Arctic Melting: Greenland Ice Cap
Arctic Melting: Greenland Ice Cap
Greenland is an island in the North Atlantic Ocean. With an area of about 836,000 sq mi (2,166,000 sq km), mostly above the Arctic Circle, Greenland is by far the world's largest island, over twice the size of New Guinea. Politically, it is a territory of Denmark and has a population of about 57,000 people. Over 80% of the island is covered by an ice sheet with an average thickness of about 1 mi (1.6 km), but the ice sheet is twice this thick in its central area. There are also a number of large glaciers and smaller ice caps around the periphery of the island. The total amount of ice stored in the cap is 964,000 cubic mi (4 million cubic km), 10% of all the ice on the planet (most of the rest is in the Antarctic ice sheet). If the Greenland ice cap were to melt completely, scientists estimate that the resulting meltwater would raise global sea levels by about 21 ft (6.5 m, with many experts estimating 7 m). Although most scientists presently assert that the complete melting of the Greenland ice cap is not likely to occur in the next century or so, in recent years greatly accelerated melting of this ice cap due to global warming has been measured.
Historical Background and Scientific Foundations
The Greenland ice sheet, like that of Antarctica, is formed of layers of annual snowfall that pack down into solid ice, each year building on the previous year. Eventually, after many millennia, the ice sheet becomes heavy enough to spread or flow under the force of its own weight, causing glaciers to flow downhill to the sea. As of 2007, the oldest ice that had been recovered by drilling into the Greenland ice sheet was about 123,000 years old, but the ice cap as a whole is much older. It is a leftover from the Pleistocene glaciation, which covered a large part of the Northern Hemisphere with thick glaciers from about 1,800,000 years ago to 11,500 years ago.
Scientific knowledge of Greenland's ice was limited until the 1970s, when Denmark, the United States, and Switzerland organized the Greenland Ice Sheet Program to obtain ice cores from the sheet. Ice cores are cylinders of ice obtained by drilling vertically downward with a hollow device especially designed to return undamaged pieces of ice to the surface. As of 2007, the longest such cores from Greenland recorded 123,000 years of snow layering; in Antarctica, the longest cores recorded some 880,000 years. Ice bubbles trapped in these cores give data on local snowfall rates, and ratios of atomic isotopes in the trapped water indicate global warmth over time. Evidence of global lead pollution by the Greek and Roman civilizations has even been found in Greenland ice cores.
Ice floating on the seas, such as the north polar ice pack, does not raise sea level when it melts. Each piece of melting ice can be visualized as simply filling in the hollow it makes in the water as it floats. However, the ice locked in the Greenland and Antarctic ice sheets, because it is sitting on land, does raise sea levels when it melts. The amount of freshwater running off of Greenland into the sea can also affect the thermohaline or conveyor-belt circulation of the oceans, which transports warm water in surface currents to the Arctic and Antarctic and returns it to the tropics in cool currents deeper in the ocean. This, in turn, affects climate patterns worldwide.
Measuring the amount of ice in Greenland is not an easy matter. The ice sheet is vast (almost as large as Mexico), accessible only by air over large areas, and of uneven thickness. In the 1990s, surface-height changes of the Greenland ice mass, which show gain or loss of ice, were made using laser altimeters (altitude-measuring devices) carried in airplanes. Repeated measurements were compared to each other to detect changes over time. As of 2000, these data indicated that the higher-altitude (above 6,560 ft/2,000 m), thicker part of the ice sheet—which totals 70% of the ice sheet—was in approximate balance, gaining mass from snow and losing it from melting at about equal rates. Seventy percent of the sheet area below 6,560 ft (2,000 m) was losing mass, but not very rapidly. These data from the 1990s indicated that the ice mass of Greenland was in approximate balance, with snowfall gains in the southwest balanced by melting in the southeast. The amount of water added to the oceans by Greenland ice loss was calculated to be contributing about 7% of observed sea-level rise, that is, about 0.005 in (0.13 mm) per year.
Gaining More Accurate Data
More accurate measurements of the changing situation in Greenland became possible starting in 2002, when the U.S. National Aeronautics and Space Administration (NASA) launched its Gravity Recovery and Climate Experiment (GRACE). GRACE, which remained in operation as of late 2007, is a pair of Earth-orbiting satellites that fly a common orbit about 136 mi (220 km) apart, one satellite ahead of the other. As the leading satellite dips into and out of regions of stronger gravity, such as exist in the vicinity of mountain ranges or thicker areas of ice sheets, the distance between the satellites changes. The satellites continuously measure this distance to extremely high accuracy. Computer analysis of the distance measurements recorded over many orbits provides a sensitive picture of gravity anomalies (areas of higher- or lower-than-average gravity) over Earth's surface. Not only does GRACE allow continuous surveying of Greenland's ice sheet with high sensitivity, but it covers the whole sheet at equal resolution, which had been impractical with airplanes.
GRACE's gravity measurements are sensitive enough to detect Greenland's seasonal changes in ice mass, with gains from snowfall in the winter and losses from melting in the summer. The results from GRACE startled scientists when enough data had been accumulated to show a trend. The new data revealed that the situation in Greenland has changed rapidly in just a few years. While the high-elevation interior has continued to gain mass from increased snowfall—about 2–2.36 in (5–6 cm) per year over the area above 6,560 ft (2,000 m) elevation—the coastal regions are losing far more than the interior gains. From 2002 to 2006, Greenland's ice sheet lost a total of between 211 million and 284 million tons (192 million and 258 million metric tons) of ice each year (about 50–68 mi3 or 212–284 km3).
WORDS TO KNOW
CALVING: Process of iceberg formation when huge chunks of ice break free from glaciers, ice shelves, or ice sheets due to stress, pressure, or the forces of waves and tides.
GLACIER: A multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.04 mi2(0.1 km2) in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of freshwater on Earth and is second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
GROUNDING LINE: Underwater boundary or line along which a glacier that is flowing into the sea floats free of the ground. Grounding lines are typically many miles from the nominal shoreline. Retreat of grounding lines toward land accompanies speeded glacial flow. Retreat of grounding lines has occurred recently for some glaciers in Greenland and Antarctica.
ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.
ICEBERG: A large piece of floating ice that has broken off a glacier, ice sheet, or ice shelf.
PALEOCLIMATE: The climate of a given period of time in the geologic past.
PLEISTOCENE EPOCH: Geologic period characterized by ice ages in the Northern Hemisphere, from 1.8 million to 10,000 years ago.
THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.
This loss contributed to an annual sea-level rise of 0.02 in (0.5 mm) per year, much more than previous studies had found. Furthermore, it found that the mass loss was accelerating rapidly. The loss rate during 2004 to 2006 was 2.5 times the rate during 2002 to 2004. Other satellite data showed that the Greenland ice sheet had experienced summertime surface melting over a greater part of its surface in 2006 than at any time at least since continuous satellite surveillance began in 1979.
Mass loss from the ice sheet occurs not only from melting, but also from speedier flow of coastal glaciers to the sea, where they calve off as floating icebergs that eventually melt. Many of Greenland's outlet glaciers have accelerated their transport of ice to the sea. Ice thinning along Greenland's periphery is concentrated along the deep channels in the ground that the glaciers flow along. The three largest glaciers—Jakobshavn Isbrae, Kangerdlugssuaq, and Helheim—have all accelerated and thinned. Jakobshavn Isbrae on the west coast, the largest, has been thinning by 49 ft (15 m) per year since 1997, while the other two, on the east coast, have been thinning by 131 ft (40 m) and 82 ft (25 m) per year, respectively. Jakobshavn Isbrae accelerated by 95% from 1996 to 2005; ice discharged almost doubled from 1996 to 2005, from 5.7 mi3(24 km3) of ice per year to 11 mi3(46 km3) per year. Kangerdlugssuaq flowed at a stable speed from 1962 to 1996, but from 2000 to 2005 it accelerated by 210%, at which time it was flowing at about 8.7 mi (14 km) per year at the calving front, the area where the glacier breaks up into icebergs.
So far, little velocity change in glacier outflow has been seen in northern Greenland. The changes described earlier are all in the south. However, as global warming proceeds, similar changes are expected to extend farther and farther northward in Greenland.
Impact of Global Warming
Global warming is accelerating the discharge of ice from both Greenland and Antarctica by several means. First, there is direct melting. Second, as floating ice tongues or shelves break up due to warmer water, they uncork or unblock the glaciers behind them, allowing them to flow more quickly to the sea. Third, downward infiltration of surface meltwater in the summer months can lubricate the glacial channel, accelerating it seasonally. Fourth, there is a lubricant effect from warmer ocean waters getting under the glacier. Many large glaciers, such as Jakobshavn Isbrae and the other large glaciers of Greenland, run along deep channels in the earth that are actually below sea level near their outlets. (In eastern Antarctica, submarine flow beds extend all the way to the center of the ice sheet.) Eventually, the glacial ice flowing down the channel floats free of the ground along a front called the grounding line. Scientists have proposed that warmer sea waters are melting ice along the grounding lines of Greenland's glaciers, pushing the grounding lines upstream. This makes the glaciers' channels more slippery and accelerates the ice flow.
Surface water temperatures in the vicinity of Greenland, however, are not high enough to account for the shifting of glacial grounding lines. Scientists explain how warmer water can nevertheless get to Greenland's glaciers in the following way. Efforts to account for Earth's heat budget—how much comes in from the sun, and where it goes—have shown that atmospheric warming cannot account for all the heat Earth is absorbing because of its increased greenhouse gases. The extra heat has been absorbed by the oceans, mostly in the top 3,280 ft (1,000 m). However, in the North Atlantic Ocean, the heat is carried to greater depths. Warm, salty water is denser than cold, freshwater from glacial melting, so at the outflow of the glacier the warmer, mid-depth ocean water slips down to the grounding line.
Glacial acceleration has been observed not only in Greenland but in the western peninsula of Antarctica, which is the only part of the world to have been warmed as much by global climate change as the Arctic (at about twice the rate of the rest of the planet).
So heavy are the ice flows of glaciers that when they slip suddenly, moving forward several feet or more in a quick jerk, a small earthquake is produced. The existence of these glacial earthquakes was only detected in 2003. By 2006, careful analysis of recordings of vibrations in Earth's crusthad revealedthatfrom1993to the late 1990s, there was a modest increase in the number of glacial earthquakes. This was followed by a rapid increase from 2002 onward, with nearly as many glacial earthquakes in 2005 as in all of 1993–1996. Seventy-two percent of the earthquakes came from Greenland's three largest glaciers: Jakobshavns Isbrae, Kangerdlugssuaq, and Helheim. Although it is not clear whether the accelerated motion of these glaciers is producing more glacial earthquakes or simply more glacial earthquakes that are easy to detect, scientists take this activity as another sign of changing conditions under the Greenland icesheet.
The earthquake and ice-flow data do indicate that modest changes in temperature in the area of the ice sheet, such as have already been seen, may lead to large increases in the amount of glacial ice discharged to the sea. Computer models of ice sheets used until 2006, including those used by contributors to the Intergovern-mental Panel on Climate Change (IPCC), the world's largest and most authoritative body of climate and weather scientists, have not taken this newly discovered sensitivity into account. As such, they may have underestimated how much future warming may accelerate Greenland's melting and thus its contribution to sea-level rise.
Finally, another form of melting has recently been observed in Greenland on an unprecedented scale— superficial melting, the direct melting of ice on the surface of the sheet as opposed to the shifting of ice directly into the sea by glaciers. A re-analysis of satellite and other data in 2007 showed that over the last 25 years, superficial melting in Greenland has accelerated twice as fast as was previously thought. From 1979 to 2005, the average year-round temperature rose 3.6°F (2°C), and the area of Greenland that saw superficial melting at least one day per year grew by 42%. The earlier studies had underestimated the melting because they were based on microwave radar waves beamed at Earth from satellites. However, clouds weaken such reflections, and the increased melting was releasing more water into the air, thus creating more clouds. In effect, the melting was partly hiding behind a screen of clouds. As with accelerated glacial movement, the increased superficial-melt area is around the edges of the ice cap, not at the center. Surface melt area set an all-time record in 2002, then broke that record in 2005. The annual difference between snowfall (which adds mass) and superficial melting (which removes it) accounted for about a third of Greenland's current mass loss; the rest is from glaciers, which dump icebergs into the sea. Like an ice cube dropped in a drink, an iceberg raises sea level as soon as it is put in, not when it finally melts.
Impacts and Issues
The impact of complete melting of Greenland's ice could be a 21–23 ft (6.5–7 m) rise in sea levels that would radically alter coastlines and force the migration of hundreds of millions of people. Although complete melting of the cap in the near future seems unlikely, there is scientific dispute over just how much of the cap may melt and when, under the conditions of global warming likely to be seen.
The IPCC's Third Assessment Report, in 2001, suggested a sea-level rise of 0.3–2.9 ft (0.09–0.88 m) by 2100 without large reductions in greenhouse-gas emissions. Continuing unabated emissions, the report said, could lead to a sea-level rise of over 16 ft (5 m) over the next thousand years. In subsequent years, other models showed that the Greenland ice sheet could melt completely over the next thousand years or so. In 2007, the IPCC issued an even more conservative estimate of sea-level rise, lowering the upper end of its estimate of sea-level rise by 2100 to 1.9 ft (0.59 m).
A number of scientists have criticized the IPCC's reports as being too conservative, that is, understating the likely or possible effects of climate change. For example, the rules for drafting the 2007 report excluded all scientific papers published after December 2005. This means that those studies which used satellite gravity measurements to reveal much faster melting of the Greenland ice cap from 2004–2006 were not taken into account in the new, lower sea-level rise estimates of 2007.
Even the higher 2001 estimate was criticized as too conservative. In a 2006 paper in Science, Jonathan T. Overpeck and colleagues examined the paleoclimate (prehistoric climate) record and found that warming of the Arctic and Antarctic regions might reach levels seen 130,000 to 127,000 years ago by 2100. At that time, sea levels were several meters higher than present-day levels, so much higher sea-level rises than a half-meter or so should not be ruled out or characterized as extremely unlikely.
The IPCC's computer models contain only a small contribution from glaciers' dynamic response, that is, the ability of glaciers to respond to warming by flowing faster, which turns out to be important in Greenland.Overpeck and his co-workers therefore argue that the record of “past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.” A threshold might be crossed, they warned, before the end of the twenty-first century, a point beyond which ice-sheet melting would accelerate greatly beyond what is already seen. Human-released soot is darkening the ice, making it absorb more solar energy and melt faster, and climate models indicate that the Antarctic might melt much more than it did 129,000 years ago. Further, climate scientists agree that warming will continue for many years even if humans greatly reduce their greenhouse-gas emissions, since the gases are already in the atmosphere.
Primary Source Connection
Data gathered by NASA satellites show that the melting of Greenland's ice sheet more than doubled from 2002– 2004 to 2004–2006, causing oceans to rise faster worldwide. This NASA press release describes the technology used to gather the data and presents a lead scientist's view of the melting's significance.
NASA'S GRACE FINDS GREENLAND MELTING FASTER, ‘SEES’ SUMATRA QUAKE
In the first direct, comprehensive mass survey of the entire Greenland ice sheet, scientists using data from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (Grace) have measured a significant decrease in the mass of the Greenland ice cap. Grace is a satellite mission that measures movement in Earth's mass.
In an update to findings published in the journal Geophysical Research Letters, a team led by Dr. Isabella Velicogna of the University of Colorado, Boulder, found that Greenland's ice sheet decreased by 162 (plus or minus 22) cubic kilometers a year between 2002 and 2005. This is higher than all previously published estimates, and it represents a change of about 0.4 millimeters (.016 inches) per year to global sea level rise.
“Greenland hosts the largest reservoir of freshwater in the northern hemisphere, and any substantial changes in the mass of its ice sheet will affect global sea level, ocean circulation and climate,” said Velicogna. “These results demonstrate Grace's ability to measure monthly mass changes for an entire ice sheet—a breakthrough in our ability to monitor such changes.”
Other recent Grace-related research includes measurements of seasonal changes in the Antarctic Circumpolar Current, Earth's strongest ocean current system and a very significant force in global climate change. The Grace science team borrowed techniques from meteorologists who use atmospheric pressure to estimate winds. The team used Grace to estimate seasonal differences in ocean bottom pressure in order to estimate the intensity of the deep currents that move dense, cold water away from the Antarctic. This is the first study of seasonal variability along the full length of the Antarctic Circumpolar Current, which links the Atlantic, Pacific and Indian Oceans.
Dr. Victor Zlotnicki, an oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., called the technique a first step in global satellite monitoring of deep ocean circulation, which moves heat and salt between ocean basins. This exchange of heat and salt links sea ice, sea surface temperature and other polar ocean properties with weather and climate-related phenomena such as El Niños. Some scientific studies indicate that deep ocean circulation plays a significant role in global climate change.
The identical twin Grace satellites track minute changes in Earth's gravity field resulting from regional changes in Earth's mass. Masses of ice, air, water and solid Earth can be moved by weather patterns, seasonal change, climate change and even tectonic events, such as this past December's Sumatra earthquake. To track these changes, Grace measures micron-scale changes in the 220-kilometer (137-mile) separation between the two satellites, which fly in formation. To limit degradation of Grace's satellite antennas due to atomic oxygen exposure and thereby preserve mission life, a series of maneuvers was performed earlier this month to swap the satellites' relative positions in orbit.
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