Abstract As part of the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS), ice velocities were measured on the Ross Ice Shelf (RIS) during 1973–78. Comparisons of these with velocity estimates at the same locations derived from RADARSAT synthetic aperture radar (SAR) measurements in 1997 and 2009 show velocity reduction in the southeast quadrant of the ice shelf by almost 200 m a−1, with deceleration rates increasing with time. Large areas of ice shelf in this region are lightly grounded, forming an ‘ice plain’ that increases local buttressing of the ice streams. ICESat measurements show this ice plain to be thickening. The observed decrease in ice-shelf velocities implies a total reduction in the mass of ice flowing into the RIS from the West Antarctic ice sheet (WAIS) by ∼23 Gt a−1, shifting the mass balance of the WAIS drainage basin from strongly negative in the 1970s to strongly positive in 2009. The resulting decrease in ice advection should lead to ice-shelf thinning further seaward of the ice plain. This thinning would reduce the lateral drag and back-stress of the shelf ice, further contributing to thinning through an increase in spreading rate. ICESat measurements show recent thinning of most of the freely floating ice shelf.

Description of NASA Airborn Topographic Mapper (ATM) lidar calibration procedures including analysis of the accuracy and consistancy of various ATM instrument parameters and the resulting influence on topographic elevation measurements. The ATM elevations measurements from a nominal operating altitude 500 to 750 m above the ice surface was found to be: Horizontal Accuracy 74 cm, Horizontal Precision 14 cm, Vertical Accuracy 6.6 cm, Vertical Precision 3 cm.

Aircraft laser-altimeter surveys during the 1990s showed near-coastal parts of the Greenland ice sheet to be thinning; despite slow thickening at higher elevations, the ice sheet lost mass to the ocean. Many outlet glaciers thinned more rapidly than could be explained by increased melting during the recent warmer summers, indicating dynamic imbalance between glacier velocity and upstream snow accumulation. Results from more recent surveys, presented here, show that thinning rates have increased in most coastal regions. For almost half of the surveys, these increases might have resulted from increases in summer melting, but rapid thinning on others is indicative of dynamic changes that increased with time. In particular, thinning rates on the three fastest glaciers increased to tens of m a(-1) after 2000, and other observations show an approximate doubling in their velocities. The deep beds of these glaciers appear to have a strong influence on rates of grounding-line retreat and thickness change, with periods of glacier acceleration and rapid thinning initiated by flotation and break-up of lightly grounded glacier snouts or break-up of floating ice tongues. Near-simultaneous thinning of these widely separated glaciers suggests that warming of deeper ocean waters might be a common cause. Nearby glaciers without deep beds are thinning far more slowly, suggesting that basal lubrication as a result of increased surface melting has only a marginal impact on Greenland outlet-glacier acceleration

AbstractWe compare rates of surface-elevation change on the Greenland ice sheet derived from European Remote-sensing Satellite-2 (ERS-2) radar-altimeter data with those obtained from laser-altimeter data collected over nearly the same time periods. Radar-altimeter data show more rapid thickening (9 ± 1 cm a−1 above 1500 m elevation in the north, and 3 ± 1 cm a−1 above 2000 m in the south) than the laser estimates, possibly caused by a lifting of the radar-reflection horizon associated with changes in the snowpack, such as those caused by progressively increased surface melting, as summer temperatures rise. Over all the ice sheet above 2000 m, this results in an ERS-derived volume balance ∼75 ± 15 km3 a−1 more positive than that from laser data. This bias between laser and radar estimates of elevation change varies spatially and temporally, so cannot at present be corrected without independent surveys such as those presented here. At lower elevations, comparison of detailed repeat laser surveys over Jakobshavn Isbræ with ERS results over the same time interval shows substantial ERS underestimation of ice-thinning rates. This results partly from missing data because of ‘bad’ radar waveforms over the very rough surface topography, and partly from the tendency for large radar footprints to sample preferentially local high points in the topography, thus missing regions of most rapid thinning along glacier depressions.

Results are presented from topographic surveys of the Assateague Island National Seashore using an airborne scanning laser altimeter and kinematic Global Positioning System [GPS) technology. The instrument used was the Airborne Topographic Mapper (ATM), developed by the NASA Arctic Ice Mapping [AIM) group from the Goddard Space Flight Center's Wallops Flight Facility. In November, 1995, and again in May, 1996, these topographic surveys were flown as a functionality check prior to conducting missions to measure the elevation of extensive sections of the Greenland Ice Sheet as part of NASA's Global Climate Change program. Differences between overlapping portions of both surveys are compared for quality control. An independent assessment of the accumcy of the ATM survey is provided by comparison to surface surveys which were conducted using standard techniques. The goal of these projects is to make these measurements to an accuracy of 2 10 cm. Differences between the fall 1995 and 1996 surveys provides an assessment of net changes in the beach morphology over an annual cycle. Introduction Beaches are one of the most dynamic geologic (sedimentary) features on Earth. Fluxes in beach morphology occur over a wide spectrum of time scales ranging from periods of hours associated with diurnal tides and storm events to years and decades in response to longer term erosional trends. On a geological scale, beaches follow gross changes in sea level during periods of glaciation and glacial retreat. However, anthropogenic activities, especially during the past century, have created a situation where erosion of beaches has severe economic consequences. Thirty of the nation's 50 states have coastlines on the Atlantic or Pacific Oceans, the Gulf of Mexico, or the Great Lakes. These 30 states contain approximately 85 percent of the nation's population, and about half of this population resides within the coastal zone (Leatherman and Dean, 1991). The U.S. coastline is more than 20,000 kilometers in length. Remote sensing offers the only possibility for producW.B. Krabill and C.W. Wright are with the National Aeronautics 1 and Space Administration, Laboratory for Hydrospheric Processes, Wallops Flight Facility, Wallops Island, VA 23337 (krabill@osb.wff.nasa.gov). R.N. Swift, E.B. Frederick, S.S. Manizade, and J.K. Yungel are 1 with EGM: Services, Wallops Island, VA 23337. C.E Martin and J.G. Sonntag are with EG&G Services, Gaithersburg, MD 20878. M. Duffy and W. Hulslander are with the National Park Service, Assateague Island National Seashore, Berlin, MD 21811. J.C. Brock was with the NOAA Coastal Services Center, Charleston, SC 29407. He is presently with the USGS Center for Coastal Geology, St. Petersburg, FL 33701. ing a time series of elevation surveys of sufficient density to permit these valuable resources to be monitored. Airborne scanning laser altimetry currently offers a strong potential to provide such accurate, detailed, and comprehensive surveys. Annual surveys could be repeated to facilitate an understanding of long-term erosional trends or gauge effects of dredging, beach replenishment, and erosion control structures such as groins. Regional surveys could be conducted following the passage of major storms such as hurricanes and northeasters to quantify the resulting erosion/deposition and permit rapid identification of beach areas which are at risk due to the removal of sand from protective dunes. Survey input could be used by the National Flood hsurance Program (NFIP) of the Federal Emergency Management Agency to make decisions on property coverage regulations. NFIP currently offers flood insurance protection to -1,200 coastal communities amounting to 1.4 million policies and over $120 billion in coverage. The survey of beaches along the entire expanse of the U.S. coast could be accomplished with a modest number of airborne scanning laser altimeters. A comprehensive review of available commercial airborne scanning laser altimeters were given by Flood and Gutelius (1997), and academic researchers have used the technology to study changes in beaches caused by tropical storms and hurricanes (Carter and Shrestha, 1997). Additionally, the federal government operates several such sensors both for research and operational applications (Krabill et al., 1995; Lillycrop eta]., 1996). General Discussion As a demonstration of the application of airborne remote sensing for beach monitoring, the northern portion of Assateague Island has been topographically mapped using an airborne scanning laser altimeter combined with kinematic Global Positioning System (GPS) technology. The site, shown in Figure 1, was initially surveyed with the NASA Airborne Topographic Mapper (ATM) in November, 1995, to evaluate the sensor for use in the Arctic Ice Mapping (AIM] Project (Krabill et al., 19961, a NASA Earth Science program that monitors changes in the height of the large ice sheet that covers most of Greenland. The ATM group, based at Goddard Space Flight Center's Wallops Flight Facility (WFF), has been gathering baseline elevation measurements in surveys regionally distributed over the Greenland ice sheet in annual field deployments between 1993 and 1998. Assateague Island was selected as a test site because of its proximity to WFF and because the beach sand has a Photogrammetric Engineering & Remote Sensing Vol. 66, No. 1, January 2000, pp. 65-71. 0099-1 112/00/6601-00OS3.00/0 tl 2000 Anlerican Society for Photogrammetry

Laser altimeter measurements over Greenland show increasing thickening rates above 2000 m, reflecting increasing snowfall in a warming climate. But near‐coastal thinning rates have increased substantially since the mid 1990s, and net mass loss more than doubled from an average of 4–50 Gt yr−1 between 1993/4 and 1998/9 to 57–105 Gt yr−1 between 1998/9 and 2004. This increasing trend is very similar to findings from independent mass‐budget studies, but differs widely from ERS radar altimeter results. This may result from limitations associated with the large ERS footprint over sloping and undulating surfaces that typify fast, narrow glaciers where thinning is most pronounced.

The Arctic Ice Mapping group (Project AIM) at the NASA Goddard Space Flight Center Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet (GIS) since 1993, using scanning airborne laser altimeters combined with Global Positioning System (UPS) technology. Earlier surveys showed the ice sheet above 2000-rn elevation to be in balance, but with localized regions of thickening or thinning. Thinning predominates at lower elevations and thinning rates have recently increased, resulting in a negative mass balance for the entire ice sheet. Recently, critical segments of near-coastal flight lines in Greenland were resurveyed. Results from the new data will be presented.

Precise measurements of surface elevation on the Greenland ice sheet have been made almost every year since 1991 by an airborne scanning laser altimeter operated by NASA/Wallops Flight Facility. Results show substantial thinning over large areas near the coast, with a general increase in thinning rates since 1997, in the drainage basins of thinning glaciers, and a recent thickening in the southeast associated with very high snowfall in this region during 2003. Here, we present first results from the comparison of the aircraft data with similar measurements from the laser altimeter aboard NASA's Ice, Cloud and land Elevation Satellite (ICESat), which was launched in January 2003. These show very close agreement with results inferred solely from the aircraft measurements, indicating that accuracies are similar for both datasets. Broad spatial coverage by satellite, together with the baseline dataset of aircraft measurements, offers the prospects of routine surveys of ice-sheet elevation changes by ICESat and follow-on missions.

Prior to the launch of the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat) in January 2003, topographic surveys were made by NASA's Airborne Topographic Mapper (ATM) over regions of the western United States and the Antarctic Dry Valleys to support calibration and validation of the range and pointing errors of GLAS lasers. Surveyed areas included terrain with large slopes, allowing pointing‐bias estimation with as little as a few seconds of ICESat data. Range errors over sloping irregular surfaces are calculated by computing the expected GLAS return waveform and comparing it with the actual waveform. We conclude that the range bias is less than 2 cm and that pointing errors for the best available data set (Laser 2a) have rss errors less than 2 arcsec.

Repeated laser‐altimeter surveys and modelled snowfall/summer melt show average ice loss from Greenland between 1997 and 2003 was 80 ± 12 km3 yr−1, compared to about 60 km3 yr−1 for 1993/4–1998/9. Half of the increase was from higher summer melting, with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation. Velocities of one large glacier almost doubled between 1997 and 2003, resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003.

Precise repeat airborne laser surveys were conducted over the major ice caps in the Canadian Arctic Archipelago in the spring of 1995 and 2000 in order to measure elevation changes in the region. Our measurements reveal thinning at lower elevations (below 1600 m) on most of the ice caps and glaciers but either very little change or thickening at higher elevations in the ice cap accumulation zones. Recent increases in precipitation in the area can account for the slight thickening where it was observed but not for the thinning at lower elevations. For the northern ice caps on the Queen Elizabeth Islands, thinning was generally <0.5 m yr −1 , which is consistent with what would be expected from the warm temperature anomalies in the region for the 5 year period between surveys, and appears to be a continuation of a trend that began in the mid‐1980s. Farther south, however, on the Barnes and Penny ice caps on Baffin Island, this thinning was much more pronounced at over 1 m yr −1 in the lower elevations. Here temperature anomalies were very small, and the thinning at low elevations far exceeds any associated enhanced ablation. The observations on Barnes, and perhaps Penny, are consistent with the idea that the observed thinning is part of a much longer term deglaciation, as has been previously suggested for Barnes ice cap. On the basis of the regional relationships between elevation and elevation change in our data, the 1995–2000 mass balance for the archipelago is estimated to be −25 km 3 yr −1 of ice, which corresponds to a sea level increase of 0.064 mm yr −1 . This places it among the more significant sources of eustatic sea level rise, though not as substantial as the Greenland ice sheet, Alaskan glaciers, or the Patagonian ice fields.

Airborne laser altimetry has been used during the past decade to measure the surface elevation of the Greenland ice sheet. These measurements have been made using a scanning laser on a NASA P-3 aircraft which was positioned by differential GPS and flown approximately 500 m above the surface. Flights have been made over major portions of the ice sheet and reflown 5 years later in order to obtain estimates of the rate of overall change of surface elevation. The accuracy with which differential elevations can be made depends upon (a) the GPS positioning accuracy, (b) the instrument calibration accuracy, (c) the stability of the laser and, (d) the accuracy of the aircraft inertial navigation system's estimation of aircraft attitude. Overall, the accuracy of an elevation change estimate is computed to be 8.5 cm over small areas and 7.1 cm when averaged over tens of kilometers as is needed for estimating ice volume changes. This effort supports±1.4 cm/year resolution for long period surface elevation changes from data acquired which are separated by 5 years. Results of inflight data analyses are consistent with these accuracy estimates.

In 1998 and 1999, the Arctic Ice Mapping (AIM) program completed resurveys of flight lines occupied 5 years earlier revealing elevation changes of the Greenland ice sheet and identifying areas of significant thinning, thickening and balance. In planning these surveys, consideration had to be given to the spatial constraints associated with aircraft operation, the spatial nature of ice sheet behavior, and limited resources, as well as temporal issues, such as seasonal and interannual variability in the context of measurement accuracy. The surveys covered the entire ice sheet with an average distance of 21.4 km between each location on the ice sheet and the nearest flight line. For most of the ice sheet, the elevation changes show relatively little spatial variability, and their magnitudes are significantly smaller than the observed elevation change signal. As a result, we conclude that the density of the sampling and the accuracy of the measurements are sufficient to draw meaningful conclusions on the state of balance of the entire ice sheet over the 5-year survey period. Outlet glaciers, however, show far more spatial and temporal variability, and each of the major ones is likely to require individual surveys in order to determine its balance.

Repeat surveys by aircraft laser altimeter in 1993/1994 and 1998/1999 have revealed significant thinning along many parts of the Greenland ice sheet at elevations below about 2000 m. In this paper we examine elevation changes from 29 repeat aircraft surveys over the lower portions of some of the larger outlet glaciers and parts of the ice sheet margin. Here thinning rates in excess of 1 m/yr are common in the lower sections of the flight lines, but in some cases, this rate is measured at elevations as high as 1500 m. Warmer summers along parts of the coast may have caused a few tens of cm/yr additional melting, but the magnitudes and character of the elevation changes suggest that in many cases they are more likely a result of glacier dynamics and creep thinning. The most extreme thinning was observed near the terminus of the Kangerdlugssuaq Glacier in southeastern Greenland where rates as high as 10 m/yr were measured. There are a few areas of significant thickening (over 1 m/yr), which is probably related to higher than normal accumulation rates during the observation period; but one location L, Bistrup Brae, had local regions of thickening of 8 to 9 m/yr. Three glaciers in the northeast show patterns of thickness change that may suggest surging behavior, and one has been independently documented as a surging glacier. Overall, the lowest reaches of the outlet glaciers and ice sheet edges appear to be changing significantly, with thinning observed more frequently than thickening.

The Arctic Ice Mapping group (Project AIM) at NASA's Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet since 1993, using scanning airborne laser altimeters combined with GPS positioning technology. Flight lines were planned to cover all major ice drainage basins, with the intention to repeat the surveys after a 5-year interval in order to detect changes in the ice-sheet volume. The first resurvey was completed in June/July, 1998, along flight lines in the southern half of Greenland which had been first surveyed in 1993. The northern half of the ice sheet will be resurveyed in 1999. The resulting data sets will provide the first comprehensive examination of regional changes in the the surface elevation of the World's second largest ice sheet. This analysis includes the ice sheet fringe areas, which are expected to be much more climatically sensitive than the interior. Data will be presented which demonstrate the combination of the ATM (Airborne Topographic Mapper) sensors and GPS (Global Positioning System) yields measurements which are repeatable at the 10 cm level over baselines in excess of 1000 km.

Results are presented from topographic surveys of the Assateague Island National Seashore using an airborne scanning laser altimeter and kinematic Global Positioning System (GPS) technology. The instrument used was the Airborne Topographic Mapper (ATM), developed by the NASA Arctic Ice Mapping (AIM) group from the Goddard Space Flight Center's Wallops Flight Facility. In November, 1995, and again in May, 1996, these topographic surveys were flown as a functionality check prior to conducting missions to measure the elevation of extensive sections of the Greenland Ice Sheet as part of NASA's Global Climate Change program. Differences between overlapping portions of both surveys are compared for quality control. An independent assessment of the accuracy of the ATM survey is provided by comparison to surface surveys which were conducted using standard techniques. The goal of these projects is to make these measurements to an accuracy of +/- 10 cm. Differences between the fall 1995 and 1996 surveys provides an assessment of net changes in the beach morphology over an annual cycle.

Aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999 have been coupled with previously reported data from southern Greenland to analyze the recent mass-balance of the Greenland Ice Sheet. Above 2000 meters elevation, the ice sheet is in balance on average but has some regions of local thickening or thinning. Thinning predominates at lower elevations, with rates exceeding 1 meter per year close to the coast. Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeter per year-approximately 7% of the observed rise.