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Topic Name: The Surface Temperature of Greenland's Massive Ice Sheet has been Rising, According to A New Study

Category: Environmental engineering

Research persons: NASA Scientists

Location: National Aeronautics and Space Administration (NASA), United States

Details

A new National Aeronautics and Space Administration study confirms that the surface temperature of Greenland's massive ice sheet has been rising, stoked by warming air temperatures, and fueling loss of the island's ice at the surface and throughout the mass beneath.

Greenland's enormous ice sheet is home to enough ice to raise sea level by about 23 feet if the entire ice sheet were to melt into surrounding waters. Though the loss of the whole ice sheet is unlikely, loss from Greenland's ice mass has already contributed in part to 20th century sea level rise of about two millimeters per year, and future melt has the potential to impact people and economies across the globe. So NASA scientists used state-of-the-art NASA satellite technologies to explore the behavior of the ice sheet, revealing a relationship between changes at the surface and below. The new NASA study appears in the January issue of the quarterly Journal of Glaciology.

"The relationship between surface temperature and mass loss lends further credence to earlier work showing rapid response of the ice sheet to surface meltwater," said Dorothy Hall, a senior researcher in Cryospheric Sciences at NASA's Goddard Space Flight Center, in Greenbelt, Md., and lead author of the study.

A team led by Hall used temperature data captured each day from 2000 through 2006 from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra satellite. They measured changes in the surface temperature to within about one degree of accuracy from about 440 miles away in space. They also measured melt area within each of the six major drainage basins of the ice sheet to see whether melt has become more extensive and longer lasting, and to see how the various parts of the ice sheet are reacting to increasing air temperatures.

The team took their research at the ice sheet's surface a step further, becoming the first to pair the surface temperature data with satellite gravity data to investigate what internal ice changes occur as the surface melts. Geophysicist and co-author, Scott Luthcke, also of NASA Goddard, developed a mathematical solution, using gravity data from NASA's Gravity Recovery and Climate Experiment (GRACE) twin satellite system. "This solution has permitted greatly-improved detail in both time and space, allowing measurement of mass change at the low-elevation coastal regions of the ice sheet where most of the melting is occurring," said Luthcke.

The paired surface temperature and gravity data confirm a strong connection between melting on ice sheet surfaces in areas below 6,500 feet in elevation, and ice loss throughout the ice sheet's giant mass. The result led Hall's team to conclude that the start of surface melting triggers mass loss of ice over large areas of the ice sheet.

The beginning of mass loss is highly sensitive to even minor amounts of surface melt. Hall and her colleagues showed that when less than two percent of the lower reaches of the ice sheet begins to melt at the surface, mass loss of ice can result. For example, in 2004 and 2005, the GRACE satellites recorded the onset of rapid subsurface ice loss less than 15 days after surface melting was captured by the Terra satellite.

"We're seeing a close correspondence between the date that surface melting begins, and the date that mass loss of ice begins beneath the surface," Hall said. "This indicates that the meltwater from the surface must be traveling down to the base of the ice sheet -- through over a mile of ice -- very rapidly, where its presence allows the ice at the base to slide forward, speeding the flow of outlet glaciers that discharge icebergs and water into the surrounding ocean."

Hall underscores the importance of combining results from multiple NASA satellites to improve understanding of the ice sheet's behavior. "We find that when we look at results from different satellite sensors and those results agree, the confidence in the conclusions is very high," said Hall.

Hall and her colleagues believe that air temperature increases are responsible for increasing ice sheet surface temperatures and thus more-extensive surface melt. "If air temperatures continue rising over Greenland, surface melt will continue to play a large role in the overall loss of ice mass." She also noted that the team's detailed study using the high-resolution MODIS data show that various parts of the ice sheet are reacting differently to air temperature increases, perhaps reacting to different climate-driven forces. This is important because much of the southern coastal area of the ice sheet is already near the melting point (0 degrees Celsius) during the summer.

Changes in Greenland's ice sheet surface temperature have been measured by satellites dating back to 1981. "Earlier work has shown increasing surface temperatures from 1981 to the present," said Hall. "However, additional years with more accurate and finer resolution data now available using Terra's imager are providing more information on the surface temperature within individual basins on the ice sheet, and about trends in ice sheet surface temperature. Combining this data with data from GRACE, arms us with better tools to establish the relationship between surface melting and loss of ice mass."

Note for Greenland Ice Sheet
The Greenland Ice Sheet is a vast body of ice covering 1.71 million km², roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost 2,400 kilometers long in a north-south direction, and its greatest width is 1,100 kilometers at a latitude of 77° N, near its northern margin. The mean altitude of the ice is 2,135 meters.  The thickness is generally more than 2 km (see picture) and over 3 km at its thickest point. It is not the only ice mass of Greenland - isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometers around the periphery. Some scientists believe that global warming may be about to push the ice sheet over a threshold where the entire ice sheet will melt in less than a few hundred years. If the entire 2.85 million km³ of ice were to melt, it would lead to a global sea level rise of 7.2 m (23.6 ft.). This would inundate most coastal cities in the world and remove several small island countries from the face of Earth, since island nations such as Tuvalu and Maldives have a maximum altitude below or just above this number.
The Greenland Ice Sheet is also sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis. It is also sometimes referred to as an ice cap. Ice sheet, however, is considered the more correct term as ice cap generally refers to less extensive ice masses.
The ice in the current ice sheet is as old as 110,000 years However, it is generally thought that the Greenland Ice Sheet formed in the late Pliocene or early Pleistocene by coalescence of ice caps and glaciers. It did not develop at all until the late Pliocene, but apparently developed very rapidly with the first continental glaciation.
The massive weight of the ice has depressed the central area of Greenland; the bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery, confining the sheet along its margins. If the ice were to disappear, Greenland would most probably appear as an archipelago. The ice surface reaches its greatest altitude on two north-south elongated domes, or ridges. The southern dome reaches almost 3,000 metres at latitudes 63° - 65° N; the northern dome reaches about 3,290 metres at about latitude 72° N. The crests of both domes are displaced east of the centre line of Greenland. The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, so that no large ice shelves occur. The ice margin just reaches the sea, however, in a region of irregular topography in the area of Melville Bay southeast of Thule. Large outlet glaciers, which are restricted tongues of the ice sheet, move through bordering valleys around the periphery of Greenland to calve off into the ocean, producing the numerous icebergs that sometimes occur in North Atlantic shipping lanes. The best known of these outlet glaciers is the Jakobshavn Isbræ, which, at its terminus, flows at speeds of 20 to 22 metres per day.
On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The lowest mean annual temperatures, about -31°C (-24°F), occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about -20°C (-4°F).
During winter the ice sheet takes on a strikingly clear blue/green color. During summer the top ice layer melts leaving pockets of air in the ice that makes the ice look all white.

Note for Sea-level Rise
Sea-level rise is an increase in sea level. Multiple complex factors may influence this change.
Sea-level has risen about 130 metres (400 ft) since the peak of the last ice age about 18,000 years ago. Most of the rise occurred before 6,000 years ago. From 3,000 years ago to the start of the 19th century sea level was almost constant, rising at 0.1 to 0.2 mm/yr. Since 1900 the level has risen at 1 to 2 mm/yr; since 1993 satellite altimetry from TOPEX/Poseidon indicates a rate of rise of 3.1 ± 0.7 mm yr–1. Church and White (2006) found a sea-level rise from January 1870 to December 2004 of 195 mm, a 20th century rate of sea-level rise of 1.7 ±0.3 mm per yr and a significant acceleration of sea-level rise of 0.013 ± 0.006 mm per year per yr. If this acceleration remains constant, then the 1990 to 2100 rise would range from 280 to 340 mm,. Sea-level rise can be a product of global warming through two main processes: thermal expansion of sea water and widespread melting of land ice . Global warming is predicted to cause significant rises in sea level over the course of the twenty-first century.
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
“Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.

About Goddard Space Flight Center
The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately 6.5 miles northeast of Washington, D.C. in Greenbelt, Maryland, USA.
GSFC has the largest combined organization of scientists and engineers dedicated to increasing knowledge of the Earth, the Solar System, and the Universe via observations from space in the United States. GSFC is a major U.S. laboratory for developing and operating unmanned scientific spacecraft. GSFC conducts scientific investigation, development and operation of space systems, and development of related technologies. Goddard scientists can develop and support a mission, and Goddard engineers and technicians can design and build the spacecraft for that mission. Goddard scientist John C. Mather shared the 2006 Nobel Prize in Physics for his work on COBE.
GSFC also operates spaceflight tracking and data acquisition networks, develops and maintains advanced space and Earth science data information systems, and develops satellite systems for the National Oceanic and Atmospheric Administration (NOAA).
GSFC manages operations for many NASA and international missions including the Hubble Space Telescope (HST), the Explorer program, the Discovery Program, the Earth Observing System (EOS), INTEGRAL, the Solar and Heliospheric Observatory (SOHO), the Rossi X-ray Timing Explorer (RXTE) and Swift. Past missions managed by GSFC include the Compton Gamma Ray Observatory, SMM, COBE, IUE, and ROSAT. Typically, unmanned earth observation missions and observatories in Earth orbit are managed by GSFC, while unmanned planetary missions are managed by the Jet Propulsion Laboratory (JPL).
The Goddard Space Flight Center is named in recognition of Dr Robert H. Goddard, the pioneer of modern rocket propulsion in the United States.
The GSFC Greenbelt facility encompasses the Main Site and adjacent outlying sites. The main campus includes 50 buildings. Additional GSFC facilities are located in New York City, Virginia, and West Virginia. The Greenbelt facility contains two campuses, which were formerly divided by Soil Conservation Road. Soil Conservation Road has recently been diverted to go around the GSFC, and the section that divided the two campuses was turned into Hubble Road.

About Gravity Recovery And Climate Experiment
The goal of the Gravity Recovery And Climate Experiment (GRACE) space mission is to obtain accurate global and high-resolution determination of both the static and the time-variable components of the Earth's gravity field. GRACE is intended to enable precise measurement of Earth's shifting water masses by detecting their effects on our planet's gravity field, allowing the study of global climatic issues by enabling a better understanding of ocean surface currents and heat transport, measuring changes in sea-floor pressure, watching the mass of the oceans change, and by monitoring changes in the storage of water and snow on the continents. Also data on ocean and deep sea currents as well on tectonics are derived from the data.
GRACE maps variations in the Earth's gravity field over its five-year spacetime (extended to eight years in 2005) with its two identical spacecraft flying about 220 kilometers apart in a polar orbit 500 kilometers above the Earth. The twin GRACE satellites were launched from Plesetsk Cosmodrome, Russia on a Rockot (SS-19 + Breeze upper stage) launch vehicle, on March 17, 2002.
The two satellites (nicknamed "Tom" and "Jerry") constantly maintain a two-way microwave-ranging link between them, as well as measuring their own movements using accelerometers and star cameras and by listening to GPS satellite broadcasts. All of this information is then downloaded to ground stations. The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations.

In figure 1, Sea surface temperature plays a vital role in the behavior of the Earth's climate and weather. It is both a causal factor and a resulting effect of complex interactions of natural forces on Earth. NASA not only measures sea surface temperature from space using powerful scientific instruments, but it also studies temperature processes in advanced computer models, as shown in this animation.

In figure 2, Aerial view of Goddard Space Flight Center

In figure 3, The MODIS instrument acquired this image of melt ponds on Greenland's western coast in June, 2006. The ponds appear as dark blue dots on the aqua blue background.

In figure 4, Orbiting Twins - The GRACE satellites

In figure 5, Oceans change. Beyond merely the sloshing of waves that we all recognize along the beaches of the world, sea level describes a complex array of conditions, from chemistry to temperature to changes in the shape of the basins that hold the world's water. In this visualization, we look at changes in sea level measured from space using data from the TOPEX/Poseidon and Jason satellites.

In figure 6, This MODIS Terra image, acquired August 23, 2006, shows the southern portion of Greenland. The Greenlandic ice cap covers about 80% of the island's surface.

In figure 7, Melt water puddles on the Greenland ice sheet and drains through cracks to the surface below. This water lubricates the underlying bedrock, causing the ice to flow faster toward the sea.