Waleed Abdalati

ICESat's laser measurements of polar ice, atmosphere, ocean, and land

The Ice, Cloud and Land Elevation Satellite (ICESat) mission will measure changes in elevation of the Greenland and Antarctic ice sheets as part of NASA’s Earth Observing System (EOS) of satellites. Timeseries of elevation changes will enable determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. Other scientific objectives of ICESat include: global measurements of cloud heights and the vertical structure of clouds and aerosols; precise measurements of land topography and vegetation canopy heights; and measurements of sea ice roughness, sea ice thickness, ocean surface elevations, and surface reflectivity. The Geoscience Laser Altimeter System (GLAS) on ICESat has a 1064 nm laser channel for surface altimetry and dense cloud heights and a 532 nm lidar channel for the vertical distribution of clouds and aerosols. The predicted accuracy for the surfaceelevation measurements is 15 cm, averaged over 60 m diameter laser footprints spaced at 172 m alongtrack. The orbital altitude will be around 600 km at an inclination of 94 � with a 183-day repeat pattern. The on-board GPS receiver will enable radial orbit determinations to better than 5 cm, and star-trackers will enable footprints to be located to 6 m horizontally. The spacecraft attitude will be controlled to point

Large fluctuations in speed on Greenland's Jakobshavn Isbræ glacier

It is important to understand recent changes in the velocity of Greenland glaciers because the mass balance of the Greenland Ice Sheet is partly determined by the flow rates of these outlets. Jakobshavn Isbræ is Greenlands largest outlet glacier, draining about 6.5 per cent of the ice-sheet area, and it has been surveyed repeatedly since 1991 (ref. 2). Here we use remote sensing data to measure the velocity of Jakobshavn Isbræ between 1992 and 2003. We detect large variability of the velocity over time, including a slowing down from 6,700 m yr-1 in 1985 to 5,700 m yr-1 in 1992, and a subsequent speeding up to 9,400 m yr-1 by 2000 and 12,600 m yr-1 in 2003. These changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter. Our observations indicate that fast-flowing glaciers can significantly alter ice discharge at sub-decadal timescales, with at least a potential to respond rapidly to a changing climate.

Recent Greenland ice mass loss by drainage system from satellite gravity observations.

Mass changes of the Greenland Ice Sheet resolved by drainage system regions were derived from a local mass concentration analysis of NASA–Deutsches Zentrum für Luftund Raumfahrt Gravity Recovery and Climate Experiment (GRACE mission) observations. From 2003 to 2005, the ice sheet lost 101 ± 16 gigaton/year, with a gain of 54 gigaton/year above 2000 meters and a loss of 155 gigaton/year at lower elevations. The lower elevations show a large seasonal cycle, with mass losses during summer melting followed by gains from fall through spring. The overall rate of loss reflects a considerable change in trend (–113 ± 17 gigaton/year) from a near balance during the 1990s but is smaller than some other recent estimates.

Greenland Ice Sheet: Increased coastal thinning

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. Copyright 2004 by the American Geophysical Union.

Snowmelt on the Greenland Ice Sheet as Derived From Passive Microwave Satellite Data

Abstract The melt extent of the snow on the Greenland ice sheet is of considerable importance to the ice sheet’s mass and energy balance, as well as Arctic and global climates. By comparing passive microwave satellite data to field observations, variations in melt extent have been detected by establishing melt thresholds in the cross-polarized gradient ratio (XPGR). The XPGR, defined as the normalized difference between the 19-GHz horizontal channel and the 37-GHz vertical channel of the Special Sensor Microwave/Imager (SSM/I), exploits the different effects of snow wetness on different frequencies and polarizations and establishes a distinct melt signal. Using this XPGR melt signal, seasonal and interannual variations in snowmelt extent of the ice sheet are studied. The melt is found to be most extensive on the western side of the ice sheet and peaks in late July. Moreover, there is a notable increasing trend in melt area between the years 1979 and 1991 of 4.4% per year, which came to an abrupt halt in 1...

The ICESat-2 Laser Altimetry Mission

Satellite and aircraft observations have revealed that remarkable changes in the Earths polar ice cover have occurred in the last decade. The impacts of these changes, which include dramatic ice loss from ice sheets and rapid declines in Arctic sea ice, could be quite large in terms of sea level rise and global climate. NASAs Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), currently planned for launch in 2015, is specifically intended to quantify the amount of change in ice sheets and sea ice and provide key insights into their behavior. It will achieve these objectives through the use of precise laser measurements of surface elevation, building on the groundbreaking capabilities of its predecessor, the Ice Cloud and Land Elevation Satellite (ICESat). In particular, ICESat-2 will measure the temporal and spatial character of ice sheet elevation change to enable assessment of ice sheet mass balance and examination of the underlying mechanisms that control it. The precision of ICESat-2s elevation measurement will also allow for accurate measurements of sea ice freeboard height, from which sea ice thickness and its temporal changes can be estimated. ICESat-2 will provide important information on other components of the Earth System as well, most notably large-scale vegetation biomass estimates through the measurement of vegetation canopy height. When combined with the original ICESat observations, ICESat-2 will provide ice change measurements across more than a 15-year time span. Its significantly improved laser system will also provide observations with much greater spatial resolution, temporal resolution, and accuracy than has ever been possible before.

Investigation of surface melting and dynamic thinning on Jakobshavn Isbræ, Greenland

Jakobshavn Isbrae is the most active glacier in Greenland, with an annual discharge of about 30 km 3 of ice, and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998, despite locally warm summers. Repeated airborne laser-altimeter surveys along a 120 km profile in the glacier basin show slow, sporadic thickening between 1991 and 1997, suggesting a small positive mass balance, but since 1997 there has been sustained thinning of several m a -1 within 20 km of the ice front, with lower rates of thinning further inland. Here, we use weather-station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates, and thus to infer the temporal and spatial patterns of dynamic thinning. These show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning, initially confined to the lower reaches of the glacier (below about 500 m elevation), but progressively spreading inland until, between 1999 and 2001, thinning predominated over the entire surveyed region, up to 2000 m elevation. If this continues, the glacier calving front and probably its grounding line will retreat substantially in the very near future.

Aircraft laser altimetry measurement of elevation changes of the greenland ice sheet: technique and accuracy assessment

Abstract 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 systems 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.

Greenland Ice Sheet melt extent: 1979–1999

Analysis of melt extent on the Greenland ice sheet is updated to span the time period 1979-1999 and examined along with its spatial and temporal variability using passive microwave satellite data. To acquire the full record, the issue of continuity between the previous passive microwave sensors (SMMR, SSM/I F-8, and SSM/I F-11) and the most recent SSM/I F-13 sensor is addressed. The F-13 cross-polarized gradient ratio melt-classification threshold is determined to be -0.0154. Results show that for the 21-year record, a positive melt trend of nearly 1 %/yr is observed, but this trend falls just below the 90% significance level. The observed melt increase does appear to be driven by conditions in the western portion of the ice sheet, rather than the east where melt appears to have decreased slightly. Moreover, the eruption of Mount Pinatubo in 1991 is likely to have had some impact on the melt but not so much as previously suspected. The 1992 melt anomaly is 1.7 standard deviations from the mean. Finally, the relationship between coastal temperatures and melt extent suggests an increase in surface runoff contribution to sea level of 0.31 mm/yr for a 1°C temperature rise.

Passive microwave-derived snow melt regions on the Greenland Ice Sheet

By comparing data from the Special Sensor Microwave Imager (SSM/I) to field data, a melt threshold of the cross-polarized gradient ratio (XPGR), which is a normalized difference between the 19 GHz horizontally-polarized and 37 GHz vertically polarized brightness temperatures, is determined. This threshold, XPGR = −0.025, is used to classify dry and wet snow. The annual areal extent of melt is mapped for the years 1988 through 1991, and inter-annual variations of melt extent are examined. The results show that the melt extent varied from a low of 38.3% of the ice sheet (1990) to a high of 41.7% (1991) during the years 1988–1991.