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These Glaciers Melt at Your Fingertips

Icy barriers began melting away at the close of the last ice age, easing human passage into North America. Now scientists are piecing together some chilling details: how one corner of Arctic ice retreated over 12,000 years in response to a changing climate.

A new set of computer simulations seeks to capture the demise of the ice on the margins of southwestern Greenland. They include a public version that allows you to take control, change environmental conditions and watch the effect on the ice.

It’s an early step toward a big idea. Like workers tinkering together mechanical parts, these scientists hope eventually to create a detailed reconstruction of a vanished climate.

That could shed new light on ice loss today and in decades to come.

Using computer modeling to make predictions of how ice will melt over the coming century “is like going down a dark hallway with a candle,” said Joshua Cuzzone, an ice researcher at NASA’s Jet Propulsion Laboratory in Pasadena, California. “We kind of know where we’re going, but we don’t know the specifics.”

Part of the answer involves better understanding of the past. So Cuzzone is lead author of a new paper that seeks to illuminate ice melt since the early Holocene.

The coastal terrain of southwestern Greenland offered a nearly ideal platform for Cuzzone and his team to build and test their simulation. The coast is craggy enough, but not too craggy; and the geologic history can be seen clearly in the lines of rubble, called moraines, left behind as glaciers dwindled.

Modeling the Melt

These geological details provided the scaffolding for a powerful simulator called the Ice Sheet System Model, or ISSM. ISSM embodies, in a computer, the equations describing the thermal and mechanical physics of ice flowing on bedrock. Such computer models face a tradeoff: if they reproduce small features well – ice streams, marine-terminating glaciers, small bumps in bedrock – they cannot be run for long periods because of the excessive computer load they impose. Earlier models had to choose between detail and long simulations.

The amount of small-scale detail a model is able to reproduce is called “model resolution,” and that’s where ISSM departs from previous models. It is designed with resolution that varies spatially: more detail in areas of sharp topography, and less detail (coarser resolution) in others.

Imagine draping a flexible mesh over the landscape. It settles and conforms itself to the hills, valleys, ocean inlets and flats. Now imagine this flexible mesh having variable resolution, with threads that are closer together in some regions than in others. The closer threads better adapt to small-scale details of bedrock and ice flow – in other words, the mesh is finer for the craggy, complex parts of the landscape, including fjords, and much coarser for flatter, more featureless sections.

The model works in a similar fashion. The “mesh” defines the model’s variable resolution. Start the clock 12,000 years ago and the tiny, gridded spaces formed by the mesh begin to sink at varying rates, tracking contours as the ancient ice melts away.

While the main driver of climate change is very different today – human emission of heat-trapping gases versus slow-working, natural processes – the mechanics of ice melt are quite similar.

“If we are able to model the retreat history locked within the geological record, it might provide us with a better understanding of how sensitive these ice sheets are,” Cuzzone said. “We’ll be able to infer how anomalous our present-day ice-mass loss is compared to the past 12,000 years.”

That, in turn, could help create more precise forecasts of ice melt in the future, and corresponding rates of sea level rise.

Adapting the meshes, with high resolution for complex terrain and low resolution for the flats, yielded the best of both worlds: accurate modeling without a heavy burden on computer time. This approach allowed Cuzzone and his team to reproduce the large-scale, well-known retreat of the southwestern Greenland Ice Sheet during the Holocene, the geologic period that began some 11,650 years ago.

The next step will involve using state-of-the-art reconstructions of past climate, derived from work by paleoclimatologists at the University of Washington, as the environmental setting for the ice model to see how it responds through time.

“This will be the constraining model for past ice history across southwestern Greenland,” Cuzzone said. “We’re seeing how well it does. Using the fact that it actually does pretty well, we’ll be able to have confidence in making the claim of comparing past ice-mass loss to current ice-mass loss.”

Related Terms

• Ice & Glaciers
• Sea Level Rise

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NASA Returns to Arctic Studying Summer Sea Ice Melt

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Facts About Earth

Warming Seas and Melting Ice Sheets

NASA is applying its unique capabilities to the challenge of understanding global sea level rise.

Sea level rise is a natural consequence of the warming of our planet.

We know this from basic physics. When water heats up, it expands. So when the ocean warms, sea level rises. When ice is exposed to heat, it melts. And when ice on land melts and water runs into the ocean, sea level rises.

For thousands of years, sea level has remained relatively stable and human communities have settled along the planet's coastlines. But now Earth's seas are rising. Globally, sea level has risen about eight inches (20 centimeters) since the beginning of the 20th century and more than two inches (5 centimeters) in the last 20 years alone.

All signs suggest that this rise is accelerating.

While NASA and other agencies continue to monitor the warming of the ocean and changes to the planet's land masses, the biggest concern is what will happen to the ancient ice sheets covering Greenland and Antarctica, which continue to send out alerts that a warming planet is affecting their stability.

"Given what we know now about how the ocean expands as it warms and how ice sheets and glaciers are adding water to the seas, it's pretty certain we are locked into at least 3 feet [0.9 meter] of sea level rise," said Steve Nerem of the University of Colorado, Boulder, and lead of the Sea Level Change Team. "But we don't know whether it will happen in 100 years or 200 years."

While the expansion of warmer ocean waters and tectonic movement of land masses play key roles in both global and local sea level changes, it's the fate of the polar ice sheets that will most determine how much coastlines change in the coming decades.

"We've seen from the paleoclimate record that sea level rise of as much as 10 feet [3 meters] in a century or two is possible, if the ice sheets fall apart rapidly," said Tom Wagner, the cryosphere program scientist at NASA Headquarters in Washington. "We're seeing evidence that the ice sheets are waking up, but we need to understand them better before we can say we're in a new era of rapid ice loss."

Finding the Level

NASA has been recording the height of the ocean surface from space since 1992. That year, NASA and the French space agency, CNES, launched the first of a series of spaceborne altimeters that have been making continuous measurements ever since. The first instrument, Topex/Poseidon, and its successors, Jason-1 and Jason-2, have recorded about 2.9 inches (7.4 centimeters) of rise in sea level, averaged over the globe.

In the 21st century, two new sensing systems have proven to be invaluable complements to the satellite altimetry record. In 2002, NASA and the German space agency launched the Gravity Recovery and Climate Experiment (GRACE) twin satellites. These measure the movement of mass, and hence gravity, around Earth every 30 days. Earth's land masses move very little in a month, but its water masses move through melting, evaporation, precipitation and other processes. GRACE records these movements of water around the globe. The other new system is the multinational Argo array, a network of more than 3,000 floating ocean sensors spread across the entire open ocean.

"To study sea level rise, the Jason series, GRACE, and Argo are the big three," said oceanographer Josh Willis of NASA's Jet Propulsion Laboratory, Pasadena, California, the project scientist for the upcoming Jason-3 altimetry mission.

Observations from the Jason series have revolutionized scientists' understanding of contemporary sea level rise and its causes. We know that today's sea level rise is about one-third the result of the warming of existing ocean water, with the remainder coming from melting land ice.

And it has shown precisely that the sea, of course, is not actually level. It varies as much as six feet (two meters) from place to place. And it is not rising evenly, like a bathtub filling with water. Currently, regional differences in sea level rise are dominated by the effects of ocean currents and natural cycles such as the Pacific Ocean's El Niño phenomenon and Pacific Decadal Oscillation. As the ice sheets continue to melt, scientists predict their meltwater will overtake natural causes as the most significant source of regional variations and the most significant contributor to overall sea level rise.

Or as Willis put it: "You ain't seen nothing yet."

Watching Ice Melt

Not that long ago, in the early 1990s, scientists were not able to determine whether polar land ice was growing, shrinking or in balance. Satellite and airborne missions, complemented by field measurements, have not only answered that question, but also provided the means for scientists to determine the mechanisms that are contributing to the growth and shrinkage of polar ice.

These advances in observing the world's frozen regions have allowed scientists to accurately estimate annual ice losses from Greenland and Antarctica in only the last decade. We now know not only how much sea level is changing -- as measured by satellites for the past 23 years -- but we can also determine how much sea level rise is caused by the loss of land ice.

In addition to the launch of the GRACE satellites in 2002, NASA also deployed the Ice Cloud and land Elevation Satellite (ICESat) from 2003 to 2009 to map changes in the height of the polar ice sheets using laser pulses. Other space agencies have used radar instruments to measure glacier speeds, as well as surface topography, such as the European Space Agency's CryoSat-2 satellite. Airborne missions, like NASA's Operation IceBridge, complement these measurements with instruments that map the bedrock topography beneath the ice, determine ice thickness and characterize its internal layers, and detect the depth of overlying snow. Combining these relatively recent -- and unprecedented -- measurements with longer-term satellite records and reanalyses of regional climate data extends the record of ice sheet mass balance to more than 40 years.

Several studies have shown that different remote sensing methods for studying ice sheet mass balance agree well. GRACE's record, spanning over a decade, shows that the ice loss is accelerating in Greenland and West Antarctica. Greenland has shed, on average, 303 gigatons of ice every year since 2004, while Antarctica has lost, on average, 118 gigatons of ice per year, with most of the loss coming from West Antarctica. Greenland's ice loss has accelerated by 31 gigatons of ice per year every year since 2004, while West Antarctica has experienced an ice mass loss acceleration of 28 gigatons per year.

The Warming North

The Greenland Ice Sheet, spanning 660,000 square miles (1.7 million square kilometers) -- an area almost as big as Alaska -- and with a thickness at its highest point of almost 2 miles (3 kilometers), has the potential to raise the world's oceans by more than 20 feet (6 meters). Situated in the Arctic, which is warming at twice the rate of the rest of the planet, Greenland fell out of balance in the 1990s, and is now shedding more ice in the summer than it gains back in the winter.

"In Greenland, everything got warmer at the same time: the air, the ocean surface, the depths of the ocean," said Ian Joughin, a glaciologist at University of Washington, in Seattle. "We don't really understand which part of that warming is having the biggest effect on the glaciers."

What scientists do know is that warming Arctic temperatures -- and a darkening surface on the Greenland ice sheet -- are causing so much summer melting that it is now the dominant factor in Greenland's contribution to sea level rise.

Greenland's summer melt season now lasts 70 days longer than it did in the early 1970s. Every summer, warmer air temperatures cause melt over about half of the surface of the ice sheet -- although recently, 2012 saw an extreme event where 97 percent of the ice sheet experienced melt at its top layer.

Greenland's glaciers have sped up, too. Though many of the glaciers in the southeast, west and northwest of the island that experienced quick thinning from 2000 to 2006 have now slowed down, others haven't. A study last year showed that the northeast Greenland ice stream had increased its ice loss rate due to regional warming.

"The early 2000s was when some big things revealed themselves, such as when we saw the fastest glacier we knew of, the Jakobshavn ice stream in Greenland, double its speed," said Waleed Abdalati, director of the Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, and former NASA chief scientist. "The subsequent surprise was that these changes could be sustained for a decade -- Jakobshavn is still going fast."

To answer the questions of how these glaciers will behave, how much they will contribute to sea level rise and how fast those changes will occur, scientists need better data on the bathymetry or geography of the ocean floor surrounding Greenland, said Eric Rignot, a glaciologist with JPL and the University of California, Irvine.

"Bathymetry is critical for understanding how ocean waters circulate around Greenland, for projections and for understanding what we've been observing in the past few decades," Rignot said.

Beginning with the deployment of research buoys in the waters around Greenland this summer, NASA is embarking on a three-year airborne and ship-based campaign to answer precisely these questions. OMG, Oceans Melting Greenland, seeks to understand the role of ocean currents and ocean temperatures in melting Greenland's ice from below -- and therefore better predict the speed at which the ice sheet will raise sea level.

Changes at the Southern End of the World

The Antarctic Ice Sheet covers nearly 5.4 million square miles (14 million square kilometers), an area larger than the United States and India combined, and contains enough ice to raise the ocean level by about 190 feet (58 meters). The Transantarctic Mountains split Antarctica into two major regions: West Antarctica and the much larger East Antarctica.

Though Antarctica's contribution to sea level rise is still at less than 0.02 inches (0.5 millimeters) per year, several events over the past decade and a half have prompted experts to start warning about an the possibility of more rapid changes in the upcoming century.

The mountainous horn of the continent, the Antarctic Peninsula, gave one of the earliest warnings on the impact of a changing climate in Antarctica, when warming air and ocean temperatures led to the dramatically fast breakup of the Larsen B ice shelf in 2002. In about a month, 1,250 square miles (3,240 square kilometers) of floating ice that had been stable for over 10,000 years were gone. In the following years, other ice shelves in the peninsula, including the last remainder of Larsen B, collapsed, speeding up in the flow of the glaciers they were buttressing.

In 2014, West Antarctica grabbed the spotlight when two studies focusing on the acceleration of the glaciers in the Amundsen Sea sector showed that its collapse is underway, and that the rest of West Antarctica will follow. While one of the studies said the demise could take 200 to 1,000 years, depending on how rapidly the ocean heats up, both studies concurred that the collapse is unstoppable and will add up to 12 feet (4 meters) of sea level rise.

For the West Antarctic Ice Sheet, which largely rests on a bed that lies below sea level, the main driver of ice loss is the ocean. The waters of the Southern Ocean are layered: on top and at the bottom, the temperatures are frigid, but the middle layer is warm. The westerlies, the winds that spin the ocean waters around Antarctica, have intensified during the last decade, pushing the cold, top layer away from the land. This allows the warmer, deeper waters to rise and spill over the border of the continental shelf, flowing all the way back to the base of many ice shelves. As the ice shelves weaken from underneath, the glaciers behind them speed up.

East Antarctica's massive ice sheet, as vast as the lower continental U.S., remains the main unknown in projections of sea level rise. Though it appears to be stable, a recent study on Totten Glacier, East Antarctica's largest and most rapidly thinning glacier, hints otherwise. The research found two deep troughs that could lead warm ocean water to the base of the glacier and melt it in a similar way to what's happening to the glaciers in West Antarctica. Other sectors grounded below sea level, such as the Cook Ice Shelf, Ninnis, Mertz and Frost glaciers, are also losing mass.

"The prevailing view among specialists has been that East Antarctica is stable, but I don't think we really know," said Rignot. "Some of the signs we see in the satellite data right now are kind of red flags that these glaciers might not be as stable as we once thought. There's always a lot of attention paid by the media to the changes we see now, but as scientists our priority remains what the changes could be tomorrow."

On the other hand, weather models and reanalyses of climate model data have shown that there has been an increase in snowfall along the coastline of Dronning Maud Land, which might counteract East Antarctica's ice loss. But this may be a temporary shift -- scientists can't tell, because obtaining accurate field measurements of snow in Antarctica is extremely difficult. There are few weather stations, and they might not provide representative measurements because snowfall in Antarctica doesn't distribute evenly; the strong katabatic winds wipe some areas clean and make snow pile up in others. And satellite readings are not yet precise enough to detect small accumulation changes that would represent a difference of gigatons of mass when spread over East Antarctica's huge surface.

As is the case for Greenland, researchers also working on Antarctica need better data on the Southern Ocean bathymetry and the pathways that warm waters can follow to reach the ice. And this kind of data, as well as snow accumulation and other ocean data, can't be obtained remotely with enough precision, according to Ted Scambos, lead scientist at the University of Colorado's National Snow and Ice Data Center in Boulder.

"We've learned so much from the satellites that we've been surfing the wave of new understanding for the last 20 years," Scambos said. "But now, to go further, we have to try to get instruments on the ground while maintaining the ability we have with airborne and satellite missions to watch the ice sheet from a global perspective."

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The edge of the Thwaites glacier in Antarctica. Felton Davis via Flickr

As ‘Doomsday’ Glacier Melts, Can an Artificial Barrier Save It?

Relatively warm ocean currents are weakening the base of Antarctica’s enormous Thwaites Glacier, whose demise could raise sea levels by as much as 7 feet. To separate the ice from those warmer ocean waters, scientists have put forward an audacious plan to erect a massive underwater curtain.

By Fred Pearce • August 26, 2024

They call it the Doomsday Glacier. A chunk of Antarctic ice as big as Florida and two thirds of a mile thick, the Thwaites Glacier disgorges into the ocean in a remote region of West Antarctica. Glaciologists say it may be on the verge of total collapse, which could swamp huge areas of low-lying coastal land around the world within a few decades. Now, ambitious plans to save it are set to become an early test of whether the world is prepared to enact massive geoengineering efforts to ward off the worst effects of climate change.

Recent monitoring by uncrewed submarines and satellites, along with ice-sheet modeling, suggest that the Thwaites Glacier and its adjacent smaller twin, the Pine Island Glacier, may already be in a death spiral — eaten up by the intensifying speed and warmth of the powerful Antarctic Circumpolar Current. If they are past a point of no return, say researchers involved in the studies, then only massive human intervention can save them.

Nothing is certain. A new modeling study published last week said the risk of unstoppable retreat of the glacier may be overblown. But there is no time to waste, argues the glaciologist orchestrating the call for action, John Moore of Lapland University, in northern Finland. Within two years, he and colleagues in Europe hope to be working in a Norwegian fjord, testing prototypes for a giant submarine curtain, up to 50 miles across, that could seal off the two glaciers from the remorseless Antarctic current.

Glaciologists have discussed scary prognoses for the rapid collapse of giant Antarctic glaciers for almost half a century.

Meanwhile, some of his collaborators, fearing the logistical complications of such a task, are pondering an even more mind-bending idea. They want to substitute the physical curtain with a giant “bubble curtain,” created by a constant injection of bubbles of air or cold surface water.

Opponents of the plans, including many glaciologists, say such outlandish proposals are a dangerous diversion from the real task of mitigating climate change by curbing carbon emissions. But advocates say the two glaciers can’t wait. “We can’t mitigate our way out of this,” says Moore. “We need other tools.”

Glaciologists have discussed scary prognoses for the rapid collapse of giant Antarctic glaciers for almost half a century. Glaciers in West Antarctica are particularly vulnerable because they are not sitting on solid land; they are surrounded by ocean and pinned precariously to the peaks of submarine mountains, between which the circumpolar current swirls.

Back in 1978, glaciologist John Mercer, of Ohio State University, warned of a “major disaster – a rapid five-meter rise in sea level, caused by deglaciation of West Antarctica” — should atmospheric levels of carbon dioxide continue to rise. Three years later, glaciologist Terry Hughes, of the University of Maine, identified a “weak underbelly” to the West Antarctic Ice Sheet, where the Thwaites and Pine Island glaciers drain into the Amundsen Sea, an arm of the Southern Ocean.

European Space Agency / Adapted by Yale Environment 360

These glaciers are two of the ice continent’s five largest and are the gateway to the ocean for nearly half of the ice sheet. Hughes warned that the glaciers could easily lose their grip on the submarine mountains as warmer water melts ice directly beneath them, leading to their disintegration within a few decades. Their meltwater would raise sea levels globally by as much as seven feet. That would rise to more than 12 feet if, as the pair suspected, the glaciers’ demise dragged down the rest of the ice sheet with it.

These fears remained a theoretical concern until 20 years ago, when NASA glaciologist Eric Rignot warned that the seaward flow of these two giant glaciers was accelerating rapidly. It also became clear that the waters lapping at their submerged edges were warming as a result of climate change, and that this melting effect was much greater than the effect of warming air.

Ted Scambos of the University of Colorado, who is a coordinator of the joint U.S.-UK International Thwaites Glacier Collaboration, says that now “the [Thwaites] glacier is flowing at over a mile per year,” nearly double the speed in the 1990s. The warm ocean current is “eroding the base of the ice, erasing it as an ice cube would disappear bobbing in a glass of water.”

Scambos believes the accelerated flow is bound to continue. “By flowing faster, the glacier pulls down the ice behind it.” While shallower ice grinds on the bedrock and gets held back, he explains, thicker ice is less constrained and so flows faster, “leading to more retreat.”

“Some say it is too late to prevent [Thwaites’] collapse,” says a glaciologist. “Others say we could have 200 years.”

This concern has only heightened with the recent publication of satellite radar images revealing that the height of the Thwaites Glacier rises and falls with the tides. Rignot, now at the University of California, Irvine, says this finding shows that the warm current is not just lapping at the front of the glacier but is penetrating several miles beneath the grounded ice, further loosening its contact with solid rock.

Modelers of the West Antarctic Ice Sheet caution against assuming the worst. Much remains unknown. Last week, Mathieu Morlighem of Dartmouth College, along with British colleagues, reported that one potential cause of collapse of the Thwaites Glacier — runaway instability of the ice cliff at the front of the glacier — was less likely than some propose, at least in the short term. But he said there was a “pressing need” for further research into these potentially devastating processes.

There is, Moore agrees, no consensus among glaciologists about whether the Thwaites Glacier is past a point of no return unless there is drastic intervention. “Some say it is too late to prevent its collapse; others say we could have 200 years. But it certainly could be beyond its tipping point, and we have to be prepared.”

Time-lapse satellite imagery of ice breaking off the Pine Island Glacier from 2015 to 2020.

Last month, Moore and an international team of researchers published a “research vision” for “glacial climate intervention.” It followed workshops held last year at Stanford and the University of Chicago with fellow glaciologists, and it warned that if tipping points at the two glaciers have or will soon be crossed, then whatever happens to greenhouse gas emissions in the future “will have little effect on preserving the ice sheet.”

Ice-sheet modeling last year by Kaitlin Naughten of the British Antarctic Survey concurred. “The opportunity to preserve the West Antarctic Ice Sheet in its present-day state has probably passed,” she concluded, “and policymakers should be prepared for several metres of sea level rise over the coming centuries.”

So what can be done? Last month’s “vision” did not directly advocate for geoengineering interventions but called for research into which of them may be viable. It highlighted a proposal for a series of giant overlapping plastic or fiber curtains tethered to concrete foundations. To hold the warm current at bay, the curtain would stretch for 50 miles across the entrance to the Amundsen Sea and extend upwards for much of the 2,000 feet from the sea floor to the surface.

Some experts are confident that giant undersea curtains can be built to withstand the forces they will face in the ocean.

Moore wants to get started on testing the idea, and he and his collaborators are seeking research funding. The first experiments in a large lab tank are expected to begin within a few weeks at Cambridge University’s Centre for Climate Repair, whose mission is to advance “climate repair projects that can be rolled out at scale within the next 5-10 years.”

Real-world experiments could follow quickly, says Moore. “Within two years, we could be working at a fjord in northern Norway, testing different designs in a marine setting.” He has identified a target fjord but won’t say where. “If that goes well, we would want to scale up to a curtain as much as a kilometer across.” He envisions this being tested among the glaciers of Svalbard, the Norwegian Arctic archipelago that has become an international center for polar research. “In 10-15 years, we should have something to deploy in Antarctica,” he says.

Moore is confident that such giant curtains can be built to withstand the forces they will face in the ocean. “And installation seems feasible with existing technology,” he says. Even so, deployment and maintenance would be a huge undertaking in an environmentally hostile region some 1,500 miles from the nearest ice-free land in South America. And potential impacts on local marine ecosystems from both installation and operation remain essentially unknown, he says.

So a diminished version might be tried at the start, says Michael Wolovick of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. Much could be accomplished with a curtain just three miles wide stretching across a “choke point” in front of the most vulnerable part of the Thwaites Glacier.

Diagram of a proposed glacial curtain. Nature / Adapted by Yale Environment 360

Hugh Hunt, an engineering professor and deputy director of Cambridge’s Centre for Climate Repair, has another proposal. “We have been looking for ideas that involve less infrastructure,” he says. The most promising would replace a fabricated curtain with a more natural barrier. He proposes laying a pipe along the bed of the Amundsen Sea that would release a constant stream of either air bubbles or cold water pumped down from the surface. “A bubble barrier probably wouldn’t completely halt the flow of warm water,” he says. “But it would disturb that flow, creating turbulence that would force it to mix with the colder water above.”

Offshore civil engineering companies already use bubble barriers to contain silt and protect marine life from their operations, Hunt says. A giant bubble machine off Antarctica would require a continuous supply of energy, which would have to be renewable. “With no winter sunlight, solar power wouldn’t work,” he says. “But offshore wind farms would. And with long-distance submarine cabling improving all the time, we could even generate power far away.”

Moore calls the bubble barrier a “wild card.” But, he says, “it is great they are pursuing it, because the potential payoff is huge.” Its main problem right now, he says, is that it remains almost entirely unresearched.

An Antarctic curtain would be hugely expensive, but far less than the cost to protect coastlines from rising tides.

There are other glacier-protecting strategies that avoid the need for curtains or other barriers. Slawek Tulaczyk, a glaciologist at the University of California, Santa Cruz, has proposed stabilizing the two imperiled glaciers by draining the meltwaters that currently seep to their base, lubricating the pinning points and accelerating the glaciers’ seaward flow. By drilling holes through the glaciers and inserting pumps, engineers could dry up the lubricant and bring that flow to a halt. The extracted water could then be sprayed across the glacier surface, where it would freeze, helping to rebuild the glacier.

Are such ideas feasible, how much would they cost, and what are the ethics of all this? Moore puts the likely bill for erecting a curtain across the Amundsen Sea at up to $80 billion. That is a lot of money. But much less, he says, than the trillions of dollars that might be needed to protect coastlines from rising tides caused by the loss of the two glaciers.

Others question this analysis. “I don’t doubt we could spend a decade building the curtain,” says Twila Moon, a glaciologist at the National Snow and Ice Data Center at the University of Colorado. “It is a naturally attractive idea that one big project can make the difference. But curtains may just displace the heat elsewhere, melting other ice.” In any event, she says, sea-level rise would continue as a result of factors such as thermal warming of the oceans, land subsidence, and changes in ocean circulation, as well as the melting of other land ice, such as on Greenland. “So the question is whether this is the right place to put our resources, including limited research funding.”

The Thwaites Glacier, photographed on a research flight. U.S. Antarctic Program

Her Colorado colleague and Thwaites Glacier expert Scambos is more open to geoengineering research, but still skeptical. “I think the ideas are worth pursuing,” he says. “We could explore them at a meaningfully large scale in sites with low negative consequences if things don’t go well.” But, like Moon, he fears the impact on climate policymaking.

In an ideal world, Scambos says, “we could pursue engineering solutions for the poles while at the same time directly decarbonizing our societies.” But the world isn’t like that. Climate negotiators at the UN COP28 meeting last December “brought up the notion that decarbonizing could go slower now that these [geoengineering] ideas are out there,” Scambos says. “The idea that ‘scientists are working on the problem’ could be a death knell for the 22nd century.”

Moore has heard these criticisms. “Yes, there is opposition,” he says. “We need to address that. We need a social licence.” He agrees that there are other important causes of current and future sea-level rise. But “none of these other sources have the potential to raise sea level at the extreme rates and magnitudes that could be realized from a rapid marine ice sheet collapse.”

If the glaciers are past their tipping points, dooming the world’s coastal lands, he says, we may have no alternative but to bite the geoengineering bullet. And the sooner we get started, he says, the better.

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News | September 29, 2015

Warming seas and melting ice sheets.

By Maria-Jose Viñas and Carol Rasmussen, NASA's Earth Science News Team

An iceberg floats in Disko Bay, near Ilulissat, Greenland, on July 24, 2015. The massive Greenland ice sheet is shedding about 300 gigatons of ice a year into the ocean, making it the single largest source of sea level rise from melting ice. Credit: NASA/Saskia Madlener

Sea level rise is a natural consequence of the warming of our planet.

We know this from basic physics. When water heats up, it expands. So when the ocean warms, sea level rises. When ice is exposed to heat, it melts. And when ice on land melts and water runs into the ocean, sea level rises.

For thousands of years, sea level has remained relatively stable and human communities have settled along the planet's coastlines. But now Earth's seas are rising. Globally, sea level has risen about eight inches (20 centimeters) since the beginning of the 20th century and more than two inches (5 centimeters) in the last 20 years alone.

All signs suggest that this rise is accelerating.

While NASA and other agencies continue to monitor the warming of the ocean and changes to the planet's land masses, the biggest concern is what will happen to the ancient ice sheets covering Greenland and Antarctica, which continue to send out alerts that a warming planet is affecting their stability.

"Given what we know now about how the ocean expands as it warms and how ice sheets and glaciers are adding water to the seas, it's pretty certain we are locked into at least 3 feet [0.9 meter] of sea level rise," said Steve Nerem of the University of Colorado, Boulder, and lead of the Sea Level Change Team. "But we don't know whether it will happen in 100 years or 200 years."

While the expansion of warmer ocean waters and tectonic movement of land masses play key roles in both global and local sea level changes, it's the fate of the polar ice sheets that will most determine how much coastlines change in the coming decades.

"We've seen from the paleoclimate record that sea level rise of as much as 10 feet [3 meters] in a century or two is possible, if the ice sheets fall apart rapidly," said Tom Wagner, the cryosphere program scientist at NASA Headquarters in Washington. "We're seeing evidence that the ice sheets are waking up, but we need to understand them better before we can say we're in a new era of rapid ice loss."

Finding the level

NASA has been recording the height of the ocean surface from space since 1992. That year, NASA and the French space agency, CNES, launched the first of a series of spaceborne altimeters that have been making continuous measurements ever since. The first instrument, Topex/Poseidon, and its successors, Jason-1 and Jason-2, have recorded about 2.9 inches (7.4 centimeters) of rise in sea level, averaged over the globe.

In the 21st century, two new sensing systems have proven to be invaluable complements to the satellite altimetry record. In 2002, NASA and the German space agency launched the Gravity Recovery and Climate Experiment (GRACE) twin satellites. These measure the movement of mass, and hence gravity, around Earth every 30 days. Earth's land masses move very little in a month, but its water masses move through melting, evaporation, precipitation and other processes. GRACE records these movements of water around the globe. The other new system is the multinational Argo array, a network of more than 3,000 floating ocean sensors spread across the entire open ocean.

"To study sea level rise, the Jason series, GRACE, and Argo are the big three," said oceanographer Josh Willis of NASA's Jet Propulsion Laboratory, Pasadena, California, the project scientist for the upcoming Jason-3 altimetry mission.

Observations from the Jason series have revolutionized scientists' understanding of contemporary sea level rise and its causes. We know that today's sea level rise is about one-third the result of the warming of existing ocean water, with the remainder coming from melting land ice.

And it has shown precisely that the sea, of course, is not actually level. It varies as much as six feet (two meters) from place to place. And it is not rising evenly, like a bathtub filling with water. Currently, regional differences in sea level rise are dominated by the effects of ocean currents and natural cycles such as the Pacific Ocean's El Niño phenomenon and Pacific Decadal Oscillation. As the ice sheets continue to melt, scientists predict their meltwater will overtake natural causes as the most significant source of regional variations and the most significant contributor to overall sea level rise.

Or as Willis put it: "You ain't seen nothing yet."

Watching ice melt

Not that long ago, in the early 1990s, scientists were not able to determine whether polar land ice was growing, shrinking or in balance. Satellite and airborne missions, complemented by field measurements, have not only answered that question, but also provided the means for scientists to determine the mechanisms that are contributing to the growth and shrinkage of polar ice.

These advances in observing the world's frozen regions have allowed scientists to accurately estimate annual ice losses from Greenland and Antarctica in only the last decade. We now know not only how much sea level is changing — as measured by satellites for the past 23 years — but we can also determine how much sea level rise is caused by the loss of land ice.

In addition to the launch of the GRACE satellites in 2002, NASA also deployed the Ice Cloud and land Elevation Satellite (ICESat) from 2003 to 2009 to map changes in the height of the polar ice sheets using laser pulses. Other space agencies have used radar instruments to measure glacier speeds, as well as surface topography, such as the European Space Agency's CryoSat-2 satellite. Airborne missions, like NASA's Operation IceBridge, complement these measurements with instruments that map the bedrock topography beneath the ice, determine ice thickness and characterize its internal layers, and detect the depth of overlying snow. Combining these relatively recent — and unprecedented — measurements with longer-term satellite records and reanalyses of regional climate data extends the record of ice sheet mass balance to more than 40 years.

Several studies have shown that different remote sensing methods for studying ice sheet mass balance agree well. GRACE's record, spanning over a decade, shows that the ice loss is accelerating in Greenland and West Antarctica. Greenland has shed, on average, 303 gigatons of ice every year since 2004, while Antarctica has lost, on average, 118 gigatons of ice per year, with most of the loss coming from West Antarctica. Greenland's ice loss has accelerated by 31 gigatons of ice per year every year since 2004, while West Antarctica has experienced an ice mass loss acceleration of 28 gigatons per year.

The warming north

The Greenland Ice Sheet, spanning 660,000 square miles (1.7 million square kilometers) — an area almost as big as Alaska — and with a thickness at its highest point of almost 2 miles (3 kilometers), has the potential to raise the world's oceans by more than 20 feet (6 meters). Situated in the Arctic, which is warming at twice the rate of the rest of the planet, Greenland fell out of balance in the 1990s, and is now shedding more ice in the summer than it gains back in the winter.

"In Greenland, everything got warmer at the same time: the air, the ocean surface, the depths of the ocean," said Ian Joughin, a glaciologist at University of Washington, in Seattle. "We don't really understand which part of that warming is having the biggest effect on the glaciers."

What scientists do know is that warming Arctic temperatures — and a darkening surface on the Greenland ice sheet — are causing so much summer melting that it is now the dominant factor in Greenland's contribution to sea level rise.

Greenland's summer melt season now lasts 70 days longer than it did in the early 1970s. Every summer, warmer air temperatures cause melt over about half of the surface of the ice sheet — although recently, 2012 saw an extreme event where 97 percent of the ice sheet experienced melt at its top layer.

Greenland's glaciers have sped up, too. Though many of the glaciers in the southeast, west and northwest of the island that experienced quick thinning from 2000 to 2006 have now slowed down, others haven't. A study last year showed that the northeast Greenland ice stream had increased its ice loss rate due to regional warming.

"The early 2000s was when some big things revealed themselves, such as when we saw the fastest glacier we knew of, the Jakobshavn ice stream in Greenland, double its speed," said Waleed Abdalati, director of the Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, and former NASA chief scientist. "The subsequent surprise was that these changes could be sustained for a decade — Jakobshavn is still going fast."

To answer the questions of how these glaciers will behave, how much they will contribute to sea level rise and how fast those changes will occur, scientists need better data on the bathymetry or geography of the ocean floor surrounding Greenland, said Eric Rignot, a glaciologist with JPL and the University of California, Irvine.

"Bathymetry is critical for understanding how ocean waters circulate around Greenland, for projections and for understanding what we've been observing in the past few decades," Rignot said.

Beginning with the deployment of research buoys in the waters around Greenland this summer, NASA is embarking on a three-year airborne and ship-based campaign to answer precisely these questions. OMG, Oceans Melting Greenland, seeks to understand the role of ocean currents and ocean temperatures in melting Greenland's ice from below — and therefore better predict the speed at which the ice sheet will raise sea level.

Changes at the southern end of the world

The Antarctic Ice Sheet covers nearly 5.4 million square miles (14 million square kilometers), an area larger than the United States and India combined, and contains enough ice to raise the ocean level by about 190 feet (58 meters). The Transantarctic Mountains split Antarctica into two major regions: West Antarctica and the much larger East Antarctica.

Though Antarctica's contribution to sea level rise is still at less than 0.02 inches (0.5 millimeters) per year, several events over the past decade and a half have prompted experts to start warning about an the possibility of more rapid changes in the upcoming century.

The mountainous horn of the continent, the Antarctic Peninsula, gave one of the earliest warnings on the impact of a changing climate in Antarctica, when warming air and ocean temperatures led to the dramatically fast breakup of the Larsen B ice shelf in 2002. In about a month, 1,250 square miles (3,240 square kilometers) of floating ice that had been stable for over 10,000 years were gone. In the following years, other ice shelves in the peninsula, including the last remainder of Larsen B, collapsed, speeding up in the flow of the glaciers they were buttressing.

In 2014, West Antarctica grabbed the spotlight when two studies focusing on the acceleration of the glaciers in the Amundsen Sea sector showed that its collapse is underway, and that the rest of West Antarctica will follow. While one of the studies said the demise could take 200 to 1,000 years, depending on how rapidly the ocean heats up, both studies concurred that the collapse is unstoppable and will add up to 12 feet (4 meters) of sea level rise.

For the West Antarctic Ice Sheet, which largely rests on a bed that lies below sea level, the main driver of ice loss is the ocean. The waters of the Southern Ocean are layered: on top and at the bottom, the temperatures are frigid, but the middle layer is warm. The westerlies, the winds that spin the ocean waters around Antarctica, have intensified during the last decade, pushing the cold, top layer away from the land. This allows the warmer, deeper waters to rise and spill over the border of the continental shelf, flowing all the way back to the base of many ice shelves. As the ice shelves weaken from underneath, the glaciers behind them speed up.

East Antarctica's massive ice sheet, as vast as the lower continental U.S., remains the main unknown in projections of sea level rise. Though it appears to be stable, a recent study on Totten Glacier, East Antarctica's largest and most rapidly thinning glacier, hints otherwise. The research found two deep troughs that could lead warm ocean water to the base of the glacier and melt it in a similar way to what's happening to the glaciers in West Antarctica. Other sectors grounded below sea level, such as the Cook Ice Shelf, Ninnis, Mertz and Frost glaciers, are also losing mass.

"The prevailing view among specialists has been that East Antarctica is stable, but I don't think we really know," said Rignot. "Some of the signs we see in the satellite data right now are kind of red flags that these glaciers might not be as stable as we once thought. There's always a lot of attention paid by the media to the changes we see now, but as scientists our priority remains what the changes could be tomorrow."

On the other hand, weather models and reanalyses of climate model data have shown that there has been an increase in snowfall along the coastline of Dronning Maud Land, which might counteract East Antarctica's ice loss. But this may be a temporary shift — scientists can't tell, because obtaining accurate field measurements of snow in Antarctica is extremely difficult. There are few weather stations, and they might not provide representative measurements because snowfall in Antarctica doesn't distribute evenly; the strong katabatic winds wipe some areas clean and make snow pile up in others. And satellite readings are not yet precise enough to detect small accumulation changes that would represent a difference of gigatons of mass when spread over East Antarctica's huge surface.

As is the case for Greenland, researchers also working on Antarctica need better data on the Southern Ocean bathymetry and the pathways that warm waters can follow to reach the ice. And this kind of data, as well as snow accumulation and other ocean data, can't be obtained remotely with enough precision, according to Ted Scambos, lead scientist at the University of Colorado's National Snow and Ice Data Center in Boulder.

"We've learned so much from the satellites that we've been surfing the wave of new understanding for the last 20 years," Scambos said. "But now, to go further, we have to try to get instruments on the ground while maintaining the ability we have with airborne and satellite missions to watch the ice sheet from a global perspective."

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Jet propulsion laboratory, more about prefire, news media contacts.

The PREFIRE mission will help develop a more detailed understanding of how much heat the Arctic and Antarctica radiate into space and how this influences global climate.

NASA’s newest climate mission has started collecting data on the amount of heat in the form of far-infrared radiation that the Arctic and Antarctic environments emit to space. These measurements by the Polar Radiant Energy in the Far-Infrared Experiment ( PREFIRE ) are key to better predicting how climate change will affect Earth’s ice, seas, and weather — information that will help humanity better prepare for a changing world.

One of PREFIRE’s two shoebox-size cube satellites, or CubeSats, launched on May 25 from New Zealand, followed by its twin on June 5. The first CubeSat started sending back science data on July 1. The second CubeSat began collecting science data on July 25, and the mission will release the data after an issue with the GPS system on this CubeSat is resolved.

The PREFIRE mission will help researchers gain a clearer understanding of when and where the Arctic and Antarctica emit far-infrared radiation (wavelengths greater than 15 micrometers) to space. This includes how atmospheric water vapor and clouds influence the amount of heat that escapes Earth. Since clouds and water vapor can trap far-infrared radiation near Earth’s surface, they can increase global temperatures as part of a process known as the greenhouse effect . This is where gases in Earth’s atmosphere — such as carbon dioxide, methane, and water vapor — act as insulators, preventing heat emitted by the planet from escaping to space.

“We are constantly looking for new ways to observe the planet and fill in critical gaps in our knowledge. With CubeSats like PREFIRE, we are doing both,” said Karen St. Germain, director of the Earth Science Division at NASA Headquarters in Washington. “The mission, part of our competitively-selected Earth Venture program, is a great example of the innovative science we can achieve through collaboration with university and industry partners.”

Earth absorbs much of the Sun’s energy in the tropics; weather and ocean currents transport that heat toward the Arctic and Antarctica, which receive much less sunlight. The polar environment — including ice, snow, and clouds — emits a lot of that heat into space, much of which is in the form of far-infrared radiation. But those emissions have never been systematically measured, which is where PREFIRE comes in.

“It’s so exciting to see the data coming in,” said Tristan L’Ecuyer, PREFIRE’s principal investigator and a climate scientist at the University of Wisconsin, Madison. “With the addition of the far-infrared measurements from PREFIRE, we’re seeing for the first time the full energy spectrum that Earth radiates into space, which is critical to understanding climate change.”

This visualization of PREFIRE data (above) shows brightness temperatures — or the intensity of radiation emitted from Earth at several wavelengths, including the far-infrared. Yellow and red indicate more intense emissions originating from Earth’s surface, while blue and green represent lower emission intensities coinciding with colder areas on the surface or in the atmosphere.

The visualization starts by showing data on mid-infrared emissions (wavelengths between 4 to 15 micrometers) taken in early July during several polar orbits by the first CubeSat to launch. It then zooms in on two passes over Greenland. The orbital tracks expand vertically to show how far-infrared emissions vary through the atmosphere. The visualization ends by focusing on an area where the two passes intersect, showing how the intensity of far-infrared emissions changed over the nine hours between these two orbits.

The two PREFIRE CubeSats are in asynchronous, near-polar orbits, which means they pass over the same spots in the Arctic and Antarctic within hours of each other, collecting the same kind of data. This gives researchers a time series of measurements that they can use to study relatively short-lived phenomena like ice sheet melting or cloud formation and how they affect far-infrared emissions over time.

The PREFIRE mission was jointly developed by NASA and the University of Wisconsin-Madison. A division of Caltech in Pasadena, California, NASA’s Jet Propulsion Laboratory manages the mission for NASA’s Science Mission Directorate and provided the spectrometers. Blue Canyon Technologies built and now operates the CubeSats, and the University of Wisconsin-Madison is processing and analyzing the data collected by the instruments.

To learn more about PREFIRE, visit: https://science.nasa.gov/mission/prefire/

Jane J. Lee / Andrew Wang Jet Propulsion Laboratory, Pasadena, Calif. 818-354-0307 / 626-379-6874 [email protected] / [email protected]

Related Terms

• PREFIRE (Polar Radiant Energy in the Far-InfraRed Experiment)

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NASA mission gets its first snapshot of polar heat emissions

NASA's newest climate mission has started collecting data on the amount of heat in the form of far-infrared radiation that the Arctic and Antarctic environments emit to space. These measurements by the Polar Radiant Energy in the Far-Infrared Experiment (PREFIRE) are key to better predicting how climate change will affect Earth's ice, seas, and weather—information that will help humanity better prepare for a changing world.

One of PREFIRE's two shoebox-size cube satellites, or CubeSats, launched on May 25 from New Zealand, followed by its twin on June 5. The first CubeSat started sending back science data on July 1. The second CubeSat began collecting science data on July 25, and the mission will release the data after an issue with the GPS system on this CubeSat is resolved.

The PREFIRE mission will help researchers gain a clearer understanding of when and where the Arctic and Antarctica emit far-infrared radiation (wavelengths greater than 15 micrometers) to space. This includes how atmospheric water vapor and clouds influence the amount of heat that escapes Earth.

Since clouds and water vapor can trap far-infrared radiation near Earth's surface, they can increase global temperatures as part of a process known as the greenhouse effect. This is where gases in Earth's atmosphere—such as carbon dioxide , methane, and water vapor—act as insulators, preventing heat emitted by the planet from escaping to space.

"We are constantly looking for new ways to observe the planet and fill in critical gaps in our knowledge. With CubeSats like PREFIRE, we are doing both," said Karen St. Germain, director of the Earth Science Division at NASA Headquarters in Washington. "The mission, part of our competitively-selected Earth Venture program, is a great example of the innovative science we can achieve through collaboration with university and industry partners."

Earth absorbs much of the sun's energy in the tropics; weather and ocean currents transport that heat toward the Arctic and Antarctica, which receive much less sunlight. The polar environment—including ice, snow, and clouds—emits a lot of that heat into space, much of which is in the form of far-infrared radiation. But those emissions have never been systematically measured, which is where PREFIRE comes in.

"It's so exciting to see the data coming in," said Tristan L'Ecuyer, PREFIRE's principal investigator and a climate scientist at the University of Wisconsin, Madison. "With the addition of the far-infrared measurements from PREFIRE, we're seeing for the first time the full energy spectrum that Earth radiates into space, which is critical to understanding climate change."

This visualization of PREFIRE data shows brightness temperatures—or the intensity of radiation emitted from Earth at several wavelengths, including the far-infrared. Yellow and red indicate more intense emissions originating from Earth's surface, while blue and green represent lower emission intensities coinciding with colder areas on the surface or in the atmosphere.

The visualization starts by showing data on mid-infrared emissions (wavelengths between 4 to 15 micrometers) taken in early July during several polar orbits by the first CubeSat to launch. It then zooms in on two passes over Greenland. The orbital tracks expand vertically to show how far-infrared emissions vary through the atmosphere. The visualization ends by focusing on an area where the two passes intersect, showing how the intensity of far-infrared emissions changed over the nine hours between these two orbits.

The two PREFIRE CubeSats are in asynchronous, near-polar orbits, which means they pass over the same spots in the Arctic and Antarctic within hours of each other, collecting the same kind of data. This gives researchers a time series of measurements that they can use to study relatively short-lived phenomena like ice sheet melting or cloud formation and how they affect far-infrared emissions over time.

Provided by NASA

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News | August 25, 2015

Warming seas and melting ice sheets.

By Maria-Jose Viñas and Carol Rasmussen, NASA's Earth Science News Team

An iceberg floats in Disko Bay, near Ilulissat, Greenland, on July 24, 2015. The massive Greenland ice sheet is shedding about 300 gigatons of ice a year into the ocean, making it the single largest source of sea level rise from melting ice. Credit: NASA/Saskia Madlener

Sea level rise is a natural consequence of the warming of our planet.

We know this from basic physics. When water heats up, it expands. So when the ocean warms, sea level rises. When ice is exposed to heat, it melts. And when ice on land melts and water runs into the ocean, sea level rises.

All signs suggest that this rise is accelerating.

While NASA and other agencies continue to monitor the warming of the ocean and changes to the planet's land masses, the biggest concern is what will happen to the ancient ice sheets covering Greenland and Antarctica, which continue to send out alerts that a warming planet is affecting their stability.

While the expansion of warmer ocean waters and tectonic movement of land masses play key roles in both global and local sea level changes, it's the fate of the polar ice sheets that will most determine how much coastlines change in the coming decades.

"We've seen from the paleoclimate record that sea level rise of as much as 10 feet [3 meters] in a century or two is possible, if the ice sheets fall apart rapidly," said Tom Wagner, the cryosphere program scientist at NASA Headquarters in Washington. "We're seeing evidence that the ice sheets are waking up, but we need to understand them better before we can say we're in a new era of rapid ice loss."

Finding the level

NASA has been recording the height of the ocean surface from space since 1992. That year, NASA and the French space agency, CNES, launched the first of a series of spaceborne altimeters that have been making continuous measurements ever since. The first instrument, Topex/Poseidon, and its successors, Jason-1 and Jason-2, have recorded about 2.9 inches (7.4 centimeters) of rise in sea level, averaged over the globe.

In the 21st century, two new sensing systems have proven to be invaluable complements to the satellite altimetry record. In 2002, NASA and the German space agency launched the Gravity Recovery and Climate Experiment (GRACE) twin satellites. These measure the movement of mass, and hence gravity, around Earth every 30 days. Earth's land masses move very little in a month, but its water masses move through melting, evaporation, precipitation and other processes. GRACE records these movements of water around the globe. The other new system is the multinational Argo array, a network of more than 3,000 floating ocean sensors spread across the entire open ocean.

"To study sea level rise, the Jason series, GRACE, and Argo are the big three," said oceanographer Josh Willis of NASA's Jet Propulsion Laboratory, Pasadena, California, the project scientist for the upcoming Jason-3 altimetry mission.

Observations from the Jason series have revolutionized scientists' understanding of contemporary sea level rise and its causes. We know that today's sea level rise is about one-third the result of the warming of existing ocean water, with the remainder coming from melting land ice.

And it has shown precisely that the sea, of course, is not actually level. It varies as much as six feet (two meters) from place to place. And it is not rising evenly, like a bathtub filling with water. Currently, regional differences in sea level rise are dominated by the effects of ocean currents and natural cycles such as the Pacific Ocean's El Niño phenomenon and Pacific Decadal Oscillation. As the ice sheets continue to melt, scientists predict their meltwater will overtake natural causes as the most significant source of regional variations and the most significant contributor to overall sea level rise.

Or as Willis put it: "You ain't seen nothing yet."

Watching ice melt

Not that long ago, in the early 1990s, scientists were not able to determine whether polar land ice was growing, shrinking or in balance. Satellite and airborne missions, complemented by field measurements, have not only answered that question, but also provided the means for scientists to determine the mechanisms that are contributing to the growth and shrinkage of polar ice.

These advances in observing the world's frozen regions have allowed scientists to accurately estimate annual ice losses from Greenland and Antarctica in only the last decade. We now know not only how much sea level is changing — as measured by satellites for the past 23 years — but we can also determine how much sea level rise is caused by the loss of land ice.

Several studies have shown that different remote sensing methods for studying ice sheet mass balance agree well. GRACE's record, spanning over a decade, shows that the ice loss is accelerating in Greenland and West Antarctica. Greenland has shed, on average, 303 gigatons of ice every year since 2004, while Antarctica has lost, on average, 118 gigatons of ice per year, with most of the loss coming from West Antarctica. Greenland's ice loss has accelerated by 31 gigatons of ice per year every year since 2004, while West Antarctica has experienced an ice mass loss acceleration of 28 gigatons per year.

The warming north

The Greenland Ice Sheet, spanning 660,000 square miles (1.7 million square kilometers) — an area almost as big as Alaska — and with a thickness at its highest point of almost 2 miles (3 kilometers), has the potential to raise the world's oceans by more than 20 feet (6 meters). Situated in the Arctic, which is warming at twice the rate of the rest of the planet, Greenland fell out of balance in the 1990s, and is now shedding more ice in the summer than it gains back in the winter.

"In Greenland, everything got warmer at the same time: the air, the ocean surface, the depths of the ocean," said Ian Joughin, a glaciologist at University of Washington, in Seattle. "We don't really understand which part of that warming is having the biggest effect on the glaciers."

What scientists do know is that warming Arctic temperatures — and a darkening surface on the Greenland ice sheet — are causing so much summer melting that it is now the dominant factor in Greenland's contribution to sea level rise.

Greenland's summer melt season now lasts 70 days longer than it did in the early 1970s. Every summer, warmer air temperatures cause melt over about half of the surface of the ice sheet — although recently, 2012 saw an extreme event where 97 percent of the ice sheet experienced melt at its top layer.

Greenland's glaciers have sped up, too. Though many of the glaciers in the southeast, west and northwest of the island that experienced quick thinning from 2000 to 2006 have now slowed down, others haven't. A study last year showed that the northeast Greenland ice stream had increased its ice loss rate due to regional warming.

"The early 2000s was when some big things revealed themselves, such as when we saw the fastest glacier we knew of, the Jakobshavn ice stream in Greenland, double its speed," said Waleed Abdalati, director of the Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, and former NASA chief scientist. "The subsequent surprise was that these changes could be sustained for a decade — Jakobshavn is still going fast."

To answer the questions of how these glaciers will behave, how much they will contribute to sea level rise and how fast those changes will occur, scientists need better data on the bathymetry or geography of the ocean floor surrounding Greenland, said Eric Rignot, a glaciologist with JPL and the University of California, Irvine.

"Bathymetry is critical for understanding how ocean waters circulate around Greenland, for projections and for understanding what we've been observing in the past few decades," Rignot said.

Beginning with the deployment of research buoys in the waters around Greenland this summer, NASA is embarking on a three-year airborne and ship-based campaign to answer precisely these questions. OMG, Oceans Melting Greenland, seeks to understand the role of ocean currents and ocean temperatures in melting Greenland's ice from below — and therefore better predict the speed at which the ice sheet will raise sea level.

Changes at the southern end of the world

The Antarctic Ice Sheet covers nearly 5.4 million square miles (14 million square kilometers), an area larger than the United States and India combined, and contains enough ice to raise the ocean level by about 190 feet (58 meters). The Transantarctic Mountains split Antarctica into two major regions: West Antarctica and the much larger East Antarctica.

Though Antarctica's contribution to sea level rise is still at less than 0.02 inches (0.5 millimeters) per year, several events over the past decade and a half have prompted experts to start warning about an the possibility of more rapid changes in the upcoming century.

The mountainous horn of the continent, the Antarctic Peninsula, gave one of the earliest warnings on the impact of a changing climate in Antarctica, when warming air and ocean temperatures led to the dramatically fast breakup of the Larsen B ice shelf in 2002. In about a month, 1,250 square miles (3,240 square kilometers) of floating ice that had been stable for over 10,000 years were gone. In the following years, other ice shelves in the peninsula, including the last remainder of Larsen B, collapsed, speeding up in the flow of the glaciers they were buttressing.

In 2014, West Antarctica grabbed the spotlight when two studies focusing on the acceleration of the glaciers in the Amundsen Sea sector showed that its collapse is underway, and that the rest of West Antarctica will follow. While one of the studies said the demise could take 200 to 1,000 years, depending on how rapidly the ocean heats up, both studies concurred that the collapse is unstoppable and will add up to 12 feet (4 meters) of sea level rise.

For the West Antarctic Ice Sheet, which largely rests on a bed that lies below sea level, the main driver of ice loss is the ocean. The waters of the Southern Ocean are layered: on top and at the bottom, the temperatures are frigid, but the middle layer is warm. The westerlies, the winds that spin the ocean waters around Antarctica, have intensified during the last decade, pushing the cold, top layer away from the land. This allows the warmer, deeper waters to rise and spill over the border of the continental shelf, flowing all the way back to the base of many ice shelves. As the ice shelves weaken from underneath, the glaciers behind them speed up.

East Antarctica's massive ice sheet, as vast as the lower continental U.S., remains the main unknown in projections of sea level rise. Though it appears to be stable, a recent study on Totten Glacier, East Antarctica's largest and most rapidly thinning glacier, hints otherwise. The research found two deep troughs that could lead warm ocean water to the base of the glacier and melt it in a similar way to what's happening to the glaciers in West Antarctica. Other sectors grounded below sea level, such as the Cook Ice Shelf, Ninnis, Mertz and Frost glaciers, are also losing mass.

"The prevailing view among specialists has been that East Antarctica is stable, but I don't think we really know," said Rignot. "Some of the signs we see in the satellite data right now are kind of red flags that these glaciers might not be as stable as we once thought. There's always a lot of attention paid by the media to the changes we see now, but as scientists our priority remains what the changes could be tomorrow."

On the other hand, weather models and reanalyses of climate model data have shown that there has been an increase in snowfall along the coastline of Dronning Maud Land, which might counteract East Antarctica's ice loss. But this may be a temporary shift — scientists can't tell, because obtaining accurate field measurements of snow in Antarctica is extremely difficult. There are few weather stations, and they might not provide representative measurements because snowfall in Antarctica doesn't distribute evenly; the strong katabatic winds wipe some areas clean and make snow pile up in others. And satellite readings are not yet precise enough to detect small accumulation changes that would represent a difference of gigatons of mass when spread over East Antarctica's huge surface.

As is the case for Greenland, researchers also working on Antarctica need better data on the Southern Ocean bathymetry and the pathways that warm waters can follow to reach the ice. And this kind of data, as well as snow accumulation and other ocean data, can't be obtained remotely with enough precision, according to Ted Scambos, lead scientist at the University of Colorado's National Snow and Ice Data Center in Boulder.

"We've learned so much from the satellites that we've been surfing the wave of new understanding for the last 20 years," Scambos said. "But now, to go further, we have to try to get instruments on the ground while maintaining the ability we have with airborne and satellite missions to watch the ice sheet from a global perspective."

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