Does the retreat of glaciers on the West Antarctic Ice Sheet mean we're in for an imminent climate catastrophe? The short answer is no. Is the ice sheet collapsing? That depends on your definition of "collapse." Are the two studies documenting Antarctica's meltdown worth all the buzz they've generated? Yes.
Here's a reality check on some of the questions raised by this week's studies:
What's happening to Antarctica's ice?
The continent's glaciers are in a constant state of flux, Penn State glaciologist Sridhar Anandakrishnan told NBC News.
"You can think of a glacier like a bank account," he explained. "You get deposits every year as snowfall, and you get withdrawals as the glacier melts and flows out into the ocean. If they're equal, then there's no change in the size of the glacier or the ocean. When your withdrawals are greater than your deposits, the glacier shrinks, and the oceans rise. It really is a zero-sum game."
Documenting the income and outgo of ice can be difficult for the West Antarctic Ice Sheet because the most significant "withdrawals" are made from beneath, as warmer ocean waters melt glacial ice. The researchers behind this week's studies relied on radar surveys that gauged where the ice was in relation to the underlying land mass. They found that in some areas, the "grounding line" where glacier meets land has retreated tens of kilometers over the course of less than two decades.
So where's the 'collapse'?
The collapse refers to this glacial retreat, but that doesn't mean there'll be a catastrophic splash of ice sheets into the sea anytime soon. In fact, The New York Times' Andrew Revkin takes issue with the word "collapse" because it implies a sudden, catastrophic breakdown. Ice is being lost. However, the computer models suggest that one of the ice sheet's key glaciers, Thwaites Glacier, won't disappear for another 200 to 1,000 years.
The problem is that the models indicate there's nothing to hold back the continuation of the glacier's retreat. "It looks like all the feedbacks tend to point toward it actually accelerating over time; there's no real stabilizing mechanism we can see," the University of Washington's Ian Joughin said in a news release.
But how can the glacial ice be disappearing when Antarctic sea ice is reaching record levels?
Anandakrishnan said the ebb and flow of sea ice is a different phenomenon. "Really, it's the difference between year-to-year variations in the local weather, and the long-term driving forces that control the glaciers," he said. Skeptical Science provides a detailed explanation of the distinction between sea ice and glacial ice, and how wind and ocean currents lead to increased sea ice production.
Sea ice does not contribute to rising sea levels, but the land-based glacial ice does, Anandakrishnan noted. "You can take a bucket of water and turn it into ice. That chunk of ice melts and turns back into water, and there's no net change in sea level," he said. "It's the land ice that's locked, or trapped — when it starts to flow out more rapidly than it usually does, it adds to the net amount in the ocean."
How much is Antarctica's glacial retreat contributing to rising sea levels?
Right now, not much. The National Ocean Service says the main contributors to global sea level rise are thermal expansion — that is, the fact that water expands as it warms — and the loss of glacial ice.
Even when it comes to glaciers, Antarctica isn't the only game in town: Recent studies have shown a faster-than-expected thaw of the Greenland Ice Sheet, which has been linked to natural weather variations as well as greenhouse-gas emissions.
"How much water could you get out of Greenland in the next century?" Anandakrishnan asked. "It's not been a big number over the past few decades, but it looks as if it's going to get bigger in the century ahead."
The Intergovernmental Panel on Climate Change estimated last year that global sea level might rise somewhere between 4 inches and 3 feet (10 to 90 centimeters) by 2100. Anandakrishnan said the latest findings would probably put future estimates toward the high end of that scale.
Computer models suggest that the Antarctic meltdown would become much more of a factor centuries from now, when entire glaciers disappear. Joughin and his colleagues said the loss of Thwaites Glacier would raise sea level by about 2 feet (60 centimeters), while the rest of the West Antarctic Ice Sheet would add 10 to 13 feet (3 to 4 meters).
Those levels aren't unprecedented: The U.S. Geological Survey notes that 21,000 years ago, sea levels were 410 feet (125 meters) lower than they are today. About 2.2 million years ago, they were 80 to 160 feet (25 to 50 meters) higher. But even a 3- to 5-foot rise would have a significant impact on today's coastal urban areas. To get a sense of what would happen to U.S. cities, check out this interactive graphic from The New York Times.
Is the meltdown really 'unstoppable'?
That's what climate scientists say, based on their analysis of Antarctic topography. Anandakrishnan says it's difficult to have an impact on glacial change. "I compare it to an aircraft carrier vs. a toy boat on a lake," he said. "Small changes can affect the boat, whereas with an aircraft carrier, it takes a lot to change its direction."
Will the Antarctic meltdown turn into a crisis in the year 2200, or the year 3000? Could there be enough time to turn an unstoppable trend into a solvable problem? It's too early to say.
"We're heading into a world for which we don't have good analogs," Anandakrishnan said. "We're going to have to learn a lot, and fairly quickly. But I think we can do it. I think in 10 years, we'll have a smaller range of numbers."