The front of the Thwaites Ice Shelf as seen from the NASA DC-8 on Oct. 16, 2012.
The glaciers that drain ice from the massive West Antarctic Ice Sheet into the sea are becoming increasingly unstable, and are poised for an inevitable dramatic acceleration that could prompt the "rapid collapse" of this region as soon as 200 years from now, according to new scientific findings released on Monday. The loss of these glaciers is "unstoppable," one of the studies shows.
More ominously, the studies found that as these glaciers go, so too may broader sections of the larger ice sheet, which would dramatically increase the amount of sea level rise.
The glaciers of West Antarctica are already responsible for the majority of the Antarctic continent's contribution to global sea level rise, and if these glaciers were to completely collapse, sea levels could rise by at least 4 feet, potentially inundating coastal megacities around the world. Even greater sea level rise of possibly up to 15 feet would result if other parts of the West Antarctic Ice Sheet were to destabilize as well.
The findings come from two separate studies released on Monday. Together they provide a more detailed view of recent ice losses in western Antarctica, and for the first time, projections for when glaciers in this region will shift from present-day melting rates to rates that are more consistent with rapid collapse.
Both of the new studies show that it is unlikely that the loss of ice seen in recent years will slow or stop, even with a projected increases in snowfall at the surface of the glaciers. This is due in part to the physical structure of the glaciers, including the slope of the terrain the glacier is flowing over, as well as the fact that once rapid retreat of these glaciers has begun, it can be difficult or impossible to stop.
One of these studies, which comes from a team of researchers from the University of Washington, examines the recent ice loss and movement of the Thwaites Glacier in West Antarctica. The researchers used computer modeling tools to estimate ice losses during the next several centuries. The study, which is to be published in the journal Science on Thursday, found that "early-stage collapse has begun."
Glaciers along the Amundsen Coast of Antarctica, such as the Thwaites Glacier, end in the ocean, with floating ice shelves that buttress the ice sheet behind them. During the past decade or more, warm ocean water driven to the surface by increased winds surrounding Antarctica have destabilized the ice sheet from below, thinning the ice shelves and causing the grounding line, which is where grounded ice meets the ocean and becomes floating ice, to retreat inland. As the grounding lines have retreated, the Thwaites Glacier and others in the region have slid faster into the ocean and become increasingly unstable.
Illustrating the high stakes involved in the fate of West Antarctica, a previous study found that the six glaciers in the Amundsen Sea Embayment — which includes the Thwaites Glacier — contributed about 10% of all the global average sea level rise that occurred between 2005 and 2010.
The new study found that the Thwaites Glacier's grounding line rests on a coastal sill about 2,000 feet below sea level, and that as one travels further inland by about 40 miles, the grounding line steepens to more than 4,000 feet below sea level. This layout helps ocean waters creep inland simply by the force of gravity, deep beneath the surface of the glacier, similar to plaque and tartar eroding the underside of a tooth and creating a cavity that, if left untreated, will cause the tooth to rot from below.
"... With ongoing thinning and only tens of kilometers separating the grounding line from the marine basin's deepest regions, collapse of Thwaites Glacier may have already begun, albeit for now at a relatively moderate rate," the study says.
The projections showed that, in a worst-case scenario, "rapid collapse" of the Thwaites Glacier could begin within the next 200 years, or it could proceed more slowly, at closer to 1,000 years from now. However, recent melt rates suggest that higher-melt rate scenarios may be more realistic, the study says, while noting that the simulations show large uncertainties in the timing of rapid collapse. The rate at which sea levels increase will help determine how well society is able to adapt, study author Ian Joughin, a glaciologist at the University of Washington's Applied Physics Laboratory, told Mashable.
"It makes a huge difference if it happens in 200 years or 800 years, because it’s the rate of sea level change that really matters,” Joughin says.
What is clear from the projections, though, is that the future will bring faster melt rates, rather than a pause or even a reversal.
"Previously, when we saw thinning we didn't necessarily know whether the glacier could slow down later, spontaneously or through some feedback," Joughin said in a press release. "In our model simulations it looks like all the feedbacks tend to point toward it actually accelerating over time; there's no real stabilizing mechanism we can see."
Once the rapid collapse of this glacier, and others along the Amundsen Coast, begins, the study says it will likely destabilize the entirety of West Antarctica.
A separate new study that examined the retreat in glaciers' grounding lines between 1992 and 2011, found that as West Antarctic glaciers have become more and more unstable, their grounding lines have deepened, which provides warm ocean waters with easier access to the ice's underbelly. In addition, the study showed that ice loss rates have not only sped up at the grounding lines, but also more than 100 miles inland, compromising the entire ice sheet that the glacier helps hold back from the sea. Rather than computer models, this study used observational data gathered from satellites, aircraft surveys and ground-based research.
The data shows that there is no high ground, or hills, underneath the upstream areas of the glaciers that could slow or stop the flow of these glaciers into the sea, said Eric Rignot, the lead author of the study and a glaciologist at NASA's Jet Propulsion Laboratory in Pasadena, California. In addition, Rignot said "it is very unlikely" that the warm water flowing toward these glaciers will cease, since projections show even more warm water moving into this area as a result of continued manmade global warming.
Studies have tied the transport of warm water to Antarctica to both manmade global warming and natural climate variability, and there is some evidence it may be related to the depletion of the ozone layer high above Antarctica, which has altered air circulation in the atmosphere.
In a conference call with reporters, Rignot said there are now enough observations to conclude that "the retreat of ice in that area of Antarctica is unstoppable."
“This retreat will have major consequences for sea level rise worldwide,” he said. As the glaciers in West Antarctica collapse, Rignot says, “It will affect other basins in the West Antarctic ice sheet” and further boost global sea levels.
Rignot cautioned, however, that the word "collapse" can mean something different to glaciologists and the public.
"Collapse in the imagination of most people sounds like a catastrophic event that is going to happen in the next few years. We are talking about a retreat that is unstoppable because we think we have enough evidence to say that these glaciers will keep retreating for decades and even centuries to come," he said. "We’re talking about a slow degradation of ice in this part of Antarctica."
Rignot's research is to be published in the journal Geophysical Research Letters..