At one point several hundred thousand years ago, snow began falling over the center of the earth’s largest island. The snow did not melt, and in the years that followed, storms brought even more. All around Greenland, the arctic temperatures remained low enough for the snow to last past spring and summer. It piled up, year after year, century after century, millennium after millennium. Eventually, the snow became the Greenland ice sheet, a blanket of ice so huge that it covered 650,000 square miles and reached a thickness of 10,000 feet in places. Meanwhile, in Antarctica, a similar process was well underway. There, as snow fell upon snow for years without end, the ice sheet spread out over a much vaster area: 5.4 million square miles, an expanse far larger than the lower 48 states. By the start of the modern era, when power plants and electric lights began illuminating the streets of Manhattan, about 75 percent of the world’s freshwater had been frozen into the ice sheets that lay over these lands at opposite ends of the earth.  The ice sheets covering Greenland and large areas of Antarctica are now losing more ice every year than they gain from snowfall. The loss is evident in the rushing meltwater rivers, blue gashes that crisscross the ice surface in warmer months and drain the sheets’ mass by billions of tons annually. Another sign of imbalance is the number of immense icebergs that, with increasing regularity, cleave from the sheets and drop into the seas. In late August, for instance, a highly active glacier in Greenland named Jakobshavn calved one of the largest icebergs in its history, a chunk of ice about 4,600 feet thick and about five square miles in area.  If the ice sheets on Greenland and Antarctica were to collapse and melt entirely, the result would be a sea-­level rise of 200 feet or so. This number, though fearsome, is not especially helpful to anyone but Hollywood screenwriters: No scientist believes that all that ice will slide into the oceans soon. During the last year, however, a small contingent of researchers has begun to consider whether sea-­level-­rise projections, increased by the recent activity of collapsing glaciers on the periphery of the ice sheets, point toward a potential catastrophe. It would not take 200 feet to drown New Orleans. Or New York. A mere five or 10 feet worth of sea-­level rise due to icebergs, and a few powerful storm surges, would probably suffice.  How soon could that happen? When it comes to understanding the implications of ice-­sheet collapse, the speed of that breakdown is everything. It could mean sea levels that rise slowly and steadily, perhaps a foot or two per century, which might allow coastal communities to adapt and adjust. Or it could mean levels that rise at an accelerating pace, perhaps five feet or more per century — forcing the evacuation of the earth’s great coastal cities and producing millions of refugees and almost unimaginable financial costs. The difference between slowly and rapidly is a crucial distinction that one scientist recently described to me as ‘‘the trillion-­dollar question.’’  Evaluating these dangers is a thorny scientific undertaking, because the Greenland and Antarctic ice sheets are not just big blocks of frozen water. They are giant plumbing systems made from gigatons of ice, snow and slush, honeycombed with buried lakes and hidden rivers. Their size and complexity have long made their behavior difficult to assess and even harder to predict. A few years ago, a NASA-­supported researcher seeking to shed light on the ice sheet’s interior pipes dropped 90 yellow rubber ducks into a Greenland moulin, a deep hole in the ice sheet, with his contact information on them, in case they were found. Only two ducks were recovered — the next year — thanks to a fisherman working in a nearby bay. Somewhere, trapped deep in the Greenland ice sheet, or floating in the waters nearby, or who knows where, really, there are 88 more rubber ducks.  Further complicating matters is the sheets’ constant motion, as their immense weight causes them to flow outward from thick center toward thinner periphery. And the movement isn’t always slow and steady; glaciers extending out from the ice sheet can start, stop, speed up — erratically — as the ice rolls over bumps in bedrock or slides along on sheets of meltwater. More important, a glacier’s edges can break off suddenly and catastrophically into the sea, at times and places that remain difficult to anticipate. Arresting the big problems associated with climate change — punishing droughts, more powerful storms, lethal heat waves — often relates directly to whether human societies can scale back carbon emissions within a reasonable time. But ice sheets are a good example of how rising temperatures can tip natural systems into unknown territory. The warmth leads to repeating loops of cause and effect that can force ice sheets to flow faster, break faster, melt faster. Glaciologists may not be able to decipher the mysteries of ice sheets, to model their behavior and sensitivity, before calamity becomes inevitable.  At the moment, there are encouraging indications that the global community is poised to put decades of inaction behind it and address the carbon-­dioxide emissions that have been driving air and ocean temperatures relentlessly upward. A United Nations-­sponsored conference in Paris, scheduled for December, seeks to set emission targets that would keep the planet’s temperatures from rising two degrees Celsius (or about four degrees Fahrenheit) above the average for the industrial era, a benchmark thought to ensure that we will avoid the most catastrophic impacts of a warming world.  But there are clear warnings that the ice sheets have entered a phase of dangerous and unknown instability. To assess what this means for tomorrow requires looking back to long ago. The current research on sea-­level rises during ancient eras — findings that are rarely discussed outside scientific circles — suggests that to regard the prospect of a future glacial collapse with only modest concern is to disregard what has happened in earth’s past, and what might happen again. ‘‘We know the ice can change fast,’’ Eric Rignot, a professor of earth sciences at the University of California, Irvine, told me in May, as we talked at a campus picnic table on a sunny afternoon. ‘‘We’ve never seen it. No human has ever seen it.’’ Rignot is fairly confident, however, that we are seeing it now — a conclusion borne out by the ice-­sheet data he scrutinizes every week. A few decades from now, he said, we may look back with regret, wondering why more of us didn’t acknowledge the signs all around us, why we didn’t see ‘‘that the collapse had already started.’’  Sea-­level rise has three primary causes. The first is that at warmer temperatures, water expands — meaning oceans literally get bigger. The second is that the world’s mountain glaciers, numbering around 200,000 and located everywhere from Argentina to Alaska, are rapidly melting and draining to the seas. The third is that the ice sheets are shedding meltwater and icebergs at an accelerating rate. Greenland, for instance, is now experiencing an average net loss of about 303 billion tons of ice every year.  Taking all these into consideration, the Intergovernmental Panel on Climate Change, the group of scientists that periodically issues reports on global warming, projects that the sea will rise between a half meter (1.6 feet) and a full meter (3.2 feet) by the end of the century. The outcome will depend on how much carbon finds its way into the atmosphere: The more carbon dioxide we produce by burning fossil fuels over the coming decades, the more ocean and air temperatures will increase. And the higher those temperatures rise, the faster oceans should rise. At the moment, sea levels — measured by satellites and tidal gauges in harbors around the world — are going up on average by about 3.3 millimeters, or one-­tenth of an inch, a year.  The panel’s most recent report on sea-­level rise is filled with footnotes, hedges and caveats. Many concern the deep uncertainty that surrounds the fate of the ice sheets over the next several hundred years, and whether their complexity might translate into unexpected events that push sea levels drastically higher — through a sudden, widespread calving of icebergs, for instance. At the moment, the biggest questions in glaciology focus on the fate of what are known as marine-­terminating glaciers. These wide rivers of ice flow out from the edges of the ice sheets and end at the sea. They are inherently unpredictable, as that giant iceberg calved at Jakobshavn demonstrates.  Because the panel’s official outlook tends toward the conservative (in large part because that’s what it takes to get thousands of scientists from around the world to agree), several prominent researchers have challenged its take on the future. With his recent work at U.C. Irvine, Rignot has assumed an outsize profile among this peer group, thanks to his scientific ingenuity and his personal audacity. Born in France and awarded a Ph.D. in engineering from the University of Southern California, Rignot has published a series of research papers that have systematically challenged conventional assumptions about the durability of the ice sheets. The works are not for casual readers (a recent title: ‘‘Antarctic Grounding Line Mapping From Differential Satellite Radar Interferometry’’). Yet taken together, they identify a host of new and disturbing vulnerabilities in the ice sheets. ‘‘Some people think he might be going too far,’’ his U.C. Irvine colleague Mathieu Morlighem told me. ‘‘But everything he has said has turned out to be correct.’’ Last week in the journal Science, a striking article by Rignot and his colleague Jeremie Mouginot called attention to the rapid decay of an enormous marine-­terminating glacier in northeast Greenland called Zachariae Isstrom, which holds ice equivalent to half a meter of sea-­level rise. Its recent destabilization, they note, ‘‘will increase sea-­level rise from the Greenland ice sheet for decades to come.’’  Describing the tendency among scientists to avoid speaking bluntly about ice-­sheet collapse, Rignot told me: ‘‘You can fiddle around and say, ‘It’s going to take a long time’ or ‘We don’t know.’ But even the most conservative people in our community will tell you: ‘We warm the climate by two or three degrees C? Greenland’s ice is gone.’ ’’ He was suggesting to me, in other words, that the Paris targets of two degrees Celsius would not halt the glacial decline. Rignot continued: ‘‘And five degrees? There’s no way Antarctica is going to stay if we warm up by five degrees.’’  Rignot, who is 53, is prone to dark expostulations. Several times he said to me: ‘‘We are teasing a giant here.’’ Twice, he declared: ‘‘We have blown the fuse.’’ What drives his work is quantifying the potential of a worst-­case scenario. ‘‘My long-­term objective,’’ he told me, ‘‘is to try to come up with ways we can put upper bounds on these changes — upper bounds on how fast the ice sheets could change and how fast sea level could rise. If we can get there and tell people, ‘Look, it’s going to be very hard for nature to go faster than this,’ I think I’ll be able to say we got our job done. Nobody will come back in 50 years and say, ‘You guys were just so conservative and did a big disservice to society.’ ’’  There is fearlessness in his approach: Rignot is willing to be wrong by pointing to an outcome that is more extreme than might actually happen. When I mentioned that some climate scientists had told me in confidence over the years that they held more pessimistic views than they felt comfortable expressing in public, Rignot shrugged with a smile and said, ‘‘They are chicken.’’  It’s essential not to overlook Rignot’s caveat — that nature may create certain speed limits for the deterioration of ice sheets. The problem is that we don’t know those limits. In late July, James Hansen, a scientist whose testimony before Congress in the 1980s called national attention to global warming, declared in a paper that sea-­level changes driven by ice-­sheet collapses could well exceed six feet within 50 to 150 years. Rignot does not think the oceans will rise quite so considerably, but as a co-­author, he contributed some insights to Hansen’s paper, which was greeted with skepticism among some climate scientists. The controversy did not bother Rignot, who told me approvingly, just after the paper came out, ‘‘Jim Hansen really stirred the pot this week.’’  Whether Hansen is overstating the dangers depends on the ice sheets’ marine-­terminating glaciers. These days, Rignot’s main work is devoted to analyzing satellite images of Greenland and Antarctica, a task he can handle from Irvine. For the past few summers, though, he has also chartered a boat to measure glaciers himself. Making his way around the cold coast of Greenland, he checks the depths and temperatures of the fjords — the deep ocean inlets that penetrate the island’s coastline — where the glaciers meet the sea. The work is punishing and somewhat risky; the crew barely sleeps. He is mapping places that have never been scientifically explored. ‘‘It can be somewhat unpleasant,’’ Rignot admitted. But the data have proved startling. His first paper on this work, published this summer, revealed that the fjords are much deeper than existing charts indicated, which means more of the glaciers’ submerged ice is exposed to seawater and can be eroded by it. The resulting interaction, he concluded bluntly, ‘‘will raise sea levels around the world much faster than previously estimated.’’ NASA recently expanded Rignot’s efforts into a broad, five-­year investigative mission designated ‘‘O.M.G.,’’ for ‘‘Oceans Melting Greenland.’’ The name suggests that someone at NASA has a sense of humor and that understanding the ocean-­ice interplay is fairly urgent — especially as warming ocean waters also erode Antarctica’s marine glaciers, many of which are far larger than Greenland’s.  Yet measuring the impact of fjords won’t come close to settling the matter of how fast an ice sheet collapses. There are too many other unknowns. Glaciologists remain vexed, for instance, by the physics of how ice cleaves off the edge of the sheet. As Rignot told me, ‘‘We don’t have a set of mathematical rules to put in a numerical model to tell you how fast a glacier breaks into icebergs.’’ He emphasized that discovering these rules, known as calving laws, could be all-­important. Richard Alley, a glaciologist at Penn State, told me: ‘‘Problems that deal with fracture mechanics — volcanic eruptions, or earthquakes, or things that involve the question ‘Will it break or not?’ — tend to be difficult. You ask, Will the ice shelf break off a lot or a little bit? Will the cliff left behind crumble? Will it crumble fast? Will it crumble slow?’’ So far, Alley says, we can’t be sure. But a formula might tell us in advance how fast the ice sheets might crash into the sea.  Rignot’s prediction of future sea-­level rise now takes us to 1.2 meters by 2100. ‘‘That’s including all the ice and the thermal expansion of the ocean,’’ he told me. ‘‘But I’ve always said that this is a lower bound.’’ There are too many glaciers, now poised at the edge of the polar seas, that could change his calculus. It is hard right now to even set an upper bound, he added: Many things are possible, and nothing is certain.  Rignot grew up in south-­central France, near a rural town called Chambon-­Sur-­Lignon. Interested in both technol­ogy and the natural world as a teenager, he didn’t imagine that his career would involve Arctic exploration. But his engineering education at U.S.C. in the late 1980s coincided with a technological shift in the study of ice. Traditionally, glaciology has involved painstaking, labor-­intensive work at the most remote locations in order to measure changes in glaciers. In Greenland and Antarctica, the work has usually been limited to confined geographic regions, so that a broader understanding of the ice sheet itself proved elusive. When he was hired to work on a radar project at NASA’s Jet Propulsion Laboratory in the late 1980s, Rignot’s boss told him, ‘‘You’re coming to the right place at the right time.’’ Engineers were then adapting radar, laser and camera equipment to gather data and images of the earth from space.  These instruments have made possible what is referred to as ‘‘remote sensing.’’ Tom Wagner, a scientist at NASA who oversees the agency’s polar programs (and much of Rignot’s work), told me: ‘‘In the 1980s, biologists got the ability to work with DNA, and that revolutionized biology. For us in glaciology, it was the remote-­sensing tools.’’ Early on, American and European satellites gave scientists sitting at their desks — a new generation of ‘‘armchair glaciologists’’ — a weekly stream of data on what was happening to glaciers all over the world. In the 1990s, nobody was actually sure whether Greenland was losing or gaining ice; remote-­sensing imagery, coupled with precise measurements from a new NASA satellite called Grace, settled that question. By the early 2000s, it was becoming clear that the Greenland ice sheet was in decline and that rapid changes were underway in Antarctica.  In recent years, specially equipped airplanes have been able to gather even more detailed information about crucial regions in Antarctica or Greenland. A month before I met Rignot in California, I joined a NASA team in Greenland on its annual mission called Operation IceBridge. For eight hours a day, six days a week, the team manned a C-­130 former military plane that flew over the ice at 1,500 feet. The aircraft had been outfitted under its wings, belly and nose cone with cameras and sensors; inside, several rows of seats for the science team were arranged behind a bank of gleaming computer consoles, high-­resolution screens and radar instruments. On the outside, the plane, which was manufactured in the 1960s, looked positively antique; on the inside, it was state of the art.  The flights took off each morning from the tiny village of Kangerlussuaq, on the west coast of Greenland just north of the Arctic Circle. The first day’s trip was to the southeast, toward the glaciers of Greenland’s rugged eastern coast. It was a long ride — Greenland is three times the size of Texas — across the island’s midsection, where the ice sheet runs in some places to a depth of two miles. During the flight, I often wandered up to the pilots’ flight deck; taking in the vast expanse of Greenland from there felt like surveying the landscape of a frozen exoplanet. The occasional derelict Cold War radar station was a mere blip on the endless blankness. Ice and rock, ice and rock, ice and rock: You could easily let yourself believe that humans had made no appreciable impact here at all.  As we flew above the ice, the data on the sheet’s thickness (measured by radar) and precise height (from an instrument called a laser altimeter) flowed into the IceBridge computer banks. It would eventually make its way to a Web portal so scientists around the world could parse its significance. Rignot, who happens to be a leader of the project, refers to IceBridge as a ‘‘game changer’’ for measuring ice thickness, because it can capture changes impossible to discern by older glaciological techniques or by the naked eye. ‘‘That can only be done from airborne platforms,’’ he says. ‘‘We can say we’re going to map ice thickness in Greenland — not just a few glaciers, but everywhere. And then we try to do the same in Antarctica.’’  Rignot has large maps of Greenland and Antarctica on his office wall color-­coded in blue, red and green to show the rates at which the ice is flowing toward the sea. On his Antarctica map, he focuses most on a remote sector above the Amundsen Sea where two enormous glaciers, known as Pine Island and Thwaites, drain ice from the West Antarctic ice sheet. It is a stroke of terrible cosmic luck that both glaciers are sitting on a ridge of land below sea level, making them potentially unstable.  As far back as 1997, Rignot was examining satellite images that convinced him that Pine Island was pulling back from its unstable perch and beginning a rapid retreat. He spent a year reviewing the information, because the implications seemed so unsettling. ‘‘When I presented the results, I had my graph upside down, because I was a bit nervous,’’ he recalls. But the journal Science published the research in 1998. ‘‘And I think at that point,’’ he says, ‘‘I realized that we were on to some very big changes.’’  Last year, Rignot updated his findings, again relying on remote-­sensing data, and declared that the deterioration of these glaciers had continued and was now ‘‘unstoppable.’’ He had known this for a while, he says, and the paper reflected ‘‘a point of exasperation.’’ As he puts it, ‘‘It was time to speak a little more loudly about it.’’  Pine Island and Thwaites are not likely to collapse as gutted buildings do. They will slowly disintegrate until they have calved enough icebergs to produce about four feet of sea-­level rise. The fundamental question is how slowly: decades or centuries? Only a small number of men and women in the world worry about this distinction, yet the future of thousands of cities hinges upon it. When I asked Richard Alley, almost certainly the most respected glaciologist in the United States, whether he would be surprised to see Thwaites collapse in his lifetime, he drew a breath. Alley is 58. ‘‘Up until very recently, I would have said, ‘Yes, I’d be surprised,’ ’’ he told me. ‘‘Right now, I’m not sure. I’m still cautiously optimistic that in my life, Thwaites has got enough stability on the ridge where it now sits that I will die before it does. But I’m not confident about that for my kids. And if someday I have grandkids, I’m not at all confident for them.’’  Rignot answers the same question more bluntly. ‘‘That’s what we’re seeing right now — they are in a state of collapse,’’ he says. ‘‘We’ve never seen it before, so it’s hard to identify it and say, ‘We know exactly what it looks like, and this is what it looks like.’ We’re still in the early stage.’’ Rignot predicts that in 30 or 40 years, people will be accustomed to watching Thwaites and Pine Island disintegrate constantly, iceberg by iceberg, into the ocean. And by then, he adds, their collapse ‘‘will be a part of everyday life.’’  CreditOlaf Otto Becker for The New York Times  Earth’s geological history, together with the contours of ancient shorelines, tell us two things about ice-­sheet collapse. The first is that great quantities of ice can fall into the ocean rapidly, at rates far exceeding what is happening today. The second is that even if sea-­level rises don’t happen quickly in the near future, they will happen eventually. As Alley told me, the historical record points in two directions: ‘‘Sea-­level rise could be scary in magnitude but not in rate. Or it could be scary in magnitude and rate if our warming reproduces what happened in the past.’’  We might imagine a rapid collapse of the ice sheets in B-­movie terms: sudden and terrifying, with enormous waves of water cresting over beachside homes in Malibu, Calif., and nature reclaiming the Rockaways. In truth, the remoteness of the sources of new icebergs means no devastating tsunamis. Because the calving would happen over the course of many decades rather than weeks, the catastrophe would manifest over time. First there is water in the basements, gutters and subways; then, storms regularly bringing water into the streets. Year by year, the rise accelerates. Brine infiltrates drinking-­water systems and sewer plants; electrical grids spark out. Flood-­insurance policies are discontinued, and home values plummet. Row by row, seaside homes are abandoned. Still the rise continues. Large-­scale evacuation then becomes imperative — as long as inland cities and funds are available for relocation. In low-­lying countries, however, the implications of significant sea-­level rise, and the occasional storm surges that amplify the floodwaters, move beyond the economic to the existential. ‘‘On these longer time scales,’’ says Anders Levermann, a sea-­level expert at the Potsdam Institute for Climate Impact Research, ‘‘the magnitude of the sea-­level rise could get so big that we have to evacuate New York, Calcutta, Hong Kong, Shanghai, Hamburg and most of the Netherlands.’’  When Rignot talks about catastrophic events happening over a short period, he sometimes mentions an event in earth’s past known as ‘‘Meltwater Pulse 1A.’’ This occurred about 14,500 years ago and lasted roughly three centuries. As global temperatures warmed — the last ice age was just ending — parts of the Antarctic ice sheet, along with an ice sheet that once covered parts of northeastern Canada, shattered into the ocean as icebergs. Over this period, ocean levels probably increased at around nine or 10 feet per century, many times even the extreme rate projected by the international panel. Rignot isn’t certain that this is our destiny; large sea-­level rises in the past can differ from today in terms of location and the forces acting upon the ice sheets. But 1A makes Rignot believe that we can’t rule out the possibility of a very fast event.  As for the question of how high oceans will eventually rise, the answer is less speculative. Ice sheets are what might be called lagging indicators; they can take hundreds or thousands of years to adjust to a new environment. But the laws of physics ultimately bring them into equilibrium. We know from climatology research and the residue of ancient shorelines that past carbon concentrations similar to today’s concentrations are associated with higher temperatures in the Arctic and Antarctic, ice-­sheet collapse and higher oceans. Scientists who study sea-­level rise sometimes call these longer-­term effects a ‘‘commitment’’ — that is, sea levels that we are committed to over the long haul if our current rates of carbon-­dioxide emissions continue. The effects of CO₂ are akin to rolling a large stone down a big hill: Eventually, it gains so much momentum that it cannot be stopped.  In July, a research team led by Andrea Dutton, a professor at the University of Florida, published a study on commitments that concluded that even a tiny swing in global average temperatures ultimately leads to substantial rises in sea level. For instance, she told me, if we compare today to earth’s last warm period — one that occurred about 120,000 years ago — ‘‘we do know the poles were at least a few degrees warmer.’’ Earth isn’t quite at that point, but projections for the Arctic regions suggest that it will be that warm in a few decades. ‘‘So we’re not saying that we’re committed yet, but we are nearing temperatures where we’ve repeatedly seen about 20 feet of sea-­level rise.’’  At some point soon, in other words, 20 feet may not be a possible outcome, but the world’s destiny. The puzzle to solve then becomes: What produces that rise in sea level? How much of the 20 feet will come from Antarctica’s ice falling into the ocean? How much from Greenland’s?  Here is one possible sequence of events: When Rignot and others point to four feet of sea-­level rise resulting from the collapse of Pine Island and Thwaites, that comes with the implication of losing, as Rignot puts it, ‘‘the back wall of West Antarctica.’’ Afterward, larger areas of West Antarctica would become vulnerable, too. And that, in turn, would lead to a more complete collapse of that part of the ice sheet, producing an additional 12 feet of sea-­level rise. The more ice you lose, the more ice you lose.  These sorts of chain reactions and feedback loops figure prominently in Rignot’s grim scenarios. Greenland’s ice sheet, meanwhile, has additional vulnerabilities. One day in Irvine, I watched him teach a class of undergraduates about another such loop, involving the color of Greenland’s ice sheet, which is growing darker as the climate warms. The darkening ice sheet absorbs more solar radiation, he said, which causes the ice sheet to melt more — and that in turn causes the ice sheet to darken more, absorb more solar radiation and melt more. ‘‘Most of the time, positive feedback is good,’’ he told the class. ‘‘If you’re a teacher, you love positive feedback. But for climate, positive feedbacks are bad.’’  The darkening of Greenland is known as the albedo-­melt feedback (albedo is Latin for ‘‘whiteness’’). There is another positive feedback in Greenland, too, known as the temperature-­elevation feedback. Simply put, at lower elevations, the temperature is warmer, and as the Greenland ice sheet melts, it gets lower in elevation. Then it melts more, and gets even lower in elevation — and so on. Alexander Robinson, a scientist at the Complutense University of Madrid, told me: ‘‘With these two feedbacks — the albedo-­melt feedback and the temperature-­elevation feedback — you’re imposing a process that won’t stop, that becomes irreversible, unless you really cool the climate in the other direction. And I think we’re getting close to temperatures where it becomes a danger that Greenland becomes auto-­sustainable, that it can just melt itself.’’ Robinson thinks we will cross the threshold within the next few decades.  Commitments are worrisome because they are hard to undo. And they are hard to undo because the waxing and waning of ice sheets is asymmetrical. It takes tens to hundreds of thousands of years to grow an ice sheet. But it may take only a few thousand or even hundreds of years to collapse it. Rignot says: ‘‘What is going to happen by 2100 with the ice sheets is, in my opinion, already locked in.’’ That could mean a sea-­level rise closer to what some climate scientists expect — perhaps a meter. Or it could mean an outcome closer to two meters. ‘‘But the world doesn’t end in 2100, either,’’ Rignot says. Then he asks: What kind of world do we want to live in, in 2100 and beyond? ‘‘Do we want to take the risk to see five, six, seven meters of sea-­level rise? Or do we want to say: No, we don’t want to go there. It’s going to be too painful?’’  In early June, a couple of months after the IceBridge flights, I passed the Jakobshavn Glacier on my way to attend a conference in the nearby village of Ilulissat about the present condition and future of the Greenland ice sheet. In the same manner that archaeologists might make a pilgrimage to Machu Picchu, glaciologists go to Jakobshavn to study its speed, ferocity and physics. Between 2000 and 2010, the glacier’s icebergs were alone responsible for about a millimeter of global sea-­level rise. Seeing the wreckage of shattered ice near the glacier’s calving front, I could understand why: From the air, it looks as if every window in the world had suddenly broken and been swept into piles here.  At the conference, I sought out an American scientist from the University of Washington named Ian Joughin. It is wise to balance Rignot’s conclusions against Joughin’s. The two men are friends, going back to their days together at the Jet Propulsion Lab in the 1990s, and both are trained as engineers, which inclines them to view ice sheets as complex physical systems rather than geological artifacts. (‘‘I can’t tell one rock from another,’’ Joughin once told me.) Moreover, on the same day last year that Rignot declared, during a news conference, the retreat of the West Antarctica glaciers to be ‘‘unstoppable,’’ Joughin and his team published a paper in Science with a similar conclusion.  Rignot, compact and athletic, can seem tensioned like a taut wire. Joughin (pronounced ‘‘JOCK-­in’’) has a genial calm about him and often dresses in a baseball cap and a fleece jacket. There are contrasts in their outlook, too. Rignot’s views on sea-­level rise qualify him as a maximalist. But Joughin’s position is more cautious. Despite his conviction that West Antarctica will eventually break down, Joughin can dismiss in a mere instant most predictions of catastrophic ice-­sheet collapse. ‘‘I would be very surprised if sea-­level rise is much more than a meter by the end of the century,’’ he told me. His view, one that seems shared by a number of his colleagues, is that his friend Rignot’s possibilities for sea-­level rise are too high: More ice would have to move into the ocean in a shorter time than is physically plausible. Or to put it another way: The world’s ice sheets are deteriorating faster than they have for thousands of years, but they’re not yet moving at a civilization-­destroying pace.  Before the conference, I emailed Joughin to ask if he would like to take a closer look with me at icebergs from Jakobshavn, and he quickly wrote back to say, ‘‘I will never turn down an opportunity to gaze upon the monster that is Jakobshavn.’’ A couple of days later, we were on a boat headed into Disko Bay. We soon began threading between battleship-­size icebergs, looming 100 feet high and 500 feet long, which had broken off Jakobshavn’s calving front 40 miles upstream.  ‘‘These are probably a year or two old, and they drifted down here slowly from the calving front,’’ said Joughin, who has been researching Jakobshavn for nearly two decades. But the ice within them was probably many thousands of years old. When the iceberg had finally broken off from the end of the flowing glacier, it had most likely released an explosive power equivalent to several atomic bombs.  Every 11 days or so for the past five years, Joughin has tracked Jakobshavn’s movement — usually through radar images taken by German satellites that he views on his work computer in Seattle. Seeing the glacier’s handiwork up close was an altogether different experience, though. ‘‘Jakobshavn is the fastest glacier we know of,’’ Joughin said. Its movement has sped up by as much as a factor of four over the past few years, to the point where it recently hit a peak of 17 kilometers, or about 10.5 miles, per year. Many of Greenland’s big glaciers are also accelerating their production of icebergs as the Arctic warms, and as they do, more and more ice breaks from the ice sheet. In Jakobshavn’s prolific calving of icebergs, you not only encounter a dazzling natural tableau. You also glimpse the future of glaciers everywhere.  As we turned back to the harbor, Joughin said he imagines that Greenland’s total collapse will take many hundreds or thousands of years. He perceives various physical constraints on how much ice can get off the island at the edges, which are ringed by mountains and narrow passes through deep fjords. ‘‘The fjords are actually like nozzles on a toothpaste tube squeezing out ice,’’ he said. Joughin sees West Antarctica’s Thwaites and Pine Island Glaciers taking a long time to disintegrate, too, from 200 to 900 years. ‘‘I think of collapse as being slow, like the decline of the Roman Empire,’’ he said. ‘‘If you want to be alarmist, you could say Jakobshavn is putting out huge amounts of ice — a millimeter of sea-­level rise in a decade. But if you want to be dismissive you can say: Well, there’s seven meters of ice in the Greenland ice sheet, and even with all these changes, it’s losing one millimeter per year. So that’s 7,000 years to go.’’  Almost certainly, Joughin added, the melt will speed up as the climate warms. But his larger point is that imminent disaster from rising seas is unlikely. What’s more, the ice sheets can be variable. In 2012, for instance, Greenland suffered an extraordinary melt season, resulting in the net loss of about 771 billion tons of ice. But 2013 was colder, and over all the ice sheet lost little or no ice.  It would be reassuring to learn that Joughin is more correct than Rignot. Tom Wagner, the NASA administrator for polar programs who helps fund the work of both men, as well as that of dozens of other American scientists, told me: ‘‘These are very intelligent people. A lot of them are my friends. These are people I greatly respect.’’ But no one knows what future sea levels are going to be, Wagner says. Still, that Joughin and Rignot disagree on such a crucial scientific uncertainty perhaps matters less than it seems. The difference between their views amounts to asking if the future for earth’s coastal regions will be bad or very bad. A few months ago, a researcher at Penn State named David Pollard created a computer model of the Antarctic ice sheet that showed that Thwaites could indeed collapse within a century. Although the model indicates that the process might not begin until West Antarctica is warmer than today, to Rignot it backs up his argument that society should consider what is possible, and not what is probable.  ‘‘If Ian is right, and Thwaites collapses in 300 years, maybe that’s O.K.,’’ Rignot told me. ‘‘But what if David Pollard is right?’’ What if, he said, its collapse takes only a hundred years? And this, he pointed out, was only one glacier system, representing just a fraction of the meltwater that could be added to the world’s seas.  A road connects the coastal town of Kangerlussuaq, Greenland, to the edge of the central ice sheet. It rolls for 21 miles over rocky, empty, breathtaking terrain, the domain of reindeer, musk ox and arctic hares. Built around 2001 by Volkswagen to test vehicles in wintry conditions, the road dead-­ends at a cluster of large gray hills. These are glacial moraines of clay, gravel and stones that were pushed across Greenland by the relentless snowplow force of the flowing ice. Beyond the moraines, the immense ice sheet stretches north, south and east.  When approached from the ground, the ice sheet gives a first impression of an ocean — not only because it seems to capture the entire horizon, but also because it is sculpted into hillocks and hollows, like a roiling sea on a day of serious weather. On some visits, though, the ice sheet strikes you as the photographic negative of an ocean: Rather than darkness streaked with white foam, it is lightness streaked with silt and dust. The sheet in this region has been darkening noticeably over the past few years, apparently from the silt underneath and particulates in the air. In the meantime, it has been thinning and dropping in elevation, signs that feedbacks are in play. Looking ahead, you can see that as the ice recedes, the road from Kangerlussuaq will need to be extended.  You wonder if the ice will ever stop receding. If there is a concern that mobilizes Rignot above all others, it may be this. Rignot is seeking attention, something he readily admits, but it does not seem to be a case of narcissism; audacity is the best strategy for capturing the attention of a younger generation that might tackle a problem that an older generation failed to solve. He has four children, which he often says influences his longer-­term thinking. ‘‘I won’t be here in 2100 anyway,’’ he told me one day. ‘‘But I will make sure when I leave this world that my kids or the young people feel that I did as much as I could.’’  I wondered if his motivations, though, were slightly more complicated than that. In our conversations, there was a running theme, a trope, of economic havoc, of drowned cities, of time running out. The blown fuse. The idea of crossing a climate threshold now or very soon, while understanding both the physics and the folly of it, is too intellectually repugnant for him to accept quietly.  ‘‘It’s probably in human nature that we’re going to react once we have our back to the wall,’’ Rignot said, ‘‘and some of the changes we project are actually seen and we can’t deny them.’’ The obvious way to avoid a grim and flooded future, he said, would be to drastically curtail our carbon-­dioxide emissions. It may be difficult to halt the most vulnerable glaciers from collapsing into the sea. But computer models of Greenland show that melting would proceed more slowly if temperature increases are minimized. A slower rise in sea levels would give coastal regions the opportunity to construct barriers to counter storm surges and give society the time to reduce the cost (and increase the scale) of clean-­energy technologies. It might even give us enough room to figure out cost-­effective ways to remove excess carbon dioxide from the atmosphere, slowing the climb in temperatures.  More ambitious schemes are being discussed, too. For instance, one University of California glaciologist, Slawek Tulaczyk, has asked whether it might be worth considering the construction of a physical barrier to protect some of Antarctica’s glaciers from warming ocean water. This is a kind of technological fix, of course, that may exceed by a large degree any engineering project ever undertaken. Then again, to see the ice sheets up close is to see that our predicament is not how much is melting now, but how much is destined to melt. That was the feeling I got watching from the cockpit on the IceBridge flights, as we flew for hours over Greenland’s blank expanses. It is something Rignot realized, too, during his first glimpse of the Pine Island glacier in Antarctica: ‘‘I flew across the front, and I was thinking, I knew this was big, but I can’t believe how long it takes at 400 knots to cross the glacier. You are there for 10 full minutes.’’  Walking for a few hours last summer on the ice sheet near Kangerlussuaq, its crystals crunching underfoot, I got the impression that I could walk forever without reaching the other side. A slightly discernible incline, where the ice sheet rises to higher elevations, was barely perceivable in the distance. At the same time, by bending down near a small creek, carved into translucent white ice by a rushing stream, I could cup my hands and drink meltwater from snow that fell thousands of years ago, water as cold and pure as any that exists in nature. All around on the ice sheet were pools and rivulets, too many to count — each of them an omen of the larger melt to come.  Jon Gertner is editor at large at Fast Company and writes frequently on innovation and technology. He has written features for The Times Magazine since 2003. Olaf Otto Becker is a photographer based in Munich. He has published two photography books on Greenland, ‘‘Broken Line’’ and ‘‘Above Zero.’’  Sign up for our newsletter to get the best of The New York Times Magazine delivered to your inbox every week.  A version of this article appears in print on November 15, 2015, on page MM48 of the Sunday Magazine with the headline: The Secrets in the Ice. Today's Paper|Subscribe
By studying the largest glaciers on earth, scientists hope to determine whether we’ll have time to respond to climate change or whether it’s already too late.