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.
Abstract. 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. IntroductionOutlet glaciers are particularly important to the mass balance of the Greenland ice sheet because they are the means by which the ice is discharged into the surrounding seas. Such discharge comprises nearly half of the ice sheet mass loss [Weidick, 1985]. An understanding of the balance of these glaciers and the mechanisms that affect this balance is essential to understanding the present and future states of the ice sheet. The factors affecting outlet glacier mass balance are complex, because this balance is not simply a result of accumulation and surface ablation, but rather it is largely influenced by flow characteristics, which are often nonlinear.Until recently, large-scale assessment of glacier and ice sheet mass balance was not possible by conventional means. While in situ measurements can provide detailed information about specific locations, large-scale coverage is often not feasible because of the costs, difficulties, and dangers associated with work on these remote glaciers. Satellite radar altimetry facilitated study of the higher portions of the Greenland ice sheet through elevation-change measurements from which mass balance of the surveyed areas can be estimated. This technique, however, is limited to shallow-sloped and relatively smooth areas and provides no useful information about steeper areas
Aircraft laser-altimeter surveys over southern Greenland in 1993 and 1998 show three areas of thickening by more than 10 centimeters per year in the southern part of the region and large areas of thinning, particularly in the east. Above 2000 meters elevation the ice sheet is in balance but thinning predominates at lower elevations, with rates exceeding 1 meter per year on east coast outlet glaciers. These high thinning rates occur at different latitudes and at elevations up to 1500 meters, which suggests that they are caused by increased rates of creep thinning rather than by excessive melting. Taken as a whole, the surveyed region is in negative balance.
Technology is changing how scientists and natural resource managers describe and study streams and rivers. A new generation of airborne aquatic‐terrestrial lidars is being developed that can penetrate water and map the submerged topography inside a stream as well as the adjacent subaerial terrain and vegetation in one integrated mission. A leading example of these new cross‐environment instruments is the Experimental Advanced Airborne Research Lidar (EAARL), a NASA‐built sensor now operated by the U.S. Geological Survey (USGS) [Wright and Brock, 2002]. Standard airborne terrestrial lidars, which currently produce the highest‐resolution maps of extensive land areas, use reflected near‐infrared laser pulses to make millions of point measurements of ground and vegetation elevations. However, near‐infrared energy is absorbed by water, which limits the use of these systems to mapping features outside of water bodies. EAARL uses a narrow‐beam green, rather than near‐infrared, laser with a footprint of only 15 centimeters from the nominal flying height (for system technical specifications, see Table S1 in the electronic supplement to this Eos issue (http://www.agu.org/eos_elec/).
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