ES 767 Quaternary Geology, Emporia State University
Final Presentation, Fall Semester 2013

Remote Sensing Techniques Applied to the Western Antarctic Ice Sheet

by Marco Allain

Western Antarctica image from Wikimedia commons

Introduction
Western Ice Sheet
Landsat and ICESat
Applied Remote Sensing
Tracking Change
References


Introduction

West Antarctica’s ice sheet is an issue of active research and societal concern. Its instability has the potential to cause significant economic impact resulting from an increase in global sea level. West Antarctica is the largest remaining ice-filled marine basin on Earth. Antarctica’s ice sheet is drained by fast-moving ice streams that extend far into the ice-sheet interior. The West Antarctic ice sheet has lost nearly two thirds of its mass, since the peak of the last ice age nearly 20,000 years ago. This is a volume sufficient to raise sea level by ten meters. The overall state of the Antarctic Ice Sheet is difficult to assess. Although the Eastern Antarctic ice sheet is generally considered to be stable, the marine nature of the western sheet has made it the focus of concern for many years (Bamber, 2009).

The West Antarctic Ice Sheet is riddled with deep crevasses and flowing ice. Changes like these are difficult to monitor from the ground because of the harsh conditions. Measurements taken with bore-hole drilling, and other methods, are often limited because of the inaccessible terrain. Therefore Landsat data is often utilized in a variety of ways to make more information available for determining the ice sheet’s behavior. Landsat has high spatial resolution which allows crevasses to be tracked individually over time (Scambos et al., 2004). Landsat data enables the use of natural markers which allow more velocity data to be collected. It also concentrates the measurements in the most rapidly moving and most dangerous areas.

Western Antarctic Ice Sheet image from NASA imagery database

Flowstripes are another surface feature mapped by satellites. Flowstripes are longitudinal surface structures. They develop as relatively wide flow stripes within glacier flow units. Sometimes flowstripes are narrow and occur where there is convergent flow around glacier confluence zones. Flowstripes and other surface features formed by rapid ice flow are being used to uncover the history of the Ross Ice Shelf using satellite imagery. Landsat data has been used to create useful maps for scientific research. Landsat data has been combined with past surveys of the latitude, longitude, and elevation of known locations around the Antarctic continent to create a library of image control points. With the help of control points and Landsat data, glaciologists intend to understand the history of the Antarctic ice sheet so they can better predict how the ice sheet may respond to climate changes in the future (Glasser and Gudmundsson, 2012).

The largest single controller of rapid sea level changes is the Western Antarctic Ice Sheet. However, glaciologists cannot yet predict how the ice sheet will respond to climatic warming. The climate record is preserved in certain parts of ice sheets and ice caps. In the ice masses there are thin layers of snow deposited on top of each other annually and are mostly undisturbed. Because the snow is undisturbed, ice cores of the area can help determine the atmosphere of the past. The ratios of oxygen isotopes, such as heavy oxygen and light oxygen, indicate air temperatures. Trapped air bubbles also provide clues. Carbon dioxide levels get preserved in the trapped air. Low carbon dioxide levels imply glaciated conditions, whereas high levels suggest a lack of glaciation (Hambrey and Alean, 2004).

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Western Ice Sheet

The Antarctic Ice Sheet comprises the East Antarctic Ice Sheet (EAIS) and the West Antarctic Ice Sheet (WAIS). They are separated by the Transantarctic Mountains. Nearly the entire land surface of Antarctica is covered by ice (Bindschadler and Scambos, 1992). The mean thickness in the EAIS is more than 2600 meters and it contains the majority of Antarctica’s ice. It is more than 5 times the size of the WAIS which measures around about 1800 meters in thickness. Unlike the EAIS, the WAIS is considered a marine ice sheet since it resides on bedrock below sea level. WAIS was likely a sea floor at some point in its history. As stated earlier, the WAIS is marine in nature and therefore interacts regularly with warm ocean currents (Bamber, 2009).

The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into Antarctica’s Amundsen Sea. Isostatic depression from the weight of the ice has caused the underlying bedrock to sink. It is estimated that the volume of the Antarctic ice sheet is about 25.4 million kilometers squared.Satellite monitoring has revealed the WAIS to have the largest loss of ice across the entire continent of Antarctica. Predictive models suggest that weakening of this area could lead to the loss of the entire WAIS. This would raise sea level by nearly 17 meters or 55 feet (Scambos et al., 2004). Warming air and ocean temperatures have already led to the dramatic breakup of numerous ice shelves. One of the more recent and spectacular breakups occurred in early 2008, with the disintegration of the Wilkins Ice Shelf.

Image from the National Snow and Ice Data Center-Colorado

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Landsat and ICESat

Landsat is a collection of 8 separate satellite missions. In 1965 the director of the U.S. Geological Survey (USGS) proposed the idea of a remote sensing satellite program to gather facts about Earth’s natural resources. The program was mentally conceived in 1966. But while weather satellites had been monitoring Earth’s atmosphere since 1960 and were largely considered useful, there was no demand for space-borne terrain data until the mid-1960s. Due to a lack of demand, there were many hurdles for the Landsat program to overcome. When Landsat 1 was proposed, it met with intense opposition from the Bureau of Budget and those who argued high-altitude aircraft would be the fiscally responsible choice for Earth remote sensing. Another complaint from the Department of Defense claimed that a civilian program such as Landsat would compromise the secrecy of their reconnaissance missions, and raise geopolitical concerns about photographing foreign countries without permission (NASA, 2013). It was not until 1970 that NASA had a green light to build a satellite. Only two years later, Landsat 1 was launched. Landsat’s launch heralded a new age of space-based remote sensing.

Table 1. History of Landsat Missions
Dates from NASA database (2013)
Landsat 1 Launched July 23, 1972 and terminated operations in 1978
Landsat 2 Launched January 22, 1975 and terminated operations in 1981
Landsat 3 Launched March 5, 1978 and terminated operations in 1983
Landsat 4 Launched July 16, 1982 and terminated operations in 1993
Landsat 5 Launched March 1, 1984 and decommissioned by the USGS in 2012
Landsat 6 Launched October 5, 1993 but failed to reach orbit
Landsat 7 Launched April 15, 1999 and still functioning
Landsat 8 Launched February 11, 2013 and functioning

The second Landsat was considered merely an experimental project and was operated by NASA. Landsat 2 carried the same sensors as Landsat 1: the Return Beam Vidicon (RBV) and the Multispectral Scanner System (MSS). In an attempt to commercialize an operational Landsat project, responsibility was slated to shift from NASA (a research and development agency) to the National Oceanic and Atmospheric Administration (NOAA). NOAA was already the agency charged with operating the weather satellites. NOAA assumed control of Landsat missions beginning with Landsat 3.

In addition to the Multispectral Scanner System (MSS) instrument, Landsat 4 and Landsat 5 carried a sensor with improved spectral and spatial resolution. This new instrument was known as the Thematic Mapper (TM). It allowed the new satellites to see a wider portion of the electromagnetic spectrum. They could also see the ground in greater detail. The Landsat 4 TM instrument had seven spectral bands. Data was collected from the blue, green, red, near-infrared, middle infrared and thermal infrared portions of the electromagnetic spectrum. After only one year in orbit, Landsat 4 lost the use of two of its solar panels. Landsat 4 was kept in orbit for routine command and tracking data until 2001. Following Landsat 4, NASA launched Landsat 5. It was the agency’s last originally mandated Landsat satellite. Landsat 5 was designed and built at the same time as Landsat 4 and carried the same MSS and TM instruments. The Earth Observation Satellite Company owned and launched Landsat 6. The satellite failed at launch after not reaching the velocity necessary to obtain orbit. Landsat 6 did not achieve orbit because of a ruptured manifold.

The Enhanced Thematic Mapper Plus (ETM+) was designed for Landsat 7 and Landsat 8. It replicates the capabilities of the Thematic Mapper instruments on Landsats 4 and 5. The ETM+ also includes additional features that make it a more versatile and efficient instrument for global change studies, land cover monitoring and large area mapping. The data quality, consistent global archiving scheme, and reduced pricing of Landsat 7 greatly increased the number of Landsat data users. In 2008 the USGS made all Landsat 7 data free to the public. All Landsat MSS and TM data were made free in January 2009. After the launch of Landsat 7, the program has been jointly managed by the U.S. Geological Survey and NASA (NASA, 2013). The name of the most recent satellite changed from the Landsat Data Continuity Mission to Landsat 8, and was launched in 2013.

Landsat 8 satellite in orbit

Ice, Cloud, and land Elevation Satellite (ICESat) is a part of the Earth Observing System mission for measuring ice sheet mass balance, cloud and aerosol heights. Land topography and vegetation characteristics are also primarily observed by ICESat. From 2003 to 2009, the ICESat mission provided multi-year elevation data needed to determine ice sheet mass balance as well as cloud property information. It also provided topography and vegetation data around the globe, especially polar-specific coverage of the Greenland and Antarctic ice sheets. ICESat 2 is the 2nd generation of the laser altimeter mission and is scheduled for launch in 2016 (NASA, 2013).

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Applied Remote Sensing

The United States Geological Survey (USGS) has recently composed The Landsat Image Mosaic of Antarctica (LIMA). It is the first true-color, high-spatial-resolution image of the seventh continent. LIMA was constructed from nearly 1100 individually selected Landsat images. Each image was orthorectified and adjusted for geometric, sensor and illumination variations. Mosaicing was done to avoid clouds in the images and it produced a high quality, nearly cloud-free image database. Scenes range from 1999 to 2003. Multiple years had to be used in order to find enough cloud free dates. LIMA is useful for discrimination between snow and bare ice; reflectance variations within bright snow; recovered reflectance values in regions of sensor saturation, and detecting subtle topographic variations associated with ice flown (Bindschadler, et al., 2008). LIMA is viewable and individual scenes are downloadable at visit LIMA

LIMA Western Ice Sheet scene downloaded from the USGS database

Tracking glacial change has been the primary usage of remotely sensed data in glaciology studies. Generally, predicting future glacier behavior requires detailed measurements of surface elevation and ice thickness. Ice coring is the primary method to accomplish this but the remoteness of the western ice sheet often prevents direct coring. For this reason, information derived from satellite laser altimeter data acquired by NASA’s ICESat is often used in place of field measurements. ICESat’s radar altimetry data show all surveyed Western Antarctic glaciers to have thinned rapidly during the 1990’s and early 2000’s. Imagery shows many areas with changes greater than one meter per year and a pattern of rapid thinning (Thomas et al., 2004). In all cases, glacier thinning dominates, indicating that thinning by longitudinal stretching exceeds advection thickening.

Over the past half-century, mean Antarctic air temperatures have doubled to four degrees Celsius from absolute zero Kelvin. Unlike the WAIS itself, floating ice shelves are responding rapidly to climate warming in the Antarctic Peninsula (Alley and Whillans, 1991). Landast 7 Enhanced Thematic Mapper (ETM+) and ICESat are used to map the velocity of moving and collapsing ice shelves. Such satellite imagery suggests rapid speed increases for flowing glaciers, and many western ice masses are actually surging.

Many critics of the use of remotely sensed data in glaciology studies cite inaccuracy as their primary concern. Errors from the ICESat image feature-tracking algorithm come from two sources: image mis-registration and noise in the correlation algorithms. Under clear sky conditions, ICESat measurements are accurate to ±20 cm and thin cloud cover can cause elevation errors of up to two meters. However, the majority of the error here comes from assuming constant slope between the profiles, so is therefore user related. Despite partial errors, the scope and potential for additional data gathering can make remotely sensed data a valuable tool in glacial studies.

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Tracking Change

Recent satellite measurements of the WAIS raise further concerns about the future contribution of Antarctica to sea level rise. Recent measurements suggest that the western part of the ice sheet is experiencing nearly twice as much warming as previously thought (UT Austin, 2009). Even without generating significant mass loss directly, surface melting on the WAIS could contribute to sea level indirectly, by weakening the West Antarctic ice shelves that restrain the region's natural ice flow into the ocean. Researchers consider the WAIS especially sensitive to climate change (Cole and Vinas, 2013). Since the base of the ice sheet rests below sea level, it is vulnerable to direct contact with warm ocean water. WAIS melting contributes to signifigant sea level rise each year (UT Austin, 2009).

Western Antarctica demands global attention from the scientific and policy making community. The WAIS is the second leading cause of global sea level rise behind only the receding Greenland ice sheet. Due to the remoteness and harsh conditions of Antarctica, practical means of monitoring the WAIS must be utilized. ICESat and Landsat data have been valuable to the glaciologists and climate scientists analyzing the ice sheets. Using satellite remote sensing techniques to support ground research teams and small format aerial photography can help better understand the Western Antarctic Ice Sheet and its response to global climate change.

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References

Website created by Marco Allain on September 22, 2013

ESU Earth Science Department homepage

For more information on the Western Antarctic Ice Sheet, visit NASA

For the ES 767 Quaternary Geology course homepage, visit Ice Age Environments
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