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A distant look at the cryosphere
Adu. Space Res. Vol. 5, No. 6, pp.263-274,
1985 Printed in Great Britain. All rights reserved.
0273-I177/85 $0.00 + .50 Copyright © COSPAR
A D I S T A N T L O O K A T THE CRYOSPHERE Charles Swithinbank British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET, U.K.
ABSTRACT Ninety nine per cent of all the fresh water on the surface of the Earth is in the form of ice. Observations from space have revealed more about the ice than about most other parts of the environment because at the dawn of the satellite era, less was known about it. The cryosphere includes all forms of naturally occurring ice but here we review what space science has done for knowledge of glaciers and ice sheets. Whereas in global terms the cryosphere exists as a response to climate, over large areas it controls climate. While imaging spacecraft systems have proved easiest to interpret, microwave sensors with poor spatial resolution are able to distinguish transient and stable surface features that are invisible to the eye. Imaging radars quite effectively describe sea ice, but precision altimetry is the only practicable method for monitoring changes in the total mass of ice on land. INTRODUCTION The cryosphere includes all forms of naturally occurring ice. In the context of space observations for climate studies we are chiefly concerned with snow cover on land, sea ice, ice sheets and smaller glaciers. Most of us and certainly all novice skiers have taken a close look at the cryosphere. But distance lends perspective. Observations from space have contributed substantially to our knowledge of the cryosphere because at the dawn of the satellite era, less was known about it than about most other parts of the environment. Whereas in global terms the cryosphere exists as a response to climate, over large areas it cnntrols climate. The high albedo of ice and snow interacts with other components to produce feedbacks that determine the overall sensitivity of the climate system. Ice sheets represent enormous planetary heat sinks whose effectiveness varies with their areal extent and interior surface elevation /I/. The biggest ice sheet that of the Antarctic is intimately linked to the circulation of the ocean and the depth of the world's seas. Modelling the cryosphere is difficult because the freezing point of water lles about half way between the highest and lowest temperatures commonly found on the Earth. Thus ice is the most changeable of all physical constituents of the Earth's surface. At the seasonal maximum, snow and ice together cover about forty per cent of the land area and fifteen per cent of the total surface area of the globe. Excluding water stored underground, ninety nine per cent of the fresh water on the surface of the Earth is in the form of ice, so ice is the principal component of the world water balance. Response times to changing weather and climate vary from days in the case of seasonal snow cover to months in sea ice, decades in valley glaciers, centuries in ice caps, and millennia in ice sheets. GLACIERS AND ICE SHEETS An accompanying review paper by Kfinzi /2/ addresses snow cover, a n d another paper by Cavalieri and Zwally /3/ deals with sea ice, so here we discuss the remaining, and in terms of mass, the principal element of the eryosphere - glaciers and ice sheets. Permanent ice covers about ten per cent of the land surface area of the Earth. Table i summarizes estimates of area and volume. Because new glaciers are still being discovered on satellite images, particularly in poorly-mapped parts of the world, it is likely that the area of mountain glaciers will be revised upward during the next decade. One of the principal objectives of the International Hydrological Decade 1965-74 (IHD) was to reach an understanding of the world water balance. Among the programmes initiated by the IHD but still unfinished are a World Glacier inventory and the development of methods for surveillance of glacier fluctuations /7,8,9/. Because fluctuations can only be measured with respect to some datum, the programmes are necessarilylinked. Although progress with respect to glaciers outside the polar regions is already substantial, very little has been done to 263
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determine the exact dimensions or to record variations of the Greenland and Antarctic ice sheets. The principal reason is that some of the problems involved have seemed to be intractable. Moreover, because the major stimulus for a world glacier inventory came from people concerned with water resources, polar ice sheets have been considered of less significance to mankind than the glaciers of mare populated countries. Indeed, funds devoted to the preparation of glacier inventories appear to have been inversely proportional to the size of the glaciers described /i0/. TABLE i.
Area and Volume of G l a c i e r s
Area (km 2)
Per cent
Volume (km 3)
Per cent
Mountain glaciers Greenland Antarctica
500,000 1,725,000 13,918,000
3.1 10.7 86.2
240,000 2,600,000 30,110,000
0.7 7.9 91.4
Total
16,143,000
i00.0
32,950,000
i00.0
Source /4/ /5/ /6/
THE CRYOSPHERE AND CLIMATE There is a growing awareness of the climatic importance of ice sheets /11,12,13,14,15/. Mercer predicted that "One of the warning signs that a dangerous warming trend is under way in Antarctica will be the breakup of ice shelves on both coasts of the Antarctic Peninsula, starting with the northernmost and extending gradually southward". Doake /16/ has recently assembled evidence of a progressive retreat of the ice shelves in this very area. A total of more than 5000 km 2 of ice shelf is now known to have disintegrated since the nineteen forties. Bentley /15/, however, suggests that the ice sheet, if it is changing at all, is growing rather than shrinking. While it seems to be agreed that the West Antarctic ice sheet is probably the part most sensitive to climatic change /17,18,19/, there is as yet no general agreement on the sign, let alone the magnitude, of the response to a given change of climate /20/. A warming of climate might increase snow accumulation, raise the surface of the ice sheet, and thus lower sea level /14/; but if melting at the margins should exceed the effect of increasing snowfall, the opposite would occur /ii/. Unfortunately, so little is understood about the likely net response of the great ice sheets to climatic change that is it not now possible to select truly representative experimental areas for detailed studies on the ground. Moreover, there are records of opposite glacier responses within neighbooring drainage basins /21/, so really representative sites may never be found. While the amount of fresh water stored in the polar regions has aroused little interest because of its remoteness from the pressing water problems of populated continents, changes in polar ice sheets, through their effect on sea level, could have a greater impact on mankind than all climateinduced changes in lower latitudes. Any change in the mass of ice on land will lead to a change in global sea level. In view of the importance of understanding climatic change there is an urgent need to detect even a small mass imbalance in the polar regions so that its long-term consequences can be considered /22,23/. Paradoxically, the largest ice mass on Earth is in terms of precipitation also the world's largest desert. Annual precipitation over more than 6,000,000 km 2 of the Antarctic ice sheet averages less than i00 mm per annum. Over a further 5,000,000 km 2 it averages between i00 mm and 200 mm. The average snow accumulation rate for the entire continent is 15.6 ± 2.3 g cm 2 a -1, representing a total of about 2,000 km 3 of water. Although error estimates in our present state of knowledge are large (± 30%) about the same volume of icebergs (2,000 km 3) may calve into the sea from the margins of the ice sheet /24/. IMAGING SYSTEMS FOR THE CRYOSPHERE Manned Spacecraft No manned spacecraft has flown in a sufficiently high-inclination orbit to make an important contribution to knowledge of the great ice sheets. However, every manned spacecraft from the Soviet Salyut and US Mercury series onwards has returned magnificent photographs of ice caps and mountain glaciers in lower latitudes. These represent an invaluable archive of the linear extent of glaciers at the time and have led to the discovery of many new glaciers in poorly-mapped regions. On-board observers, moreover, have been able to recognize transient phenomena such as avalanches, glacier floods, surging glaciers, and the calving of icebergs. These observations were of immediate interest to people in the vicinity and w o u l d have been of significantly less value if new features or changes had gone unrecognized until photographs taken from the spacecraft had subsequently been analysed on the ground. Kotlyakov /26/ reports that for this reason, Soviet cosmonauts undergo formal training in glaciology. Part of the course involves flights in TU-134 aircraft over the Pamirs in Tadzhikistan, where there have been subsatellite experiments involving fixed-wing aircraft and helicopters /27/.
A Distant Look at the Cryosphere
265
Vidicon systems and scanning radiometers. The power of observing significant snow and ice features from satellites was first demonstrated when television cameras with relatively modest spatial resolution, mounted on the TIROS-2 weather satellite, monitored the break-up of sea ice in 1961 /28/. It rapidly became clear that one of the great advantages of remote sensing from satellites is the ability to obtain almost instantaneously a synoptic view over large areas. Observations in many other spectral regions have been made since the visible wavelength sensors of early satellites. These have made it possible to monitor repetitively over extensive areas many different properties of snow and ice. Observations from the Nimbus-3 infrared radiometer in the 0.7-1.3 ,m spectral region revealed areas in which active melting was occurring on ice and snow and thus decreasing the reflectance in this band. The spectral range of environmental satellite sensors has progressively increased throughout the TIROS, Nimbus, ESSA, Kosmos, Meteor, ITOS, and NOAA series, such that imaging systems with different engineering designs have now operated in many bands between 0.2 ~m and 50 Bm. Despite the low resolution of early imaging systems, icebergs were observed calving or calved from the Antarctic ice sheet /29,30/ and drift tracks of individual icebergs were monitored over periods of several years /31/. Several extensive recurring polynyas were found, revealing that katabatic winds from the ice sheet follow distinct channels and consistently force pack ice away from the shore even in the depth of winter /32,33,34/. Clouds were a problem in interpreting imagery because their reflectance characteristics are similar to those of ice and led to confusion in interpreting the extent of ice. Provided that the user was interested in a weekly or monthly rather than a synoptic view, the effect of clouds could be minimized by combining a series of consecutive images to produce what was known as a composite minimum brightness (CMB) chart /35/. The resolution of imaging systems improved during the nineteen sixties but only with the launch in 1972 of the NOAA-2 satellite could it be said that unmanned satellites were beginning to yield data on glaciers and ice sheets that was useful for other than meteorological purposes. NOAA-2 carried the so-called very high resolution radiometer (VHRR) which yielded images in the visible and thermal infrared bands w~th a nadir resolution of about 1 km. An image of the Ross Sea area recorded in January 1973 showed what may have been the first artifact seen in polar satellite pictures a 300-m-wide channel broken by US coast Guard icebreakers through the fast ice in McMurdo Sound /36/. An unexpected discovery on the infrared images was the extent to which gravity winds from outlet glaciers flowing through the Transantarctic Mountains were able to break down the surface temperature inversion over the Ross Ice Shelf /36/. Swith~nbank /37/ was disconcerted to find that Latady Island was 50 km longer than it was depicted on the latest maps. He speculated that this might be the last time in the long history of exploration that any coastline on earth would b e found so far from where it was supposed to be. Subsequent changes to the map of ~tim~sm was premature /38/. Even at small scales, m a p s of the ary in places that Wiesnet /39/ was able to contribute new data examining mosaics of NOAA-5 visible and infrared VHRR images. an image mosaic of Antarctica at a scale of 1:5,000~000; there purposes this will be a more useful product than the best of iced.
Zwally
and
Gloersen
/45/
For the earth scientist viewing the cryosphere from space, ~o often. This difficulty was partially overcome by the e Radiometer (ESMR) on Nimbus 5 launched in 1972. Thin crowave (1.55 cm) emissions which can pass through cloud. o n g the satellite's path, successive strips of images were ~_ mosaic of the whole surface of the Earth from pole to pole. element) size was about 30 x 30 km, the resolution was adequate lobal sea-lee distribution /28,41,42,43/. While the amount of :ould be deduced from the computer products was astonishing, the is of emissivity over the inland ice sheets allowed extrapolation ill sparse network of observations on the ground to the whole area se the mean annual temperature and the accumulation rate of dry the grain sizes upon which the microwave emission depends, these for the main features of the patterns observed from Greenland and particle sizes normally encountered, most of the radiation has a layer up to i0 m in thickness. A marked increase in emission allows the recognition of regions affected by sum~mer melting. found remarkably low brightness temperatures over the dry desert ~nland ice sheets, and were able to detect ice-stream mnd from major outlet glaciers. At the time, some of kind of ground or aircraft observations. Zwally /46/i mce temperature on the one hand and snow accumulation on the accumulation rate to an accuracy of the order of 20 lies /47,48,49/ have refined the understanding of these data available from the Scanning Microwave Spectrometer The emerging possibility not simply of detecting quite v the great ice sheets but also of understanding them can
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C. Swithinbank
be judged from studying series of images over periods of years /42/. The Scanning Multichannel Microwave Radiometers (SMMR) launched with Nimbus-7 and Seasat in 1978 have opened up new possibilities but also new needs for theoretical studies before the cryosphere data can be fully interpreted. Landsat Multi-spectral scanner. Twelve years have elapsed since the launch of Landsat-i on 23 July 1972. The Landsat series of satellites has contributed more to knowledge of the great ice sheets than has any other spacecraft before or since. Landsats i, 2, and 3 have carried two types of imaging sensor: Return Beam Vidicon (RBV) cameras and a Multi-Spectral Scanner (MSS). Landsats 4 and 5 carried the MSS and a thematic mapper (TM), a scanner with increased resolution and greater spectral sensitivity. However, TM data have not yet been acquired over ice sheets. Although the RBV offered greater geometrical fidelity, the MSS images have proven to be more useful. The MSS is an optlcal-mechanical llne-scannlng device which employs an oscillating mirror to scan the terrain passing beneath the spacecraft simultaneously in four or more spectral bands. Although each of the bands can be used for identifying rock exposures (nunataks), only the near-lnfrared bands (0.7-1.1 pm) show significant detail on the surface of the ice sheets. While professional photogrammetrists accustomed to mapping from air photographs poured scorn on the idea of mapmaking from MSS imagery because of its lack of geometrical fidelity /50/, students of the eryosphere were euphoric about the possibilities /51,52/. The difference was due to the fact that mapmakers on populated continents had access to almost complete vertical air photo coverage of their territories whereas such photography is still a rare luxury in the polar regions. Only about 20 per cent of the area of Antarctica is covered by oblique air photography and a much smaller proportion by vertical photography. The far side of the Moon is actually better mapped than parts of Antarctica. Thus Landsat image maps are vastly preferable to having no maps at all. The first person to recognize this was W.R. MacDonald of the U.S. Geological Survey, and it is as a result of his initiative that thousands of Landsat images of Antarctica are now available from the period 1972-1974. These constitute a priceless archive and baseline survey of the position of the ice-sheet margin at that time. Published image maps quickly followed /52,53/. Because the United States and United Kingdom led the field in the early nineteen seventies, Landsat image maps of Antarctica, most of them at a scale of 1:250,000, have been published by Australia, France, Japan, New Zealand, Norway, and South Africa /54/. Landsat
has
the potential
for
imaging
about
ii,000,000
km 2 or
79 per cent of the area of About 70 per cent of the Landsat imaging area of Antarctica (7,700,000 km 2) or about 55 per cent of the continent has now been covered by at least one cloud-free image, while 90 per cent of the Greenland ice sheet has been covered. Whereas the geographical exploration of the Moon at least sufficient for mapping at a scale of 1:250,000 is virtually complete, the same cannot be said of our own planet. Using Landsat imagery, Swithinbank /37/ found the mapped position of Charcot Island to be 30 km in error, MacDonald /52/ discovered "major changes in coastal features", and Swithinbank and others /38/ added 38,000 km 2 to the land area of Antarctica. Many other revisions of maps of Antarctica have resulted from later examination of Landsat imagery /55/. Considering only features relevant to fluctuations of glaciers, Landsat has been used to identify and provide for planimetric mapping of the contemporary position of floating ice fronts /16/, grounded ice walls, under-ice coastlines /56/, surface features which can be used to define glacier units, ice streams /57/, the direction of ice movement /58/, crevassed areas which indicate regions undergoing rapid deformation /59/, ice divides as defined by ridges /60,61/, superimposed ice, surface equilibrium zones, and ablation areas /62/, and glacier surges /63/. The more that glacier surges have been studied, the more glaciers that surge have been discovered /64/. The probable reason why we have not yet found periodically surging glaciers in the polar regions is the lack of an effective means for monitoring the perimeter of the ice sheets; Landsat imagery now fills the gap /65/. The long-term average rate of movement of ice streams /66/ and ice fronts /67/ has been determined from time-series of Landsat images. The net mass balance of glaciers has been deduced /68/ and areas of blue (bare) ice likely to be fruitful in the Search for meteorites have been identified /69/. However, very many potentially useful images of the cryosphere have been lost due to overexposure and saturation of the sensor systems /70/, although partial restoration by selective enhancement can sometimes recover data which otherwise would have been lost /71/.
Antarctica and 1,700,000 km 2 or 98 per cent of the Greenland ice sheet.
Return beam vidicon. In terms of cryosphere science, the exciting potential of RBV cameras has not been realized. Landsats 1 and 2 recorded in three spectral bands covering the range 0.475-0.750 pm but did not yield images of the cryosphere that approached the quality of most MSS images. The Landsat 3 RBV recorded images in one spectral band covering the range 0.505-0.750 Bm and had the advantage over earlier RBV and MSS systems of a 30 m rather than 80 m pixel spatial resolution. However, it was neither designed nor operated in the interests of imaging the cryosphere. On the basis of an archive search in November 1982, 41/2 years after the launch of Landsat 3 on 5 March 1978, Ferrigno /72/ found 1735 98 x 98 km subscenes of Antarctica which covered only 164 of the 2514 nominal scene centres on the
A Distant Look at the Cryosphere
267
!
continent. Of these images, less than i per cent were excellent and 71 per cent contained no visible data at all. A further 2036 subscenes were acquired after November 1982 but did not seem to offer any greater percentage of useful imagery. The main problem is due to saturation of the RBV sensor by the high reflectivlty of snow and ice. W o r s e , the sensor was blinded by the snow and took many seconds to recover. As a result, surface detail was obliterated except when the sun was low in the sky. Scenes recorded with a sun angle between i° and i0° show good surface detail, the optimum being 5-7 ° . The low sun casts long shadows in mountain areas hut on glaciers, shadows reveal a multitude of surface features which are of incomparable glaclologlcal interest (Fig. i).
Fig. i. Landsat 3 RBV image of Lillle Glacier, Antarctica. The area across (NASA image from EROS Data Center, U.S. Geological Survey).
shown
is 98 km
Imaging Radar Imaging radars, and in particular synthetic aperture radars (SAR), have a tremendous potential for yielding valuable data on the cryosphere /73/. However, the data flow rate from the spacecraft is so high that it cannot be stored on board and must be transmitted in real time to receiving stations within llne of sight. Given the location of all currently planned receiving stations, the effect of this constraint is that the SAR system on board the European Space Agency's Remote Sensing Satellite ERS-I (to be launched in 1988) will not yield images of any part of Antarctica and will only see about half of the Greenland ice sheet. By way of consolation, it may be that SAR will be most useful for identifying dynamic changes in mountain and piedmont glaciers in lower latitudes. Hall and Ormsby /74/ investigated the use in Alaskan glaclologlcal studies of Seasat SAR and Landsat MSS imagery. The 25 m spatial resolution of the SAR was a distinct asset. Separately, both the SAR and MSS data were useful in identifying the distinctive medial moraines of surging glaciers. On the
268
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Malaspina Glacier, the SAR was able to reveal heavily crevassed areas beneath moraine, whereas the MSS showed the extent of debris resting on the ice. Use of the MSS alone could result in an inaccurate assessment of the extent of the glacier because the very low reflectance of debris-covered ice in interlobate areas could be mistaken for land. The SAR, however, was responsive to surface roughness and the dielectric properties of the material, giving a bright return in the interlobate areas as well as in the main piedmont lobe. Future Satellites Developments in satellite imagery will soon overtake the need for air photographs to provide the basis for photogrammetric mapping at medium scales. The French Systeme Probatoire d'Observation de la Terre (SPOT) satellite will be the first civil satellite capable of providing overlapping stereoscopic images with a resolution of about I0 m. Other satellite designs feature automatic mapping systems /75/76/ but are not yet being built. The SPOT system uses a multilinear array of charge-coupled silicon detectors which should prove superior to earlier scanning systems. With a ground resolution of I0 m it seems probable that we may have reached the point at which further improvements in resolution become uneconomic; remote sensing from aircraft rather than satellites can offer higher resolution more cheaply. However, the practical possibilities of spacecraft remote sensing of unforeseen aspects of both the terrestrial /77/ and planetary cryospheres /78/ are escalating, so it would be rash to speeulate on how best we will monitor changes in the cryosphere by the end of the century. NON-IMAGING
SYSTEMS
Altimetry for Glacio-Eustasy Tens of millions of people will be affected if world sea levels change faster in future than they have done in the past. Recent reports /81,82/ estimate that sea level will rise from 0.7 m to 1.8 m and possibly more by the end of the next century. This is expected to result from the melting of land ice and thermal expansion of the upper layer of the ocean. A global rise in sea level of 1.8 m by the year 2100 suggests a rise some 4 times faster than has been allowed for in the Thames Flood Prevention defence levels. The cost of direct damage in the event of catastrophic flooding of London has been estimated at billions of pounds /83/. The indirect costs due to disruption of business and industry would be greater. In spite of almost three decades of intensive field work on the Greenland and Antarctic ice sheets by up to a dozen nations ever since the International Geophysical Year of 1957-58, there is today no reliable assessment of the contribution of the cryosphere to the contemporary rise in sea level /84/. Radio-echo sounding has led to a series of upward revisions of estimates of the amount of ice on Earth and hence of the potential of eustatic changes in sea level. Little progress can be made in understanding the climatic response and possible instability of polar ice masses until we can detect small changes in ice volume and deduce their causes. Within the realms of current technology, the most effective way to monitor the behaviour of ice sheets is by means of satellite altimetry /85/. Satellite radar altimeters have been used to construct small scale contoured maps of parts of the great ice sheets /86,87,88/ and to obtain an instantaneous view of the position of the ice margin /89/. Brooks /90/ reports an elevation repeatability better than ±i m after waveform retracking. This is a substantial improvement over all other methods of heighting that have yet been developed. After adjustment of the radial components of the orbits, Zwally and others /91/ achieved an rms difference of 0.25 m over relatively flat regions. However, the precision is substantially degraded as the slope of the surface or amplitude of the undulations increases /92/. The equipment has not yet been fitted to any satellite flying in a really high-latitude orbit. This limitation can be overcome in future satellites, and when it is, we shall be in a position to contour all but steeply sloping areas of the ice sheets to an accuracy comparable with that achieved by large-scale photogrammetric mapping on other continents. Accurate surface elevation maps could be constructed from accumulated precessing passes. Repeated over a period of years in relation to an unchanging reference ellipsoid, it would be possible to detect changes in surface level. The completion of radar altimetry would be a major step forward. But what can be done to reduce slope-induced errors near the margin of the ice sheets? Zwally and others /93/ offer a substantial improvement in accuracy with satellite laser altimetry, and design parameters have been carefully considered /94/. Bufton and others /95/ describe a system with a range precision of 0.i m and a footprint diameter of 70 m. There is bound to be a loss of accuracy in steeply sloping regions due to the laser pointing error, but this should be acceptably small over a large proportion of the area of the ice sheets and much less significant after averaging. Over large areas it should be possible to detect changes in average elevation of 0.2 m. No single experiment could yield more baseline data of value to understanding fluctuations of the great ice sheets. Given some knowledge of ice thickness, surface elevation and surface slope are basic parameters which go a long way towards describing the dynamics of an ice
A Distant Look at the Cryosphere
269
sheet. Laser altimetry, or better still a combination of narrow-beam radar and laser altimetry, would reveal the ice divides that define drainage basins and smaller glacier units; the topography associated with flow patterns that reveal ice streams; the position of grounding lines between inland ice sheets and floating ice shelves as marked by a break in slope; and the contemporary position of ice fronts and ice walls. The ability to see subtle variations in surface slope would allow detection of subglacial lakes /96/, zones of rapid basal sliding, and data on the roughness of the glacier bed. Repetitive mapping of the surface elevation would offer a unique opportunity to study the time-dependent behaviour of ice sheets. Within a few years it would provide a measure of local or general changes in ice volume brought about by changes in mass balance. The magnitude and spatial distribution of changes would provide important clues as to their causes. Changes in surface elevation or migrating grounding lines could give early warning of non steady-state flow. Surges would be detected as kinematic waves on the surface of ice sheets. No other survey technique can measure ice sheet elevation with sufficient accuracy to detect volume changes within a reasonable time scale. Zwally and others /93/ have assembled data suggesting that a typical rate of thickness change may be a few centlmetres per year over the ice sheet summit but some tens of centimetres per year in the lower reaches of ice streams. Summit changes could therefore be detected in a decade and changes in the lower reaches in one to three years. The dynamic changes which might take place as a response to the CO 2 greenhouse effect /II/ could readily be detected. A contlnent-wide surface elevation change of 0.5 m would correspond with a volume change of 1 part in 5,000 with respect to the 2,500 m average thickness of the Antarctic ice sheet. But the practical sensitivity of the method would be much greater. For example, an elevation change of 0.5 m detected over one tenth of the area of the ice sheet would correspond with an overall volume change of 1 part in 50,000. While over short periods the lower d e n s i t y of surface snow could affect the results by a factor of 2, the precision of measurement is such that with comparative ease we could monitor the prlncfpal component of the world water balance far more accurately than the remaining components can be determined on any other continent. More important is that changes could be detected long before an unambiguous sea level signal was recognizable. T h e higher the latitude scanned by the satellite, the easier it will be to estimate the extent to which changes observed near the periphery of the ice sheets might be compensated ~ by equal and opposite changes in surface level at higher latitudes. CONCLUSION Significant issues of public policy emerge from this brief review of the contribution of space science to knowledge of the eryosphere. Just as there has been a crescendo of voices calling for operational satellite imagery for earth resources monitoring, today there is a crescendo of voices calling for satellite altimetry of the great ice sheets /97,98,99,100/. Some part of the blame for a contemporary lack of public understanding and government response to the needs of ice science must lie with the glaciologists themselves. Their numhers, admittedly, are small. But their failure to communicate the relevance of their science to the man in the street has led to chronic underfunding of some relatively simple experiments that could go a long way towards measuring the response of ice sheets to changing climate. Although the Scientific Committee on Antarctic Research (SCAR) of the International Council of Scientific Unions rates satellite altimetry as a priority requirement /i01/, it notes that "no laser altimeter with resolution sufficient to detect small ice sheet elevation changes is scheduled on any satellite to be launched in the foreseeable future". Worse, of all the radar-altlmeter equipped satellites currently planned, none is programmed for launch into a sufficiently high latitude orbit to assess the ice-sheet contribution to the contemporary rise in sea level. Few acts of generosity in the history of science can have been greater than the U.S. Government's decision to allow all nations free access to Landsat imagery without any direct compensation to the U.S. taxpayer. This policy has brought unprecedented benefits to many different branches of science /102,103/. But while other continents bask in the luxury of repetitive Landsat coverage spread over a period of 12 years, there is still not a single cloud-free image for 70 per cent of the nominal MSS scenes of Antarctica /104/. Because of overlapping coverage between adjacent orbital paths, the situation is not as bad as it seems, but we still lack usable images for 30 per cent of the imaging area /69/. The only reason why the whole imaging area of Antarctica has not yet been covered is because of the low priority a~corded to it by the U.S. Government. Surely it is not too much to ask that strenuous efforts now be made to complete a basic data set (once-only image) of all the world's glaciers and ice sheets. Fewer than i000 scenes could complete the job. With current L~ndsat MSS acquisitions running at about 6000 per month and with more than one million scenes already in archives, glaciologists can hardly be accused of unjustified special pleading. Some other points arise with respect to the potential as distinct from the performance of Landsat in imaging the cryosphere. It was noted above that 77 per cent of all the RBV scenes o~ Antarctica ( t o November 1982) contained no visible data at all /72/. The pity of the RBV story is that a few simple experiments in the early d~ys of Landsat 3 could have determined JASR 5:6-R
270
C. Swithinbank
the optimum shutter speed and optimum sun angle for good imagery. As it turned out, thousands of useless images were collected at a cost of millions of dollars. Perhaps the lesson here is in the time-lag of the response of the operational management to perceived mistakes, or alternatively, to the tlme-lag in perceiving mistakes. However, the polar science community alone is to blame for not establishing a real time Landsat receiving station in Antarctica. If there had been one, the RBV problem would have been diagnosed sooner than it was. It has become evident that the evaluation of cloud cover by Goddard Space Flight Center was often erroneous because of the difficulty the technicians had in distinguishing between snow and clouds. The MSS 0.6-0.7 ~m band was used in the evaluations whereas the best band to discriminate between snow and clouds is the near-infrared (0.8-1.1 Mm). A review of Landsat images of Antarctica /104/ revealed that many superb cloud-free images were listed in the data base as having I00 per cent cloud cover. Thus many a fine picture was born to blush unseen and to this day remains wrongly-described on index maps. Another problem concerns the deterioration of magnetic tapes holding early Landsat images. A large number of these have not been transferred to the current computer format and are now unusable /105/. Accustomed to dealing in ephemera, space technologists may have undervalued the enormous archival significance of baseline data on natural phenomena which have a response time that is very slow compared with other features of the environment. One more fact that must be faced is that about 20 per cent of the Antarctic continent lies beyond the Landsat imaging area. Sooner or later there will be mounting pressure for a truly polar imaging satellite. With the rising cost of space science projects, there will also be pressure for the release, after a reasonable lapse of time, of military satellite data. Here too the danger is that most military authorities are preoccupied with ephemera and may not appreciate the value to science of what they are observing in the strategically most insensitive regions of the globe. If any area is ripe for leading the way to declassification, surely it must be Antarctica, where all other data are freely exchanged between nations. REFERENCES i.
G.H. Denton and T.J. Hughes, Milankovitch theory of ice ages: Hypothesis of ice-sheet linkage between regional insolation and global climate, Quaternary Res. 20, 125 (1983).
2.
K.F. K~nzi, this issue.
3.
D.J. Cavalieri and H.J. Zwally, this issue.
4.
N. Untersteiner, Sea ice and ice sheets and their role in climatic variations, in: physical basis of climate and climatic modellin$, GARP Publication Series No. WMO/ICSU 1975, p. 206.
5.
W.F. Budd, T.H. Jacka, D. Jenssen, U. Radok, and N.W. Young, Derived physical characteristics of the Greenland ice sheet, Mark i, University of Melbourne Meteorology Dept. (Publ. 23), 1982.
6.
D.J. Drewry, S.R. Jordan, and E. Jankowski, Measured properties of the Antarctic ice sheet: surface configuration, ice thickness, volume and bedrock characteristics, Annals of Glacio!o~y 3, 83 (1982).
7.
UNESCO, Variations Hydrology 3 (1969).
8.
UNESCO, Perennial ice and snow masses, Paris, UNESCO/IASH Technical Papers in Hydrology i (1970).
9.
K. Scherler, Report ZUrich (1980).
of
existing
glaciers,
on World Glacier
Paris,
Inventory,
UNESCO/IASH
Eidgenossische
Technical
Technische
Papers
The 16,
in
Hochschule,
i0.
C. Swithinbank, The problem of a glacier inventory of the Antarctic, in: World Glacier Inventory~ Proceedings of the Riederalp Workshop~ September 1978, 229 ( 1 9 8 0 ) , International Association of Hydrological Sciences (Publication No. 126).
ii.
J.H. Mercer, West Antarctic Nature 271, 321 (1978).
12.
S. Manabe and R.J. Stouffer, Sensitivity of a global climate model to an increase of CO 2 concentration in the atmosphere, J. Geophys. Res. 85 (CI0), 5529 (1980).
13.
J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. Russell, Climate impact of increasing atmospheric carbon dioxide, Science 213, 957 (1981).
ice sheet and CO 2 greenhouse effect: a threat of disaster,
A Distant Look at the Cryosphere
271
14.
J. Oerlemans, Response of the Antarctic ice sheet to a climate warming: a model study, J. Climatology 2, 1 (1982).
15.
C.R. Bentley, The West Antarctic ice sheet: diagnosis and prognosis, in: Carbon Dioxide, Science, and Consensus: Proceedings~ Carbon Dioxide Research Conference~ Berkeley Springs~ West Verginia~ September 1982 (1983), p. IV.3.
16.
C.S.M. Doake, State of balance of the ice sheet in the Antarctic Peninsula, Glaciology 3, 77 (1982).
17.
T.J. Hughes, Is the West Antarctic ice sheet disintegrating?, J. Geophys. Res. 78, 7884 (1973).
18.
J.H. Mercer, West Antarctic ice volume: the interplay of sea level and temperature, and a strandline test for absence of the ice sheet during the last interglacial, in: Sea level~ ice, and climatic chanse, ed. I. Allison, International Association of Hydrological Sciences (Publication No. 131), 323 (1981).
19.
C.R. Bentley, Environmental and societal consequences of a possible CO~-induced climatic chan~e 2, Pt i, Dept of Energy, Washington DC, 1982.
20.
R.H. Thomas, T.J.O. Sanderson, and K.E. Rose, Effect of climate warming on the West Antarctic ice sheet, Nature 277, 355 (1979).
21.
W.S.B. Paterson, The Physics of Glaciers, Pergamon (1981).
22.
R. Etkins and E.S. Epstein, The rise of global mean climatic change, Science 215, 287 (1982).
23.
J. Hansen, V. Gornitz, S. Lebedeff, and E. Moore, Global mean sea level: climatic change, Science 219, 996 (1983).
24.
V.M. Kotlyakov, K.S. Losev Geography 2, 251 (1978).
25.
R.N. Colwell, ed., Manual of Remote Sensing 2, 2nd edition. American Society of Photogrammetry, 1983.
26.
V.M. Kotlyakov, Space glaciology: its establishment and prospects, Remote Sensing i, i, Harwood Academic Publishers (1981).
27.
L.V. Desinov, G.A. Nosenko, G.M. Grechko, A.S. Ivanchenkov, and V.M. Kotlyakov, Glaciological studies and experiments aboard the Salyut-6 orbital station (in Russian), Issiedovanie Zemli iz Kosmosa i, 25 (1980).
28.
P. iGloersen and V.V. Salomonson, Satellites - new global observing techniques for ice and snow, J. Glaeiolo~y 15, 373 (1975).
29.
C. Swithinbank, Giant icebergs in the Weddell Sea, Polar Record 14, 477 (1969).
30.
J.E. Sissala, Observations of an Antarctic satellite, Nature 224, 1285 (1969).
31.
C. Swithinbank, P. McClain, Record 18, 495 (1977).
32.
C. Swithinbank, Satellite photographs of the Antarctic Peninsula area, Polar Record 15, 19 (1970).
33.
D.H. Bromwich and D.D. Kurtz, Experiences of Scott's Northern Party: evidence for a relationship between winter katabatic winds and the Terra Nova Bay polynya, Polar Record 21, 137 (1982).
34.
D.H. Bromwich and D.D. Kurtz, Katabatic wind forcing of the Terra Nova Bay polynya, J. Geophys. Res. 89, 3561 (1984).
35.
A.L. Booth and V.R. Taylor, Meso-scale archive and computer products of digitized video data from ESSA satellites, Bull. Amer. Met. Soc. 50, 431 (1969).
36.
C. Swithinbank, Higher resolution satellite pictures, Polar Record 16, 739 (1973).
37.
C. Swithinbank, A new map of Alexander Island, Antarctica, Polar Record 17, 155 (1974).
and
I.A.
Loseva,
The
Ocean
ice
Annals of
sea level as an indication of
budget
of
indicator of
Antarctica,
Polar
Falls Church, Virginia,
tabular iceberg
Soviet Journal of
from
and P. Little, Drift tracks of Antarctic
the Nimbus
icebergs,
2
Polar
272
C. Swithinbank
38.
C. Swithinbank, C. Doake, A. Wager, and Antarctica, Polar Record 18, 295 (1976).
R.D.
Crabtree,
Major
change
in
the map
of
39.
D.R. Wiesnet, A satellite mosaic of the Greenland ice sheet, in: World Inventory~ Proceedings of the Riederalp Workshop~ September 1978, 343 International Association of Hydrological Sciences (Publication No, 126).
40.
C.P. Berg, D.R. Wiesnet, and R. Legeckis, The NOAA-6 satellite mosaic of Antarctica: progress report, Annals of Glaciology 3, 23 (1982).
41.
P. Gloersen, T.T. Wilheit, T.C. Chang, W. Nordberg, and W.J. Campbell, Microwave maps of the polar ice of the Earth, Bull. Amer. Met. Soc. 55, 1442 (1974).
42.
H.J. Zwally, J.C. Camiso, C.L. Parkinson, W.J. Campbell, R.D. Carsey, and P. Gloersen, Antarctic sea ice~ 1973-1976: satellite passive microwave observations, National Aeronautics and Space Administration, Washington DC, 1983 (NASA SP-459).
43.
H.J. Zwally, Observing polar ice variability, Annals of Glaciology 5, 191 (1984).
44.
T.C. Chang, P. Gloersen, T. Schmugge, T.T. Wilheit, and H.J. Zwally, Microwave emission from snow and glacier ice, J. Glaciology 16, 23 (1976).
45.
H.J. Zwally and P. Gloersen, Passive microwave images of the polar regions and research applications, Polar Record 18, 431 (1977).
46.
H.J. Zwally, Microwave emissivity and accumulation rate of polar firn, J. Glaciology 18, 195 (1977).
47.
A.T.C. Chang, B.J. Choudhury, and P. Gloersen, Microwave brightness measured by Nimbus 5 and 6 ESMR, J. Glaciology 25, 85 (1980).
48.
S.R. Rotman, A.D. Fisher, and D.H. Staelin, Analysis of multiple angle microwave observations of snow and ice using cluster-analysis techniques, J. Glaciology 27, 89
Glacier (1980),
a
of polar firn as
(1981). 49.
H.J. Zwally, J.L. Saba, and J.C. Comiso, Relationship and microwave emissivity, preprinted abstract (1984).
50.
G. Petrie, Some considerations regarding mapping from earth satellites, Photogrammetric Record 6, 590 (1970) and 7, 55 (1971).
51.
C. Swithinbank, (1973).
52.
W.R. MacDonald, Antarctic cartography, in: ERTS-I, a New Window on our Planet, eds. R.S. Williams Jr., and W.D. Carter, U.S. Geological Survey Professional Paper 929, 37 (1976).
53.
C. Swithinbank and C. Lane, Antarctic mapping from satellite imagery, in: Remote sensing of the terrestrial environment, eds. R.F. Peel, L.F. Curtis, and E.C. Barrett, Butterworths, London 1977, p. 212.
54.
R.S. Williams Jr, J.G. Ferrigno, T.M. Kent, and J.W. Schoonmaker, Landsat images and mosaics of Antarctica for mapping and glaciological studies, Annals of Glaciology 3, 321 (1982).
55.
D.J. Drewry, ed., Antarctica: Glaciological and Geophysical Folio, Scott Polar Research Institute, Cambridge, 1983.
56.
C. Swithinbank, Glaciological Lond. B. 279, 161 (1977).
57.
T. Hughes, West Antarctic ice streams, J. Geophys. Res. 15, i (1977).
58.
R.D. Crabtree and C.S.M. Doake, Flow lines on Antarctic ice shelves, Polar Record 20, 31
Satellite
view
of McMurdo
Sound,
between
Antarctica,
firn accumulation
Polar Record
16,
rates
851
research in the Antarctic Peninsula, Phil. Trans. R. Soc.
(1980). 59.
G. de Q. Robin, C.S.M. Doake, H. Kohnen, R.D. Crabtree, S.R. Jordan, and D. M~lier, Regime of the Filchner-Ronne ice shelves, Antarctica, Nature 302, 582 (1983).
60.
P.J. Martin, Ridges on Antarctic ice rises, J. Glaciology 17, 141 (1976).
61.
P.J. Martin and T.J.O. 25, 33 (1980).
Sanderson,
Morphology
and dynamics of ice rises,
J. GlaQiology
A Distant Look at the Cryosphere
273
62.
D.C. Rundquist, S.G. Collins, R.B. Barnes, D.E. Bussom, S.A. Samson, and J.S. Peake, The use of Landsat digital information for assessing glacier inventory parameters, in: World Glacier Inventory~ Proceedings of the Riederalp Workshopl September 1978, 321 (1980), International Association of Hydrological Sciences (Publication No. 126).
63.
R.M. Krimmel and M.F. Meier, Glacier applications of ERTS images, J. Glaciology 15, 391 (1975).
64.
M.F. Meier, Monitoring the motion of surging glaciers in the Mount McKinley massif, Alaska, in: ERTS-I: A new window on our planet, R.S. Williams Jr., and W.D. Carter (eds.), U.S. Geological Survey Professional Paper 929, 185 (1976).
65.
W.R. Macdonald, Glaciology in Antarctica, in: ERTS-I: A new window on our planet, R.S. Williams Jr., and W.D. Carter (eds.), U.S. Geological Survey Professional Paper 929, 194 (1976).
66.
R.D. Crabtree and C.S.M. Doake, Pine Island Glacier and its drainage basin: results from radio echo-sounding, Annals of Glaciolo~ 3, 65 (1982).
67.
A.J. Colvill, Movement of Antarctic ice fronts measured from satellite imagery, Polar Record 18, 390 (1977).
68.
G. #strem, ERTS data in glaciology - an effort to monitor glacier mass balance from satellite imagery, J. Glaciology 15, 403 (1975).
69.
R.S. Williams Jr., T.K. Meunier, and J.G. Ferrigno, Blue ice, meteorites, and satellite imagery in Antarctica, Polar Record 21, 493 (1983).
70.
J.G. Ferrigno and R.S. Williams Jr., Limitations in the use of Landsat images for mapping and other purposes in snow- and ice-covered regions: Antarctica, Iceland, and Cape Cod, Massachusetts, in: Proceedings of the Seventh International Symposium on Remote Sensing of the Envlronment~ Ann Arbor~ Michigan, Environmental Research Institute of Michigan, Ann Arbor 1983, vol. i, p. 335.
71.
B.K. Lucchitta, E.M. Eliason and S. Southworth, Multlspectral digital Antarctica with Landsat images, Antarctic J. of the U.S., in press (1984).
72.
J.G. Ferrigno, R.S. Williams Jr., and T.M. Kent, Evaluation of Landsat 3 RBV images for earth science studies in Antarctica, in: Antarctic Earth Science, eds. R.L. Oliver, P.R. James, and J.B. Jago, Cambridge University Press 1983, p. 446.
73.
G. Weller, F. Carsey, B. Holt, D.A. Rothrock, and W.F. Weeks, Science program for an imaging radar receiving station in Alaska, NASA/JPL, 1983.
74.
D.K. Hall and J.P. Ormsby, Use of Seasat synthetic aperture radar and Landsat multispectral scanner subsystem data for Alaskan glaciology studies, J. Geophys. Res. 88, 1597 (1983).
75.
Itek. Conceptual design of an automated mapping satellite system, Itek Optical Systems 81-8449A-I (January 1981).
76.
A.P. Colvocoresses, An automated mapping satellite Engineering and Remote Sensing 48, 1585 (1982).
77.
D.P. Rublncam, Postglacial rebound observed by Lageos and the effective viscosity of the lower mantle, J. Geophys. Res. 89, 1077 (1984).
78.
B.K. Lucchltta, Ice sculpture in the Martian outflow channels, J. Geophys. Res. 87, 9951 (1982).
79.
N.W. Young, Measured velocities of interior East Antarctica balance within the I.A.G.P. area, J. Glaclolog~ 24, 77 (1979).
80.
W.F. Budd, D. Jenssen, and U. Radok, Derived physical characteristics of the Antarctic ice sheet, ANARE Interim Reports, Series A (IV) Glaciology 120, Antarctic Division, Department of Supply, Melbourne (1971).
81.
Changing cllmate, Report of the Carbon Dioxide Assessment Committee, National Academy Press, Washington D.C., 1983.
82.
J.S. Hoffman, D. Keyes, and J.G. Titus, Projecting future sea level rise, methodology, estimates to the year 2100, and research needs, EPA 230-09-007, Environmental Protection Agency, Washington D.C. (October 1983).
system
(Mapsat),
and
the
mapping
of
Photogrammetrlc
state of mass
274
C. Swithinbank
83. D. Ayres, private communication (1984). 84. V. Gornitz, S. Lebedeff, Science 215, 1611 (1982). 85. C. Swithinbank, 289 (1983).
and J. Hansen,
Global
sea level
trend in the past century,
Towards an inventory of the great ice sheets,
86. R.L. Brooks, W.J. Campbell, R.O. Ramseier, H.R. Stanley, topography by satellite altimetry, Nature 274, 539 (1978).
Geo~rafiska Annaler 65A,
and H.J.
Zwally,
Ice
sheet
87. T.V. Martin, H.J. Zwally, A.C. Brenner, and R.A. Bindschadler, Analysis and retracking of continental ice sheet radar altimeter waveforms, J" Geophys. Res. 88, 1608 (1983). 88. R.L. Brooks, R.S. Williams Jr., J.G. Ferrigno, and W.B. Krabill, Amery topography from satellite radar altimetry, in: Antarctic Earth Science, Oliver, P.R. James, and J.B. Jago, Cambridge University Press 1983, p. 441. 89. R.H. Thomas, T.V. Martin, and H.J. Zwally, Mapping altimetry data, Annals of Glaciolo~y 4, 283 (1983). 90. R.L. Brooks, 32 (1982).
ice-sheet
margins
Ice Shelf eds. R.L.
from
radar
Satellite altimeter results over East Antarctica, Annals of Glaciolosy 3,
91. H.J. Zwally, R.A. Bindsehadler, A.C. Brenner, T.V. Martin, and R.H. Thomas, Surface elevation contours of Greenland and Antarctic ice sheets, J. Geophys. Res. 88, 1589 (1983). 92. A.C. Brenner, R.A. Bindschadler, R.H. Thomas, and H.J. Zwally, Slope-induced errors in radar altimetry over continental ice sheets, J. Geophys. Res. 88, 1617 (1983). 93. H.J. Zwally, R.H. Thomas, and R.A. Bindschadler, Ice-sheet dynamics by laser altimetry, NASA Goddard Space Flight Center, Greenbelt, Tech. Memo. 82128, 1981. 94. ICEX Ice and Climate Experiment, Report of Science and Applications Working Group, NASA Goddard Space Flight Center, Greenbelt, 1979. 95. J,L. Bufton, J.E. Robinson, M.D. Femiano, and F.S. Flatow, Laser altimetry measurement of ice-sheet topography, NASA Goddard Space Flight Center, 1981.
for
96. G.K.A. Oswald, Investigations of sub-ice bedrock characteristics by radio echo sounding, J. Glaciology 15, 75 (1975). 97. Ocean-cryosphere topography advanced radar altimeter proposal for Radarsat, Natural Environment Research Council-Science and Engineering Research Council, Swindon, England (April 1983). 98. A study of satellite radar altimeter operation over Ice-covered surfaces, European Space Agency Contract Report 5182/82/F/CG(SC) (May 1983). 99. A study of ocean and ice topography from space, a joint NERC-SERC proposal for development of satellite altimetry, Natural Environment Research Council-Science Engineering Research Council, Swindon, England (September 1983). i00. International Union Resolution 8.
of Geodesy
and Geophysics,
XVIII General Assembly,
Hamburg
the and
1983,
i01. I. Allison, ed., Antarctic Climate Research, Scientific Committee on Antarctic Research (1983). 102. R.S. Williams Jr., and W.D. Carter (eds.), ERTS-I, a new window Geological Survey Professional Paper 929, Washington D.C. (1976).
on
our
planet,
103. N.M. Short, P.D. Lowman Jr., S.C. Freden, and W.A. Finch Jr., Mission to Earth: Landsat views the world, National Aeronautics and Space Administration, Washington D.C., NASA SP-360 (1976). 104. R.S. Williams Jr., private communication (1984). 105. B.K. Lucchitta, private communication (1984).
1985 Printed in Great Britain. All rights reserved.
0273-I177/85 $0.00 + .50 Copyright © COSPAR
A D I S T A N T L O O K A T THE CRYOSPHERE Charles Swithinbank British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET, U.K.
ABSTRACT Ninety nine per cent of all the fresh water on the surface of the Earth is in the form of ice. Observations from space have revealed more about the ice than about most other parts of the environment because at the dawn of the satellite era, less was known about it. The cryosphere includes all forms of naturally occurring ice but here we review what space science has done for knowledge of glaciers and ice sheets. Whereas in global terms the cryosphere exists as a response to climate, over large areas it controls climate. While imaging spacecraft systems have proved easiest to interpret, microwave sensors with poor spatial resolution are able to distinguish transient and stable surface features that are invisible to the eye. Imaging radars quite effectively describe sea ice, but precision altimetry is the only practicable method for monitoring changes in the total mass of ice on land. INTRODUCTION The cryosphere includes all forms of naturally occurring ice. In the context of space observations for climate studies we are chiefly concerned with snow cover on land, sea ice, ice sheets and smaller glaciers. Most of us and certainly all novice skiers have taken a close look at the cryosphere. But distance lends perspective. Observations from space have contributed substantially to our knowledge of the cryosphere because at the dawn of the satellite era, less was known about it than about most other parts of the environment. Whereas in global terms the cryosphere exists as a response to climate, over large areas it cnntrols climate. The high albedo of ice and snow interacts with other components to produce feedbacks that determine the overall sensitivity of the climate system. Ice sheets represent enormous planetary heat sinks whose effectiveness varies with their areal extent and interior surface elevation /I/. The biggest ice sheet that of the Antarctic is intimately linked to the circulation of the ocean and the depth of the world's seas. Modelling the cryosphere is difficult because the freezing point of water lles about half way between the highest and lowest temperatures commonly found on the Earth. Thus ice is the most changeable of all physical constituents of the Earth's surface. At the seasonal maximum, snow and ice together cover about forty per cent of the land area and fifteen per cent of the total surface area of the globe. Excluding water stored underground, ninety nine per cent of the fresh water on the surface of the Earth is in the form of ice, so ice is the principal component of the world water balance. Response times to changing weather and climate vary from days in the case of seasonal snow cover to months in sea ice, decades in valley glaciers, centuries in ice caps, and millennia in ice sheets. GLACIERS AND ICE SHEETS An accompanying review paper by Kfinzi /2/ addresses snow cover, a n d another paper by Cavalieri and Zwally /3/ deals with sea ice, so here we discuss the remaining, and in terms of mass, the principal element of the eryosphere - glaciers and ice sheets. Permanent ice covers about ten per cent of the land surface area of the Earth. Table i summarizes estimates of area and volume. Because new glaciers are still being discovered on satellite images, particularly in poorly-mapped parts of the world, it is likely that the area of mountain glaciers will be revised upward during the next decade. One of the principal objectives of the International Hydrological Decade 1965-74 (IHD) was to reach an understanding of the world water balance. Among the programmes initiated by the IHD but still unfinished are a World Glacier inventory and the development of methods for surveillance of glacier fluctuations /7,8,9/. Because fluctuations can only be measured with respect to some datum, the programmes are necessarilylinked. Although progress with respect to glaciers outside the polar regions is already substantial, very little has been done to 263
264
C. Swithinbank
determine the exact dimensions or to record variations of the Greenland and Antarctic ice sheets. The principal reason is that some of the problems involved have seemed to be intractable. Moreover, because the major stimulus for a world glacier inventory came from people concerned with water resources, polar ice sheets have been considered of less significance to mankind than the glaciers of mare populated countries. Indeed, funds devoted to the preparation of glacier inventories appear to have been inversely proportional to the size of the glaciers described /i0/. TABLE i.
Area and Volume of G l a c i e r s
Area (km 2)
Per cent
Volume (km 3)
Per cent
Mountain glaciers Greenland Antarctica
500,000 1,725,000 13,918,000
3.1 10.7 86.2
240,000 2,600,000 30,110,000
0.7 7.9 91.4
Total
16,143,000
i00.0
32,950,000
i00.0
Source /4/ /5/ /6/
THE CRYOSPHERE AND CLIMATE There is a growing awareness of the climatic importance of ice sheets /11,12,13,14,15/. Mercer predicted that "One of the warning signs that a dangerous warming trend is under way in Antarctica will be the breakup of ice shelves on both coasts of the Antarctic Peninsula, starting with the northernmost and extending gradually southward". Doake /16/ has recently assembled evidence of a progressive retreat of the ice shelves in this very area. A total of more than 5000 km 2 of ice shelf is now known to have disintegrated since the nineteen forties. Bentley /15/, however, suggests that the ice sheet, if it is changing at all, is growing rather than shrinking. While it seems to be agreed that the West Antarctic ice sheet is probably the part most sensitive to climatic change /17,18,19/, there is as yet no general agreement on the sign, let alone the magnitude, of the response to a given change of climate /20/. A warming of climate might increase snow accumulation, raise the surface of the ice sheet, and thus lower sea level /14/; but if melting at the margins should exceed the effect of increasing snowfall, the opposite would occur /ii/. Unfortunately, so little is understood about the likely net response of the great ice sheets to climatic change that is it not now possible to select truly representative experimental areas for detailed studies on the ground. Moreover, there are records of opposite glacier responses within neighbooring drainage basins /21/, so really representative sites may never be found. While the amount of fresh water stored in the polar regions has aroused little interest because of its remoteness from the pressing water problems of populated continents, changes in polar ice sheets, through their effect on sea level, could have a greater impact on mankind than all climateinduced changes in lower latitudes. Any change in the mass of ice on land will lead to a change in global sea level. In view of the importance of understanding climatic change there is an urgent need to detect even a small mass imbalance in the polar regions so that its long-term consequences can be considered /22,23/. Paradoxically, the largest ice mass on Earth is in terms of precipitation also the world's largest desert. Annual precipitation over more than 6,000,000 km 2 of the Antarctic ice sheet averages less than i00 mm per annum. Over a further 5,000,000 km 2 it averages between i00 mm and 200 mm. The average snow accumulation rate for the entire continent is 15.6 ± 2.3 g cm 2 a -1, representing a total of about 2,000 km 3 of water. Although error estimates in our present state of knowledge are large (± 30%) about the same volume of icebergs (2,000 km 3) may calve into the sea from the margins of the ice sheet /24/. IMAGING SYSTEMS FOR THE CRYOSPHERE Manned Spacecraft No manned spacecraft has flown in a sufficiently high-inclination orbit to make an important contribution to knowledge of the great ice sheets. However, every manned spacecraft from the Soviet Salyut and US Mercury series onwards has returned magnificent photographs of ice caps and mountain glaciers in lower latitudes. These represent an invaluable archive of the linear extent of glaciers at the time and have led to the discovery of many new glaciers in poorly-mapped regions. On-board observers, moreover, have been able to recognize transient phenomena such as avalanches, glacier floods, surging glaciers, and the calving of icebergs. These observations were of immediate interest to people in the vicinity and w o u l d have been of significantly less value if new features or changes had gone unrecognized until photographs taken from the spacecraft had subsequently been analysed on the ground. Kotlyakov /26/ reports that for this reason, Soviet cosmonauts undergo formal training in glaciology. Part of the course involves flights in TU-134 aircraft over the Pamirs in Tadzhikistan, where there have been subsatellite experiments involving fixed-wing aircraft and helicopters /27/.
A Distant Look at the Cryosphere
265
Vidicon systems and scanning radiometers. The power of observing significant snow and ice features from satellites was first demonstrated when television cameras with relatively modest spatial resolution, mounted on the TIROS-2 weather satellite, monitored the break-up of sea ice in 1961 /28/. It rapidly became clear that one of the great advantages of remote sensing from satellites is the ability to obtain almost instantaneously a synoptic view over large areas. Observations in many other spectral regions have been made since the visible wavelength sensors of early satellites. These have made it possible to monitor repetitively over extensive areas many different properties of snow and ice. Observations from the Nimbus-3 infrared radiometer in the 0.7-1.3 ,m spectral region revealed areas in which active melting was occurring on ice and snow and thus decreasing the reflectance in this band. The spectral range of environmental satellite sensors has progressively increased throughout the TIROS, Nimbus, ESSA, Kosmos, Meteor, ITOS, and NOAA series, such that imaging systems with different engineering designs have now operated in many bands between 0.2 ~m and 50 Bm. Despite the low resolution of early imaging systems, icebergs were observed calving or calved from the Antarctic ice sheet /29,30/ and drift tracks of individual icebergs were monitored over periods of several years /31/. Several extensive recurring polynyas were found, revealing that katabatic winds from the ice sheet follow distinct channels and consistently force pack ice away from the shore even in the depth of winter /32,33,34/. Clouds were a problem in interpreting imagery because their reflectance characteristics are similar to those of ice and led to confusion in interpreting the extent of ice. Provided that the user was interested in a weekly or monthly rather than a synoptic view, the effect of clouds could be minimized by combining a series of consecutive images to produce what was known as a composite minimum brightness (CMB) chart /35/. The resolution of imaging systems improved during the nineteen sixties but only with the launch in 1972 of the NOAA-2 satellite could it be said that unmanned satellites were beginning to yield data on glaciers and ice sheets that was useful for other than meteorological purposes. NOAA-2 carried the so-called very high resolution radiometer (VHRR) which yielded images in the visible and thermal infrared bands w~th a nadir resolution of about 1 km. An image of the Ross Sea area recorded in January 1973 showed what may have been the first artifact seen in polar satellite pictures a 300-m-wide channel broken by US coast Guard icebreakers through the fast ice in McMurdo Sound /36/. An unexpected discovery on the infrared images was the extent to which gravity winds from outlet glaciers flowing through the Transantarctic Mountains were able to break down the surface temperature inversion over the Ross Ice Shelf /36/. Swith~nbank /37/ was disconcerted to find that Latady Island was 50 km longer than it was depicted on the latest maps. He speculated that this might be the last time in the long history of exploration that any coastline on earth would b e found so far from where it was supposed to be. Subsequent changes to the map of ~tim~sm was premature /38/. Even at small scales, m a p s of the ary in places that Wiesnet /39/ was able to contribute new data examining mosaics of NOAA-5 visible and infrared VHRR images. an image mosaic of Antarctica at a scale of 1:5,000~000; there purposes this will be a more useful product than the best of iced.
Zwally
and
Gloersen
/45/
For the earth scientist viewing the cryosphere from space, ~o often. This difficulty was partially overcome by the e Radiometer (ESMR) on Nimbus 5 launched in 1972. Thin crowave (1.55 cm) emissions which can pass through cloud. o n g the satellite's path, successive strips of images were ~_ mosaic of the whole surface of the Earth from pole to pole. element) size was about 30 x 30 km, the resolution was adequate lobal sea-lee distribution /28,41,42,43/. While the amount of :ould be deduced from the computer products was astonishing, the is of emissivity over the inland ice sheets allowed extrapolation ill sparse network of observations on the ground to the whole area se the mean annual temperature and the accumulation rate of dry the grain sizes upon which the microwave emission depends, these for the main features of the patterns observed from Greenland and particle sizes normally encountered, most of the radiation has a layer up to i0 m in thickness. A marked increase in emission allows the recognition of regions affected by sum~mer melting. found remarkably low brightness temperatures over the dry desert ~nland ice sheets, and were able to detect ice-stream mnd from major outlet glaciers. At the time, some of kind of ground or aircraft observations. Zwally /46/i mce temperature on the one hand and snow accumulation on the accumulation rate to an accuracy of the order of 20 lies /47,48,49/ have refined the understanding of these data available from the Scanning Microwave Spectrometer The emerging possibility not simply of detecting quite v the great ice sheets but also of understanding them can
266
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be judged from studying series of images over periods of years /42/. The Scanning Multichannel Microwave Radiometers (SMMR) launched with Nimbus-7 and Seasat in 1978 have opened up new possibilities but also new needs for theoretical studies before the cryosphere data can be fully interpreted. Landsat Multi-spectral scanner. Twelve years have elapsed since the launch of Landsat-i on 23 July 1972. The Landsat series of satellites has contributed more to knowledge of the great ice sheets than has any other spacecraft before or since. Landsats i, 2, and 3 have carried two types of imaging sensor: Return Beam Vidicon (RBV) cameras and a Multi-Spectral Scanner (MSS). Landsats 4 and 5 carried the MSS and a thematic mapper (TM), a scanner with increased resolution and greater spectral sensitivity. However, TM data have not yet been acquired over ice sheets. Although the RBV offered greater geometrical fidelity, the MSS images have proven to be more useful. The MSS is an optlcal-mechanical llne-scannlng device which employs an oscillating mirror to scan the terrain passing beneath the spacecraft simultaneously in four or more spectral bands. Although each of the bands can be used for identifying rock exposures (nunataks), only the near-lnfrared bands (0.7-1.1 pm) show significant detail on the surface of the ice sheets. While professional photogrammetrists accustomed to mapping from air photographs poured scorn on the idea of mapmaking from MSS imagery because of its lack of geometrical fidelity /50/, students of the eryosphere were euphoric about the possibilities /51,52/. The difference was due to the fact that mapmakers on populated continents had access to almost complete vertical air photo coverage of their territories whereas such photography is still a rare luxury in the polar regions. Only about 20 per cent of the area of Antarctica is covered by oblique air photography and a much smaller proportion by vertical photography. The far side of the Moon is actually better mapped than parts of Antarctica. Thus Landsat image maps are vastly preferable to having no maps at all. The first person to recognize this was W.R. MacDonald of the U.S. Geological Survey, and it is as a result of his initiative that thousands of Landsat images of Antarctica are now available from the period 1972-1974. These constitute a priceless archive and baseline survey of the position of the ice-sheet margin at that time. Published image maps quickly followed /52,53/. Because the United States and United Kingdom led the field in the early nineteen seventies, Landsat image maps of Antarctica, most of them at a scale of 1:250,000, have been published by Australia, France, Japan, New Zealand, Norway, and South Africa /54/. Landsat
has
the potential
for
imaging
about
ii,000,000
km 2 or
79 per cent of the area of About 70 per cent of the Landsat imaging area of Antarctica (7,700,000 km 2) or about 55 per cent of the continent has now been covered by at least one cloud-free image, while 90 per cent of the Greenland ice sheet has been covered. Whereas the geographical exploration of the Moon at least sufficient for mapping at a scale of 1:250,000 is virtually complete, the same cannot be said of our own planet. Using Landsat imagery, Swithinbank /37/ found the mapped position of Charcot Island to be 30 km in error, MacDonald /52/ discovered "major changes in coastal features", and Swithinbank and others /38/ added 38,000 km 2 to the land area of Antarctica. Many other revisions of maps of Antarctica have resulted from later examination of Landsat imagery /55/. Considering only features relevant to fluctuations of glaciers, Landsat has been used to identify and provide for planimetric mapping of the contemporary position of floating ice fronts /16/, grounded ice walls, under-ice coastlines /56/, surface features which can be used to define glacier units, ice streams /57/, the direction of ice movement /58/, crevassed areas which indicate regions undergoing rapid deformation /59/, ice divides as defined by ridges /60,61/, superimposed ice, surface equilibrium zones, and ablation areas /62/, and glacier surges /63/. The more that glacier surges have been studied, the more glaciers that surge have been discovered /64/. The probable reason why we have not yet found periodically surging glaciers in the polar regions is the lack of an effective means for monitoring the perimeter of the ice sheets; Landsat imagery now fills the gap /65/. The long-term average rate of movement of ice streams /66/ and ice fronts /67/ has been determined from time-series of Landsat images. The net mass balance of glaciers has been deduced /68/ and areas of blue (bare) ice likely to be fruitful in the Search for meteorites have been identified /69/. However, very many potentially useful images of the cryosphere have been lost due to overexposure and saturation of the sensor systems /70/, although partial restoration by selective enhancement can sometimes recover data which otherwise would have been lost /71/.
Antarctica and 1,700,000 km 2 or 98 per cent of the Greenland ice sheet.
Return beam vidicon. In terms of cryosphere science, the exciting potential of RBV cameras has not been realized. Landsats 1 and 2 recorded in three spectral bands covering the range 0.475-0.750 pm but did not yield images of the cryosphere that approached the quality of most MSS images. The Landsat 3 RBV recorded images in one spectral band covering the range 0.505-0.750 Bm and had the advantage over earlier RBV and MSS systems of a 30 m rather than 80 m pixel spatial resolution. However, it was neither designed nor operated in the interests of imaging the cryosphere. On the basis of an archive search in November 1982, 41/2 years after the launch of Landsat 3 on 5 March 1978, Ferrigno /72/ found 1735 98 x 98 km subscenes of Antarctica which covered only 164 of the 2514 nominal scene centres on the
A Distant Look at the Cryosphere
267
!
continent. Of these images, less than i per cent were excellent and 71 per cent contained no visible data at all. A further 2036 subscenes were acquired after November 1982 but did not seem to offer any greater percentage of useful imagery. The main problem is due to saturation of the RBV sensor by the high reflectivlty of snow and ice. W o r s e , the sensor was blinded by the snow and took many seconds to recover. As a result, surface detail was obliterated except when the sun was low in the sky. Scenes recorded with a sun angle between i° and i0° show good surface detail, the optimum being 5-7 ° . The low sun casts long shadows in mountain areas hut on glaciers, shadows reveal a multitude of surface features which are of incomparable glaclologlcal interest (Fig. i).
Fig. i. Landsat 3 RBV image of Lillle Glacier, Antarctica. The area across (NASA image from EROS Data Center, U.S. Geological Survey).
shown
is 98 km
Imaging Radar Imaging radars, and in particular synthetic aperture radars (SAR), have a tremendous potential for yielding valuable data on the cryosphere /73/. However, the data flow rate from the spacecraft is so high that it cannot be stored on board and must be transmitted in real time to receiving stations within llne of sight. Given the location of all currently planned receiving stations, the effect of this constraint is that the SAR system on board the European Space Agency's Remote Sensing Satellite ERS-I (to be launched in 1988) will not yield images of any part of Antarctica and will only see about half of the Greenland ice sheet. By way of consolation, it may be that SAR will be most useful for identifying dynamic changes in mountain and piedmont glaciers in lower latitudes. Hall and Ormsby /74/ investigated the use in Alaskan glaclologlcal studies of Seasat SAR and Landsat MSS imagery. The 25 m spatial resolution of the SAR was a distinct asset. Separately, both the SAR and MSS data were useful in identifying the distinctive medial moraines of surging glaciers. On the
268
C. Swithinbank
Malaspina Glacier, the SAR was able to reveal heavily crevassed areas beneath moraine, whereas the MSS showed the extent of debris resting on the ice. Use of the MSS alone could result in an inaccurate assessment of the extent of the glacier because the very low reflectance of debris-covered ice in interlobate areas could be mistaken for land. The SAR, however, was responsive to surface roughness and the dielectric properties of the material, giving a bright return in the interlobate areas as well as in the main piedmont lobe. Future Satellites Developments in satellite imagery will soon overtake the need for air photographs to provide the basis for photogrammetric mapping at medium scales. The French Systeme Probatoire d'Observation de la Terre (SPOT) satellite will be the first civil satellite capable of providing overlapping stereoscopic images with a resolution of about I0 m. Other satellite designs feature automatic mapping systems /75/76/ but are not yet being built. The SPOT system uses a multilinear array of charge-coupled silicon detectors which should prove superior to earlier scanning systems. With a ground resolution of I0 m it seems probable that we may have reached the point at which further improvements in resolution become uneconomic; remote sensing from aircraft rather than satellites can offer higher resolution more cheaply. However, the practical possibilities of spacecraft remote sensing of unforeseen aspects of both the terrestrial /77/ and planetary cryospheres /78/ are escalating, so it would be rash to speeulate on how best we will monitor changes in the cryosphere by the end of the century. NON-IMAGING
SYSTEMS
Altimetry for Glacio-Eustasy Tens of millions of people will be affected if world sea levels change faster in future than they have done in the past. Recent reports /81,82/ estimate that sea level will rise from 0.7 m to 1.8 m and possibly more by the end of the next century. This is expected to result from the melting of land ice and thermal expansion of the upper layer of the ocean. A global rise in sea level of 1.8 m by the year 2100 suggests a rise some 4 times faster than has been allowed for in the Thames Flood Prevention defence levels. The cost of direct damage in the event of catastrophic flooding of London has been estimated at billions of pounds /83/. The indirect costs due to disruption of business and industry would be greater. In spite of almost three decades of intensive field work on the Greenland and Antarctic ice sheets by up to a dozen nations ever since the International Geophysical Year of 1957-58, there is today no reliable assessment of the contribution of the cryosphere to the contemporary rise in sea level /84/. Radio-echo sounding has led to a series of upward revisions of estimates of the amount of ice on Earth and hence of the potential of eustatic changes in sea level. Little progress can be made in understanding the climatic response and possible instability of polar ice masses until we can detect small changes in ice volume and deduce their causes. Within the realms of current technology, the most effective way to monitor the behaviour of ice sheets is by means of satellite altimetry /85/. Satellite radar altimeters have been used to construct small scale contoured maps of parts of the great ice sheets /86,87,88/ and to obtain an instantaneous view of the position of the ice margin /89/. Brooks /90/ reports an elevation repeatability better than ±i m after waveform retracking. This is a substantial improvement over all other methods of heighting that have yet been developed. After adjustment of the radial components of the orbits, Zwally and others /91/ achieved an rms difference of 0.25 m over relatively flat regions. However, the precision is substantially degraded as the slope of the surface or amplitude of the undulations increases /92/. The equipment has not yet been fitted to any satellite flying in a really high-latitude orbit. This limitation can be overcome in future satellites, and when it is, we shall be in a position to contour all but steeply sloping areas of the ice sheets to an accuracy comparable with that achieved by large-scale photogrammetric mapping on other continents. Accurate surface elevation maps could be constructed from accumulated precessing passes. Repeated over a period of years in relation to an unchanging reference ellipsoid, it would be possible to detect changes in surface level. The completion of radar altimetry would be a major step forward. But what can be done to reduce slope-induced errors near the margin of the ice sheets? Zwally and others /93/ offer a substantial improvement in accuracy with satellite laser altimetry, and design parameters have been carefully considered /94/. Bufton and others /95/ describe a system with a range precision of 0.i m and a footprint diameter of 70 m. There is bound to be a loss of accuracy in steeply sloping regions due to the laser pointing error, but this should be acceptably small over a large proportion of the area of the ice sheets and much less significant after averaging. Over large areas it should be possible to detect changes in average elevation of 0.2 m. No single experiment could yield more baseline data of value to understanding fluctuations of the great ice sheets. Given some knowledge of ice thickness, surface elevation and surface slope are basic parameters which go a long way towards describing the dynamics of an ice
A Distant Look at the Cryosphere
269
sheet. Laser altimetry, or better still a combination of narrow-beam radar and laser altimetry, would reveal the ice divides that define drainage basins and smaller glacier units; the topography associated with flow patterns that reveal ice streams; the position of grounding lines between inland ice sheets and floating ice shelves as marked by a break in slope; and the contemporary position of ice fronts and ice walls. The ability to see subtle variations in surface slope would allow detection of subglacial lakes /96/, zones of rapid basal sliding, and data on the roughness of the glacier bed. Repetitive mapping of the surface elevation would offer a unique opportunity to study the time-dependent behaviour of ice sheets. Within a few years it would provide a measure of local or general changes in ice volume brought about by changes in mass balance. The magnitude and spatial distribution of changes would provide important clues as to their causes. Changes in surface elevation or migrating grounding lines could give early warning of non steady-state flow. Surges would be detected as kinematic waves on the surface of ice sheets. No other survey technique can measure ice sheet elevation with sufficient accuracy to detect volume changes within a reasonable time scale. Zwally and others /93/ have assembled data suggesting that a typical rate of thickness change may be a few centlmetres per year over the ice sheet summit but some tens of centimetres per year in the lower reaches of ice streams. Summit changes could therefore be detected in a decade and changes in the lower reaches in one to three years. The dynamic changes which might take place as a response to the CO 2 greenhouse effect /II/ could readily be detected. A contlnent-wide surface elevation change of 0.5 m would correspond with a volume change of 1 part in 5,000 with respect to the 2,500 m average thickness of the Antarctic ice sheet. But the practical sensitivity of the method would be much greater. For example, an elevation change of 0.5 m detected over one tenth of the area of the ice sheet would correspond with an overall volume change of 1 part in 50,000. While over short periods the lower d e n s i t y of surface snow could affect the results by a factor of 2, the precision of measurement is such that with comparative ease we could monitor the prlncfpal component of the world water balance far more accurately than the remaining components can be determined on any other continent. More important is that changes could be detected long before an unambiguous sea level signal was recognizable. T h e higher the latitude scanned by the satellite, the easier it will be to estimate the extent to which changes observed near the periphery of the ice sheets might be compensated ~ by equal and opposite changes in surface level at higher latitudes. CONCLUSION Significant issues of public policy emerge from this brief review of the contribution of space science to knowledge of the eryosphere. Just as there has been a crescendo of voices calling for operational satellite imagery for earth resources monitoring, today there is a crescendo of voices calling for satellite altimetry of the great ice sheets /97,98,99,100/. Some part of the blame for a contemporary lack of public understanding and government response to the needs of ice science must lie with the glaciologists themselves. Their numhers, admittedly, are small. But their failure to communicate the relevance of their science to the man in the street has led to chronic underfunding of some relatively simple experiments that could go a long way towards measuring the response of ice sheets to changing climate. Although the Scientific Committee on Antarctic Research (SCAR) of the International Council of Scientific Unions rates satellite altimetry as a priority requirement /i01/, it notes that "no laser altimeter with resolution sufficient to detect small ice sheet elevation changes is scheduled on any satellite to be launched in the foreseeable future". Worse, of all the radar-altlmeter equipped satellites currently planned, none is programmed for launch into a sufficiently high latitude orbit to assess the ice-sheet contribution to the contemporary rise in sea level. Few acts of generosity in the history of science can have been greater than the U.S. Government's decision to allow all nations free access to Landsat imagery without any direct compensation to the U.S. taxpayer. This policy has brought unprecedented benefits to many different branches of science /102,103/. But while other continents bask in the luxury of repetitive Landsat coverage spread over a period of 12 years, there is still not a single cloud-free image for 70 per cent of the nominal MSS scenes of Antarctica /104/. Because of overlapping coverage between adjacent orbital paths, the situation is not as bad as it seems, but we still lack usable images for 30 per cent of the imaging area /69/. The only reason why the whole imaging area of Antarctica has not yet been covered is because of the low priority a~corded to it by the U.S. Government. Surely it is not too much to ask that strenuous efforts now be made to complete a basic data set (once-only image) of all the world's glaciers and ice sheets. Fewer than i000 scenes could complete the job. With current L~ndsat MSS acquisitions running at about 6000 per month and with more than one million scenes already in archives, glaciologists can hardly be accused of unjustified special pleading. Some other points arise with respect to the potential as distinct from the performance of Landsat in imaging the cryosphere. It was noted above that 77 per cent of all the RBV scenes o~ Antarctica ( t o November 1982) contained no visible data at all /72/. The pity of the RBV story is that a few simple experiments in the early d~ys of Landsat 3 could have determined JASR 5:6-R
270
C. Swithinbank
the optimum shutter speed and optimum sun angle for good imagery. As it turned out, thousands of useless images were collected at a cost of millions of dollars. Perhaps the lesson here is in the time-lag of the response of the operational management to perceived mistakes, or alternatively, to the tlme-lag in perceiving mistakes. However, the polar science community alone is to blame for not establishing a real time Landsat receiving station in Antarctica. If there had been one, the RBV problem would have been diagnosed sooner than it was. It has become evident that the evaluation of cloud cover by Goddard Space Flight Center was often erroneous because of the difficulty the technicians had in distinguishing between snow and clouds. The MSS 0.6-0.7 ~m band was used in the evaluations whereas the best band to discriminate between snow and clouds is the near-infrared (0.8-1.1 Mm). A review of Landsat images of Antarctica /104/ revealed that many superb cloud-free images were listed in the data base as having I00 per cent cloud cover. Thus many a fine picture was born to blush unseen and to this day remains wrongly-described on index maps. Another problem concerns the deterioration of magnetic tapes holding early Landsat images. A large number of these have not been transferred to the current computer format and are now unusable /105/. Accustomed to dealing in ephemera, space technologists may have undervalued the enormous archival significance of baseline data on natural phenomena which have a response time that is very slow compared with other features of the environment. One more fact that must be faced is that about 20 per cent of the Antarctic continent lies beyond the Landsat imaging area. Sooner or later there will be mounting pressure for a truly polar imaging satellite. With the rising cost of space science projects, there will also be pressure for the release, after a reasonable lapse of time, of military satellite data. Here too the danger is that most military authorities are preoccupied with ephemera and may not appreciate the value to science of what they are observing in the strategically most insensitive regions of the globe. If any area is ripe for leading the way to declassification, surely it must be Antarctica, where all other data are freely exchanged between nations. REFERENCES i.
G.H. Denton and T.J. Hughes, Milankovitch theory of ice ages: Hypothesis of ice-sheet linkage between regional insolation and global climate, Quaternary Res. 20, 125 (1983).
2.
K.F. K~nzi, this issue.
3.
D.J. Cavalieri and H.J. Zwally, this issue.
4.
N. Untersteiner, Sea ice and ice sheets and their role in climatic variations, in: physical basis of climate and climatic modellin$, GARP Publication Series No. WMO/ICSU 1975, p. 206.
5.
W.F. Budd, T.H. Jacka, D. Jenssen, U. Radok, and N.W. Young, Derived physical characteristics of the Greenland ice sheet, Mark i, University of Melbourne Meteorology Dept. (Publ. 23), 1982.
6.
D.J. Drewry, S.R. Jordan, and E. Jankowski, Measured properties of the Antarctic ice sheet: surface configuration, ice thickness, volume and bedrock characteristics, Annals of Glacio!o~y 3, 83 (1982).
7.
UNESCO, Variations Hydrology 3 (1969).
8.
UNESCO, Perennial ice and snow masses, Paris, UNESCO/IASH Technical Papers in Hydrology i (1970).
9.
K. Scherler, Report ZUrich (1980).
of
existing
glaciers,
on World Glacier
Paris,
Inventory,
UNESCO/IASH
Eidgenossische
Technical
Technische
Papers
The 16,
in
Hochschule,
i0.
C. Swithinbank, The problem of a glacier inventory of the Antarctic, in: World Glacier Inventory~ Proceedings of the Riederalp Workshop~ September 1978, 229 ( 1 9 8 0 ) , International Association of Hydrological Sciences (Publication No. 126).
ii.
J.H. Mercer, West Antarctic Nature 271, 321 (1978).
12.
S. Manabe and R.J. Stouffer, Sensitivity of a global climate model to an increase of CO 2 concentration in the atmosphere, J. Geophys. Res. 85 (CI0), 5529 (1980).
13.
J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. Russell, Climate impact of increasing atmospheric carbon dioxide, Science 213, 957 (1981).
ice sheet and CO 2 greenhouse effect: a threat of disaster,
A Distant Look at the Cryosphere
271
14.
J. Oerlemans, Response of the Antarctic ice sheet to a climate warming: a model study, J. Climatology 2, 1 (1982).
15.
C.R. Bentley, The West Antarctic ice sheet: diagnosis and prognosis, in: Carbon Dioxide, Science, and Consensus: Proceedings~ Carbon Dioxide Research Conference~ Berkeley Springs~ West Verginia~ September 1982 (1983), p. IV.3.
16.
C.S.M. Doake, State of balance of the ice sheet in the Antarctic Peninsula, Glaciology 3, 77 (1982).
17.
T.J. Hughes, Is the West Antarctic ice sheet disintegrating?, J. Geophys. Res. 78, 7884 (1973).
18.
J.H. Mercer, West Antarctic ice volume: the interplay of sea level and temperature, and a strandline test for absence of the ice sheet during the last interglacial, in: Sea level~ ice, and climatic chanse, ed. I. Allison, International Association of Hydrological Sciences (Publication No. 131), 323 (1981).
19.
C.R. Bentley, Environmental and societal consequences of a possible CO~-induced climatic chan~e 2, Pt i, Dept of Energy, Washington DC, 1982.
20.
R.H. Thomas, T.J.O. Sanderson, and K.E. Rose, Effect of climate warming on the West Antarctic ice sheet, Nature 277, 355 (1979).
21.
W.S.B. Paterson, The Physics of Glaciers, Pergamon (1981).
22.
R. Etkins and E.S. Epstein, The rise of global mean climatic change, Science 215, 287 (1982).
23.
J. Hansen, V. Gornitz, S. Lebedeff, and E. Moore, Global mean sea level: climatic change, Science 219, 996 (1983).
24.
V.M. Kotlyakov, K.S. Losev Geography 2, 251 (1978).
25.
R.N. Colwell, ed., Manual of Remote Sensing 2, 2nd edition. American Society of Photogrammetry, 1983.
26.
V.M. Kotlyakov, Space glaciology: its establishment and prospects, Remote Sensing i, i, Harwood Academic Publishers (1981).
27.
L.V. Desinov, G.A. Nosenko, G.M. Grechko, A.S. Ivanchenkov, and V.M. Kotlyakov, Glaciological studies and experiments aboard the Salyut-6 orbital station (in Russian), Issiedovanie Zemli iz Kosmosa i, 25 (1980).
28.
P. iGloersen and V.V. Salomonson, Satellites - new global observing techniques for ice and snow, J. Glaeiolo~y 15, 373 (1975).
29.
C. Swithinbank, Giant icebergs in the Weddell Sea, Polar Record 14, 477 (1969).
30.
J.E. Sissala, Observations of an Antarctic satellite, Nature 224, 1285 (1969).
31.
C. Swithinbank, P. McClain, Record 18, 495 (1977).
32.
C. Swithinbank, Satellite photographs of the Antarctic Peninsula area, Polar Record 15, 19 (1970).
33.
D.H. Bromwich and D.D. Kurtz, Experiences of Scott's Northern Party: evidence for a relationship between winter katabatic winds and the Terra Nova Bay polynya, Polar Record 21, 137 (1982).
34.
D.H. Bromwich and D.D. Kurtz, Katabatic wind forcing of the Terra Nova Bay polynya, J. Geophys. Res. 89, 3561 (1984).
35.
A.L. Booth and V.R. Taylor, Meso-scale archive and computer products of digitized video data from ESSA satellites, Bull. Amer. Met. Soc. 50, 431 (1969).
36.
C. Swithinbank, Higher resolution satellite pictures, Polar Record 16, 739 (1973).
37.
C. Swithinbank, A new map of Alexander Island, Antarctica, Polar Record 17, 155 (1974).
and
I.A.
Loseva,
The
Ocean
ice
Annals of
sea level as an indication of
budget
of
indicator of
Antarctica,
Polar
Falls Church, Virginia,
tabular iceberg
Soviet Journal of
from
and P. Little, Drift tracks of Antarctic
the Nimbus
icebergs,
2
Polar
272
C. Swithinbank
38.
C. Swithinbank, C. Doake, A. Wager, and Antarctica, Polar Record 18, 295 (1976).
R.D.
Crabtree,
Major
change
in
the map
of
39.
D.R. Wiesnet, A satellite mosaic of the Greenland ice sheet, in: World Inventory~ Proceedings of the Riederalp Workshop~ September 1978, 343 International Association of Hydrological Sciences (Publication No, 126).
40.
C.P. Berg, D.R. Wiesnet, and R. Legeckis, The NOAA-6 satellite mosaic of Antarctica: progress report, Annals of Glaciology 3, 23 (1982).
41.
P. Gloersen, T.T. Wilheit, T.C. Chang, W. Nordberg, and W.J. Campbell, Microwave maps of the polar ice of the Earth, Bull. Amer. Met. Soc. 55, 1442 (1974).
42.
H.J. Zwally, J.C. Camiso, C.L. Parkinson, W.J. Campbell, R.D. Carsey, and P. Gloersen, Antarctic sea ice~ 1973-1976: satellite passive microwave observations, National Aeronautics and Space Administration, Washington DC, 1983 (NASA SP-459).
43.
H.J. Zwally, Observing polar ice variability, Annals of Glaciology 5, 191 (1984).
44.
T.C. Chang, P. Gloersen, T. Schmugge, T.T. Wilheit, and H.J. Zwally, Microwave emission from snow and glacier ice, J. Glaciology 16, 23 (1976).
45.
H.J. Zwally and P. Gloersen, Passive microwave images of the polar regions and research applications, Polar Record 18, 431 (1977).
46.
H.J. Zwally, Microwave emissivity and accumulation rate of polar firn, J. Glaciology 18, 195 (1977).
47.
A.T.C. Chang, B.J. Choudhury, and P. Gloersen, Microwave brightness measured by Nimbus 5 and 6 ESMR, J. Glaciology 25, 85 (1980).
48.
S.R. Rotman, A.D. Fisher, and D.H. Staelin, Analysis of multiple angle microwave observations of snow and ice using cluster-analysis techniques, J. Glaciology 27, 89
Glacier (1980),
a
of polar firn as
(1981). 49.
H.J. Zwally, J.L. Saba, and J.C. Comiso, Relationship and microwave emissivity, preprinted abstract (1984).
50.
G. Petrie, Some considerations regarding mapping from earth satellites, Photogrammetric Record 6, 590 (1970) and 7, 55 (1971).
51.
C. Swithinbank, (1973).
52.
W.R. MacDonald, Antarctic cartography, in: ERTS-I, a New Window on our Planet, eds. R.S. Williams Jr., and W.D. Carter, U.S. Geological Survey Professional Paper 929, 37 (1976).
53.
C. Swithinbank and C. Lane, Antarctic mapping from satellite imagery, in: Remote sensing of the terrestrial environment, eds. R.F. Peel, L.F. Curtis, and E.C. Barrett, Butterworths, London 1977, p. 212.
54.
R.S. Williams Jr, J.G. Ferrigno, T.M. Kent, and J.W. Schoonmaker, Landsat images and mosaics of Antarctica for mapping and glaciological studies, Annals of Glaciology 3, 321 (1982).
55.
D.J. Drewry, ed., Antarctica: Glaciological and Geophysical Folio, Scott Polar Research Institute, Cambridge, 1983.
56.
C. Swithinbank, Glaciological Lond. B. 279, 161 (1977).
57.
T. Hughes, West Antarctic ice streams, J. Geophys. Res. 15, i (1977).
58.
R.D. Crabtree and C.S.M. Doake, Flow lines on Antarctic ice shelves, Polar Record 20, 31
Satellite
view
of McMurdo
Sound,
between
Antarctica,
firn accumulation
Polar Record
16,
rates
851
research in the Antarctic Peninsula, Phil. Trans. R. Soc.
(1980). 59.
G. de Q. Robin, C.S.M. Doake, H. Kohnen, R.D. Crabtree, S.R. Jordan, and D. M~lier, Regime of the Filchner-Ronne ice shelves, Antarctica, Nature 302, 582 (1983).
60.
P.J. Martin, Ridges on Antarctic ice rises, J. Glaciology 17, 141 (1976).
61.
P.J. Martin and T.J.O. 25, 33 (1980).
Sanderson,
Morphology
and dynamics of ice rises,
J. GlaQiology
A Distant Look at the Cryosphere
273
62.
D.C. Rundquist, S.G. Collins, R.B. Barnes, D.E. Bussom, S.A. Samson, and J.S. Peake, The use of Landsat digital information for assessing glacier inventory parameters, in: World Glacier Inventory~ Proceedings of the Riederalp Workshopl September 1978, 321 (1980), International Association of Hydrological Sciences (Publication No. 126).
63.
R.M. Krimmel and M.F. Meier, Glacier applications of ERTS images, J. Glaciology 15, 391 (1975).
64.
M.F. Meier, Monitoring the motion of surging glaciers in the Mount McKinley massif, Alaska, in: ERTS-I: A new window on our planet, R.S. Williams Jr., and W.D. Carter (eds.), U.S. Geological Survey Professional Paper 929, 185 (1976).
65.
W.R. Macdonald, Glaciology in Antarctica, in: ERTS-I: A new window on our planet, R.S. Williams Jr., and W.D. Carter (eds.), U.S. Geological Survey Professional Paper 929, 194 (1976).
66.
R.D. Crabtree and C.S.M. Doake, Pine Island Glacier and its drainage basin: results from radio echo-sounding, Annals of Glaciolo~ 3, 65 (1982).
67.
A.J. Colvill, Movement of Antarctic ice fronts measured from satellite imagery, Polar Record 18, 390 (1977).
68.
G. #strem, ERTS data in glaciology - an effort to monitor glacier mass balance from satellite imagery, J. Glaciology 15, 403 (1975).
69.
R.S. Williams Jr., T.K. Meunier, and J.G. Ferrigno, Blue ice, meteorites, and satellite imagery in Antarctica, Polar Record 21, 493 (1983).
70.
J.G. Ferrigno and R.S. Williams Jr., Limitations in the use of Landsat images for mapping and other purposes in snow- and ice-covered regions: Antarctica, Iceland, and Cape Cod, Massachusetts, in: Proceedings of the Seventh International Symposium on Remote Sensing of the Envlronment~ Ann Arbor~ Michigan, Environmental Research Institute of Michigan, Ann Arbor 1983, vol. i, p. 335.
71.
B.K. Lucchitta, E.M. Eliason and S. Southworth, Multlspectral digital Antarctica with Landsat images, Antarctic J. of the U.S., in press (1984).
72.
J.G. Ferrigno, R.S. Williams Jr., and T.M. Kent, Evaluation of Landsat 3 RBV images for earth science studies in Antarctica, in: Antarctic Earth Science, eds. R.L. Oliver, P.R. James, and J.B. Jago, Cambridge University Press 1983, p. 446.
73.
G. Weller, F. Carsey, B. Holt, D.A. Rothrock, and W.F. Weeks, Science program for an imaging radar receiving station in Alaska, NASA/JPL, 1983.
74.
D.K. Hall and J.P. Ormsby, Use of Seasat synthetic aperture radar and Landsat multispectral scanner subsystem data for Alaskan glaciology studies, J. Geophys. Res. 88, 1597 (1983).
75.
Itek. Conceptual design of an automated mapping satellite system, Itek Optical Systems 81-8449A-I (January 1981).
76.
A.P. Colvocoresses, An automated mapping satellite Engineering and Remote Sensing 48, 1585 (1982).
77.
D.P. Rublncam, Postglacial rebound observed by Lageos and the effective viscosity of the lower mantle, J. Geophys. Res. 89, 1077 (1984).
78.
B.K. Lucchltta, Ice sculpture in the Martian outflow channels, J. Geophys. Res. 87, 9951 (1982).
79.
N.W. Young, Measured velocities of interior East Antarctica balance within the I.A.G.P. area, J. Glaclolog~ 24, 77 (1979).
80.
W.F. Budd, D. Jenssen, and U. Radok, Derived physical characteristics of the Antarctic ice sheet, ANARE Interim Reports, Series A (IV) Glaciology 120, Antarctic Division, Department of Supply, Melbourne (1971).
81.
Changing cllmate, Report of the Carbon Dioxide Assessment Committee, National Academy Press, Washington D.C., 1983.
82.
J.S. Hoffman, D. Keyes, and J.G. Titus, Projecting future sea level rise, methodology, estimates to the year 2100, and research needs, EPA 230-09-007, Environmental Protection Agency, Washington D.C. (October 1983).
system
(Mapsat),
and
the
mapping
of
Photogrammetrlc
state of mass
274
C. Swithinbank
83. D. Ayres, private communication (1984). 84. V. Gornitz, S. Lebedeff, Science 215, 1611 (1982). 85. C. Swithinbank, 289 (1983).
and J. Hansen,
Global
sea level
trend in the past century,
Towards an inventory of the great ice sheets,
86. R.L. Brooks, W.J. Campbell, R.O. Ramseier, H.R. Stanley, topography by satellite altimetry, Nature 274, 539 (1978).
Geo~rafiska Annaler 65A,
and H.J.
Zwally,
Ice
sheet
87. T.V. Martin, H.J. Zwally, A.C. Brenner, and R.A. Bindschadler, Analysis and retracking of continental ice sheet radar altimeter waveforms, J" Geophys. Res. 88, 1608 (1983). 88. R.L. Brooks, R.S. Williams Jr., J.G. Ferrigno, and W.B. Krabill, Amery topography from satellite radar altimetry, in: Antarctic Earth Science, Oliver, P.R. James, and J.B. Jago, Cambridge University Press 1983, p. 441. 89. R.H. Thomas, T.V. Martin, and H.J. Zwally, Mapping altimetry data, Annals of Glaciolo~y 4, 283 (1983). 90. R.L. Brooks, 32 (1982).
ice-sheet
margins
Ice Shelf eds. R.L.
from
radar
Satellite altimeter results over East Antarctica, Annals of Glaciolosy 3,
91. H.J. Zwally, R.A. Bindsehadler, A.C. Brenner, T.V. Martin, and R.H. Thomas, Surface elevation contours of Greenland and Antarctic ice sheets, J. Geophys. Res. 88, 1589 (1983). 92. A.C. Brenner, R.A. Bindschadler, R.H. Thomas, and H.J. Zwally, Slope-induced errors in radar altimetry over continental ice sheets, J. Geophys. Res. 88, 1617 (1983). 93. H.J. Zwally, R.H. Thomas, and R.A. Bindschadler, Ice-sheet dynamics by laser altimetry, NASA Goddard Space Flight Center, Greenbelt, Tech. Memo. 82128, 1981. 94. ICEX Ice and Climate Experiment, Report of Science and Applications Working Group, NASA Goddard Space Flight Center, Greenbelt, 1979. 95. J,L. Bufton, J.E. Robinson, M.D. Femiano, and F.S. Flatow, Laser altimetry measurement of ice-sheet topography, NASA Goddard Space Flight Center, 1981.
for
96. G.K.A. Oswald, Investigations of sub-ice bedrock characteristics by radio echo sounding, J. Glaciology 15, 75 (1975). 97. Ocean-cryosphere topography advanced radar altimeter proposal for Radarsat, Natural Environment Research Council-Science and Engineering Research Council, Swindon, England (April 1983). 98. A study of satellite radar altimeter operation over Ice-covered surfaces, European Space Agency Contract Report 5182/82/F/CG(SC) (May 1983). 99. A study of ocean and ice topography from space, a joint NERC-SERC proposal for development of satellite altimetry, Natural Environment Research Council-Science Engineering Research Council, Swindon, England (September 1983). i00. International Union Resolution 8.
of Geodesy
and Geophysics,
XVIII General Assembly,
Hamburg
the and
1983,
i01. I. Allison, ed., Antarctic Climate Research, Scientific Committee on Antarctic Research (1983). 102. R.S. Williams Jr., and W.D. Carter (eds.), ERTS-I, a new window Geological Survey Professional Paper 929, Washington D.C. (1976).
on
our
planet,
103. N.M. Short, P.D. Lowman Jr., S.C. Freden, and W.A. Finch Jr., Mission to Earth: Landsat views the world, National Aeronautics and Space Administration, Washington D.C., NASA SP-360 (1976). 104. R.S. Williams Jr., private communication (1984). 105. B.K. Lucchitta, private communication (1984).