The scale of Antarctica is startling. Miles of ice stretch to the horizon, growing thicker as you move toward the South Pole. This line represents 20 miles. ROSS SEA Cape Crozier Cape Tennyson ROSS ICE SHELF Mount Terror Lewis Bay ROSS ISLAND MOUNT Erebus Cape MacKay Fog Bay The southernmost active volcano on Earth. Aurora Glacier Scientists at McMurdo Station are working to understand the continent’s history and to predict its future. The scale of the task is enormous.This flat expanse of white is the Ross Ice Shelf, a floating chunk of ice nearly as large as Texas. Windless Bight Hut Point Peninsula Dellbridge Islands Erebus Ice Tongue This is Williams Field, an airfield on the ice shelf. McMURDO SOUND McMurdo Station This is the edge of the ice shelf, where it meets the thinner, seasonal sea ice. Mount Heine McMURDO ICE SHELF WHITE ISLAND Mount Nesos Cape Hodgson BLACK ISLAND ROSS ICE SHELF Brown Peninsula Hahn Island Minna Bluff MOUNT Discovery Heald Island Eady Ice Piedmont Koettlitz Glacier Lake Morning Mason Spur MOUNT MORNING This line represents the route of the New York City Marathon. On average, runners finish it in about four and a half hours. Shults Peninsula SKELTON GLACIER Fishtail Point Glaciers flow into the Ross Ice Shelf Evteev Glacier Teall Island ROSS ICE SHELF MULOCK GLACIER Buntley Bluff Cape Lankester Hoffman Point Bertoglio Glacier Brosnahan Island Mount Keltie CONWAY RANGE Cape Murray ROSS ICE SHELF CARLYON GLACIER MacDonald Point Reeves Bluffs Bowling Green Plateau Goorkha Craters COOK MOUNTAINS Dot Peak Gawn Ice Piedmont Foggydog Glacier Mulgrew Nunatak Cape Kerr Diamond Hill “The Nozzle” Brown Hills Tentacle Ridge Cranfield Icefalls DARWIN GLACIER Horney Bluff Junction Spur BYRD GLACIER Merrick Glacier Island Arena Ragotzkie Glacier Darwin Mountains Yancey Glacier Mount Ash Forbes Ridge Midnight Plateau Dusky Ridge HATHERTON GLACIER Hinton Glacier Lieske Glacier Derrick Peak Judith Glacier MOUNT OLYMPUS Peckham Glacier BRITANNIA RANGE Mount Henderson Mount Quackenbush Haven Mountain Three Nunataks Vantage Hill Remseyer Glacier BYRD GLACIER Mount Rummage Darnell Nunatak Rundle Peaks Sefton Glacier Bates Nunataks Zeller Glacier Byrd Névé There are 2,300 more miles of ice to cross between this spot on the East Antarctic ice sheet and the continent’s opposite shore. EAST ANTARCTIC ICE SHEET The scale of Antarctica is startling. Miles of ice stretch to the horizon, growing thicker as you move toward the South Pole. ROSS SEA Cape Tennyson ROSS ICE SHELF Lewis Bay Scientists at McMurdo Station are working to understand the continent’s history and to predict its future. The scale of the task is enormous.This flat expanse of white is the Ross Ice Shelf, a floating chunk of ice nearly as large as Texas. McMurdo Station ROSS ICE SHELF Brown Peninsula MOUNT Discovery Mason Spur MOUNT MORNING ROSS ICE SHELF This line represents the route of the New York City Marathon. On average, runners finish it in about four and a half hours. ROSS ICE SHELF CONWAY RANGE DARWIN GLACIER BYRD GLACIER Judith Glacier Peckham Glacier Mount Henderson Haven Mountain Vantage Hill BYRD GLACIER Darnell Nunatak Bates Nunataks Zeller Glacier Byrd Névé There are 2,300 more miles of ice to cross between this spot on the East Antarctic ice sheet and the continent’s opposite shore. EAST ANTARCTIC ICE SHEET The scale of Antarctica is startling. Miles of ice stretch to the horizon, growing thicker as you move toward the South Pole. ROSS SEA Cape Crozier Cape Tennyson ROSS ICE SHELF Mount Terror Lewis Bay ROSS ISLAND MOUNT Erebus Cape MacKay Scientists at McMurdo Station are working to understand the continent’s history and to predict its future. The scale of the task is enormous.This flat expanse of white is the Ross Ice Shelf, a floating chunk of ice nearly as large as Texas. Hut Point Peninsula McMURDO SOUND McMurdo Station Cape Hodgson BLACK ISLAND ROSS ICE SHELF Brown Peninsula Hahn Island Minna Bluff MOUNT Discovery Heald Island Eady Ice Piedmont Koettlitz Glacier Lake Morning Mason Spur MOUNT MORNING ROSS ICE SHELF This line represents the route of the New York City Marathon. On average, runners finish it in about four and a half hours. Shults Peninsula SKELTON GLACIER Glaciers feed Ross Ice Shelf ROSS ICE SHELF Evteev Glacier Teall Island MULOCK GLACIER Buntley Bluff Cape Lankester Hoffman Point Bertoglio Glacier Brosnahan Island Mount Keltie CONWAY RANGE Cape Murray ROSS ICE SHELF CARLYON GLACIER MacDonald Point COOK MOUNTAINS Gawn Ice Piedmont “The Nozzle” Brown Hills Tentacle Ridge Cranfield Icefalls DARWIN GLACIER Horney Bluff Junction Spur BYRD GLACIER Merrick Glacier Ragotzkie Glacier Darwin Mountains Yancey Glacier Forbes Ridge HATHERTON GLACIER Hinton Glacier Lieske Glacier Derrick Peak Judith Glacier Peckham Glacier BRITANNIA RANGE Mount Henderson Haven Mountain Three Nunataks Remseyer Glacier BYRD GLACIER Darnell Nunatak Rundle Peaks Sefton Glacier Bates Nunataks Zeller Glacier Byrd Névé There are 2,300 more miles of ice to cross between this spot on the East Antarctic ice sheet and the continent’s opposite shore. EAST ANTARCTIC ICE SHEET

This is the last of three dispatches from a New York Times reporting trip to Antarctica.

From the air, the Ross Ice Shelf looks like a vast white plain extending to the horizon. The monochromatic landscape is relieved only occasionally by rocks poking through, or by deep crevasses in the ice itself.

Only at its edge does the ice shelf become something more dramatic: a spectacular sheer cliff rising 100 feet above the ocean and extending 900 feet below the surface. From that cliff edge, icebergs occasionally calve away, completing a thousand-year journey of the ice from land to sea.

Scientists are racing to understand what is happening to the Ross Ice Shelf — and the rest of Antarctica — as the planet warms around it. They are trying to map the thickness of the ice and the shape of the sea floor beneath it in an effort to gauge how vulnerable the shelf may be to collapse, and how soon that could happen.

Scientists are also trying to measure the role of human-caused climate change in weakening some parts of the West Antarctic ice sheet, and to fathom how damaging the seas around the continent might prove to be as they warm over time.

The edge of the Ross Ice Shelf.

The answers carry profound implications for humanity. In the scientists’ worst-case computer simulations, continued global warming will cause the Ross Ice Shelf to weaken and collapse starting as early as the middle of this century.

Right now, the shelf works like a giant bottle-stopper that slows down ice trying to flow from the land into the sea. If it collapses, the ice could flow into the ocean more rapidly, an effect that has already happened on a much smaller scale in other areas of Antarctica.

The most vulnerable parts of the West Antarctic ice sheet could raise the sea level by 10 to 15 feet, inundating many of the world’s coastal cities, though most scientists think that would take well over a century, or perhaps longer. They are worried about a possible rise of as much as six feet by the end of this century.

Whether these alarming forecasts ultimately prove right depends in part on the shape of the sea floor beneath the ice shelf, and in particular on whether it has deep channels that can funnel warming ocean water under the ice shelf and attack the West Antarctic ice sheet from below. A different undersea topography — high ridges of rock, for example — may keep warmer water out, stabilizing the ice sheet, possibly for hundreds of years.

“We’re hoping to figure out how warm water can get to the edge of the ice sheet,” said Robin E. Bell, head of the Columbia University laboratory that sent a team to survey the Ross Ice Shelf late last year. “What are the sort of hidden roads it can go on?”

As they flew back and forth across the vast white landscape in December, the Columbia scientists used some of the world’s most sophisticated geophysical instruments to see into and beneath the ice. As the hours-long flights wore on, the scientists took turns napping, knitting or eating cold pizza, but at all times, somebody kept close watch on the instruments.

Kirsty J. Tinto, the scientist leading the field team, loved watching as the measurements stripped away the illusion that the ice was just a flat, boring pancake.

“You take a slice through it and you can see a thousand years of history, a hundred million years of history,” she said.

Kirsty J. Tinto steps out of an LC-130 Hercules on the Ross Ice Shelf.

The Ross Ice Shelf appears stable now, so the Columbia project will function somewhat like a doctor’s baseline X-ray — a starting point for comparison if the ice starts to deteriorate.

Satellite evidence suggests that this is already happening in other parts of West Antarctica, and many scientists believe that relatively warm ocean water is the culprit. “It’s kind of a blowtorch on the underside of the ice shelf,” said Robert A. Bindschadler, a retired NASA climate scientist.

But the story is not straightforward, and the warmer water attacking the ice has not been linked to global warming — at least not directly. The winds around the continent seem to be strengthening, stirring the ocean and bringing up a layer of warmer water that has most likely been there for centuries.

Are those stronger winds tied to human-caused global warming? Some scientists think so, but others say the case is unproven. “We’re not sure because we don’t have enough data, for long enough, to separate signal from noise,” said Eric J. Steig, a scientist at the University of Washington who has studied temperature trends in Antarctica.

Though the role of global warming is unclear now, it is likely to be a factor in the relatively near future. Many experts think warmer air temperatures could start to weaken the ice of West Antarctica from above, even as warmer ocean water attacks it from below.

The warmer water seems to be doing the most damage to a series of glaciers that flow into a region of West Antarctica called the Amundsen Sea. Satellites have identified the most rapid loss of ice there, raising a critical question: Has an unstoppable collapse of the ice sheet already begun?

Ross Ice Shelf Pine Island Glacier Thwaites Glacier Ross Sea Amundsen Sea Pine Island Glacier Thwaites Glacier Amundsen Sea Red areas have lost significant amounts of ice since 2010.

The Amundsen Sea region is one of the remotest parts of the continent, far from American and British research bases. Working together, the two countries are planning to devote tens of millions of dollars to getting better measurements there, having decided that it is imperative to begin answering questions about the region’s potential collapse.

For instance, scientists need to know a lot more about the ground beneath the glaciers there. Is it slick mud that may allow the ice to flow much faster, or hard rock that may slow down the ice even in a warmer world?

“What we need to know is really the details of what is occurring where the ice, ocean and land all come together,” said Ted A. Scambos, a University of Colorado scientist who is helping to plan the joint research effort.

Unraveling the answers, and gaining a better understanding of how Antarctica’s ice has waxed and waned in the past, may offer a rough guide to the changes that human-caused global warming could wreak in the future.

Already, scientists know enough to be concerned. About 120,000 years ago, before the last ice age, the planet went through a natural warm period, with temperatures similar to those expected in coming decades.

The sea level was 20 to 30 feet higher than it is today, implying that the ice sheets in both Greenland and Antarctica must have partly disintegrated, a warning of what could occur in the relatively near future if the heating of the planet continues unchecked.

But some research suggests that a catastrophe might not yet be inevitable. In a study last year, Robert M. DeConto of the University of Massachusetts, Amherst, and David Pollard of Pennsylvania State University used their computer model to predict what would happen if emissions were reduced sharply over the next few decades, in line with international climate goals.

Under the most ambitious scenarios, they found a strong likelihood that Antarctica would remain fairly stable.

“There’s still a chance that all hell will break loose,” Dr. DeConto said. “But the model is suggesting there’s a way to reduce the risk of a big sea-level rise from Antarctica.”