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A 30-kilometer crack angles across the Pine Island Glacier, a vulnerable part of the West Antarctic Ice Sheet.
NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team/Flickr
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Scientists have now identified the source of all that water: a collapse of the West Antarctic Ice Sheet. Glaciologists worry about the present-day stability of this formidable ice mass. Its base lies below sea level, at risk of being undermined by warming ocean waters, and glaciers fringing it are retreating fast. The discovery, teased out of a sediment core and reported last week at a meeting of the American Geophysical Union in Washington, D.C., validates those concerns, providing evidence that the ice sheet disappeared in the recent geological past under climate conditions similar to today’s. “We had an absence of evidence,” says Anders Carlson, a glacial geologist at Oregon State University in Corvallis, who led the work. “I think we have evidence of absence now.”
If it holds up, the finding would confirm
that “the West Antarctic Ice Sheet might not need a huge nudge to
budge,” says Jeremy Shakun, a paleoclimatologist at Boston College.
That, in turn, suggests “the big uptick in mass loss observed there in
the past decade or two is perhaps the start of that process rather than a
short-term blip.” If so, the world may need to prepare for sea level to
rise farther and faster than expected: Once the ancient ice sheet
collapse got going, some records suggest, ocean waters rose as fast as
some 2.5 meters per century.
As an analogy for the present, the Eemian, from 129,000 to 116,000
years ago, is “probably the best there is, but it’s not great,” says
Jacqueline Austermann, a geophysicist at Columbia University’s
Lamont-Doherty Earth Observatory. Global temperatures were some 2°C
above preindustrial levels (compared with 1°C today). But the cause of
the warming was not greenhouse gases, but slight changes in Earth’s
orbit and spin axis, and Antarctica was probably cooler than today. What
drove the sea level rise, recorded by fossil corals now marooned well
above high tide, has been a mystery.
Scientists once blamed the melting of Greenland’s ice sheet. But in 2011, Carlson and colleagues exonerated Greenland
after identifying isotopic fingerprints of its bedrock in sediment from
an ocean core drilled off its southern tip. The isotopes showed ice
continued to grind away at the bedrock through the Eemian. If the
Greenland Ice Sheet didn’t vanish and push up sea level, the vulnerable
West Antarctic Ice Sheet was the obvious suspect. But the suspicion rested on little more than simple subtraction, Shakun says. “It’s not exactly the most compelling or satisfying argument.”Carlson and his team set out to apply their isotope technique to Antarctica. First, they drew on archived marine sediment cores drilled from along the edge of the western ice sheet. Studying 29 cores, they identified geochemical signatures for three different bedrock source regions: the mountainous Antarctic Peninsula; the Amundsen province, close to the Ross Sea; and the area in between, around the particularly vulnerable Pine Island Glacier.
Armed with these fingerprints, Carlson’s team then analyzed marine sediments from a single archived core, drilled farther offshore in the Bellingshausen Sea, west of the Antarctic Peninsula. A stable current runs along the West Antarctic continental shelf, picking up ice-eroded silt along the way. The current dumps much of this silt near the core’s site, where it builds up fast and traps shelled microorganisms called foraminifera, which can be dated by comparing their oxygen isotope ratios to those in cores with known dates. Over a stretch of 10 meters, the core contained 140,000 years of built-up silt.
For most of that period, the silt contained geochemical signatures from all three of the West Antarctic bedrock regions, the team reported, suggesting continuous ice-driven erosion. But in a section dated to the early Eemian, the fingerprints winked out: first from the Pine Island Glacier, then from the Amundsen province. That left only silt from the mountainous peninsula, where glaciers may have persisted. “We don’t see any sediments coming from the much larger West Antarctic Ice Sheet, which we’d interpret to mean that it was gone. It didn’t have that erosive power anymore,” Carlson says.
He concedes that the dating of the core is not precise, which means the pause in erosion may not have taken place during the Eemian. It is also possible that the pause itself is illusory—that ocean currents temporarily shifted, sweeping silt to another site.
More certainty is on the way. Next month, the International Ocean Discovery Program’s JOIDES Resolution research ship will begin a 3-month voyage to drill at least five marine cores off West Antarctica. “That’s going to be a great test,” Carlson says. Meanwhile, he hopes to get his own study published in time to be included in the next United Nations climate report. In the 2001 and 2007 reports, West Antarctic collapse was not even considered in estimates of future sea level; only in 2013 did authors start to talk about an Antarctic surprise, he says. Research is due by December 2019. “We gotta beat that deadline.”
Links
- Ancient Antarctic Ice Sheet Collapse Could Happen Again, Triggering A New Global Flood
- Nations falling short of emissions cuts set by Paris climate pact, analysis finds
- Absence of the West Antarctic ice sheet during the last interglaciation
- In just 12 days, the world gets half of its annual rainfall
- Massive crater under Greenland’s ice points to climate-altering impact in the time of humans
- New climate report actually understates threat, some researchers argue
- Key climate panel, citing impending crisis, urges crash effort to reduce emissions
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