Large parts of the massive East Antarctic Ice Sheet did not retreat significantly during a time when atmospheric carbon dioxide concentrations were similar to today's levels, according to a team of researchers funded by the National Science Foundation (NSF). The finding could have significant implications for global sea level rise.
The East Antarctic Ice Sheet is one of three major ice sheets closely watched by scientists as global temperatures and carbon dioxide levels increase, glaciers melt and sea levels rise. Of the three, the East Antarctic Ice Sheet is the largest potential contributor to sea level rise.
To gauge the ice sheet's stability, researchers took ultra-sensitive analytical measurements of chemical signatures in sediment samples taken from the ice sheet's sea bed. They concluded that some ice on the southernmost part of the continent could be stable in a warming climate, as was the case during the Pliocene Epoch.
But they also caution that ongoing, rising carbon emissions mean that carbon dioxide levels will soon surpass the benchmark set during the Pliocene -- approximately 5.3 to 2.6 million years ago -- the last time Earth experienced carbon dioxide levels higher than 400 parts per million.
The findings were published today in the journal Nature.
The researchers relied on samples taken as part of the international ANtarctic geological DRILLing (ANDRILL) project. NSF funded the U.S. participation in ANDRILL.
Examining sediment samples delivered from land-based sections of the ice sheet, the researchers found those areas that drain into Antarctica's Ross Sea have been stable for the past 8 million years.
The geological history of the massive ice sheet -- frozen both above and, in many places, below the ocean's surface -- has been difficult to pinpoint. The absence of data about the East Antarctic Ice Sheet's response to warming in the past have hindered efforts to predict its role in future sea level rise.
This study focused on the portion of the ice sheet that sits above the ocean. It holds enough water to cause as much as 34 meters of sea level rise if the ice sheet were to melt completely.
Another component of the East Antarctic Ice Sheet, marine-based ice, sits below sea level and is thus directly affected by the ocean.
"Based on this evidence from the Pliocene, today's current carbon dioxide levels are not enough to destabilize the land-based ice on the Antarctic continent," said Jeremy Shakun, an assistant professor of Earth and Environmental Science at Boston College and a lead author on the paper.
"This does not mean that at current atmospheric carbon dioxide levels Antarctica won't contribute to sea level rise. Marine-based ice is already starting to add to sea level rise and alone could contribute as much as 20 meters. We're saying that the terrestrial segment of the ice sheet is more resilient at current carbon-dioxide levels."
Shakun's co-authors on the paper include Carling C. Hay, also of Boston College; researchers Lee B. Corbett, Paul R. Bierman, Kristen Underwood and Donna M. Rizzo of the University of Vermont; Susan R. Zimmerman of the Lawrence Livermore National Laboratory; Marc W. Caffee of Purdue University; and Tim Naish and Nicholas R. Golledge of Victoria University of Wellington, New Zealand.
Estimates on global sea level rise due to melt from the East Antarctic Ice Sheet during the Pliocene vary from 5 meters to more than 40 meters higher than today. The upper end of this range would imply that most of the ice on the planet melted, enough to raise sea levels by 63 meters.
If the land-based East Antarctic Ice Sheet was stable during the Pliocene, as Shakun and colleagues suggest, the Pliocene total could have been at most 30 meters.
The researchers analyzed sediment contained in drill cores taken from the sea floor. These cores contain geological records, but also chemical signatures -- in particular, the rare isotopes beryllium-10 and aluminum-26, which were extracted at the NSF-funded Community Cosmogenic Facility at the University of Vermont and measured using particle accelerators at Purdue University and Lawrence Livermore National Laboratory.
Both isotopes are found in rock surfaces that have been exposed to cosmic radiation bombarding the Earth from outer space. Researchers usually examine rock samples from hillsides, mountain tops and rivers to determine where and when ice retreated during prior geological eras.
Shakun and some of the co-authors of the latest report used a different approach two years ago to offer one of the most comprehensive climatological accounts ever compiled of the Greenland Ice Sheet, dating back 7.5 million years.
In the Greenland study, levels of beryllium-10 found in sandy deposits brought out to sea in icebergs suggested the ice sheet has been a "persistent and dynamic" presence that melted and re-formed periodically in response to temperature fluctuations. The findings helped confirm that the Greenland Ice Sheet is a sensitive responder to global climate change.
Earlier studies of the East Antarctic Ice Sheet indicated that some marine-based portions of the ice sheet and its neighboring West Antarctic Ice Sheet retreated during the Pliocene. But it was unclear whether terrestrial ice also retreated.
Their analysis found "extremely low" concentrations of beryllium and aluminum isotopes in quartz sand in the marine sediment samples taken in the region, which leads to the conclusion that the ice sheet has been stable for millions of years.
While the sediment was the product of erosion from the continent, the low levels of tell-tale chemical signatures reveal that the sediment experienced only minimal exposure to cosmic radiation, leading the team to conclude East Antarctica must have remained covered in ice.
According to the paper's authors, "the findings indicate that atmospheric warming during the past eight million years was insufficient to cause widespread and/or long-lasting meltback of the EAIS margin onto land."
The findings not only clarify the past impact of rising temperatures on East Antarctic ice, said Shakun, but confirm the accuracy of models scientists are using to assess past and future consequences of a warming planet.