‘Once you pull the stopper out of the bottle, the glaciers move much faster …’
An Antarctic ice sheet about the size of Colorado disintegrated suddenly and quickly at the end of the last global ice age, scientists concluded in a new study, showing what might happen in other parts of the cryosphere as Earth’s atmosphere continues to warm.
Analyzing detailed data from a 2015 research expedition, U.S. and Japanese oceanographers showed that a 100,000-square-mile section of the Ross Ice Shelf broke apart within 1,500 years during a warming period after the last ice age.
The Ross Ice Shelf is the world’s largest ice shelf, a vast floating extension of the West Antarctic Ice Sheet that is about the size of France. But at the end of the last ice age, it extended much farther north and covered the entire Ross Sea.
The new paper was co-authored by Rice University oceanographer John Anderson, postdoctoral research associate Lauren Simkins, graduate student Lindsay Prothro and colleagues at the University of Tokyo. It was published this month in the Proceedings of the National Academy of Sciences.
“At the height of the last ice age, we know that the sheet of ice covering the Antarctic continent was larger and thicker than it is today,” said Anderson, Rice’s Maurice Ewing Professor of Oceanography and professor of Earth science. “This continent-enveloping ice sheet extended all the way to the continental shelf, and in western Antarctica it filled the entire Ross Sea basin.”
“We found that about 10,000 years ago, this thick, grounded ice sheet broke apart in dramatic fashion,” Anderson said. “The evidence shows that an armada of icebergs — each at least twice as tall as the Empire State Building — was pushed out en masse. We know this because this part of the Ross Sea is about 550 meters (1,804 feet) deep, and the icebergs were so large and so tightly packed that they gouged huge furrows into the seafloor as they moved north.”
Researchers measured the furrows using a seafloor mapping system — the most sensitive ever employed in the Antarctic — during a 2015 cruise by the U.S. research vessel Nathaniel B. Palmer, which is operated by the National Science Foundation.
Simkins, who helped gather data during the 56-day cruise, said other features preserved on the seafloor, formed by the retreating ice, showed that the margin of the grounded ice sheet retreated rapidly after the initial collapse and fell back hundreds of miles in stair-step fashion.
The Ross Ice Shelf appeared after the breakup of the ice sheet. An ice shelf is the floating, seaward extension of an ice sheet and marks the point at which the ice is thin enough to float.
“The grounding line is the location where the ice actually sits on the seafloor,” Simkins said. “Following ice-shelf break up, the grounding line is left exposed to marine processes, such as ocean warming, which can erode the grounding line and cause it to move back toward the shore.”
The retreat was halted when the grounding line reached a series of shallow banks that acted as anchors and stabilized the ice shelf for about 5,000 years.
Anderson said, “Throughout this period, the ice shelf was pinned atop these shallow banks. On the surface, ice still covered large portions of the Ross Sea, but there was open water beneath the ice shelf.”
The detailed reconstruction of ice shelf history was also aided by geochemical analyses of seafloor sediments that were overseen by study lead author Yusuke Yokoyama, a professor at the University of Tokyo who was also a Wiess Visiting Professor in Rice’s Department of Earth Science in 2014-2015.
“The really big breakup began around 3000 B.C.,” Anderson said. “We believe it was similar, in many respects, to the breakup of the Larsen B Ice Shelf in 2002. The Larsen is far smaller than the Ross Ice Shelf, but satellite imagery that year showed the Larsen dramatically breaking apart in just a few weeks. We believe the large breakup of the Ross Ice Shelf occurred at roughly this same pace, but the area involved was so much larger — about the size of the state of Colorado — that it took several centuries to complete.”
By 1500 B.C. the breakup had exposed about 100,000 square miles of the Ross Sea that had been either wholly or largely ice-covered for many millennia, Anderson said.
Anderson said knowledge about the past behavior of the ice sheet and ice shelf, in particular their rate of response to atmospheric and oceanic warming, informs scientists about how present-day ice sheets and ice shelves may respond to future warming.
“There are similarities to what we see the modern Ross Ice Shelf doing,” Anderson said. “The farthest boundary of the ice shelf extends nearly 1,000 kilometers (621 miles) from the grounding line, where the ice sheet is grounded in about 800 meters (2,625 feet) of water. That’s a condition that most glaciologists consider unstable, and it is not unlike the situation that existed prior to the big breakup that began 5,000 years ago.”
The present Ross Ice Shelf is about 500 miles wide and several hundred feet thick. Because the ice shelf is already floating, its breakup and melting would not, by itself, pose a risk of raising global sea level, Anderson said. However, he pointed out that the ice shelf acts as a brake to dozens of Antarctic ice streams and outlet glaciers, and ice flowing into the ocean from those would contribute to global sea level rise.
“The ice shelf slows the flow of grounded ice from the glaciers, and as we saw after the Larsen B breakup, once you pull the stopper out of the bottle, the glaciers move much faster, in some cases about 10 times faster,” Anderson said.