West Antarctic ice sheet faces ‘unavoidable’ melting, a warning for sea level rise

ACCELERATING ice losses are all but "unavoidable" this century in vulnerable West Antarctic ice shelves as waters warm around them, according to new research. And the analysis could mean scientists were too conservative in predicting about 1 to 3 feet of sea level rise by 2100.

A study found that regardless of how aggressively humans act to reduce fossil fuel emissions - and thus limit how much the planet warms - waters around some of West Antarctica's glaciers are forecast to warm at a pace three times faster than they have in the past.

That is forecast to cause "widespread increases in ice-shelf melting, including in regions crucial for ice-sheet stability," according to a study published in the journal Nature Climate Change on Monday. Unlike relatively thin and floating sea ice, the ice shelves are thicker and hold back massive glaciers that contain far more ice.

"It appears we may have lost control of the West Antarctic ice shelf melting over the 21st century," Kaitlin Naughten, the study's lead author and an ocean modeller with the British Antarctic Survey, told reporters in a media briefing. "That very likely means some amount of sea level rise that we cannot avoid."

The research helps to solidify an understanding that humans have likely already pushed some polar ice systems past a tipping point and into escalating decline.

Arctic sea ice has been decreasing for decades, with data suggesting an "irreversible" thinning around the North Pole since 2007. And while Antarctic sea ice has been more stable, it now may be showing signs of dramatic declines, as well. Sea ice cover hit a record low around the South Pole in February and last month reached a winter maximum that was its smallest ever observed, by a wide margin.

As the Southern Ocean warms, thinning the floating sea ice, it is also increasingly threatening the ice shelves. The new research underscores what dozens of studies have suggested for three decades, said glaciology researcher Ted Scambos: The West Antarctic ice sheet appears to be headed for an eventual "collapse".

"It is the opposite of resilient," Scambos, a senior research scientist at the Earth Science & Observation Center at the University of Colorado at Boulder, said in an e-mail. "It takes an ice age to build it, but in a warm period, like now, it teeters on instability."

The new research focused on the Amundsen Sea, an area of the Southern Ocean that surrounds some of Antarctica's largest glaciers, which are buttressed by the thinning and retreating ice shelves. That includes Thwaites Glacier, which scientists have nicknamed the "doomsday" glacier because if it retreats far enough, it would essentially compromise the centre of West Antarctica.

Scientists have estimated losses to Thwaites could eventually trigger as much as 10 feet of sea level rise, with recent research suggesting the glacier is already showing signs of disintegration.

In the study, simulations of future warming in the Amundsen show sea temperatures rising dramatically in any of a number of scenarios for future global warming. The researchers explored how the seas would warm in a world where global warming is limited to 1.5 degrees Celsius (2.7 degrees Fahrenheit) - the ambitious target global leaders agreed upon in Paris in 2015 - but also two scenarios that allow for more middle-range pathways for emissions and the resulting planetary warming.

In each of those scenarios, projections of how much the Amundsen would warm were "statistically indistinguishable," Naughten said.

The researchers only noted significant changes in their projections of Amundsen warming under the most pessimistic scenario for global warming, one featuring massive fossil fuel use throughout the century and strong warming. In that case, Amundsen Sea temperatures would be expected to rise more than 2 degrees Celsius (3.6 degrees Fahrenheit) per century at some depths, close to twice as quickly as predicted in the other scenarios.

In a Nature column published alongside the study, one scientist called it "the most comprehensive set of future projections of warming in the Amundsen Sea so far." Taimoor Sohail, a post-doctoral researcher at the University of New South Wales in Australia, wrote that the study highlights the urgency of not just reducing greenhouse gas emissions, but adapting communities for now-inevitable effects of climate change, including sea level rise.

Other scientists not involved in the research also called its approach sound. Scambos said the analysis is "about as good as the state of the art at present." That the researchers described the melting as "unavoidable" in the title of their paper "means they're pretty confident," he added.

IMPLICATIONS FOR GLOBAL SEA LEVEL RISE

Because the ice shelves are floating, their melting does not directly add to sea level rise. But their vulnerability raises questions about how much of the grounded ice cover across Antarctica, which is frequently over a mile thick, could eventually flow into the Southern Ocean.

The latest report of the UN Intergovernmental Panel on Climate Change, released in March, might give the impression that Antarctica will make a modest contribution to sea level rise by 2100.

The panel considers multiple emissions scenarios and gives a range of possible outcomes, including some severe ones. Yet its central predictions only show around a third of a foot of sea level rise by the end of this century because of ice losses from the enormous frozen continent. That's despite a vast range in possible scenarios for human greenhouse gas emissions.

While there's little doubt that the new research worsens this outlook, it isn't clear by how much, because the loss of ice from West Antarctica will still take a long time to play out, and will undoubtedly be much more dramatic after 2100.

Naughten said the study's findings are not yet incorporated into the IPCC's sea level rise projections. Translating the expected warming in the Amundsen Sea into an estimate of sea level rise involves a complex research undertaking of its own, factoring in melting as well as snowfall and the flow of glaciers, she said.

Thwaites glacier is showing quickening ice losses, but has thus far only contributed a few millimetres to sea level rise since the late 1970s, according to data shared by Eric Rignot, an expert on Antarctic glaciers at the University of California at Irvine. Scientists generally fear that this could get a lot worse, but also think it may take a few more decades to reach this dire point.

Rignot notes that scientists first detected Thwaites' retreat inland along the sea floor in 1992. The glacier has since moved its moorings about 18 miles toward the centre of Antarctica, he said. But it still has another 12 to 18 miles to go until it reaches "the very deep part when the retreat will go orders of magnitude faster," Rignot says.

In other words, there's a difference between saying that something can't be stopped, and saying that it has already arrived. While coastal planners still have time for a lot preparation, the world contains an enormous amount of infrastructure and millions of people living in vulnerable, low-lying areas.

What the study stresses is that much of the ice shelf losses are already locked in given how much the planet has warmed since humans began consuming fossil fuels - more than 1 degree Celsius (1.8 degrees Fahrenheit).

"To stop or slow down the retreat, we have to go back to a cooler climate," Rignot said in an email.

Naughten acknowledged the research would likely contribute to pessimistic views of global warming, but said it still demonstrates the importance of action to reduce greenhouse gas emissions and limit global warming. While the ice shelves near the Amundsen Sea could eventually contribute to sea level rise, the region accounts for just 10 percent of Antarctic ice, she said.

"Even if we can't avoid melting this region, we could still avoid the melting of East Antarctica," she said. "We could still avoid damage to coral reefs. We could still avoid heat waves."

- THE WASHINGTON POST