99% of Earth’s freshwater ice is locked up in the Antarctic and Greenland ice caps. Now, a growing number of studies are raising the possibility that as those ice sheets melt, sea levels could rise by six feet this century, and far higher in the next, flooding many of the world’s populated coastal areas.
5 May 2016 from Yale 360 by Nicola Jones
In April 2016 in Greenland, more than a tenth of the ice sheet’s surface was melting in the unseasonably warm spring sun, smashing 2010’s record for a thaw so early in the year. In the Antarctic, warm water licking at the base of the continent’s western ice sheet is, in effect, dissolving the cork that holds back the flow of glaciers into the sea; ice is now seeping like wine from a toppled bottle.
The planet’s polar ice is melting fast, and recent satellite data, models, and fieldwork have left scientists sobered by the speed of the sea level rise we should expect over the coming decades. Although researchers have long projected that the planet’s biggest ice sheets and glaciers will wilt in the face of rising temperatures, estimates of the rate of that change keep going up. When the Intergovernmental Panel on Climate Change (IPCC) put out its last report in 2013, the consensus was for under a meter (3.3 feet) of sea level rise by 2100. In just the last few years, at least one modeling study suggests we might need to double that.
Eric Rignot at the University of California, Irvine says that study underscores the possible speed of ice sheet melt and collapse. “Once these processes start to kick in,” he says, “they’re very fast.”
The Earth has seen sudden climate change and rapid sea level rise before. At the end of the planet’s last glaciation, starting about 14,000 years ago, sea levels rose by more than 13 feet a century as the huge North American ice sheet melted.
Greenland is losing some 200 billion tons of ice each year. That rate doubled from the 1900s to the 2000s.
But researchers are hesitant about predicting similarly rapid climate shifts in our future given the huge stakes involved: The rapid collapse of today’s polar ice sheets would erase densely populated parts of our coastlines.
“Today, we’re struggling with 3 millimeters [0.1 inch] per year [of sea level rise],” says Robert DeConto at the University of Massachusetts-Amherst, co-author of one of the more sobering new studies. “We’re talking about centimeters per year. That’s really tough. At that point your engineering can’t keep up; you’re down to demolition and rebuilding.”
Antarctica and Greenland hold the overwhelming majority of the world’s ice: Ninety percent of the planet’s freshwater ice is locked up in Antarctica’s ice cap and nine percent in Greenland’s. Today, the ice sheet that’s inarguably melting fastest is Greenland. That giant block of ice, which has the potential to raise global sea levels by 23 feet if it melts in its entirety, is losing some 200 billion tons of ice each year. That rate has doubled from the 1900s to the 2000s.
“We are seeing changes in Greenland in all four corners, even in the far north,” says Rignot. Many of the outlet glaciers that flow down fjords into the sea, which were “on the fence” about retreating or advancing over the past decade, are now “starting to fall apart,” he says.
And they’re moving fast. “The flow speeds we talk about today would have been jaw-dropping in the 1990s,” says Ted Scambos of the University of Colorado’s National Snow and Ice Data Center. Greenland’s Jakobshavn Glacier dumped ice into the sea at the astonishing rate of 150 feet per day in the summer of 2012. The most dramatic action in Greenland is simply from surface melting, as temperatures there and across the Arctic have soared in the last four decades. In 2012, Greenland lost a record 562 billion tons of ice as more than 90 percent of its surface melted in the summer sun.
Many questions remain about the physics of Greenland’s ice loss, such as whether meltwater gets soaked up by a ‘sponge’ of snow and ice, or trickles down to lubricate the base of the ice sheet and speed its seaward movement. Most modeling work has been about how Greenland’s melt tracks rising air temperatures; far less is known about how warming waters might eat away at the edges of its ice sheet. Rignot is part of a team now launching the Oceans Melting Greenland project (with the intentionally punny acronym OMG) to investigate that. These uncertainties make Rignot think that estimates of Greenland’s melt — contributing as much as 9 inches of global sea level rise by 2100, according to the 2013 IPCC report — have been far too conservative. Assuming that the Greenland ice sheet’s demise “will be slow is wishful thinking,” Rignot says.
“Greenland is more predictable and straightforward,” at least, says DeConto. Although the ice sheet can be expected to steadily melt in the face of rising temperatures, Greenland’s ice cap shouldn’t rapidly collapse, because most of its ice sits safely on rock far above sea level. For fear of rapid, runaway collapse, the research community turns its eyes south.
Antarctica is, for now, losing ice more slowly than Greenland. The latest data from the GRACE project — twin satellites that measure mass using gravity data — say Antarctica is losing about 92 billion tons of ice per year, with that rate having doubled from 2003 to 2014.
The sizeable western half of Antarctica holds some of the fastest-warming areas on the planet.
But Antarctica is vast — 1.5 times the size of the United States, with ice three miles thick in places — and holds enough ice to raise global sea levels by roughly 200 feet.
The larger, eastern half lies mostly above sea level and remains very cold; researchers have typically considered its ice stable, though even that view is beginning to change. The sizeable western half of the Antarctic, by contrast, has its base lying below sea level, and holds some of the fastest warming areas on the planet. “You look at West Antarctica and you think: How come it’s still there?” says Rignot.
Warming ocean water licking at the underside of the floating edges of the Western Antarctic Ice Sheet is eating away at the line where the ice rests on solid rock. Much of the bedrock of the Antarctic slopes downward toward the center of the continent, so as the invading water flows downhill it seeps further and further inland, causing ever-larger chunks of glaciers to flow faster into the sea. This so-called “grounding line” has been eroding inland rapidly, in some parts of West Antarctica at rates of miles per year. In 2014, satellite radar images revealed just how vulnerable five massive glaciers flowing into the Admundsen Sea are from this effect. And a 2015 paper showed that the same thing is happening more slowly to Totten Glacier, one of the biggest glaciers in the east.
Such dramatic processes have been the bane of Antarctic modeling and the reason why scientists have been loathe to put a number on sea level contributions from a melting southern continent. Then in March 2016 came a report in Nature that some say represents a step change in our ability to do that. DeConto and David Pollard of Pennsylvania State University put into their ice sheet model two basic phenomena: meltwater trickling down to lubricate glacier flow, and giant walls of ice (created when the ends of glaciers snap off) simply collapsing under their own weight. These new modeling parameters gave DeConto and Pollard a better understanding of past sea level rise events. For the Pliocene era 3 million years ago, for example — when seas were dozens of feet higher than today — older models estimated that a partially melting Antarctic added about 23 feet to global sea level rise. The new model increased Antarctica’s contribution to sea level rise during the Pliocene to 56 feet.
Even DeConto admits that, under the model used in his paper, the timing and pace of Antarctica’s ice loss is “really uncertain” — it could be a decade or two, or three or four, before these dramatic processes start to kick in, he says. “The paper just shows the potentials, which are really big and really scary,” says DeConto. But Scambos and other observers call DeConto’s numbers “perfectly plausible.”
Researchers could better pin down their models if they could track the rate of sea level rise from polar ice sheet collapse in the past, but this has proven hard to do. When seas rose a whopping 13 feet per century at the end of the last glaciation (the current record-holder for known rates of sea level rise in the past), much of the water came from an ice sheet over North America, where there isn’t one today. “I wouldn’t use that as an analogue for the future,” says paleo-geologist Andrea Dutton of the University of Florida, who wrote a recent review of past records of sea level rise. “But it has important lessons for us nonetheless — that ice sheets can retreat suddenly and in steps instead of gradually.”
For a better analogue of what’s going on today, researchers often look to the last interglacial period, about 120,000 years ago, when temperatures were about a degree warmer than pre-industrial levels and seas were 20 to 30 feet higher than today. Ice cores from Greenland have suggested that much of that water must have come from the Antarctic. To find out just how fast sea levels rose at that time, Dutton is now looking at old corals in Mexico, Florida, and Australia; corals can be used to track sea level, since they grow in shallow waters to capture sunlight.
A map of sea level rise around the world, and how it was higher in one place than another, could be used to infer where the water came from. Success isn’t guaranteed; corals are notoriously difficult to date. And whatever they find, notes Scambos, it will still be hard to draw a parallel to the modern world. “That was a natural warming period in Earth’s history,” Scambos says. “We’re putting our pedal to the metal today; we’re driving the system very hard.”
James Hansen, a climatologist at Columbia University, summarized the evidence for rapid sea level rise in a recent controversial paper, raising some eyebrows at its stark warnings of catastrophe. Though many researchers have taken issue with the dramatic tone and specific details of that paper, its conclusion — that multi-meter sea level rise is possible in the next 50, 100, or 200 years — does not seem so alarmist in the face of other recent work.
“I think a lot of us who work on paleo records are all aware that a lot of change can happen very quickly — I’m always looking at big numbers,” says Dutton, who hasn’t been startled by recent studies like DeConto’s. “It’s always going to be a difficult question to answer. Maybe we need to accept we’re always going to have this uncertainty and just prepare for the worst.”
ALSO FROM YALE e360: A Quest to Document Earth’s Disappearing Glaciers
Eric Rignot gives some more background on last week’s “holy shit moment” – his new paper on accelerated melt of the Antarctic ice sheet – and adds some new holy shit moments in the process.
Last Monday, we hosted a Nasa conference on the state of the West Antarctic ice sheet, which, it could be said, provoked something of a reaction. “This Is What a Holy Shit Moment for Global Warming Looks Like,” ran a headline in Mother Jones magazine.
We announced that we had collected enough observations to conclude that the retreat of ice in the Amundsen sea sector of West Antarctica was unstoppable, with major consequences – it will mean that sea levels will rise one metre worldwide. What’s more, its disappearance will likely trigger the collapse of the rest of the West Antarctic ice sheet, which comes with a sea level rise of between three and five metres. Such an event will displace millions of people worldwide.
Two centuries – if that is what it takes – may seem like a long time, but there is no red button to stop this process. Reversing the climate system to what it was in the 1970s seems unlikely; we can barely get a grip on emissions that have tripled since the Kyoto protocol, which was designed to hit reduction targets. Slowing down climate warming remains a good idea, however – the Antarctic system will at least take longer to get to this point.
The Amundsen sea sector is almost as big as France. Six glaciers drain it. The two largest ones are Pine Island glacier (30km wide) and Thwaites glacier (100km wide). They stretch over 500km.
Many impressive scientists have gone before us in this territory. The concept of West Antarctic instability goes back to the 1970s following surveys by Charles Bentley in the 1960s that revealed an ice sheet resting on a bed grounded well below sea level and deepening inland. Hans Weertman had shown in 1974 that a marine-based ice sheet resting on a retrograde bed was unstable. Robert Thomas extended his work to pursue the instability hypothesis. Terry Hughes suggested that the Pine Island sector of West Antarctica was its weak underbelly and that its retreat would collapse the West Antarctic ice sheet. Considerable uncertainty remained about the timescale, however, due to a lack of observation of this very remote area.
Things changed with the launch of the ERS-1 satellite which allowed glaciers in this part of Antartica to be observed from space. In 1997, I found that the grounding line (where the glacier detaches from its bed and becomes afloat) of Pine Island glacier had retreated five kilometres in the space of four years, between 1992 and 1996. Stan Jacobs and Adrian Jenkins had found a year earlier that the glacier was bathing in unusually warm waters, which suggested the ocean had a major influence on the glacier. Duncan Wingham and others showed that the glacier was thinning. In 2001, I found that Thwaites glacier was retreating too .
At that point, the scientific community took a different look at the region. Work by the British Antarctic Survey, NASA and Chile led to more detailed observations, a monitoring programme was initiated, instruments were placed on the ice, in the ocean and scientific results started to pile up from a variety of research programmes. From that point, we all sought to find out whether this was really happening. Now, two decades after this process started, we have witnessed glacier grounding lines retreat by kilometres every year, glaciers thinning by metres every year hundreds of kilometres inland, losing billions of tons of water annually, and speeding up several percent every year to the flanks of topographic divides.
Thwaites glacier started to accelerate after 2006 and in 2011 we detected a huge retreat of the glacier grounding lines since 2000. Detailed reconstructions of the glacier bed further confirmed that no mountain or hill in the back of these glaciers could act as a barrier and hold them up; and 40 years of glacier flow evolution showed that the speed-up was a long story.
All these results indicate a progressive collapse of this area. At the current rate, a large fraction of the basin will be gone in 200 years, but recent modelling studies indicate that the retreat rate will increase in the future. How did this happen? A clue is that all the glaciers reacted at the same time, which suggested a common force that can only be the ocean. Ocean heat is pushed by the westerly winds and the westerlies have changed around Antarctica in response to climate warming and the depletion of the ozone. The stronger winds are caused by a world warming faster than a cooling Antarctica. Stronger westerlies push more subsurface warm waters poleward to melt the glaciers, and push surface waters northward.
Nerilie Abram and others have just confirmed that the westerlies are stronger now than at any other time in the past 1,000 years and their strengthening has been particularly prominent since the 1970s as a result of human-induced climate warming. Model predictions also show that the trend will continue in a warming climate.
What this means is that we may be ultimately responsible for triggering the fast retreat of West Antarctica. This part of the continent was likely to retreat anyway, but we probably pushed it there faster. It remains difficult to put a timescale on it, because the computer models are not good enough yet, but it could be within a couple of centuries, as I noted. There is also a bigger picture than West Antarctica. The Amundsen sea sector is not the only vulnerable part of the continent. East Antarctica includes marine-based sectors that hold more ice. One of them, Totten glacier, holds the equivalent of seven metres of global sea level.
Controlling climate warming may ultimately make a difference not only about how fast West Antarctic ice will melt to sea, but also whether other parts of Antarctica will take their turn. Several “candidates” are lined up, and we seem to have figured a way to push them out of equilibrium even before warming of air temperature is strong enough to melt snow and ice at the surface.
Unabated climate warming of several degrees over the next century is likely to speed up the collapse of West Antarctica, but it could also trigger irreversible retreat of marine-based sectors of East Antarctica. Whether we should do something about it is simply a matter of common sense. And the time to act is now; Antarctica is not waiting for us.
Eric Rignot is a glaciologist at NASA’s Jet Propulsion Laboratory. He is the lead author of last week’s (this is May 2014) landmark scientific paper on West Antartica