Time on the Shelf
 

Robert Bindschadler stood on a remote stretch of the West Antarctic Ice Sheet and searched the horizon with a growing sense of anxiety. That is, he couldn’t see the horizon! While Bindschadler focused on his work—driving flagged poles deep into the ice and carefully recording their precise coordinates—clouds gathered quickly overhead, cloaking the dazzlingly bright blue sky in a shroud of sameness that disoriented him. In a matter of minutes the world around him became a vast featureless white space where he could no longer distinguish sky from surface.

With no shadows and no contrasting color shades, humans lose depth perception. Bindschadler had to concentrate just to walk without stumbling. He gathered his gear and trudged back to his snowmobile. He knew it was dangerous to be alone in such conditions so far from base camp. He also knew it was dangerous to attempt driving back to base camp. The ice held hidden hazards that could claim his life in an instant.

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(Image in title graphic courtesy Nicolas Cullen, Cooperative Institute for Research in Environmental Sciences)

  Harsh conditions
 

“When ice moves rapidly, it can experience stresses large enough to crack it,” Bindschadler explains. “Some of these cracks, called crevasses, can be quite large and dangerous—30, 50, even 100 feet deep. Some crevasses are covered by thin bridges of snow so they are hard to spot.”

Such bridges collapse easily under the weight of a person. “Unless one is roped and connected to something or someone else, one usually doesn’t survive a fall into a crevasse,” Bindschadler says in a tone that conveys self-admonishment. He knew better than to allow himself to get caught in such a predicament. Nevertheless, he faced a difficult decision: hunker down until the weather cleared, or keep going and risk getting lost in the white-out conditions, or worse, falling into a crevasse.

In hindsight, Bindschadler admits, he should have stopped and dug himself a snow shelter. “Given that weather conditions can change rapidly in Antarctica, field workers need to always be prepared with the essentials for survival: food, fuel, and shelter,” he recites. “If you have a shovel, you can build shelter. If you have food and fuel, you’ll be okay.”

He had a shovel, food, and fuel. Yet there he was astride his snowmobile, in white-out conditions, squinting ahead at the flat expanse of ice for signs of his previous passage. He felt he would be safe retracing his path back to base camp. But it was hard to focus and the surface yielded precious few clues. What would have been a 30-minute trip under clear skies on a beeline back to base camp became a 3-hour meandering ordeal during which Bindschadler constantly second-guessed his decision to keep going. He heaved a sigh of relief when the shadowy silhouette of base camp finally appeared out of the whiteness ahead.

Crevasses and rapid, extreme shifts in the weather are just some of the hazards Bindschadler faces in his work as a glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “A glaciologist is somebody who studies ice,” he says flatly. Specifically, his job is to measure the shape of ice sheets. How thick are they? How long are they? Are they growing thicker or thinner? Are they surging forward to cover more area, or retreating to expose more of the underlying land? How fast are they moving?
 

 

Antarctica’s environment is extreme and hazardous for human visitors. The photo on the left shows field workers drilling to measure sea ice thickness. The middle photo shows an LC-130 Hercules aircraft stuck in a crevasse on Bindschadler Ice Stream in West Antarctica. The picture on the right shows two field workers out walking in near-white out conditions. (Photos courtesy Konrad Steffen, Cooperative Institute for Research in Environmental Sciences (left); Robert Bindschadler, NASA GSFC (center); Waleed Abdalati, NASA GSFC (right))

 

A glaciologist’s goal is to be able to measure the “mass balance” of a given ice sheet. That is, how much ice the sheet accumulates from snowfall over the course of a year minus how much it loses due to melt as well as the periodic calving off of large ice chunks. Ice sheets can be good indicators of what the climate is like in a given region; changes in the climate can cause changes in Earth’s ice sheets. This simple fact points to the ultimate question facing glaciologists today: As the globe warms will Antarctica’s ice mass remain in balance? Or, will the southern continent gain or lose ice mass over time?

When Bindschadler and his colleagues first visited Antarctica in 1980, they brought with them the traditional wisdom gleaned by generations of explorers and glaciologists before them that large ice sheets are slow, plodding things that wax and wane on cycles ranging from centuries to millennia. But the evidence they painstakingly pieced together over the course of 13 expeditions to Antarctica over 25 years suggests that the perceived permanence of Antarctica’s ice sheets is an illusion. They focused their studies in West Antarctica where the ice sheet is draped over a layer of marine sediments that is much more slippery than rock. They suspected that this slippery sediment layer and the above-average amount of subterranean heat the Earth vents in West Antarctica, including numerous volcanoes, made the West Antarctic Ice Sheet more prone to movement than scientists had previously thought.

  Photograph of Robert Bindschadler

Robert Bindschadler is shown here standing next to a crevasse during snow survival school on Antarctica in 1988. (Photo courtesy Robert Bindschadler, NASA GSFC)

  Map of Antarctica

Linking Land Ice to Sea Level

Twenty-five years ago glaciologists used standard surveying techniques to measure changes in ice sheets over time. They would venture out from base camp with sleds full of bamboo poles that they carefully planted every half a kilometer or so along a straight line. This way they knew what routes were safe for human passage as well as where and how much the ice was moving.

 

This RADARSAT map of Antarctica shows key features of the continent, including the Ross Ice Shelf and the Transantarctic Mountains. (Image courtesy National Snow and Ice Data Center)

 

They used theodolites to monitor the flagpoles’ movements. (A theodolite is a small, calibrated telescope mounted on a tripod that surveyors use to measure angles between objects.) Bindschadler’s team became proficient in tracking their own position relative to magnetic north, relative to the position of the Sun, and relative to those few stars that were visible in the daytime sky. Once they established their own position, they then measured the relative angles of all their flagpoles to a tiny fraction of a degree. These measures allowed them to determine how fast the ice was moving and in what direction, both vertically and horizontally. Bindschadler and his team were surprised by what they observed.

“When I first got into Antarctic studies, ice sheets were thought to be very slow to respond,” he recalls. “We had this preconception that any changes would be on scales of centuries. Yet there were rapid changes taking place. Ice streams would start and stop suddenly. And they are organized in a vast, interconnected network of streams.” Similar to tributaries on land that flow into larger rivers, Bindschadler’s team discovered a vast network of coalescing tributaries feeding into ice streams flowing toward the ocean.

Based on those discoveries and others over the past few decades, glaciologists began to suspect that Antarctica’s ice sheets are not only sensitive to global warming, but that there could be observable changes in our lifetime. What does that mean for sea level? Spread across a land area roughly equal to that of the United States, the southern continent contains about 90 percent of all ice on our planet. Enough water is locked in ice there to raise global sea level by about 60 meters (197 feet). Considering that at least one-third of humanity lives in coastal areas, it would be catastrophic should the ice sheet suddenly and totally collapse.

  Researcher using theodolite

John Scofield and Dean Lindstrom conducted a theodolite survey in West Antarctica during the team's deep field expedition in 1983. Before the advent of GPS technology and modern satellite remote sensors, glaciologists relied mainly on theodolites to track the movements of glaciers and ice streams over time. (Photo courtesy Robert Bindschadler, NASA GSFC)

  Global temperature anomalies
 

Preposterous! Conventional wisdom says it is inconceivable to melt Antarctica’s ice fast enough to significantly affect sea level in our lifetime, or even that of our great grandchildren. So why worry? Antarctica is remote and irrelevant to the lives of ordinary citizens in the world today, right?

On the contrary, Bindschadler asserts, Antarctica’s ice sheets are entirely relevant. “The West Antarctic Ice Sheet has, does, and will continue to affect sea level. We know that if this sheet changes in size, it will change sea level. That connection is direct and irrefutable.”

Over the last 18,000 years, in the wake of the last major Ice Age, Earth’s ice sheets dwindled and drove sea level up by about 127 meters (381 feet). Paleoclimate data records indicate the rate of sea level rise varied with the climate; warmer temperatures led to rapid rise while cold spells slowed the rate of rise. Over the long term, sea level has been rising at an average rate of about 2 millimeters per year. In the coming decades, as heightened levels of atmospheric greenhouse gases drive up Earth’s temperatures, how will sea level be affected? Today, locked up in the West Antarctic Ice sheet—roughly the size of Greenland—is enough water to raise global sea level by another 5 meters (16 feet). Speaking of Greenland, its ice sheet has seen dramatic melting in the last decade. Greenland contains about 9 percent of all ice on Earth—also enough water to raise sea level by 5 meters. Should either West Antarctica or Greenland surrender its ice sheet to the ocean, much of the southern half of Florida would be under water.

 

Thermometer measurements of surface air temperature recorded at many stations around the globe since the 1880s show a gradual rise in temperature over that span. According to these data, global warming accelerates noticeably in the last 25 years. Colors on this map show global mean temperature anomaly, or the difference between the long-term average and the temperature measured at a given time. Reds show warmer-than-average temperature, blues are cooler than average, and white is average. Gray areas indicate no available data. Notice that warm temperature anomalies (reds) dominate, and are most extreme at higher latitudes and in the polar regions. (Image by Robert Simmon, NASA GSFC, based upon data courtesy James Hansen, NASA GISS)

  Florida under water

Today, scientists see serious signs of stress on both ice sheets. Should they both collapse, sea level will be 10 meters (32 feet) higher, and cartographers will have to redraw the contours of all Earth’s continents. Neither ice sheet has changed much in recent centuries, but does that mean they can’t change in the near future?

 

Should either Greenland or West Antarctica surrender its ice sheet to the ocean, sea level would rise 5 meters and much of Florida would be underwater. (Illustration courtesy William Haxby)

 

A Place of Absolute Stillness

 

Visitors to our world’s southernmost land learn quickly that it’s not about the temperature, it’s about the wind. The Sun is up all the time during the 3-month-long summer there and, late in the season, the temperature climbs above freezing in many coastal areas. Away from the coast, 10 degrees Celsius (50°F) makes for a balmy summer day. On a sunny day with no wind, an adult can work in short sleeves; a 10-knot breeze would feel quite chilly; a 30-knot breeze would make you feel you were freezing to death—all at the same temperature.

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  Antarctic field camp
 

“You watch the wind carefully,” Bindschadler cautions. “The wind is the dominant factor in how you dress in Antarctica. People act like penguins there—they stand side-by-side with their backs to the wind.”

He estimates he has spent one and a half years of his life in West Antarctica and in all that time he has never seen the Sun set. Interestingly, most of Bindschadler’s descriptions of his study sites in West Antarctica focus on what isn’t there. Farther than 80 degrees south latitude, hundreds of kilometers west of the Transantarctic Mountains, deep in the heart of Marie Byrd Land in an area known as the Rockefeller Plateau, there are no mountains to be seen. There are no trees, no buildings. The ice sheet is flat and white with snow everywhere, completely devoid of contrasting scenery as far as the eye can see. There is no wildlife, except for the odd lone seabird passing high overhead on its way someplace else. (Bindschadler has seen a total of two.) When the wind isn’t howling, and your teeth aren’t chattering, there is no sound and no movement at all.

“It has been the most quiet I’ve ever heard,” Bindschadler says. He adds pensively, “I’ve had the feeling of being most close to God there. It can be a place of absolute stillness.”

 

In 1984, Bindschadler and his colleagues set up base on the Whillans Ice Stream in West Antarctica. The team slept in Scott tents (right) and traveled on snowmobiles, while setting flags to indicate avenues of safe passage. (Photo courtesy Robert Bindschadler)

  Placement of GPS devices

But flagged poles planted in the ice tell scientists their sense of stillness is an illusion. The ice sheet is moving. Parts of it are moving rapidly—more than 2 meters per day. Glaciologists refer to areas that experience such rapid movement as “ice streams.” The goal of their first expedition in 1980 was to solve the mystery of why ice streams form. They are still trying to solve the mystery today. “It can take decades to solve this type of problem,” he states. “There have been some surprises along the way.”

 

Bindschadler (kneeling, right) and his colleague Ian Joughin prepare to plant a flag pole. First, they plant a solar panel to power their equipment and then use GPS technology to establish their precise location. (Photo courtesy Robert Bindschadler)

  Antarctic ice streams
  Bindschadler Ice Stream
 

They found one stream that spends most of its time stationary, and during periods of falling tide it lurches forward suddenly, moving forward half a meter in 20 minutes before stopping suddenly. “One of our findings that was hardest to believe is that movements of some ice streams are linked to rising and falling tides,” Bindschadler explains. “We discovered that as tides go up and down a meter or so, some ice streams change speed by anywhere from plus or minus 50 percent. That finding shook our understanding of ice stream dynamics right to the core.”

Of course, not all ice stream movements are caused by tidal cycles. There remain many unanswered questions about when and why they start and stop. It became apparent to Bindschadler and his colleagues that while theodolites offer a good way to track individual glaciers and ice streams over short time spans, there was no way that ground-based survey techniques would allow them to monitor a region as immense and inhospitable as Antarctica. To survey the whole continent for a sustained period of time, they would need a much more robust technology.

 

Major ice streams in West Antarctica used to be named Ice Stream A, B, C, D, E, and F. The streams have been renamed after Antarctic researchers. (Top image courtesy National Snow and Ice Data Center, inset map in top image courtesy CIA World Fact Book) The bottom image is an aerial photo showing the crevassed margin of Bindschadler Ice Stream, West Antarctica, circa 2000. (Photo courtesy N. Nereson)

 

Perspectives from On High

 

Glaciologists received a huge technological boost with the arrival of Global Positioning System (GPS) satellites in the late 1980s. Almost overnight this technology fundamentally changed how glaciologists survey ice sheets. Affixing antennas with the ability to receive GPS signals onto their flagpoles dramatically simplified scientists’ ability to monitor the movements of ice sheets over much larger areas. The GPS technology also improved the precision of scientists’ measurements and, once deployed, the devices allowed them to collect their data from the safer confines of their workspaces with less personal risk. Handheld GPS devices also allow scientists to travel in Antarctica without the burden of planting flagged poles every half a kilometer.

“GPS has made recording data in the field much easier than it was before,” states Bindschadler. “But even that doesn’t tell us all we need to know to understand ice sheets.”
 

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New satellite technologies give glaciologists the tools to begin answering these questions. From the perspective of space, satellite sensors collect data over Earth’s entire surface almost every day and, because most climate research satellites orbit from pole to pole, they collect data over the polar regions as often as fourteen times a day. Moreover, because some satellite sensors are sensitive to regions of the electromagnetic spectrum that our eyes cannot see, satellites provide a much more complete picture of our world’s ice sheets than scientists could get any other way.

For example, the Defense Military Satellite Program’s Special Sensor Microwave Imager (SSM/I) is very good at detecting where and when snow and ice get wet. NASA developed and demonstrated this new capability with its SMMR instrument launched in 1978 aboard the Nimbus satellite. SMMR and SSM/I microwave data have helped scientists to monitor melting on the surface of Earth’s ice masses. NASA extended scientists’ ability to monitor ice melt with the May 2002 launch of the Aqua satellite, which carries Japan’s Advanced Microwave Scanning Radiometer for EOS (AMSR-E).

  Amery Ice Shelf

Satellite optical sensors viewing the surface in multiple wavelengths of light, and at multiple angles, can reveal details otherwise hidden. In this 2002 false-color image, the Terra satellite’s Multi-angle Imaging SpectroRadiometer (MISR) substituted color for viewing angle to reveal a large crack forming in East Antarctica’s Amery Ice Shelf—an iceberg early in the calving process. To make this false-color image, scientists combined MISR’s red band data from its 60-degree aft-viewing camera (displayed as blue), its downward-looking camera (green), and its 60-degree forward-viewing camera (red). The different colors represent differences in the light reflected at different angles. (Image courtesy NASA/GSFC/LaRC/JPL, MISR Team)

  Greenland melt
 

According to Waleed Abdalati, Head of the Cryospheric Sciences Branch at NASA GSFC, these satellite data confirm a trend of increased melting around Greenland. “As global temperature rises, the melting will accelerate,” states Abdalati. Bill Krabill, a glaciologist at NASA’s Wallops Flight Facility, reports an overall thinning of Greenland’s ice sheet at lower elevations, by as much as 70 meters in some places in just the last five years, which is a direct contribution to sea level change.

But melting is only half of the problem. Jay Zwally, glaciologist at NASA GSFC, observed that much of the melt water doesn’t simply run off the ice sheet; rather, the water flows downward through large and small cracks in the ice sheet and eventually reaches the bottom. This water acts as a lubricant that helps accelerate the flow of ice streams and the rate of icebergs calving off the edge of the sheet.
 

 

Like much of the Arctic, Greenland experienced record melting in 2002. In this image, colored lines represent approximate melt zones for June 2001 through June 2005. September melt zones are more extensive because they reflect the entire season’s melt. (Image by Robert Simmon, NASA Earth Observatory)

 

About half of the loss of Greenland’s ice mass into the North Atlantic Ocean is in the form of melt water, and the other half is in the form of calving icebergs. Each year, Greenland’s loss of ice contributes about 10 percent of today’s 3-millimeter rise in sea level per year. That number seems small, but scientists are concerned that the rate could increase through a positive feedback loop. Warmer temperatures cause melting, which leads to faster flow, drawing more of the ice sheet down to lower altitudes and warmer temperatures, which contributes to more melting, which leads to more ice acceleration and thinning, and so on.

Conversely, there is not a lot of melting in Antarctica because temperatures are too cold year round. There, glaciologists’ main concern centers on the flow rate within the vast network of ice streams. Is Antarctica losing ice to the ocean through these streams faster than it is accumulating ice through annual snowfall? Help answering this question has come in the form of three different types of satellite sensors.

Like sophisticated, space-based digital cameras, optical sensors aboard NASA’s Landsat, Terra, and Aqua satellites allow scientists to track changes on the surface, such as icebergs calving and drifting, and to see how those changes relate to other variables, like topography and temperature. Some instruments aboard these satellites produce high-resolution images in which subtle changes in shading and surface features reveal the locations of cracks and crevasses. Scientists can track these crevasses to determine flow rates, much like they used poles in earlier years.

While optical sensors can determine how much area is covered by ice, they cannot observe whether the ice is getting thicker or thinner. It is possible for an ice sheet to shrink in terms of area and yet gain mass through annual snowfall accumulation, thus making the sheet thicker. The latest report by the Intergovernmental Panel on Climate Change (IPCC) predicts that as average global temperature rises, Antarctica is likely to gain ice mass due to increased snowfall. A team of researchers led by Curt Davis, University of Missouri, supported the IPCC’s prediction in a May 2005 issue of the journal Science. Specifically, analyzing radar data collected by European Space Agency satellites from 1992 to 2003, Davis’ team found that East Antarctica gained about 45 billion tons of ice per year. To put this weight gain into perspective, it corresponds to removing the top 0.12 millimeters of the ocean and spreading it over an 8.5 million-square-kilometer area in Eastern Antarctica.

While it is welcome news that Eastern Antarctica is helping to slow the rate of sea level rise, Davis’ team’s finding only intensifies the mystery of what is causing sea level to rise. Satellite measurements suggest that in the last 10 years the rate of sea level rise has quickened to 3 millimeters a year (accurate to within plus or minus 0.5 millimeters), but scientists can only account for about two-thirds of this rise. Like all physical bodies, water expands when it warms, and scientists estimate that the ocean’s increased temperature has caused it to expand by 1 millimeter overall. Ice loss from Greenland, Antarctica, and smaller ice sheets and glaciers all over the world account for another millimeter in sea level rise. So where is that last millimeter coming from? Waleed Abdalati notes that while Davis’ findings show increased thickness in East Antarctica’s continental interior, scientists still don’t know whether the ice is thickening or thinning along the coastal edges of East Antarctica. If scientists don’t fully understand the mechanism then they cannot account for it accurately in their models, and they cannot say whether its effect on sea level is likely to increase, decrease, or stay the same.

To help scientists determine whether Antarctica’s ice mass is in or out of balance, in 2003 NASA launched its Ice, Cloud, and Land Elevation Satellite, or ICESat. ICESat carries a laser altimeter that beams pulses of green and infrared light straight down at Earth 40 times per second and then collects the reflected light in an onboard one-meter telescope. The time it takes the light to travel from the satellite to its target and back again directly relates to the height of the target. Such measures allow scientists to map the elevations of ice sheets all over the world more accurately than ever before—improving upon the vertical resolution of older radar technologies by about 20 times. Thus, glaciologists now have the ability to measure ice extent, flow rate (using optical sensors), and thickness (using laser altimeters).

  Ice flowing into a moulin

A common sight in Greenland, a meltwater stream flows into a large moulin. (Photo courtesy Roger J. Braithwaite, University of Manchester, United Kingdom)

  ICESat track around globe

“We discovered that parts of the West Antarctic Ice Sheet are thinning more than a meter per year—faster than anyone imagined,” Bindschadler states. The mass balance of the West Antarctic Ice Sheet is about 10 percent of all the ice on the planet, and some sections are shrinking at alarming rates. These most active sections are adding about 0.2 millimeters per year to sea level, or about 7 percent of the recent annual rise.

 

Orbiting the Earth at nearly 17,000 miles (27,360 km) per hour, NASA’s ICESat collects three-dimensional measurements of the Earth’s surface and atmosphere. (Image courtesy Scientific Visualizations Studio, NASA GSFC)

  Elevation map
 

To help glaciologists understand why the West Antarctic Ice Sheet is losing ice, the Synthetic Aperture Radar (SAR) allows them to map and monitor the continent’s ice streams and even to peer beneath the surface to locate subsurface crevasses. SAR has the added advantage of being able to penetrate clouds, which is important because the coast of West Antarctica is one of the cloudiest places on Earth. A portion of the radar signal reflects off the ice surface while the penetrating energy scatters off deeper snow and returns to space, where subtle differences in the signal are picked up by the radar’s detectors. Because different ice features yield distinct patterns in various radar wavelengths reflected back to the satellite, any fractional difference in the radar reflectance allows scientists to precisely pinpoint where each distinct part of the ice has moved as well as how far and how fast.

 

The colors on this map represent ICESat’s measurements of Antarctica’s topography, using data collected from October 3 through November 8, 2004. Red shows the highest elevations (up to 4,000 meters above sea level). Yellow, green, and turquoise show progressively lower elevations (green is 2,000 meters above sea level). Dark blue shows sea level. (Image courtesy Chris Schuman, NASA GSFC)

  SAR 3-dimentional image
 

“Now we can track the movements of the snowpack even when there aren’t observable [to the human eye] features on the surface,” Bindschadler says triumphantly. “Satellite technology has torn away the shroud and allowed us to observe detailed patterns of ice flow within an ice sheet. These new data reveal a very strongly organized network of ice streams flowing within Antarctica’s ice sheets.”

Overall, what do satellites tell scientists about the current state of Earth’s ice sheets?

 

This visualization shows ice velocity data from Synthetic Aperture Radar measures superimposed on a three-dimensional model of the surface in West Antarctica. The colored patterns represent ice streams flowing off the grounded West Antarctic Ice Sheet onto the Ross Ice Shelf. The speed of the ice increases as the colors change from light green (slow) to purple to red to yellow (fast). (Image courtesy the Scientific Visualization Studio, NASA GSFC)

 

Canaries in the Coal Mine

 

According to Bindschadler, NASA’s new satellite technology is helping scientists rewrite the textbooks on ice sheet dynamics. Until recently, most glaciologists believed ice streams require decades, if not centuries, to start and stop. But new data reveal they can start and stop in a matter of seconds. Moreover, conventional wisdom held that the bottoms of ice shelves could melt no faster than 1 to 2 meters per year. But Eric Rignot, of NASA’s Jet Propulsion Laboratory, proved recently that ice shelves can lose tens of meters of ice thickness per year due to melting on the bottom. This discovery is particularly concerning to Bindschadler and his colleagues.

Because ice shelves already float in the ocean, their melting cannot significantly influence sea level. (To test this fact, place a handful of ice cubes in a glass and then fill it with water. Just as the water level in the glass does not change as the ice cubes melt, global sea level does not change significantly as ice shelves melt.) However, cautions Bindschadler, ice shelves such as those around Antarctica are important because they buttress the much larger ice sheets poised on land. Removal of any of the continent’s ice shelves would likely accelerate the flow rate of ice streams off the land and into the ocean, which would in turn raise sea level.

Antarctica’s ice shelves are massive structures that have remained largely intact for many thousands of years. How quickly could they change?

“The Larsen Ice Shelf is the poster child for this phenomenon,” Bindschadler says ominously. “It has three major sections—A, B, and C. Most of the Larsen B Ice Shelf collapsed in just two days, and no one saw it coming.”

Scientists knew the Larsen Ice Shelf was thinning and retreating. But in March 2002 they were stunned to see most of the 2,000-square-kilometer shelf disintegrate so suddenly. And sure enough, Bindschadler and his colleagues see an increased flow of ice off the land where the Larsen B shelf used to be. Even more disturbing is the fact that other Antarctic ice shelves are showing signs similar to the Larsen just before it collapsed. ICESat data show that many of Antarctica’s major ice shelves are thinning, making them increasingly prone to forming cracks and crevasses due to temperature changes and tidal cycles. In the summertime, melt water on the surface seeps down into the cracks, helping them to grow larger and deeper. Because water is heavier than ice, enough water can continue the process, driving cracks completely through the shelf. All that’s left is for the tall, thin slabs of ice to fall over in the water, like a pack of standing dominoes. Scientists suspect this is what happened to Larsen B.

Data records reveal that our world’s southern continent has warmed by 2.5 degrees Celsius (4.5°F) over the last 50 years, making it one of the fastest-warming places on the planet. Over the last 25 years, Bindschadler and his colleagues have learned that Earth’s ice sheets are much more responsive to global warming than they used to think. “The picture that is emerging is increasingly disturbing,” Bindschadler states.

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Larsen Ice Shelf disintegration

In the largest single disintegration event in 30 years of ice shelf monitoring, approximately 3,250 square kilometers of the Larsen B shattered in 2002. The disintegration released 720 billion tons of ice into the Weddell Sea. (Images courtesy of the National Snow and Ice Data Center)

  Carbon dioxide and temperature change
 

In a global-warming scenario, scientists expect to see changes happening most noticeably and most rapidly in the polar regions. Specifically, they expect to see ice streams accelerating, more icebergs calving, and ice sheets thinning. On Greenland, and in other areas around the Arctic Circle, they expect to see earlier break-ups of sea ice packs in the spring, prolonged growing seasons, increased plant growth, thawing permafrost in the soils, shrinking glaciers, and more freshwater runoff into the ocean.

“And we do see all of those things!” Bindschadler concludes. “You might say ice sheets are the ‘canaries in the coal mine’ of climate science. And right now the canaries are chirping an alarm.”

Bindschadler pauses and then qualifies that last statement. “We can’t be certain something cataclysmic is going to happen. I’m saying the fundamental physics that control ice sheet conditions certainly allow something cataclysmic to happen, and that changes are happening at an accelerating rate. There’s ample reason to be concerned.”

  • References
  • Abdalati, W., and Steffen, K. (2001) Update on the Greenland Ice Sheet Melt Extent. Journal of Geophysical Research Atmospheres (106) 33,983-88.
  • Bindschadler, R. and Bentley, C. (2002) On Thin Ice. Scientific American (287) 98-105.
  • Bindschadler, R., King, M., Alley, R., Anandakrishnan, S., Padman, L. (2003) Tidally Controlled Stick-Slip Discharge of a West Antarctic Ice Stream. Science (301) 1087-1089.
  • David, C., Li, Y., McConnell, J., Frey, M., and Hanna, E. (2005) Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise. Sciencexpress. May 19, 2005.
  • Joughin, I., Gray, L., Bindschadler, R., Price, S., Morse, D., Hulbe, C., Mattar, K., and Werner, C. (1999) Ice Streams Revealed by RADARSAT Interferometry. Science (286) 283-286.
  • Krabill, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W., and Yungel, J. (1999) Rapid Thinning of Parts of the Southern Greenland Ice Sheet. Science (283) 1522-1524.
  • Krabill, W., Abdalati, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W., and Yungel, J. (2000) Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning. Science (289) 428-430.
  • Rignot, E., and Jacobs, S. (2002) Rapid Bottom Melting Widespread near Antarctic Ice Sheet Grounding Lines. Science (296) 2020-2023.
  • Rignot, E., and Thomas, R. (2002) Mass Balance of Polar Ice Sheets. Science (297) 1502-1506.
  • Zwally, H. J., Abdalati, W., Herring, T., Larson, K., Saba, J., and Steffen, K. (2002) Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow. Science (297), 218-222.
 

Comparison of carbon dioxide (green line) content in the atmosphere over the last 400,000 years with surface temperature (blue) during the same period reveals that these two measurements are very closely correlated over time. As atmospheric carbon dioxide concentrations rise and fall, so does temperature. Today, humans have driven up the concentration of carbon dioxide to about 380 parts per million—a value much higher than our world has seen in more than half a million years. (Graph derived from data extracted from an ice core recovered from the Vostok Station in Antarctica)