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Earth Observations from Space: The First 50 Years of Scientific Achievements
7
Cryosphere
No other single technological development has revolutionized cryosphere research as much as satellite observations. Most of Earth’s frozen regions are remote and access by land or seas involves often great risks; therefore, conducting in situ observations is logistically difficult and expensive. The synoptic view from satellites increases the data coverage by multiple orders of magnitude, and access is no longer restricted by seasons.
Understanding changes to ice sheets, sea ice, ice caps, and glaciers is important for understanding global climate change and predicting its effects. In particular, “shrinking ice sheets” and their contribution to sea-level rise were identified as the third most significant “Breakthrough of the Year” for 2006 according to Science magazine1:
Glaciologists nailed down an unsettling observation this year: The world’s two great ice sheets—covering Greenland and Antarctica—are indeed losing ice to the oceans, and losing it at an accelerating pace. Researchers don’t understand why the massive ice sheets are proving so sensitive to an as-yet-modest warming of air and ocean water. The future of the ice sheets is still rife with uncertainty, but if the unexpectedly rapid shrinkage continues, low-lying coasts around the world—including New Orleans, South Florida, and much of Bangladesh—could face inundation within a couple of centuries rather than millennia.
—Science (2006)
This breakthrough is one of the examples of major accomplishments in cryosphere science presented in this chapter. Other examples include the change in seasonal snow cover (see Chapter 6), detection of earlier spring thaw and associated lengthening of the growing season (Chapter 9, Box 9.4), the new perspective of the dynamic ice streams in Antarctica, the decrease in sea ice in the Arctic, and the change in glacier extent.
NONUNIFORM AND DYNAMIC ICE STREAMS IN ANTARCTICA
Field exploration of the Antarctic ice sheet is time consuming, logistically intensive, costly, and sometimes dangerous. Prior to satellite observations, spatial coverage was very sparse: information on the Antarctic ice sheet was acquired slowly over the years by numerous surface traverses (Figure 7.1).
In 1997, Radarsat data were used to create the first complete radar-based map of Antarctica (Figure 7.1). Analyses of radar images from various sensors through the years have enabled detailed measurements of surface velocity, and in turn these measurements have enabled calculation of strain rates and basal shear at the bed. For the first time, satellite data revealed the extent of the ice stream network, leading to the discovery of new ice streams and the ice stream tributaries (Joughin et al. 1999). For example, satellite-based measurements of surface velocity within Antarctic ice streams reveal a complex pattern of flow not apparent from previous measurements (Bindschadler et al. 1996). Furthermore, satellite observations led to the discovery that ice streams move at variable speeds, resulting in a more dynamic picture than the previously held view that ice sheets move at a constant velocity (Figure 7.2; Bindschadler and Vonberger 1998). Satellites provide improved data collection methods to increase data density and to improve velocity estimates substantially.
1
The first-ranking breakthrough of the year was the proof of the Poincaré conjecture, a long-standing problem in mathematics. The second-ranking breakthrough was in the area of paleogenomics: the sequencing of Neanderthal DNA proves that Neanderthal evolution diverged from modern humans at least 450,000 years ago.
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Earth Observations from Space: The First 50 Years of Scientific Achievements
FIGURE 7.1 Spatial coverage of data from Antarctica. (a) Surface transverses since the 1957-1958 International Geophysical Year. SOURCE: National Snow and Ice Data Center, University of Colorado. (b) Airborne surveys, over snow radio-echo sounding (RES), seismic surveys, gravimetric surveys, and ice-core missions since the 1957-1958 International Geophysical Year. SOURCE: BEDMAP consortium. (c) Satellite coverage. SOURCE: John Crawford, Canadian Space Agency, National Aeronautics and Space Administration, Jet Propulstion Laboratory.
ACCELERATING ICE SHEET FLOW IN ANTARCTICA AND GREENLAND
One of the central questions in climate change and cryosphere research is how the warming climate will affect the ice sheets because the amount of continental ice and melt water entering the ocean strongly contributes to the change in sea level. Glaciologists and climatologists have long been debating whether a warming climate would decrease ice mass. However, early research focused on how increased melting would be offset by increased precipitation. The ice mass balance and thus its contribution to sea-level rise was originally thought to be determined by the difference between melting and precipitation.
Satellite observations have revolutionized this thinking by allowing scientists to monitor precise ice sheet elevation, velocity, and overall mass. Satellite images revealed that in fact the overall mass is declining (Luthcke et al. 2006). In addition to observing great variability in the ice stream velocity over time and space, satellite images revealed that the overall velocities of the ice streams in Antarctica and Greenland have increased during the past decade, resulting in more ice flow into the ocean (Bindschadler and Vonberger 1998, Joughin et al. 2001).
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FIGURE 7.2 Velocity variations in Antarctica ice streams. SOURCE: Binschadler et al. (1996). Reprinted from the Annals of Glaciology with permission of the International Glaciological Society, copyright 1996.
The discoveries of accelerating ice loss from Antarctica and Greenland and the importance of ice sheet dynamics in their mass balances rest on measurements by a suite of satellite and airborne sensors using novel techniques (Bindschadler et al. 1996, Chen et al. 2006a, Kerr 2006, Luthcke et al. 2006, Rignot and Kanagaratnam 2006). These discoveries are possible because of decades of optical and radar images, laser and radar altimeters, and more recently the National Aeronautics and Space Administration’s (NASA) Gravity Recovery and Climate Experiment (GRACE) mission, which measures ice mass directly through its gravitational pull. In addition, airborne laser altimeter data show thinning of ice near the coastline, radar data show faster flow, Landsat data show retreat of the grounding line,2 and the Moderate Resolution Imaging Spectroradiometer (MODIS) data show calving of large icebergs. Warming ocean waters seem to have increased calving of the ice shelves, thereby allowing the ice sheet’s outlet glaciers to flow more quickly (Box 7.1). Glaciers in Greenland have also increased in velocity, perhaps from increased basal lubrication by meltwater penetrating from the surface. These new discoveries indicate that ice stream dynamics (the balance between the forcing, such as ice thickness and surface slope, and the resistance, such as internal stiffness) are the primary drivers of rapid sea-level change instead of the balance between melting and precipitation.
The ability to estimate the overall mass of ice sheets is a remarkable accomplishment of satellite observations. Numerous techniques, including radar images, measurements of surface elevation from laser altimeters, and GRACE’s gravity data, now show that both Greenland and Antarctica have been losing ice over the past 5 to 10 years. From 2003 to 2005, Greenland lost more than 155 gigatons3 per year at lower elevations and gained about 54 gigatons per year at higher elevations, with most of the losses occurring during summer (Chen et al. 2006b, Luthcke et al. 2006, Rignot and Kanagaratnam 2006, Wahr et al. 2006). In Antarctica the gravity data show mass losses of 70-200 km3 per year (60-160 gigatons per year). Most of the loss is from West Antarctica, with East Antarctica in approximate balance (Figure 7.3).
DECLINING ARCTIC SUMMER SEA ICE
Just as miners once had canaries to warn of rising concentrations of noxious gases, researchers working on climate change rely on arctic sea ice as an early warning system.
—Arctic Climate Impacts Assessment (2004)
For many reasons, observing trends in sea ice reliably has been possible only with the advent of satellite observations. Navigating the remote and frozen seas off Antarctica or in the Arctic to obtain in situ measurements of sea ice extent is treacherous, and sea ice extent is highly variable in time and space due to wind advection and localized melting. Before satellite observations became available, spatial coverage of sea ice was monitored by tracking the location of the ice edge from ships. Because the ice edge is moving with winds and ocean currents, it is not a robust indicator of basin-scale sea ice extent. Thus, accurate and quantitative interannual comparisons of the basin-scale ice coverage became only possible with the availability of the synoptic view from satellites.
Sea ice has been monitored continuously with passive microwave sensors (Electrically Scanning Microwave Radiometer [ESMR], Scanning Multichannel Microwave Radiometer [SMMR], Special Sensor Microwave/Imager [SSM/I], and Advanced Microwave Scanning Radiometer-Earth Observing System [AMSR-E]) since 1979. Not limited by weather conditions or light levels, they are particularly
2
The location along the coast where ice is no longer supported by the ground and where it begins to float.
3
1 gigaton = 1 billion metric tons.
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BOX 7.1
Ice Shelf Collapse
The observation of the collapse of the Larsen B Ice Shelf was astonishing in the sheer dimension and abruptness of change observed via satellite, and it alerted the Earth science community due to its potential implications for sea-level rise (Figure 7.3). The dynamics contributing to the collapse were documented by various satellites: the thinning of the ice shelf toward the coast by satellite altimetry, the accelerated flow by the interferometric synthetic aperture radar (InSAR), the retreat of the grounding line by Landsat, and the calving of the icebergs by MODIS.
FIGURE 7.3 Collapse of the Larson B Ice Shelf in western Antarctica, January-March 2002. Two thousand square kilometers of the Larsen Ice Shelf disintegrated in just 2 days. SOURCE: National Snow and Ice Data Center, University of Colorado.
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well suited for monitoring sea ice because of the strong contrast in microwave emission between open and ice-covered ocean. The long-term 35-year data set from the passive microwave sensors has enabled us to produce trend analyses beyond the strong interannual variability of sea ice. Recent estimates indicate that Arctic sea ice extent decreased by approximately 7.4 percent from 1978 through 2003, while multiyear ice area has decreased by approximately 7.0-11.0 percent per decade (Comiso 2002, Johannessen et al. 2004; Figure 7.4). The past several years have been nothing short of extraordinary (NRC 2007c). Since 2000, record summer ice minima have been observed during 4 out of the past 6 years in the Arctic (Stroeve et al. 2005). Moreover, most recent indications are that winter ice extent is now also starting to retreat at a faster rate, possibly as a result of the oceanic
FIGURE 7.4 Deviations in monthly sea ice extent for the northern and southern hemispheres from November 1978 through December 2004, derived from satellite passive-microwave observations. The Arctic sea-ice decreases are statistically significant, with a trendline slope of −38,200 ± 2,000 km2/year, and have contributed to much concern about the warming Arctic climate and the potential effects on the Arctic ecosystem. The Antarctic sea ice increases are also statistically significant, although at a much lower rate of +13,600 ± 2,900 km2/year. The northern hemisphere plot is extended from Parkinson et al. (1999), and the Southern Hemisphere plot is extended from Zwally et al. (2002). SOURCE: Courtesy of Claire Parkinson and Donald Cavalieri, NASA Goddard Space Flight Center, as updated from Parkinson et al. (1999) and Zwally et al. (2002).
warming associated with a thinner, less extensive ice cover. These observations of shrinking Arctic sea ice are consistent with climate model predictions of enhanced high-latitude warming, which in turn are driven in significant part by ice-albedo feedback4 (Holland and Bitz 2003). In contrast to the Arctic, no clear trend in the extent of Antarctic sea ice coverage has been detected.
Over the past few years, there have been a growing number of reports forecasting sea ice conditions, and these reports are based entirely or mostly on data from satellites. For example, the Arctic Climate Impact Assessment (ACIA 2005) concluded that continued reductions in Arctic sea ice might soon lead to a seasonally ice-free Arctic and increased maritime traffic because shipping routes through the Arctic Ocean are much shorter than routes through the Panama or Suez Canals. However, there is some evidence that a reduction in the ice cover will be accompanied by greater interannual variability, at least in certain regions (Atkinson et al. 2006); the potential combination of increased maritime traffic, high interannual variability in the ice cover, and regional variations will require improved regional sea ice forecasts for maritime operators.
GLACIER EXTENT AND POSITION OF EQUILIBRIUM LINE
The study of glacier regimes worldwide reveals widespread wastage since the late 1970s, with a marked acceleration in the late 1980s. Remote sensing is used to document changes in glacier extent (the size of the glacier) and the position of the equilibrium line (the elevation on the glacier where winter accumulation is balanced by summer melt; König et al. 2001). Since 1972, satellites have provided optical imagery of glacier extent. The synthetic aperture radar (SAR) is used to study zones of glacial snow accumulation and ice melt to determine climate forcing, and laser altimetry is used as well to measure change in glacier elevation. For example, a study in the Ak-shirak Range of the central Tien Shan plateau used aerial photographs in the 1970s, along with the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery from 2001, to document a reduction in glacier area of 20 percent between 1977 and 2001 (Figure 7.5; Khromova et al. 2003).
Because glaciers respond to past and current climatic changes, a complete global glacier inventory is being developed to keep track of the current extent as well as the rates of change of the world’s glaciers. Coordinated by the National Snow and Ice Data Center, the Global Land Ice Measurements from Space project is using data from ASTER and the Landsat Enhanced Thematic Mapper to inventory about 160,000 glaciers worldwide. This effort will likely
4
Ice-albedo feedback is a positive feedback loop whereby melting sea ice exposes more seawater (of lower albedo, or less reflective), which in turn absorbs heat and causes more sea ice to melt.
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FIGURE 7.5 Location and changes of the Ak-shirak glacier system, 1943-2001. (a) ASTER image for September 14, 2001. (b) Location map. In the other insets the green lines indicate glacier outlines in 1943: (c) decrease in glacier size through climate change and direct anthropogenic impact, (d) decrease in size of a surging glacier and appearance of new glaciers, (e) increase in area of outcrops and in the perimeters of water divides between glaciers, and (f) disappearance of former small glaciers. SOURCE: Khromova et al. (2003). Reprinted with permission from the American Geophysical Union, copyright 2003.
result in major scientific advances in the near future with important ramifications for climate research. As for the other examples of accomplishments listed in this chapter, these measurements and the resulting trend analyses are important indicators of climate change and exemplify the value and importance of long-term data sets for understanding the complex climate system.