that when atmospheric carbon dioxide concentrations reached 400 parts per million around 4 million years ago, the associated global warming amplified the effect of the Earth’s axial tilt on the stability of the ice sheet,” said Naish, who led the Naturestudy with Powell.
“Carbon dioxide concentration in the atmosphere is again approaching 400 parts per million,” Naish said. “Geological archives, such as the ANDRILL core, highlight the risk that a significant body of permanent Antarctic ice could be lost within the next century as Earth's climate continues to warm. Based on ANDRILL data combined with computer models of ice sheet behavior, collapse of the entire WAIS is likely to occur on the order of 1,000 years, but recent studies show that melting has already begun.”
A second related study in Nature – led by David Pollard of Pennsylvania State University and Rob DeConto from University of Massachusetts – reports results from a computer model of the ice sheets. The model shows that each time the WAIS collapsed, some of the margins of the East Antarctic Ice Sheet also melted, and the combined effect was a global sea-level rise of 7 meters above present-day levels.
Whether the beginnings of such a collapse could start 100 years from now or within the next millennium is hard to predict and depends on future atmospheric CO2 levels, the researchers said.
“From these combined data-modeling studies, we can say that past warming events caused West Antarctic ice shelves and ice grounded below sea level to melt and disappear. The modeling suggests these collapses took one to a few thousand years,” Pollard said.
Pollard and DeConto also underscored the role of ocean temperatures in melting of the ice.
“It’s clear from our combined research using geological data and modeling that ocean temperatures play a key role,” DeConto said. “The most substantial melting of protective ice shelves comes from beneath the ice, where it is in contact with seawater. We now need more data to determine what is happening to the underside of contemporary ice shelves.”
The ANDRILL program involves scientists from the United States, New Zealand, Italy and Germany. The National Science Foundation (NSF), which manages the U.S. Antarctic Program (USAP), provided about $20 million in support of ANDRILL.