metadata of the chapter that will be visualized online · 20 that contain water in its frozen form....

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Chapter Title Cryosphere: Spaceborne and Airborne Measurements/Monitoring

Copyright Year 2011

Copyright Holder Springer Science+Business Media, LLC

Corresponding Author Family Name Jezek

Particle

Given Name Kenneth C.

Suffix

Division/Department Byrd Polar Research Center, School of Earth

Sciences

Organization/University The Ohio State University

Postcode 1090

Street Carmack Road

City Columbus

State OH

Country USA

Phone (614) 292-7973

Fax (614) 292-7688

Email [email protected]

Comp. by: KArunKumar Stage: Galleys Chapter No.: 717 Title Name: ESSTPage Number: 0 Date:23/8/11 Time:07:06:43

1 C

2 Cryosphere: Spaceborne and3 Airborne Measurements/Monitoring

4Au1 KENNETH C. JEZEK

5 The Ohio State University

6 Columbus, OH, USA

7 Article Outline

8 Glossary

9 Definition of the Subject: The Cryosphere

10 Introduction

11 Early History of Cryospheric Remote Sensing

12 Scientific Advances from Airborne and Spaceborne

13 Remote Sensing

14 Cooperative Efforts to Observe, Monitor, and Under-

15 stand the Cryosphere

16 Future Directions

17 Bibliography

18 Glossary

19 Cryosphere Those components of the Earth system

20 that contain water in its frozen form.

21 Radar Radio detection and ranging systems.

22 Lidar Light detection and ranging systems.

23 Radiometers Radio frequency receivers designed to

24 detect emitted radiation from a surface and in

25 accordance with Planck’s law.

26 Synthetic aperture radar Radar system which

27 increases along track resolution by using the

28 motion of the platform to synthesize a large

29 antenna.

30 Permafrost Persistently frozen ground.

31 Ice sheet Continental-scale, freshwater ice cover that

32 deforms under its own weight.

33 Sea ice Saline ice formed when ocean water freezes.

34 Glaciers Long, channelized, slabs of freshwater ice

35 thick enough to deform under their own weight.

36Seasonal snow The annual snow that blankets land

37cover in the winter and melts by summer.

38Definition of the Subject: The Cryosphere Au2

39The Cryosphere broadly constitutes all the components

40of the Earth system which contain water in a frozen

41state [1]. As such, glaciers, ice sheets, snow cover, lake

42and river ice, and permafrost make up the terrestrial

43elements of the Cryosphere. Sea ice in all of its forms,

44frozen sea bed and icebergs constitute the oceanic ele-

45ments of the Cryosphere while ice particles in the upper

46atmosphere and icy precipitation near the surface are

47the representative members of the Cryosphere in atmo-

48spheric systems. This overarching definition of Earth’s

49cryosphere immediately implies that substantial por-

50tions of Earth’s land and ocean surfaces are directly

51subject in some fashion to cryospheric processes.

52Through globally interacting processes such as the

53inevitable transfer of heat from the warm equatorial

54oceans to the cold polar latitudes, it seems reasonable

55to argue that all regions of Earth are influenced by

56cryospheric processes and their integration into the

57modern climate of the planet. Observations of the

58cryosphere necessary to predict future variability in

59Earth’s ice cover and its interaction with other Earth

60systems must be made on commensurate spatial and

61temporal scales. Consequently, airborne and

62spaceborne remote sensing technologies with global

63reach play a key role in acquiring data necessary to

64understand the important physical processes and

65earth system interactions that govern the evolution of

66the Cryosphere (Fig. 1).

67Introduction

68The broad spatial and seasonally changing distribution

69of ice plays an important role in earth systems and

70human activities. At high latitudes, ice covered land

71and ocean surfaces are highly reflective thus redirecting

72incoming solar radiation back into space in the

Robert A. Meyers (ed.), Encyclopedia of Sustainability Science and Technology, DOI 10.1007/978-1-4419-0851-3,# Springer Science+Business Media, LLC 2011


73 summer months. Indeed, reductions in the spatial area

74 of snow and ice cover are believed to be an important

75 feedback mechanism that enhances warming at high

76 latitudes [2, 3]. Essentially, reduced snow and ice cover

77 exposes darker land and ocean surfaces that retain

78 rather than reflect solar energy. This results in increased

79 warming and hence a further decrease in the area of

80 snow and ice covered surfaces. Sea ice is an important

81 habitat for birds such as penguins and mammals such

82 as seals and polar bears which thrive in this icy envi-

83 ronment [4]. But the underside of sea ice is also an

84 important refuge for some of the smallest creatures

85 including the shrimp-like krill which graze upon algae

86 that grows just beneath and within the ice canopy

87 [5, 6]. Terrestrial permafrost and frozen sediments

88 beneath the oceans support an important reservoir of

89 organic carbon and gas hydrates [7]. As permafrost

90 melts, methane can be released contributing to the

91 increasing concentration of greenhouse gases in the

92 atmosphere. Glaciers and ice sheets are vast reservoirs

93 of Earth’s freshwater. As the ice sheets thin, water flows

94 from the ice sheets into the oceans raising global sea

95 level [8]. In terms of our daily activities, seasonal snow

96 and glaciers are important sources of spring runoff for

97 irrigation and power generation, while ice jams on

98 rivers constitute important obstacles to winter-time

99 navigation and can cause low-land flooding [9].

100 Thawing permafrost causes the land near surface to

101 become unstable which can result in catastrophic struc-

102 tural failures in buildings.

103 The global span of cryospheric processes and the

104 strong daily to seasonal swing in the extent of snow and

105 ice makes studying and monitoring the cryosphere an

106 especially challenging scientific objective [10]. More-

107 over the variety of forms in which ice can be manifest in

108 Earth systems means that no single observing system is

109 capable of making adequate observations. Rather an

110 ensemble of techniques is required to fully appreciate

111 and eventually understand the complexities of the

112 cryosphere and its interaction with other earth systems.

113 Locally, observing tools may include direct, field mea-

114 surements of snow pack thickness or physical temper-

115 ature. Much different sets of tools are needed to

116 characterize the cryosphere on a global and annual

117 scale where the inhospitable climate and the physical

118 remoteness of many sectors of the cryosphere represent

119 obstacles to scientific investigation. Here, aircraft and

120spacecraft mounted instruments are required to infer

121geophysical properties from a remote distance (Fig. 2).

122Compounded with these geographic challenges,

123observations of the icy surface often have to overcome

124cloud cover that frequently obscures the high latitudes,

125as well as continuing measurements during the long

126polar night. Consequently a range of instruments gen-

127erally rely on distinguishing properties evident in the

128broad-spectral electrical characteristic of icy terrain.

129For example, even the cloudy atmosphere is largely

130transparent to microwave radiation. That fact com-

131bined with the very different microwave emission of

132open ocean and sea ice enables spaceborne microwave

133observations of annual sea ice extent and concentration

134day and night and in all weather. Similarly the electrical

135contrast between rock and ice enables airborne radio

136frequency radar measurements of the thickness of polar

137ice sheets and glaciers. However some of the most

138recent advances in cryospheric science have been

139made by relying on one of the most basic properties

140of the icy cover, namely its mass. Spaceborne measure-

141ments of the changing gravitational attraction of vari-

142ably sized snow and ice bodies enables direct estimates

143of changing polar ice mass and the redistribution of

144melting ice into the liquid oceans. Indeed it can be

145argued that engineering and technological advances in

146airborne and remote sensing have had some of their

147greatest scientific impacts in the understanding the

148evolution of Earth’s ice cover.

149This article provides an overview of remote sensing

150of the cryosphere from both aircraft and spacecraft.

151A brief historical review of remote sensing of snow

152and ice is followed by a discussion of the physics of

153remote sensing of the cryosphere and how develop-

154ments in remote sensing have led to a series of impor-

155tant scientific advances. These include the realization

156of the diminishing extent of Arctic sea ice and the

157thinning of glaciers, ice caps, and ice sheets worldwide.

158Both of these observations form critical, direct evidence

159of changing world climate. The article concludes with

160a discussion of developing international collaborations

161that are aimed at pooling technologically sophisticated

162and operationally expensive international assets so as

163to obtain an integrated system of continuing remote

164sensing observations necessary to predict future changes

165in Earth’s ice and the consequent impacts on human

166activities.

2 C Cryosphere: Spaceborne and Airborne Measurements/Monitoring


167 Early History of Cryospheric Remote Sensing

168 The science and operational communities have always

169 been quick to adopt new instruments and platforms for

170 use in observing snow and ice in all of its forms. This

171 section reviews early developments in cryospheric

172 remote sensing – which were in part driven by science

173 and in part by the basic desire to explore the most

174 remote regions of Earth.

175 Airborne Photo-Reconnaissance

176 Aerial photography of ice covered terrain began during

177 early twentieth century expeditions to the high atitudes

178 and was used primarily to document the progress of the

179 expedition. Survey quality aerial mapping was adopted

180 using techniques primarily developed during World

181 War I [11]. Mittelholzer and others [12] writing

182 about the 1923 Junkers Expedition to Spitzbergen

183 offer a very complete overview of the geographic and

184 cartographic objectives for aerial photography in the

185 North including a brief discussion of glacier formation

186 as revealed by the aerial photographs. They also give an

187 interesting technical discussion of the challenges faced

188 when doing aerial photographic reconnaissance over

189 highly reflective snow-covered terrain.

190 Wilkins documented ice cover in the Antarctic Pen-

191 insula during the first successful flight in Antarctica by

192 using a hand-held, folding Kodak 3A camera [13, 14].

193 Richard E. Byrd devoted time and resources to aerial

194 photography for quantitative surveying purposes dur-

195 ing his first Antarctic Expedition of 1928–1930. In his

196 book, “Little America,” Byrd [15] writes that photo-

197 graphs from Ashley McKinley’s laboratory provided

198 “perhaps, the most important geographical informa-

199 tion from the expedition.” McKinley was third in com-

200 mand of the expedition and the aerial surveyor.

201 McKinley operated his Fairchild K-3 mapping camera

202 during Byrd’s 1929 historic flight to the South Pole.

203 These early airborne photographic records were

204 acquired with great skill and at considerable risk. The

205 quality of the photographs is often exceptional and the

206 photos themselves represent an oft under-utilized

207 resource for directly gauging century-scale changes in

208 Earth’s ice cover.

209Satellite Photography

210A mere 32 years after Byrd’s aerial photographic

211surveying in Antarctica, spaceborne cameras began

212capturing unique pictures of Earth. CORONA,

213ARGON, and LANYARD were the first three opera-

214tional imaging satellite reconnaissance systems and

215they acquired data during the early 1960s for both

216detailed reconnaissance purposes and for regional

217mapping [16–18]. Early reconnaissance satellite pho-

218tographs provide a unique view of our world as it

219appeared at the beginning of the space age. Researchers

220in the environmental science community are the most

221recent beneficiaries of these data after they were

222declassified and made publicly available in 1995

223through the efforts of Vice President Al Gore along

224with several government agencies working together

225with civilian scientists as part of the MEDEA program

226[19]. Polar researchers in particular inherited a wealth

227of detailed photography covering both of the great

228polar ice sheets. After processing with sophisticated

229digitizing instruments and subsequent analysis with

230modern photogrammetric and image processing tech-

231niques, investigators have shown that these data can be

232used to characterize local fluctuations in glacier termini

233[20, 21], investigate large-scale flow features on ice

234sheets [22, 23] and tomeasure long-term average veloc-

235ity by feature retracking techniques [24] (Fig. 3).

236Development of Depth Sounding Radar

237Early suggestions that glaciers were penetrated by radio

238signals are attributed to observations made at Little

239America during Byrd’s 2nd Antarctic Expedition [25].

240This observation, along with reports that pulsed radar

241altimeters were yielding faulty readings over glaciers,

242led to the first radar experiments to measure ice thick-

243ness in 1955. In 1960, Waite and Schmidt [25] made

244measurements over Greenland from aircraft at 110,

245220, 440, and 4,300 MHz. These results initiated a rev-

246olution in glaciology because the ice thickness and

247internal structure of glaciers and ice sheets could be

248rapidly sounded from aircraft [26].

249Although the sophistication of ice sounding radar

250systems has increased tremendously, the basic principle

251of the technique remains the same and continues to be

252a fundamental tool used by researchers (Fig. 4). Essen-

253tially, the one-way travel time of a radar pulse

3CCryosphere: Spaceborne and Airborne Measurements/Monitoring


254 transmitted through the ice or snow is multiplied by

255 the appropriate wave speed and the thickness so deter-

256 mined. More complex, two dimensional maps of the

257 glacier bed topography can be assembled frommultiple

258 profiles that are combined using travel time migration

259 techniques [27]. Several snow and ice sounding radars

260 are part of the primary instrument suite presently

261 carried aboard aircraft supporting NASA’s IceBridge

262 program [28].

263 Depth sounding radar remains one of the few tech-

264 niques available to researchers interested in probing the

265 volume of the terrestrial ice cover and the properties of

266 the underlying bed. Airborne radars have been success-

267 fully used to study glacier, ice sheets, and to a more

268 limited degree sea ice and permafrost. Seismic tech-

269 niques yield important complementary information

270 but can only be carried out only in situ. Airborne

271 gravity measurements provide important regional

272 information but are generally less accurate than the

273 radar technique for measurements on glaciers.

274 Scientific Advances from Airborne and

275 Spaceborne Remote Sensing

276 In most instances, airborne and spaceborne platforms

277 carry similar kinds of remote sensing instruments.

278 Often times, spaceborne instruments are first proto-

279 typed as part of initial airborne campaigns. However,

280 advantages of instrument installations on multiple

281 kinds of platforms go far beyond vetting the effective-

282 ness of an instrument. For example, spaceborne sys-

283 tems can provide global scale observations, with some

284 imaging instruments yielding pole-to-pole observa-

285 tions on a daily basis. Manned and increasingly

286 unmanned aircraft, with their ability to maneuver and

287 loiter over an area, can provide much denser spatial

288 and temporal coverage when profiling instruments,

289 such as altimeters, are of interest. This section begins

290 with a review of remote sensing physics and then sum-

291 marizes key applications of remote sensing to several

292 cold regions themes.

293 Basics of Remote Sensing of the Cryosphere

294 Airborne and spaceborne sensors measure local changes

295 in electromagnetic, gravitational, and magnetic force

296 fields. Disturbances can arise from the passive, thermo-

297 dynamically driven electromagnetic radiation emitted by

298the earth’s distant surface, or the reflectance of incoming

299solar radiation from the Earth’s surface, both of which

300change with the terrain type. Instruments and associ-

301ated satellites of this type include the Electrically

302Scanning Microwave Radiometer (ESMR), Scanning

303Multichannel Microwave Radiometer (SMMR), Special

304Sensor Microwave Radiometer (SSMI), Advanced Very

305High Resolution Radiometer (AVHRR), Landsat,

306Moderate Resolution Imaging Spectrometer (MODIS),

307Medium Resolution Imaging Spectrometer (MERIS),

308Satellite Pour l’Observation de la Terre (SPOT). Distur-

309bances can also arise from the active behavior of the

310sensor itself. Radars and lidars include the Airborne

311Topographic Mapper (ATM), Laser Vegetation Imaging

312Sensor (LVIS), synthetic aperture radar (SAR) and radar

313altimeters on the European Remote Sensing Satellite

314(ERS-1/2), RADARSAT 1/2, synthetic aperture radar

315and radar altimeter on Envisat, Phased Array L-band

316SAR, TerraSAR-X, GEOSAT, the Ice and Climate Exper-

317iment Satellite (ICESat) lidar, the Cryosat radar altime-

318ter. These instruments illuminate the surface with

319electromagnetic signals, which upon reflection can be

320detected back at the sensor after an elapsed time. Local

321changes in the gravity field at airborne and spaceborne

322elevations are indications of changes in the distribution

323of mass within the Earth. Such instruments include

324airborne gravimeters on NASA’s IceBridge, NASA’s

325Gravity Recovery and Climate Experiment (GRACE)

326satellite, and ESA’s Gravity field and Ocean Circulation

327Explorer (GOCE) satellite.

328The task of the remote sensing scientist is to take

329measurements of these basic force fields and infer from

330them geophysical properties about the Earth [29]. For

331the cryosphere, examples include using changes in the

332gravitational field to estimate changes in the mass of

333glaciers and ice sheets as is done with the GRACE

334satellite. Other examples include using radars and

335lidars to measure the time of flight of signals reflected

336off the ice surface and use that information to measure

337elevation and elevation changes on sea ice and ice

338sheets. Finally, scattered solar radiation can be used to

339acquire hyper-spectral images for surface characteriza-

340tion and for traditional mapping of surface features.

341Accomplishing this task requires technical informa-

342tion about the behavior of the instrument, knowledge

343about the physical relationships between the measured

344field and the surface under study, and the development


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345 of algorithms that correctly invert the measured signal

346 into some desired property [30, 31]. In some cases this

347 task can be relatively easy. Because glacier ice is nearly

348 homogeneous and because it is almost transparent at

349 radar frequencies from about 1 to 500 MHz [32], the

350 propagation time of a radar echo through the ice sheet

351 and back can be converted to ice thickness by simply

352 knowing the average ice dielectric constant, which in

353 turn is readily convertible into a propagation velocity.

354 In other cases, the task is more challenging because of

355 heterogeneities in the material. The physical and elec-

356 trical properties of sea ice change substantially as the ice

357 ages [33]. This leads to changes in the thermally driven,

358 microwave emission from the surface. The amount of

359 energy received, measured in terms of a brightness

360 temperature, is related to the product of the physical

361 temperature and the emissivity of the surface. For open

362 water, the microwave brightness temperature is cool

363 because little energy escapes from the ocean surface.

364 For sea ice, the brightness temperature is warm because

365 more energy is transmitted across the electrically less

366 reflective snow surface at these frequencies. Conse-

367 quently, brightness temperature can be used tomeasure

368 sea ice concentration and extent with high accuracy

369 (several percent). Figure 5 shows that just on the basis

370 of brightness temperature maps alone it is easy to

371 distinguish the annual cycle of sea ice growth and

372 decay across the arctic.

373 Sea Ice Extent, Concentration, Motion, and

374 Thickness

375 Sea ice modulates polar climate by restricting the flow

376 of heat from the relatively warm polar ocean into the

377 relatively cold polar atmosphere. Because freezing ice

378 preferentially rejects impurities from the crystalline

379 lattice, the growth of sea ice modulates ocean circula-

380 tion by releasing dense, cold brine that sinks beneath

381 the marginal ice zones and initiates oceanic convection.

382 Sea ice represents a natural barrier to surface naviga-

383 tion and so changes in sea ice cover have important

384 consequences for future development of the

385 Arctic. While sea ice is usually distinguishable from

386 open ocean at optical wavelengths, cloud cover and

387 the long polar night dictate the use of all-weather,

388 day/night passive microwave radiometers for monitor-

389 ing sea ice extent and concentration. As noted above,

390these radiometers measure the radiant energy emitted

391from the surface that, on one hand, can help identify ice

392type and age but, on the other hand, leads to complex-

393ities in designing more sophisticated instruments capa-

394ble of sorting out whether the observed changes are due

395to different ice types populating a scene or because of

396change in the fractions of sea ice and open water in the

397scene. Moreover, the dimensions of the area sampled

398on the surface are often quite large (Hollinger and

399others [34] quote a field of view of 37 � 28 km for

400the SSMI 37 GHz vertically polarized channel). Conse-

401quently, approaches for estimating the concentration

402within a single pixel are required. A simple algorithm

403relies on the fact that the total emitted energy or equiv-

404alently the brightness temperature must equal the

405brightness temperature of the components in the

406scene times their respective concentrations within the

407pixel [35]. The component brightness temperature as

408are selected by tie points (such as a tie point for the

409brightness temperature of open water or first-year sea

410ice) which allows for a solution. More sophisticated

411algorithms attempt to better characterize regions

412where ice concentrations are low and where the data

413may be contaminated by weather effects. The results of

414these analyses are detailed measurements of ice extent

415which document the dramatic decrease in Arctic sea ice

416cover (Fig. 6). Graphs such as these represent some of

417the most compelling and straightforward evidence for

418changing climate at high latitudes.

419Images such as those in Fig. 5 can also be used to

420document the coarse motion of the sea ice by tracking

421common features in repeat images. Better results are

422obtained with increasing resolution ranging from the

423several kilometer resolution obtainable with AVHRR to

424the very fine resolutions (tens of meters or less) achiev-

425able with SAR. Combined with airborne (Airborne

426Topographic Mapper flown as part of IceBridge) and

427spaceborne (ERS-1/2, ICESat, Cryosat-2) altimeter

428estimates of ice thickness, circulation driven changes

429in ice thickness about Antarctica [36] and the total ice

430flux across the Arctic and out into the marginal seas can

431be computed. Based on ICESat estimates of sea ice

432freeboard, Kwok and others [37] estimate a 0.6 m thin-

433ning in Arctic multiyear ice for the period from 2003 to

4342008.


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temperatures are


435 Regional Image Mapping of Glaciers and Ice Sheets

436 Airborne and spaceborne image mapping of glaciers

437 and ice sheets focuses on basic physical characteristics

438 such as glacier termini, glacier snow lines, crevasse

439 patterns, snow facies boundaries [38]. These properties

440 have been successfully monitored with optical and

441 microwave imaging instruments. As described in sec-

442 tion “Early History of Cryospheric Remote Sensing,”

443 high latitude mapping began as early as 1962 with the

444 launch of the Argon satellite. Shortly thereafter, the

445 1970s Landsat 1, 2, and 3 Multi-Spectral Scanner

446 (MSS) images constitute an important glaciological

447 resource [39, 40] and have been compiled into a series

448 of beautiful folios edited by R. Williams Jr. and J.

449 Ferrigno of the U.S. Geological Survey [see 41].

450 Compiling separate images into seamless, high-

451 resolution digital maps of continental-scale areas is

452 complicated by the data volume and the challenges in

453 accurately estimating satellite orbits and instrument

454 viewing geometries along orbit segments that might

455 span from coast to coast. Initial successes in large-

456 scale mapping were achieved through use of the mod-

457 erate spatial resolution (1–2.5 km) and wide swath

458 (2,400 km) Advanced Very High Resolution Radiome-

459 ter (AVHRR) images [42] which helped reveal details

460 about ice stream flow inWest Antarctica [43]. After the

461 original AVHRRmosaic of Antarctica, the United State

462 Geological Survey (USGS) made subsequent improve-

463 ments to the mosaic by eliminating more cloud, sepa-

464 rating the thermal band information to illustrate

465 surface features more clearly, and correcting the coast-

466 line of the mosaic to include grounded ice while

467 excluding thin, floating fast ice [44]. The European

468 Earth Resources Satellite – 1 carried on board

469 a synthetic aperture radar which allowed for large-

470 scale regional mapping. Fahnestock and others [45]

471 compiled a mosaic of Greenland which revealed the

472 existence of a long ice stream in north east Greenland.

473 In 1997, RADARSAT-1 synthetic aperture radar (SAR)

474 data were successfully acquired over the entirety of

475 Antarctica (Fig. 7). The coverage is complete and was

476 used to create the first, high-resolution (25 m) radar

477 image mosaic of Antarctica [46–48] The image was

478 used tomap the ice sheet margin [49] and to investigate

479 patterns of ice flow across Antarctica resulting in the

480 discovery of large ice streams that drain from Coates

481Land into the Filchner Ice Shelf. Other large-scale map-

482ping has been completed with MODIS for both the

483Arctic and the Antarctic [50]. Most recently, Landsat

484imagery of Antarctica has been compiled into a single,

485easily accessible map-quality data set [51] and SPOT

486stereo imagery has been used to derive digital elevation

487models of ice sheets, ice caps, and glaciers [52].

488InSAR Measurements of Glaciers and Ice Sheets

489Surface Velocity

490Glaciers and ice sheets move under the load of their

491own weight. They spread and thin in a fashion dictated

492by their thickness, the material properties of ice, and

493the environmental conditions operative on the glacier

494surface, sides and bottom. The rate and direction of

495motion reveals important information about the forces

496acting on the glacier, provides knowledge about the rate

497at which ice is pouring into the coastal seas, and enables

498scientists to predict how the ice sheet might respond to

499changing global climate.

500Since the International Geophysical Year of 1957–

5011958 and before, scientists have placed markers on the

502ice sheet and then, using a variety of navigation tech-

503niques from solar observations to GPS, have

504remeasured their positions to calculate motion. More

505recently, scientists have used high-resolution satellite

506images to track the position of crevasses carried along

507with the glacier to compute surface motion. These

508approaches are time consuming and result in patchy

509estimates of the surface velocity field. During the early

5101990s, researchers at the Jet Propulsion Laboratory

511showed that synthetic aperture radar (SAR) offered

512a revolutionary new technique for estimating the sur-

513face motion of glaciers [53]. Here, the SAR is operated

514as an interferometer. That is, the distance from the SAR

515to a point on the surface is computed by measuring the

516relative number of radar-wave cycles needed to span

517the distance between the radar and the surface. Later,

518another measurement is made from a slightly different

519position and the numbers of cycles is computed again.

520The difference in the number of cycles combined with

521control points is used to estimate relative displacement

522to about one quarter of a radar-wave cycle (just a few

523centimeters for RADARSAT-1). Given an estimate of

524surface topography, this enables measurement of even

525the slowest moving portions of glaciers [54, 55].



526 A related approach relies on the fact that images

527 formed from coherent radar signals scattered off

528 a rough surface will have a small-scale-speckly appear-

529 ance. The speckle pattern is random over scene but it is

530 stable from scene to scene over a short time. Tracking

531 the speckle pattern over time enables another measure-

532 ment of surface velocity and while less accurate than

533 the interferometric phase approach, speckle retracking

534 has the advantage of yielding estimates of two compo-

535 nents of the velocity vector [56, 57]. Typically a mixture

536 of phase interferometry and speckle retracking are used

537 in glacier motion studies.

538 Surface topography can also be estimated using

539 InSAR techniques in cases where the surface velocities

540 are small, the period between repeat observations is

541 short [58], when multiple interferometric pairs are

542 available [59], or whenmultiple antennas enable acqui-

543 sition of interferometric data in a single pass as was

544 done with NASA’s Shuttle Radar Topographic Mission

545 [60]. Although the surface elevation accuracy of InSAR

546 topography is typically on the order of a few meters, the

547 results can be used to estimate local slopes and also in

548 comparison with earlier data provide a rough estimate

549 of mass change.

550 Surface elevation and velocity data are key to stud-

551 ies of glacier dynamics [61]. InSAR data have been used

552 to investigate the relative importance of different resis-

553 tive stresses acting on ice streams and to predict the

554 future behavior of ice streams and glaciers currently in

555 retreat [62, 63]. Measurements on mostly retreating

556 glaciers in the Himalayas [64], Patagonia [65, 66],

557 European Alps [67], and Alaska (Fig. 8) document

558 changing surface elevation and internal dynamics,

559 and, together with estimates of ice thickness and sur-

560 face accumulation rate, can provide a direct estimate of

561 glacier mass loss [68].

562 Glaciers and Ice Sheets Mass Loss

563 Glaciers and ice sheets are reservoirs of freshwater with

564 over 90% of Earth’s freshwater bound in the Antarctic

565 Ice Sheet [69], whichwhen depleted have local, regional

566 and global impacts. Indirect approaches for identifying

567 whether ice sheets are losing mass include using proxy

568 indicators such as surface melt area and duration –

569 both measureable using passive microwave techniques

570 [70, 71]. More directly, there are three primary remote

571sensing techniques currently used to assess the chang-

572ing volume (or mass) of ice contained in glaciers [72].

573The first involves an estimate of the difference between

574the annual net accumulation of mass on the surface of

575the glacier and the flux of ice lost from the terminus.

576The flux from the terminus is calculated using the

577measured ice thickness from airborne radar and the

578surface velocity, which is currently best estimated

579with InSAR [73]. The second approach is to measure

580surface elevation change. This has been accomplished

581with spaceborne radar altimeters [74, 75] and airborne

582and spaceborne lasers [76–78]. Finally, the changing

583glacial mass can be estimated directly by measuring

584changes in the gravity field as has been done with

585GRACE [79–81]. Figure 9 illustrates the estimated

586mass loss rate from Alaskan Glaciers as illustrated

587using GRACE data (Updated from [82]).

588Recent analyses suggest these different techniques

589are yielding similar mass reductions for the Greenland

590Ice sheet [8, 83]. Thinning is primarily in the coastal

591regions where the rate of thinning has been increasing

592since the early 1990s and the current rates of mass loss

593estimated between 150 and 250 Gt/year. West Antarc-

594tica data indicate mass loss similar to Greenland,

595whereas East Antarctica retains a slightly positive

596(50 Gt/year) mass balance. Using surface mass balance

597estimates and GRACE gravity data, Rignot and others

598[84] report a combined ice sheet thinning rate of

599475 +- Au3158 Gt/year. Cazenave and Llovel [8] conclude

600that for the period between 1993 and 2007 about 55%

601of total sea level rise can be attributed to melting of

602glaciers and ice sheets.

603The Seasonal Snow Pack

604Seasonal snow cover plays an important role in regional

605hydrology and water resource management. Rapid

606melting of the seasonal snow pack across the northern

607Great Plains in April 1997 resulted in catastrophic

608flooding of the Red River. From a climate perspective,

609the bright snow surface also serves as an effective mir-

610ror for returning incoming solar radiation back into

611space thus modulating the planetary heat budget.

612The spectral reflectivity differences between snow,

613cloud, and other land cover types enables the seasonal

614snow cover to be routinely mapped globally using

615visible and infrared sensors such as NOAA’s AVHRR,



616 NASA’s MODIS, and ESA’s MERIS instruments. Snow

617 may be mapped based on visual inspection of multi-

618 spectral imagery. Automatic snow detection is accom-

619 plished by computing the normalized difference

620 between, for example, the MODIS visible band

621 (0.545–0.565 mm) and the near infrared band (1.628–

622 1.652 mm). Snow is detected when the normalized

623 difference exceeds a threshold value of 0.4 and when

624 other criteria on land and cloud cover are met [85]

625 (Fig. 10). Based on analysis of the NOAA 35 year data

626 record, Dery and Brown [86] conclude that springtime

627 snow extent across the northern hemisphere declined

628 by some 1.28 � 106 km2 over a 35 year period.

629 Snow thickness and hence indirectly the mass of

630 snow are key variables for estimating the volume of

631 water available in a reservoir and potentially releasable

632 as runoff. The most successful techniques to date have

633 relied on passive-microwave-based algorithms. One

634 approach for estimating snow thickness is to difference

635 19 and 37 GHz brightness temperature data that along

636 with a proportionality constant yields an estimate of

637 the snow thickness. The algorithm is based on the fact

638 that 19 GHz radiation tends to minimize variations in

639 ground temperature because it is less affected by the

640 snow pack. The 37 GHz radiation is strongly scattered

641 by the snow grains and brightness temperature at this

642 frequency decreases rapidly with snow thickness/snow

643 water equivalent. Factors which confuse this algorithm

644 include topography and changes in ground cover [87].

645 Information about the seasonal onset of snowmelt

646 can be obtained from microwave data. A few percent

647 increase in the amount of free water in the snow pack

648 causes the snow emissivity to approach unity resulting

649 in a dramatic increase in passive microwave brightness

650 temperature. This fact has been successfully used to

651 track the annual melt extent on the ice sheets and also

652 to track the springtime melt progression across the

653 arctic. Higher resolution estimates of melt extent can

654 be obtained with scatterometer and SAR data. These

655 data generally show an earlier spring time date for the

656 beginning of melt onset and a later date for the fall

657 freeze [88].

658 Lake and River Ice

659 Lake and river ice form seasonally at mid and high

660 latitudes and elevations. River ice forms under the

661flow of turbulent water which governs its thickness.

662The combination of ice jams on rivers with increased

663water flow during springtime snowmelt can result in

664catastrophic floods. Lake ice forms under less dynamic

665conditions resulting in a smoother ice surface that acts

666as an insulator to the underlying water. Hence lacus-

667trine biology is strongly influenced by the formation of

668the ice canopy. The start of ice formation and the start

669of springtime ice break up are proxy indicators for

670changes in local climate as well as impacts on the ability

671to navigate these waterways.

672Streams, rivers, lakes of all sizes dot the landscape.

673Locally, ice cover observations can be made from air-

674craft. Regionally or globally, river and lake ice moni-

675toring is challenging because physical dimensions (long

676but narrow rivers) often require high resolution instru-

677ments like medium- to high-resolution optical data

678(Fig. 11) or SAR to resolve details [89]. Moreover,

679since the exact date of key processes, such as the onset

680of ice formation or river-ice breakup are unknown,

681voluminous data sets are required to support large-

682scale studies.

683Permafrost

684Permafrost presents one of the greatest challenges for

685regional remote sensing technologies [7, 90]. The near

686surface active layer, the shallow zone where seasonal

687temperature swings allow for annual freeze and thaw, is

688complicated by different soil types and vegetative cover.

689This combination tends to hide the underlying persis-

690tently frozen ground from the usual airborne and

691spaceborne techniques mentioned above. Even in win-

692ter when the active layer is frozen, remote sensing of the

693permafrost at depth is extremely difficult because of the

694spatially variable electrical properties of the material.

695Thus far, the most successful airborne and spaceborne

696remote sensing methods involve optical photography

697to identify surface morphologies as proxy indicators of

698the presence of permafrost. Patterned ground and

699pingos are examples of the types of features visible in

700optical imagery and that are diagnostic of underlying

701permafrost. SAR interferometry has been suggested as

702another tool that can be used to monitor terrain

703for slumping associated with thawing permafrost.

704Figure 12 shows E01 Hyperion satellite, visible-band

705data collected over the north slope of Alaska. Shallow,



706 oval shaped lakes form in thermokarst, which develops

707 when ice rich permafrost thaws and forms

708 a hummocky terrain. Lakes and depressions left by

709 drained lakes are densely distributed across the tundra.

710 The long axis of the lake is oriented perpendicular to

711 the prevailing wind direction. SAR intensity images

712 have been used to determine that most of these small

713 lakes freeze completely to the bottom during the winter

714 months [91].

715 Recent Developments in Airborne Radar Ice

716 Sounding of Glaciers

717 Today, airborne radars operating between about 5 and

718 500 MHz are the primary tools used for measuring ice

719 sheet thickness, basal topography, and inferring basal

720 properties over large areas. These radars are typically

721 operated as altimeters and acquire profile data only

722 along nadir tracks that are often separated by 5 or

723 more kilometers. The along track resolution is met by

724 forming a synthetic aperture and the vertical resolution

725 of the thickness is met by transmitting high bandwidth

726 signal. Even though highly accurate thickness measure-

727 ments can be achieved, information in the third cross-

728 track dimension is absent.

729 While the surface properties of the ice sheets are

730 becoming increasing well documented, the nature of

731 the glacier bed remains obscured by its icy cover.

732 Revealing basal properties, such as the topography

733 and the presence or absence of subglacial water, is

734 important if we are to better estimate the flux of ice

735 from the interior ice sheet to the sea and to forecast

736 anticipated changes of the size of the ice sheets. Recent

737 experiments demonstrate how it is possible to go

738 beyond airborne nadir sounding of glaciers and to

739 produce three-dimensional images of the glacier bed.

740 This development represents a major step forward in

741 ice sheet glaciology by providing new information

742 about the basal boundary conditions modulating the

743 flow of the ice and revealing for the first time

744 geomorphologic processes occurring at the bed of

745 modern ice sheets. The approach relies on the applica-

746 tion of radar tomography to UHF/VHF airborne radar

747 data collected using multiple, independent antennas

748 and receivers. Tomography utilizes phase and amplitude

749information from the independent receivers to isolate

750the direction of a natural target relative to the aircraft.

751Combined with the range to the target based on the echo

752travel time and position of the aircraft, tomographic

753methods yield swaths of reflectivity and topographic

754information on each side of the aircraft [92].

755Cooperative Efforts to Observe, Monitor, and

756Understand the Cryosphere

757Several cooperative, international scientific studies of

758the high latitudes have been organized. Beginning with

759the 1897 voyage of the Belgica to the Antarctic Penin-

760sula and continuing to the 2007 International Polar

761Year, studies have relied on the most recent technolo-

762gies to increase knowledge of the polar regions. To

763realize the benefit of the growing constellation of inter-

764national satellites to the scientific objectives of the 2007

765International Polar Year (IPY) (Fig. 13), the Global

766Interagency IPY Polar Snapshot Year (GIIPSY) project

767engaged the science community to develop consensus

768polar science requirements and objectives that could

769best and perhaps only be met using the international

770constellation of earth observing satellites [93]. Require-

771ments focused on all aspects of the cryosphere and

772range from sea ice to permafrost to snow cover and

773ice sheets. Individual topics include development of

774high-resolution digital elevation models of outlet gla-

775ciers using stereo optical systems, measurements of ice

776surface velocity using interferometric synthetic aper-

777ture radar, and frequently repeated measurements of

778sea ice motion using medium resolution optical and

779microwave imaging instruments.

780The IPY Space Task Group (STG), convened by the

781World Meteorological Organization (WMO), formed

782the functional link between the GIIPSY science com-

783munity and the international space agencies. STG

784membership included representatives from the

785national space agencies of Italy, Germany, France, UK,

786US, Canada, Russia, China, Japan, and the European

787Space Agency (ESA), which in itself represents 19

788nations. The STG determined how best to satisfy

789GIIPSY science requirements in a fashion that distrib-

790uted the acquisition burden across the space agencies

791and recognized the operational mandates that guide

792the activities of each agency.



793 The STG adopted four primary data acquisition

794 objectives for its contribution to the IPY. These are:

795 ● Pole to coast multi-frequency InSARmeasurements

796 of ice-sheet surface velocity

797 ● Repeat fine-resolution SAR mapping of the entire

798 Southern Ocean sea ice cover for sea ice motion

799 ● One complete high resolution visible and thermal

800 IR (Vis/IR) snapshot of circumpolar permafrost

801 ● Pan-Arctic high and moderate resolution Vis/IR

802 snapshots of freshwater (lake and river) freeze-up

803 and breakup

804 The STG achieved most of these objectives includ-

805 ing: acquiring Japanese, ALOS L-band, ESA Envisat

806 and Canadian Radarsat C-band, and German

807 TerraSAR-X (Fig. 14) and Italian COSMO_SKYMED

808 X band SAR imagery over the polar ice sheets [94];

809 acquiring pole to coast InSAR data for ice sheet surface

810 velocity; optically derived, high-resolution digital ele-

811 vation models of the perimeter regions of ice caps and

812 ice sheets; coordinated campaigns to fill gaps in Arctic

813 and Antarctic sea ice cover; extensive acquisitions of

814 optical imagery of permafrost terrain; observations of

815 atmospheric chemistry using the Sciamachy

816 instrument.

817 Future Directions

818 Emerging sensor and platform technologies hold great

819 promise for future airborne and spaceborne remote

820 sensing of the cryosphere. Airborne programs are likely

821 to continue to use large manned aircraft in a fashion

822 similar to NASA’s IceBridge program which integrates

823 a sophisticated suite of instruments including lidars,

824 radars, gravimeters, magnetometers, optical mapping

825 systems, and GPS. But there will also be a steady shift

826 toward smaller, more dedicated unmanned aerial vehi-

827 cles capable of remaining on station for longer periods

828 and fulfilling some of the temporal coverage require-

829 ments that are difficult to satisfy with larger aircraft

830 requiring a substantial number of crewmen.

831 It is worth noting again that ice sounding radars are

832 exclusively deployed on aircraft for terrestrial research.

833 This is driven operationally by the challenges of iono-

834 spheric distortions and by governmental controls on

835 frequency allocations available for remote sensing

836applications designed to limit interference with com-

837munications and other commercial uses of the

838frequency bands. Certainly the latter is not an issue

839for other planetary studies and in fact the Martian ice

840caps have been successfully sounded from the orbiting

841MARSIS and SHARAD radars [95]. These extraterres-

842trial successes motivate continuing interest in

843deploying similar instrument for observing Earth’s ice

844cover in the future.

845As indicated in Fig. 13, there will be an ongoing

846constellation of satellites capable of collecting valuable

847cryospheric data. Here, coordination amongst the

848different space faring nations will be key to realizing

849the greatest scientific benefit. Lessons about coopera-

850tion gleaned from the IPY can be profitably extended to

851the acquisition of data and the development of geo-

852physical products beyond the polar regions to all sec-

853tors of the cryosphere. There could also be generally

854better integration of the atmospheric chemistry and

855polar meteorological communities, as well as incorpo-

856ration of gravity and magnetic geopotential missions

857into the coordination discussions. It is also possible to

858envision discussion and collaboration on emerging

859technologies and capabilities such as the Russian

860Arktika Project [96] and advanced subsurface imaging

861radars. A primary objective of continued coordination

862of international efforts is securing collections of

863spaceborne “snapshots” of the cryosphere through the

864further development of a virtual Polar Satellite Con-

865stellation [97]. A natural vehicle for adopting lessons

866learned from GIIPSY/STG into a more encompassing

867international effort could be the Global Cryosphere

868Watch [98] recently proposed by WMO to be in sup-

869port of the cryospheric science goals specified for the

870Integrated Global Observing Strategy Cryosphere

871Theme [1].

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Cryosphere: Spaceborne and Airborne Measurements/

Monitoring. Figure 1

NASA MODIS composite for July 7, 2010. Starting in the

lower left quadrant and going clockwise, the Greenland

sheet is almost cloud free as are the adjacent smaller ice

caps on the Canadian Arctic Islands. Much of northern

Canada and Alaska are cloud covered till the Bering Strait

which is also almost sea ice free. Patches of sea ice cling to

the Asian coast and there is a smattering of snow cover on

the Putorana Mountains in western Siberia. The Gulf of Ob,

Novaya Zemlya, the Franz Josef Land, and Spitsbergen

Islands are barely visible through the cloud. The central

Arctic basin is covered by sea ice. The image illustrates the

scale of the cryosphere as well as some of the

complications faced when trying to study it from space

(Image prepared by NASA’s MODIS Rapid Response team)



Illustration of NASA’s ICESat passing over the

Antarctic. ICESat collected repeat elevation data over ice

sheets and sea ice to study changing ice thicknesses.

ICESat also sounded the atmosphere to study aerosols.

NASA/Goddard Space Flight Center Scientific Visualization

Studio, RADARSAT mosaic of Antarctica (Canadian Space

Agency)



Cryosphere: Spaceborne and Airborne Measurements/Monitoring. Figure 3

European Space Agency MERIS image of Peterman Glacier, Greenland (left). The multispectral image was acquired in July,

2008 and before a large part of the floating ice tongue broke free. Argon panchromatic photograph acquired during the

spring of 1962. The clarity of this early view from space is exceptional and provides a valuable gauge for assessing changes

in glacier ice from the 1960s to the present

0

500

1000

1500

2000

250068.6 68.7 68.8 68.9 69 69.1

Latitude

Dep

th (

m)

69.2 69.3 69.4



North south airborne profile of ice thickness in west central

Greenland. The upper black line is the radar reflection from

the snow surface. The line at about 500 m depth is

a multiple echo of the surface from the aircraft itself. The

undulating line at an average depth of about 1,400m is the

reflection from the glacier bottom. The profile crosses the

upstream segment of Jakobshavn Glacier at 69.2o

N 48.1oW. There the 2,000 m thick ice has incised a deep

channel into the bedrock. The data were acquired by the

University of Kansas in 2008



280

260

240

220

200

180

160

140

120

100


19 GHz, horizontally polarized, special sensor microwave imager (SSM/I) brightness temperature data for the Arctic on

February 15, 2004 (left) and August 25, 2004 (right). The brightness temperature scale (far right) is in Kelvin. Differences

between the emissivity of ocean, sea ice, land and ice sheets enable the relatively easy identification of the seasonal retreat

in sea ice cover. Brightness temperature variations across the ice pack are caused by differences in ice type and age as well

as in ice concentration

16.0

Average Monthly Arctic Sea Ice ExtentJanuary 1979 to 2011

15.5

15.0

14.5

13.5

14.0

Ext

ent (

mill

ion

squa

re k

ilom

eter

s)

13.0

12.5

12.01978 1982 1986 1990 1994 1998 2002 2006 2010

Year


Winter-time Arctic sea ice extent computed using passive microwave data. The decreasing trend illustrates the reduction

in Arctic sea ice extent. There is a muchmore dramatic decline in ice extent during the summer months (Graph courtesy of

the National Snow and Ice Data Center)


jezek

Cross-Out

jezek

Replacement Text

degrees Kelvin




Canadian RADARSAT-1 Antarctic Mapping Project

synthetic aperture radar mosaic. The radar is sensitive to

changes in the physical structure of the near surface snow.

Bright coastal returns are scattered from subsurface ice

lenses formed during fall freeze-up. Darker interior tones

occur where snow accumulation is high. Other features

such as ice streams and ice divides are visible (Map

courtesy of K. Jezek. RADARSAT-1 data courtesy of the

Canadian Space Agency)

Velocity2

0.5

0.25

km/y

r m/day

0

1.5

1

0.5

0



Surface velocities on Hubbard Glacier Alaska measured

using Japanese PALSAR L-band interferometric pairs.

Hubbard Glacier is a tidewater glacier and ice from the

advancing snout calves directly into Disenchantment Bay

(lower center). Hubbard Glacier is one of the few in Alaska

that is currently thickening and advancing (Image courtesy

of E. W. Burgess and R. R. Forster, University of Utah)



70°

210°

60°

50°

−12 −10 −8 −6 −4 −2 0 2 4 6

cm eq. H2O / yr


Mass loss from Alaskan Glaciers in cm of water equivalent per year from GRACE (Image courtesy of S. Luthcke, NASA GSFC)



Snow cover extent from NASA’s MODIS. Increasing shades

of gray indicate greater percentages of snow cover within

each pixel. Brown denotes snow-free surface





January 2010 NASA MODIS image of ice formed on the St.

Lawrence River. Thin layers of new ice are distorted into

swirls by the surface currents (center left). Thicker ice is held

fast to the southern shore (Image courtesy of MODIS Rapid

Response Team, NASA GSFC)



EO-1 Hyperion satellite data acquired over the north coast

of Alaska (70.5 N 156.5 W) and about 50 km inland from the

coast, which is toward the right on this image. The image is

7.5 km wide (top to bottom). Color composite using

channels 16 23 and 28



02 03 04 05 06 07

Cryosphere Satellite Missions

08 09 10 11 12 13 14 15 16

PALSAR/ALOS L-band

RA, SAR & Wind Scat/ERS-2 RA2 & ASAR/Envisat C-band

RADARSAT-2 C-band

SAR/RISAT C-band

TerraSAR/Tandem-X X-band

SAR/COSMO-SKYMED X-band

ASCAT & AVHRR/MetOp -1, -2, -3

Seawinds/QuikSCAT Ku-band

ICESAT ICESAT-2

CryoSat-2

GRACE

GOCELEO missions

SMOS

AMSR2/GCOM-W1

VIIRS/NPP NPOESS C1

WindSat

OLS & SSMI/DMSP—AVHRR & AMSU/NOAA

MODIS & AMSR-E/EOS-Aqua

Aster/MODIS/EOS-Terra

COCTS/HY-1A HY-1B

VIRR/FY-1D

SPOT-4/5 & Landsat

In Orbit

Solid Colour = R & D mission; Hatched = operational mission

Approved Planned/Pending approval

MODI & MERSI/FY-3A

HY-1C

HY-2A HY-2B

PCW 1 & 2Arctica-M 1 & 2

Courtesy: M. Drinkwater

HEO missions

GMES S-3A, B

GRACE-C

Ku-Scat & MSMR/OCEANSAT-2

IPY

RADARSAT-ConstellationRADARSAT-1 C-band

HY-3 C-, X-band

GMES S-1A, B


Current, approved and planned satellites for studying and monitoring the Cryosphere (Figure courtesy of Mark

Drinkwater, European Space Agency)





German Aerospace Center TerraSAR-X observations of the

Nimrod Glacier (inset map of Antarctica). Ice floes around

a central nunatak and down toward the Ross Ice Shelf.

Crevasses appear in conjunction with the interruption of

flow by the nunatak. Cooperative use of Canadian,

German, European Space Agency, Italian, and Japanese

synthetic aperture radar (SAR) satellites along with ground

segment and data processing capabilities provided by the

United States yielded a rich and diverse SAR data set that

will be a lasting legacy of the IPY (TerraSAR-X data courtesy

of D. Floricioiu, German Aerospace Center. Inset coastline

derived from RADARSAT-1 Antarctic Mapping Project map)



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