Greenland's Ice Island Alarm
 

by Kendall Haven • design by Robert Simmon • August 28, 2007

In early spring, with the summer melt still weeks away, researchers flock like migrating birds to the Greenland Ice Sheet. The low, circling Sun reflects off the surface, making the vast ice sheet look like a gently rolling ocean with rippling waves riding over undulating swells that glide off to the horizon.

Waleed Abdalati, head of NASA Goddard Space Flight Center’s Cryospheric Sciences Branch, was one of those migrating researchers. His boots were on the ice most years from 1993 through 2004. He slept on the floating tongue of Petermann Glacier (80°N) as the ice groaned and cracked with the rise and fall of the tide. He slept at high elevations of the ice sheet interior where the bitter cold made his lungs ache with each breath. He slept on the western flank of the great ice sheet near a large melting area, where a relentless, subfreezing wind rattled his tent constantly.

 
  Photograph of researcher's tents on the Petermann Glacier, Greenland.
 

Abdalati describes life on the Greenland ice sheet as an environment of contrasts. “When the Sun’s position is just right, the scenery is stunningly beautiful,” he says. “At other times, the same scene can seem stark and lifeless.” And while the days can sometimes be a comfortable 32°F (0°C), at other times the brutal cold can be deadly, with a wind chill sometimes reaching below -50°F (-46°C). But it is the wind Abdalati recalls first and most. It’s the endless wind rolling downhill from the frozen highlands toward the distant ocean that becomes your bitter enemy as you trespass across the alien world of the ice sheet. The wind is almost always with you on the ice sheet and, when the temperature is well below freezing, it brings a stinging sensation to any exposed skin and rips away what frail body heat you create.

 

Waleed Abdalati and other researchers visited the Petermann Glacier on the northern coast of Greenland every spring. The glacier is beautiful, but remote, cold, and windy. The time spent on the ice and the area that the team could survey were limited by the hostile conditions. [NASA photograph courtesy Waleed Abdalati, Goddard Space Flight Center (GSFC).]

  Photographs of mild and severe weather on Greenland.
 

On-the-ice research in Greenland is generally limited to the short period after the worst of winter’s cold and before summer’s melt can make the ice treacherous. According to Abdalati, an ominous sound mingling with the wind ushers in each summer: gurgling water. Melt water begins to flow under the snow that blankets the ice, enlarging small cracks and fissures under the frozen skin, sometimes trickling and sometimes plunging toward the bottom of the ice sheet. For the first few weeks, the melt water sounds like a peaceful stream. Soon it takes on the menacing roar of a rushing river.

 

During warm, clear days researchers on the ice sheet could enjoy the beauty of Greenland’s ice formations. However, in bad weather the cold and high winds made research impossible. (NASA photographs courtesy Waleed Abdalati, GSFC.)

  Photograph of a Zodiak atop a Large Melt Pond on the Greenland Ice Sheet.
 

Even during these “ideal” spring months, the ice sheet is not exactly hospitable. Heavy boots, multiple layers of clothing, and thick mittens make movement awkward. Even the simplest of tasks requires thought, planning, and considerable effort.

Still, Abdalati jammed as much research as time and technology allowed into those precious few months each year to chip away at the secrets of the ice. The environment was challenging, but the data were crucial. He hoped he would find answers before it was too late.

In the early 1990s, as the Arctic began to warm noticeably, many eyes turned to Greenland. Scientists and policy makers asked: Is Greenland melting? If so, how fast, and how high will sea level rise as a result? “Through the 1990s, the only answer scientists could give was ‘We don’t know,’” Abdalati recalls.

 

As the surface of the ice sheet melts, rivers of water flow on the surface, eventually collecting in huge blue pools atop the ice or plunging into deep crevasses. (Photograph ©2005 Greenpeace/Andrew Davies.)

  Map of Recent Temperature Trend in the Arctic

What would large-scale ice loss on Greenland mean to the rest of the world? “Things we’ve come to count on could change,” he explains. Coastlines will recede as sea level rises and ocean water edges inland, threatening coastal cities. The changing ice could impact ocean and atmospheric circulation patterns and the global climate to which life today has adapted. What will be the nature and impact of these changes? Abdalati’s answer: “While the future impacts of the changing ice cover are uncertain, the potential effects are scary enough so that we better figure it out.” [See sidebar: Why Does the Greenland Ice Sheet Matter?]

 

Few places on Earth are warming faster than the Arctic. Temperatures have been rising several degrees Celsius per decade (red areas) since 1981. Among the most fundamental questions about Arctic climate change is how the warming will affect the Greenland Ice Sheet. (NASA Map by Robert Simmon, based on data from Josefino Comiso, GSFC.)

Graph of the contribution of the Greenland ice sheet to sea level rise in the twenty first century.

These uncertain, potentially dramatic effects of ice loss are what motivated Abdalati to face so many frigid springs on the Greenland Ice Sheet. Ultimately, he sought to answer the fundamental question—overall, is the Greenland Ice Sheet growing or shrinking? Computer models predicted that as Earth’s temperature rose, the ice sheet would thin and retreat around its margin. Yet, counter-intuitively, the models also predicted that the warmer temperatures would cause snowfall to increase in Greenland’s highland interior so that the ice sheet would actually grow thicker there.

Several field studies confirmed these predictions. Bill Krabill, a geoscientist at NASA’s Wallops Flight Facility, measured an inland gain of up to 1 meter of snow and up to three times average snowfall in some areas of Southeast Greenland between 2002 and 2003. In 2005, Ola Johannessen, from the University of Bergen’s Nansen Environmental and Remote Sensing Center, measured a gain of 2.4 inches per year from 1992 to 2003 for elevations above 1,500 meters, with a net ice loss below that elevation. But even if the models were right, which trend—ice gain in the interior, or melting at the margin—would prove greater, and by how much?

 

The Greenland Ice Sheet could contribute around 4 centimeters to sea level rise by the year 2100, about 10 percent of the total predicted rise. This estimate could be too low, however, since it does not account for rapid, large-scale ice loss through processes such as the accelerated flow of glaciers into the sea. (Graph by Robert Simmon, based on data from Johannes Oerlemans, Universiteit Utrecht.)

 

Chipping Away at the Truth

 

Answers require data. Through the 1990s, data on weather, climate, and ice had only been collected for short periods of time at widely scattered locations on the ice sheet. The primary goal of Abdalati and his colleagues during their Greenland expeditions was to augment their meager Greenland ice sheet datasets by collecting ground-based and aircraft data that would help them to understand the measurements made by early generations of Earth-monitoring satellites and to figure out what was happening to the ice sheet. He and other researchers set up observation stations on the ice sheet to record when the ice began to melt at particular places, how long the melt lasted, and how much ice melted during this period.

 
  Photograph of a weather station on the Petermann Glacier, Greenland.

Abdalati and his colleagues dug shallow snow pits to measure annual snowfall. They set up stations to observe ice movement and to measure how much heat and sunlight was emitted and reflected from the surface. Krabill augmented these on-the-ice studies with aircraft flights that produced laser altimetry maps of how the surface elevation was changing over large parts of the ice sheet and coastal glaciers. These aircraft measurements provided important new insights and helped NASA’s scientists make progress in understanding the behavior of the ice sheet by offering a wider perspective than ground stations. But aircraft only provide snapshots in time of a limited number of regions of the ice sheet. All of these ground and aircraft measurements combined offered only tantalizing glimpses of the enormous ice sheet and could not give Abdalati and his colleagues all the answers they sought. Prior to 2000, there were no observational estimates of Greenland’s ice sheet mass balance. Krabill’s aircraft measurements provided the first ice mass balance estimates in 2000, but scientists needed even more data to understand how representative his estimate was of the longer-term changes of the entire ice sheet.

 

Weather stations on Greenland provide detailed wind, temperature, and precipitation data, but only in limited areas. (NASA photograph courtesy Waleed Abdalati, GSFC.)

Map of weather stations in Greenland.

It was frustrating to Abdalati to know that the limited information scientists had gave only a glimpse into what was happening on the Greenland ice sheet. It would take many years of these expensive, logistically complicated studies to answer the critical question that drove him to Greenland. Abdalati knew that these studies were just “chipping away at the edges of the major questions with the tools at hand.”

 

The sparse network of weather stations (locations marked with black dots) scattered across Greenland can’t reveal all the changes that are occurring. Overland expeditions and aircraft overflights expand coverage slightly, but only provide snapshots. Only satellites can provide long-term, continuous observations over the entire surface of the ice sheet. (NASA map by Robert Simmon, based on ICESat data.)

 

A New Perspective

 

The challenge facing Abdalati and other researchers was one of perspective. Figuring out whether the entire ice sheet is losing or gaining mass based on data collected at a handful of sites is like crawling on your belly while gazing at individual blades of grass through a microscope to decide if your lawn needs mowing. How difficult is it to interpret the meaning of individual readings or to resolve discrepancies between readings? How would you convert the information you saw through the microscope into a decision to mow or not mow? How many spots must you measure to arrive at a meaningful conclusion?

If you stand up and look at the yard, you know in a single glance whether or not the grass needs to be cut. Scientists need this kind of elevated perspective of Greenland. The challenge was developing and launching satellites into orbit that were capable of collecting the data scientists need to measure the mass balance of the entire ice sheet.

Abdalati explains that the problem was that the satellites in orbit a decade ago could only provide small pieces of the equation of the ice sheet’s mass balance. Those earlier data were collected over short time spans and therefore could not be pieced together to tell the whole story of what was happening to the ice sheets. For example, Landsat data provided excellent detail of the ice margin, and a means of determining how fast the ice was flowing, but without information on the thickness of the ice sheet scientists could not relate flow rates to how much ice was being discharged from the sheet’s margin.

 
  Landsat image of Jakobshavn Glacier with the Calving Front Mapped Since 1851
 

Scientists could use microwave instruments to determine areas of melt because wet snow reflects microwave signals differently than dry snow. So, data from the Special Sensor Microwave/Imager (SSM/I) allowed scientists to map the extent of the wet snow on Greenland every summer, but they had no way to convert these maps of surface “wetness” to a measurement of the net loss of snow. Moreover, there was no reliable means of estimating how much snow was falling on the ice, nor could scientists determine how snowfall was varying over time, except from a limited sampling of ice cores at various locations on the ice sheet. To figure out whether the ice sheet was growing or shrinking, and by how much, scientists needed more. They needed a means of integrating all of these observations over long periods of time and, more critically, they needed to be able to observe directly how the ice thickness and mass were changing.

 

The Jakobshavn Glacier in western Greenland drains the central ice sheet, and it is retreating inland faster than any other. This image shows the glacier in 2001. The glacier flows from upper right to lower left. The fjord beyond the glacier terminus is packed with seasonal ice and icebergs. Terminus locations before 2001 were determined by surveys; more recent contours were derived from Landsat data. Without measurements of ice thickness, however, the picture of ice loss is incomplete. (NASA image by Cindy Starr, based on data from Ole Bennike and Anker Weidick (Geological Survey of Denmark and Greenland) and Landsat data.)

Map of Surface Melting Anomaly on the Greenland Ice Sheet for 2005.

Abdalati and his colleagues pinned their hopes on two missions that would represent a new generation of spacecraft. The first was the twin satellites of the joint NASA and German Aerospace Center (DLR) Gravity Recovery And Climate Experiment (GRACE), launched in March 2002, which detect minute shifts in Earth’s gravity field based on changes in distance between the two satellites. These shifts indicate the mass below the satellites has changed. Over Greenland, the shifts would reveal whether the ice sheet as a whole gained or lost mass.

 

Warmer temperatures are increasing the number of summer days when portions of the surface of the Greenland Ice Sheet melt. Along the margins of the ice sheet, up to 20 additional days of melting occurred in 2005 compared to the average since 1988. (NASA map by Robert Simmon and Marit Jentoft-Nilsen, based on data from Marco Tedesco, GSFC.)

  Artist's conception of the GRACE satellites.  

The Gravity Recovery and Climate Experiment (GRACE) consists of two satellites orbiting 220 kilometers (140 miles) apart, linked by a microwave ranging system. Together, the satellites measure the mass of the Earth, and can detect changes such as earthquakes, the flow of water, and the movement of ice. (NASA image courtesy JPL.)

  Artist's rendering of ICESat.

The second mission was NASA’s Ice, Cloud, and land Elevation Satellite (ICESat), launched in January 2003. ICESat uses a sophisticated laser altimetry system to make extremely accurate measurements of Earth’s surface elevation and how it changes over time. Changes in surface elevation of the ice sheet correlate with changes in its mass. Using these two new streams of space-based data—along with all the other information from earlier satellites, aircraft, and field observations—scientists hoped to finally answer the question: Overall, is Greenland’s ice sheet growing or shrinking?

 

ICESat maps elevation with a laser using a technique called Light Detection and Ranging (LIDAR). Changes in ice sheet elevation over time represent a gain or loss of mass. (NASA image courtesy GSFC.)

 

First with GRACE

 

According to Abdalati, launching those satellites wasn’t the end of the challenges faced by a scientific community hungry for data. “In fact, in some ways,” he says, “launching those missions marked a new beginning.” The data these satellites generated were brand new, and scientists had to learn how to interpret them correctly.

Enter Isabella Velicogna, a scientist with a joint appointment at NASA’s Jet Propulsion Lab and the University of Colorado. She speaks with obvious passion for her research, punctuated often with an infectious laugh. Though Velicogna has never set boots on Greenland’s ice, she was among the first to “crack the code” of what GRACE had to tell about how Greenland’s Ice Sheet is changing.

The GRACE project team at the University of Texas converted the streams of raw data from GRACE into monthly averaged maps of Earth’s gravity field, which Velicogna converted into ice mass changes. Since GRACE cannot directly calculate the total amount of ice, her study would assess the redistribution, or change, of ice mass. She knew it wouldn’t be easy to tease out the signal in Earth’s gravity field caused solely from mass changes in Greenland’s Ice Sheet. Other geophysical processes in Earth’s environment are also changing, such as ocean circulation patterns and large-scale movements of water on land. Her goal was to distinguish any changes in Earth’s gravity field caused by changes in Greenland’s Ice Sheet from all other possible geophysical changes.

  Photograph of Isabella Velicogna.

Isabella Velicogna. (NASA photograph courtesy JPL.)

  Map of Earth's gravity anomaly measured by the GRACE satellite.
 

“GRACE gave us a new type of data,” Velicogna explains. “We had to make new model assumptions. At first, no one was sure how those model assumptions and the spacecraft design would affect the results.” For example, Velicogna relied upon a global circulation model to estimate mass in the atmosphere, which she then subtracted from the total gravity signal. Likewise, she assumed that the mass of the mountains underneath the ice sheet was constant so that too could be removed from the equation. And so it went for months as she painstakingly peeled away all the probable causes of changes in Earth’s gravity field except the Greenland ice sheet, which she would gradually isolate. Once she produced a mean measurement of mass for the entire Greenland Ice Sheet in 2002, she could do so again and again with each passing year to determine whether it was changing over time. By 2005, she would finally be able to answer the burning question: Overall, is Greenland’s ice sheet growing or shrinking?

Her much-anticipated results showed a net mass loss for the ice sheet of 248 gigatons per year—enough to raise sea level about half a millimeter each year. (A gigaton is one billion metric tons, the mass of a cube of water that is 1 kilometer wide, tall, and deep.) Although other NASA researchers published similar findings shortly after Velicogna, these GRACE results showed a significantly larger loss—four to five times more ice loss—than most estimates that scientists had published before hers, based on different data from the 1990s through 2003.

A discrepancy of that magnitude raised a number of questions about the different approaches researchers took. Would the new GRACE data be as useful and reliable for ice sheet research as scientists had hoped? Were Velicogna’s basic approach and underlying research assumptions flawed? Did the analyses that preceded Velicogna’s work overlook something important? Or, did the ice sheet really change that much?

 

The GRACE satellites map anomalies (difference from average) in Earth’s gravity field. Gravity depends on mass, and mass is influenced by topography and other physical features. High mountain ranges and the Greenland Ice Sheet have greater-than-average gravity (red), while oceanic trenches have lower-than-average gravity (blue). GRACE can also measure changes in gravity over time, providing a method for calculating changes in the mass of the Greenland Ice Sheet. (NASA map by Robert Simmon, based on GRACE data.)

 

Resolving the Discrepancy

 

Abdalati said, “We didn’t regard the discrepancy in findings as an error to be corrected. Rather, we needed to understand why the discrepancy existed and to learn what information was contained in those different results.”

Scott Luthcke, of NASA’s Goddard Space Flight Center Planetary Geodynamics Laboratory, set out to determine if the discrepancy was a result of analytical assumptions and numerical manipulations. Knowing that many factors can affect GRACE’s raw gravity measurements, Luthcke embarked upon his own reprocessing of the data that provided greater spatial and temporal resolution than the original monthly averaged GRACE gravity field maps that were provided to Velicogna. His “new” technique was actually a modern update of the approach used by Apollo missions to map the moon’s gravity field. With this updated system for processing GRACE data, Luthcke was able to isolate geographic areas by drainage system and by elevation. He also aggregated the data into shorter time (ten-day) increments.

Using these finer resolutions of space and time on GRACE data, Luthcke calculated a net annual ice loss for the Greenland ice sheet of about 101 gigatons per year between July 2003 and July 2005. He also duplicated Velicogna’s analysis, and he got results similar to hers. This suggested that the discrepancy was due neither to GRACE data nor to the scientists’ analytical models, but, rather, to how GRACE data had originally been processed. Luthcke’s effort confirmed that GRACE data (like many data sources) are extremely sensitive to the methodology used to process and convert the raw data into values ready for research calculations. Luthcke’s study also confirmed that GRACE data are very reliable and well-suited for monitoring Earth’s great ice sheets.

Velicogna herself pinpointed two other important reasons why her estimates of the ice loss were larger than estimates made by other scientists. The first is model approach and methodology. Studies using GRACE data convert gravitational force to mass, whereas studies using data from laser and radar altimeters such as ICESat or the European Remote Sensing (ERS) satellites convert changes in surface elevation to mass. Different sets of assumptions are required by each of these different data sources; each approach brings its own inherent sources of error and approximation into the calculations.

The second and potentially greater source of the discrepancy is time. Different studies used data from different years. Scientists now know there was a substantial jump in ice loss from 2002 through 2004 compared to previous years. The timing of the GRACE data was such that Velicogna’s study coincided with the beginning of a period of increased ice loss on Greenland.

 
 
Researchers Mass Change (GT/year) Method Time span
Krabill et al. 2000 -47 Aircraft Surveys 1994–1999
Velicogna et al. 2006 -200 to -260 GRACE 2002–2006
Luthcke et al. 2007* -145 to -175 GRACE 2003–2006
Zwally et al. 2007* -80 to -100 ICESat 2003–2005
 

Further, two years of data is a rather short time period to be the basis for reliable model comparisons. As data for additional years has become available all the NASA research community’s estimates are converging toward an average annual net loss of ice mass loss of 150 to 180 gigatons per year. For example, Byron Tapley, an Aerospace Engineering Professor at University of Texas at Austin (2006), estimated total ice sheet loss at 209 gigatons per year. Eric Rignot, of NASA’s JPL (2006), calculated a net loss of 162 gigatons for 2005. Preliminary numbers produced by Jay Zwally (also released in 2006) using data from ICESat ranged from 80 to 100 gigatons of annual ice loss. Today, Velicogna’s new calculations adding data for 2005 and 2006 to her previous estimates show ice loss at just over 210 gigatons per year. When Luthcke and his colleagues updated their calculations with GRACE data collected through June 2006, they calculated a loss of 161 gigatons per year.

 

Scientists using different sources of data and different ways of processing raw data have generated a range of estimates for the mass balance of the Greenland Ice Sheet. Despite some differences, all the results indicate that the ice sheet has lost hundreds of gigatons of mass in recent years. (*Estimates based on unpublished data.)

Map of elevation change on the Greenland ice sheet.

While some differences remain in current estimates, Abdalati says the real story is not the discrepancies between these studies, it is the degree of their agreement. Every study over the past seven years—using a variety of assumptions, data sources, and study methodologies—has come to the same conclusion: Greenland is now losing significant amounts of ice each year.

“While differences in these studies still exist,” Abdalati concludes, “collectively, they very convincingly paint a picture of the Greenland Ice Sheet as having been close to balance in the 1990s, contributing a small amount to sea level, but becoming significantly out of balance and losing a substantial amount of ice to the sea in the last several years.”

Today, the questions facing Abdalati and his colleagues have evolved from whether the ice sheet is growing or shrinking to what will the ice sheet do in the future? Will its rate of shrinking accelerate? Slow down? Reverse? Remain the same?

Velicogna marvels at her and her colleagues’ ability to address such questions today using the new capabilities of GRACE and ICESat. “Putting all these instruments’ data together is giving us the full picture,” she states. “Now we can look at things that are happening and see what is really going on. To me, that is exciting and really cool.”

Velicogna points out that for a long time scientists thought that models would be the way they would figure out what was happening on the ice sheets. “But the models aren’t good enough now to predict the changes we are observing,” she observes. “The physics of the ice sheets aren’t fully understood, and so while those models are good, nature is more complex.”

Abdalati says he is convinced that the Greenland Ice Sheet will continue to shrink at a significant and, perhaps, accelerating rate. Already he and his colleagues have new studies underway in which they are investigating the sensitivity of the ice sheet to rising temperatures and the specific mechanisms by which the ice sheet responds to increased warmth. “As we look at Greenland through the eyes of these remarkable new satellite missions,” he says, “the ice sheet continues to surprise us.”

References

  • Abdalati, W. and K. Steffen. (2001). Greenland ice sheet melt extent: 1979-1999. Journal of Geophysical Research,106, (D24), 33983-33988. doi:10.1029/2001JD900181.
  • Herring, D. (2005). Time on the shelf. NASA Earth Observatory. Accessed: August 16, 2007.
  • Howat, I., Joughin, I., Tulaczyk, S., and Gogineni, S. (2005). Rapid retreat and acceleration of the Helheim glacier, East Greenland. Geophysical Research Letters, 32(10), L22502, doi:10.1029/2005GL024737.
  • Johanessen, O., Khvorostovsky, K., Miles, M., and Bobylev, L. (2005). Recent ice-sheet growth in the interior of Greenland. Science,310(5750), 1013-1016. doi: 10.1126/science.1115356.
  • 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(5478), 428-430. doi: 10.1126/science.289.5478.428.
  • 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(5407), 1522-1524. doi: 10.1126/science.283.5407.1522.
  • Krabill, W., Hanna, E., Huybrechts, P., Abdalati, W., Cappelen, J., Csatho, B., Frederick, E., Manzinade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., and Yungel, J. (2004). Greenland Ice Sheet: Increased coastal thinning. Geophysical Research Letters, 31, L24402, doi:10.1029/2004GL021533.
  • Luthcke, S., Zwally, H., Abdalati, W., Rowlands, D., Ray, R., Nerem, R., Lemoine, F., McCarthy, J., and Chinn, D. (2006). Recent Greenland ice mass loss by drainage system from satellite gravity observations. Science, 314(5803), 1286-1289. doi: 10.1126/science.1130776.
  • Luthcke, S., Zwally, H., Rowlands, D., Abdalati, W., Nerem, R., Ray, R., Lemoine, F., and Chinn, D. (2006). Recent ice sheet mass change observations from GRACE mascon solutions. EOS Transactions AGU,87(52), Fall Meeting Supplement, G12A-07.
  • Mosley-Thompson, E., Readinger, C., Craigmile, P., Thompson, L., and Calder, C. (2005). Regional sensitivity of Greenland precipitation to NAO variability. Geophysical Research Letters,32, L24707, doi:10.1029/2005GL024776.
  • Rignot, E., and Kanagaratnam, P. (2006). Changes in the velocity structure of the Greenland Ice Sheet. Science311(5763), 986-990. doi: 10.1126/science.1121381.
  • Rignot, E, and Thomas, R. (2002). Mass balance of the polar ice sheets. Science297(5586), 1502-1506. doi: 10.1126/science.1073888.
  • Rothrock, D., Yu, Y., and Maykut. G. (1999). Thinning of the Arctic sea-ice cover. Geophysical Research Letters, 26(33), 3469-3472. doi: 10.1029/1999GL010863.
  • Serreze, M., Maslanik, J., Scambos, T., Fetterer, F., Stroeve, J., Knowles, K., Fowler, C., Drobot, S., Barry, R, and Haran, T. (2003). A record minimum Arctic sea ice extent and area in 2002. Geophysical Research Letters, 30(3), 1110. doi: 10.1029/2002GL016406.
  • Velicogna, I., and Wahr, J. (2005). Ice mass balance in Greenland from GRACE. Geophysical Research Letters, 32(18), L18505. doi: 10.1029/2005GL023955.
  • Wahr, J., and Velicogna, I. (2003). What might GRACE contribute to studies of post glacial rebound? Space Science Reviews, 108(1-2), 319-330. doi: 10.1023/A:1026183526762.
  • Weir, J. (2003). Searching for Atlantic rhythms: winter weather and the North Atlantic Oscillation. NASA Earth Observatory. Accessed: August 16, 2007.
  • Zwally, H., Abdalati, W., Herring, T., Larson, K., Saba, J., and Steffen, K. (2002). Surface melt-induced acceleration of Greenland Ice Sheet flow. Science,297(5579), 218-222. doi: 10.1126/science.1072708.
  • Zwally, H., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J., and Yi, D. (2005). Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise:1992-2002. Journal of Glaciology, 51(175), 509-527.
 

Using ICESat data, scientists calculated the change in thickness of the Greenland Ice Sheet between 2003 and 2006. Increased melting and faster glacial flow lowered the surface along the margins of the ice sheet (brown), while increased snowfall thickened the ice in the interior of Greenland (blue). This pattern is consistent with model predictions of how global warming will affect the ice sheet. (NASA map by Robert Simmon, based on ICESat data.)

 

Why Does the Greenland Ice Sheet Matter?

 

The Greenland Ice Sheet blankets 81 percent of Greenland Island. This monstrous ice slab stretches 2,480 kilometers (1,540 miles) long and up to 750 kilometers (465 miles) wide. The ice sheet is so big it would stretch from Key West, Florida, to 100 miles beyond Portland, Maine, covering a swath as wide as from Washington, D.C., to Indianapolis, Indiana. It’s 80 percent as big as the entire United States east of the Mississippi River. It’s not only huge, it’s also thick—an average of 2.3 kilometers (1.6 miles) thick. It contains roughly 8 percent of all of Earth’s fresh water.

 
  Satellite image of the Petermann Glacier and the Greenland Ice Sheet.
 

Greenland’s Ice Sheet matters for four reasons:

Sea level: As the Greenland Ice Sheet melts, sea level rises. It is a direct, proven effect. This is the biggest reason for concern over Greenland. Scientists estimate that if the entire ice sheet melted, sea level would rise 23 feet. Depending on how rapidly such a change occurred, it could be a global-scale catastrophe because nearly one-third of the world’s population lives in or near a coastal zone. The global impact of several billion refugees and the negative impacts on coastal economic activity would be staggering.

A sea level rise of only two to three feet—the high end of current plausible scenarios for the next 20 years—would create serious global problems: increased coastal erosion, salt water encroachment, loss of barrier formations (islands, sand bars, and reefs), and increased storm surge damage. Through the 1990s, sea level rose at a rate of about 3 millimeters per year. The rate crept up to almost 4 millimeters per year by the end of last decade. For historical perspective, sea level has risen more than 380 feet since the last ice age 18,000 years ago. That’s an average rate of 2.5 inches (10 centimeters) per decade, or 10 millimeters per year. However, most of that rise occurred as the ice age ended. Sea level has been relatively stable for the past several millennia.

Ocean circulation: Sea level rise is not the only effect of a melting Greenland Ice Sheet. If the Greenland Ice Sheet melts at a faster rate, it will spread a slick of fresh water on top of the heavier salt water of the North Atlantic. This change in salinity could depress the Gulf Stream and alter North Atlantic circulation patterns that control weather in Europe. Combined with a loss of Arctic sea ice, this effect could radically change global ocean circulation patterns.

Global heat transfer: The loss of Greenland ice mass would affect global atmospheric heat movement. Any heat transfer is driven by a temperature difference. The greater that difference, the faster heat flows. As the polar regions warm, the temperature difference between the equator and the poles is reduced, altering global atmospheric circulation patterns by reducing the force that drives equatorial heat energy toward the poles. Much of the world’s current pattern of rainfall would be altered.

Regional atmospheric circulation: On average, the top 2,560 meters (8,400 feet) of Greenland is ice. Like a mountain range, this physical mass affects regional atmospheric circulation patterns. If (as) Greenland’s massive range of ice melts, regional circulation patterns will also change.

 

The Greenland Ice Sheet is a frozen reservoir for 8 percent of all freshwater on the planet. Glaciers, such as the Petermann Glacier along the island’s northern margin, drain the interior, returning this water to the sea as icebergs. Melt water can accelerate the flow of a glacier by lubricating the underside as it scrapes over the rocky terrain. Despite Greenland’s remoteness, large-scale changes to the island’s ice sheet will have global influence. This image from the Moderate Resolution Imaging Spectroradiometer aboard NASA’s Terra satellite was captured on July 5, 2003. (NASA image by Jesse Allen and Robert Simmon.)