New research finds that widespread collapse of the West Antarctic ice sheets is not inevitable: the pace of ice loss varies according to regional differences in the atmosphere and ocean circulation.
An international team of researchers has combined satellite imagery and climate and ocean records to gain the most detailed understanding yet of how the West Antarctic Ice Sheet — which contains enough ice to raise global sea level by 3.3 metres — is responding to climate change.
The researchers, from the University of Cambridge, the University of Edinburgh and the University of Washington, found that the frequency and extent of ice destabilization along the coast of West Antarctica varies with regional climate.
Their results have been published in the journal Nature Communicationsshows that while the West Antarctic ice sheet continues to retreat, the pace of retreat slowed across a vulnerable area on the coast between 2003 and 2015. This slowdown was driven by changes in the temperature of the surrounding oceans, which were in turn caused by differences in offshore wind conditions. .
The West Antarctic marine ice sheet, home to the vast and unstable Pine Island and the Thwaites glaciers, sits atop a land mass up to 2,500 meters below the ocean’s surface. Since the early 1990s, scientists have observed an abrupt acceleration in ice melt, retreat, and speed in this region, which has been attributed in part to human-induced climate change over the past century.
Other scientists have previously suggested that this type of response across a lower landmass could be the start of an irreversible collapse at the ice sheet level called sea ice sheet instability, which will persist independently of any other climate forcing.
“The idea that once a sea ice sheet passes a certain tipping point it will cause a rapid response has been widely reported,” said Dr Fraser Christie of the Scott Polar Research Institute in Cambridge, lead author of the paper. “Despite this, questions remain about the extent to which ongoing climate changes are regulating ice losses along the entire West Antarctic coast.”
Using observations collected by an array of satellites, Christie and her colleagues found clear regional differences in how the West Antarctic ice sheet has evolved since 2003 due to climate change, with the pace of retreat in the Amundsen Sea sector significantly slowing compared to the adjacent sector. The Bellingshausen sea sector is accelerated.
By analyzing climate and ocean records, the researchers linked these regional differences to changes in the strength and direction of offshore surface winds.
In this part of Antarctica, the prevailing winds come from the west. As these westerly winds become stronger, they stir up warmer, saltier waters from the ocean depths, which reach the Antarctic coast and increase the rate of ice melt.
“However, between 2003 and 2015 off the coast of the Amundsen offshore sector, the prevailing westerly winds decreased in intensity,” Christie said. “This means that deeper, warmer waters cannot infiltrate, and we saw a marked change in the behavior of the corresponding glaciers along the region: a clear decrease in the rate of melt and loss of glacier mass.”
So what caused these winds to weaken, and thus reduce snowmelt? The researchers found that the main cause was the unusual deepening of the low-pressure system of the Amundsen Sea, which reduced the intrusion of warm waters. This system is the main atmospheric circulation pattern in the region, and the location of its center of pressure—near which changes in the strength of offshore winds are greatest—is usually farther from the coast of the same name for most of the year.
Away from this center of pressure, the researchers found that the accelerating response of glaciers flowing from the Bellingshausen marine sector could be explained by relatively unchanging winds, allowing for more sustained ocean-generated melting in comparison.
Ultimately, the study demonstrates the complexity of competing interactions between ice, ocean and atmosphere that drive short-term changes across West Antarctica, and raises important questions about how quickly the icy continent can evolve in an increasingly warming world.
“Forcing mechanisms in the oceans and atmosphere are still really important in West Antarctica,” said co-author Professor Eric Steig of the University of Washington in Seattle. “This means that the collapse of the ice sheet is not inevitable. It depends on how the climate will change over the next few decades, which we can influence in a positive way by reducing greenhouse gas emissions.”
The researchers stress that more work is needed to examine the relevance of such mechanisms in the future amid a background of increasing sea ice cover instability. Co-author Professor Robert Bingham from the University of Edinburgh is now working directly on the Thwaites glacier to understand how it is affected by climate change.
“This study reinforces the urgent demand to demonstrate how rapidly the most vulnerable regions of the West Antarctic ice sheet such as Thwaites Glacier are retreating, with global ramifications for sea level rise,” said Bingham. “The new data we are currently getting from crossing the Thwaites Glacier in January will directly address this goal.”
“There is a close connection between climate and how the ice behaves,” said Christie. “We have the potential to mitigate ice losses in West Antarctica – if we can reduce carbon emissions.”
The study was supported by the Carnegie Endowment for Universities of Scotland, the Scottish Alliance for Earth, Environment and Society Sciences (SAGES), the Prince Albert II of Monaco Foundation and the Natural Environment Research Council (NERC), part of the UK Research and Innovation Foundation. (UKRI), the US National Science Foundation, and the joint UK NERC/NSF International Thwaites Glacier and European Space Agency (ESA) international collaboration project.