Elevation Change Estimates Research Articles

Bayesian estimation of glacier surface elevation changes from DEMs

Accurate estimates of glacier surface elevation changes are paramount for various aspects of the study of the cryosphere, from glacier flow and thickness estimates to hydrological forecasts and projections of sea-level-rise. We present a novel probabilistic framework to filter outliers and estimate uncertainties in glacier surface elevation changes computed from the subtraction of digital elevation models (DEM). Our methodology frames outlier filtering as a Bayesian inference problem, thus characterizing the state of knowledge on glacier surface elevation changes through the posterior distribution as the combination of glacier volume variation observations and prior knowledge arising from previously collected data and/or modeled results. We validate this technique with experiments using Gaussian random fields to generate artificial noise in glacier surface elevation variation observations and show that the model satisfactorily culls the simulated outliers. Surface elevation change estimates are consistent with results computed from widely-used outlier filtering and uncertainty estimation techniques. The Bayesian framework allows unifying DEM error models with physical considerations on glacier surface elevation changes within a simple, statistically coherent model preventing temporal correlation and additional biases in other techniques. On the basis of these results, we discuss the implications of DEM uncertainty and offer suggestions for the glaciological community.

Elevation change of the Antarctic Ice Sheet: 1985 to 2020

Abstract. The largest uncertainty in future projections of sea level change comes from the uncertain response of the Antarctic Ice Sheet to the warming oceans and atmosphere. The ice sheet gains roughly 2000 km3 of ice from precipitation each year and loses a similar amount through solid ice discharge into the surrounding oceans. Numerous studies have shown that the ice sheet is currently out of long-term equilibrium, losing mass at an accelerated rate and increasing sea level rise. Projections of sea level change rely on accurate estimates of the contribution of land ice to the contemporary sea level budget. The longest observational record available to study the mass balance of the Earth's ice sheets comes from satellite altimeters. This record, however, consists of multiple satellite missions with different life spans and inconsistent measurement types (radar and laser) of varying quality. To fully utilize these data, measurements from different missions must be cross-calibrated and integrated into a consistent record of change. Here, we present a novel approach for generating such a record that implies improved topography removal, cross-calibration, and normalization of seasonal amplitudes from different mission. We describe in detail the advanced geophysical corrections applied and the processes needed to derive elevation change estimates. We processed the full archive record of satellite altimetry data, providing a seamless record of elevation change for the Antarctic Ice Sheet that spans the period 1985 to 2020. The data are produced and distributed as part of the NASA MEaSUREs ITS_LIVE (Making Earth System Data Records for Use in Research Environments Inter-mission Time Series of Land Ice Velocity and Elevation) project (Nilsson et al., 2021, DOI: https://doi.org/10.5067/L3LSVDZS15ZV​​​​​​​).

25 years of elevation changes of the Greenland Ice Sheet from ERS, Envisat, and CryoSat-2 radar altimetry

The shape of the large ice sheets responds rapidly to climate change, making the elevation changes of these ice-covered regions an essential climate variable. Consistent, long time series of these elevation changes are of great scientific value. Here, we present a newly-developed data product of 25 years of elevation changes of the Greenland Ice Sheet, derived from satellite radar altimetry. The data product is made publicly available within the Greenland Ice Sheets project as part of the ESA Climate Change Initiative programme.Analyzing repeated elevation measurements from radar altimetry is widely used for monitoring changes of ice-covered regions. The Greenland Ice Sheet has been mapped by conventional radar altimetry since the launch of ERS-1 in 1991, which was followed by ERS-2, Envisat and currently CryoSat-2. The recently launched Sentinel-3A will provide a continuation of the radar altimetry time series. Since 2010, CryoSat-2 has for the first time measured the changes in the coastal regions of the ice sheet with radar altimetry, with its novel SAR Interferometric (SARIn) mode, which provides improved measurement over regions with steep slopes.Here, we apply a mission-specific combination of cross-over, along-track and plane-fit elevation change algorithms to radar data from the ERS-1, ERS-2, Envisat and CryoSat-2 radar missions, resulting in 25 years of nearly continuous elevation change estimates (1992–2016) of the Greenland Ice Sheet. This analysis has been made possible through the recent reprocessing in the REAPER project, of data from the ERS-1 and ERS-2 radar missions, making them consistent with Envisat data. The 25 years of elevation changes are evaluated as 5-year running means, shifted almost continuously by one year. A clear acceleration in thinning is evident in the 5-year maps of elevation following 2003, while only small elevation changes observed in the maps from the 1990s.

Terrain changes from images acquired on opportunistic flights by SfM photogrammetry

Abstract. Acquiring data to analyse change in topography is often a costly endeavour requiring either extensive, potentially risky, fieldwork and/or expensive equipment or commercial data. Bringing the cost down while keeping the precision and accuracy has been a focus in geoscience in recent years. Structure from motion (SfM) photogrammetric techniques are emerging as powerful tools for surveying, with modern algorithm and large computing power allowing for the production of accurate and detailed data from low-cost, informal surveys. The high spatial and temporal resolution permits the monitoring of geomorphological features undergoing relatively rapid change, such as glaciers, moraines, or landslides. We present a method that takes advantage of light-transport flights conducting other missions to opportunistically collect imagery for geomorphological analysis. We test and validate an approach in which we attach a consumer-grade camera and a simple code-based Global Navigation Satellite System (GNSS) receiver to a helicopter to collect data when the flight path covers an area of interest. Our method is based and builds upon Welty et al. (2013), showing the ability to link GNSS data to images without a complex physical or electronic link, even with imprecise camera clocks and irregular time lapses. As a proof of concept, we conducted two test surveys, in September 2014 and 2015, over the glacier Midtre Lovénbreen and its forefield, in northwestern Svalbard. We were able to derive elevation change estimates comparable to in situ mass balance stake measurements. The accuracy and precision of our DEMs allow detection and analysis of a number of processes in the proglacial area, including the presence of thermokarst and the evolution of water channels.

CryoSat-2 delivers monthly and inter-annual surface elevation change for Arctic ice caps

Abstract. We show that the CryoSat-2 radar altimeter can provide useful estimates of surface elevation change on a variety of Arctic ice caps, on both monthly and yearly timescales. Changing conditions, however, can lead to a varying bias between the elevation estimated from the radar altimeter and the physical surface due to changes in the ratio of subsurface to surface backscatter. Under melting conditions the radar returns are predominantly from the surface so that if surface melt is extensive across the ice cap estimates of summer elevation loss can be made with the frequent coverage provided by CryoSat-2. For example, the average summer elevation decreases on the Barnes Ice Cap, Baffin Island, Canada were 2.05 ± 0.36 m (2011), 2.55 ± 0.32 m (2012), 1.38 ± 0.40 m (2013) and 1.44 ± 0.37 m (2014), losses which were not balanced by the winter snow accumulation. As winter-to-winter conditions were similar, the net elevation losses were 1.0 ± 0.20 m (winter 2010/11 to winter 2011/12), 1.39 ± 0.20 m (2011/12 to 2012/13) and 0.36 ± 0.20 m (2012/13 to 2013/14); for a total surface elevation loss of 2.75 ± 0.20 m over this 3-year period. In contrast, the uncertainty in height change from Devon Ice Cap, Canada, and Austfonna, Svalbard, can be up to twice as large because of the presence of firn and the possibility of a varying bias between the true surface and the detected elevation due to changing year-to-year conditions. Nevertheless, the surface elevation change estimates from CryoSat for both ice caps are consistent with field and meteorological measurements.

A Comparison of Recent Elevation Change Estimates of the Devon Ice Cap as Measured by the ICESat and EnviSAT Satellite Altimeters

We have used surface elevation measurements acquired by the Ice, Cloud,and land Elevation Satellite Geoscience Laser Altimeter System (GLAS) and EnviSAT Radar Altimeter 2 (RA-2) satellite altimeters to assess the elevation change of the 13 700-km2 Devon Ice Cap (DIC) in Arctic Canada between 2002 and 2008. We present algorithms for the retrieval of elevation change rates over ice caps using data acquired from these satellites. A comparison of GLAS elevation data to those acquired by the RA-2 shows reasonable agreement between the two instruments; the root mean square elevation change difference was 56 cm, and the correlation coefficient between the two data sets was 0.68. Using only RA-2 elevation measurements, which are spatially and temporally more continuous, we determined the elevation change rate of the areas of the DIC where the surface geometry allows the RA-2 retracker to maintain lock. This includes most of the DIC, excluding large parts of the eastern half of the ice cap. The elevation change rate was found to be insignificant given a statistical estimate of the measurement error (-0.09 ± 0.29 m/a). We also present an assessment of the regional variations of the DIC elevation change, including a significant -0.71 ± 0.49 m/a elevation change rate of the 1980-km2 western arm. Furthermore, we present evidence of a localized 2-m drop in the surface elevation of the South Croker Bay Glacier during summer 2007. This drop is apparent within both satellite data sets, and we interpret this signal to reflect a sudden speedup of the glacier.

A comparison of Greenland ice-sheet volume changes derived from altimetry measurements

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.

Glacier geometry and elevation changes on Svalbard (1936–90): a baseline dataset

AbstractThis study uses older topographic maps made from high-oblique aerial photographs for glacier elevation change studies. We compare the 1936/38 topographic map series of Svalbard (Norwegian Polar Institute) to a modern digital elevation model from 1990. Both systematic and random components of elevation error are examined by analyzing non-glacier elevation difference points. The 1936/38 photographic aerial survey is examined to identify areas with poor data coverage over glaciers. Elevation changes are analyzed for seven regions in Svalbard (~5000 km2), where significant thinning was found at glacier fronts, and elevation increases in the upper parts of the accumulation areas. All regions experience volume losses and negative geodetic balances, although regional variability exists relating to both climate and topography. Many surges are apparent within the elevation change maps. Estimated volume change for the regions is –1.59±0.07km3 a–1 (ice equivalent) for a geodetic annual balance of –0.30ma–1w.e., and the glaciated area has decreased by 16% in the 54 year time interval. The 1936–90 data are compared to modern elevation change estimates in the southern regions, to show that the rate of thinning has increased dramatically since 1990.

Thickening of the ice stream catchments feeding the Filchner‐Ronne Ice Shelf, Antarctica

Earlier ice‐sheet mass‐balance estimates disagree with satellite‐altimeter‐measured thickening of the catchment area that feeds the Filchner‐Ronne Ice Shelf, Antarctica. In an effort to reconcile these differences, we generated a new flux‐gate mass‐balance estimate, using improved elevation and velocity data. Our results show a 39 ± 26 Gton/yr thickening, whereas earlier flux‐gate results indicated minor thinning. Our results are consistent with altimeter‐derived estimates of elevation change. This suggests that altimeter‐observed thickening on the East Antarctic plateau may be the result of some combination of a 20th Century accumulation increase and a continuing response to the ∼50% accumulation increase that began in the early Holocene.

Laser altimeter return pulse correlation: a method for detecting surface topographic change

Quantifying and monitoring of many natural hazards requires repeated measurements of a topographic surface whose change reflects a geological or geophysical process. Topography and topographic change measurements are routinely made using techniques such as Interferometric Synthetic Aperture Radar and GPS, but both of these techniques have limitations for these purposes. A technique attracting increasing attention for its ability to perform accurate high-resolution topographic mapping (including sub-canopy) is laser altimetry, or lidar. Here, we evaluate the feasibility of a new method for using laser altimeter return echoes, or waveforms, to detect relative elevation change. The method, dubbed the return pulse correlation method, maximizes the shape similarity of coincident laser return waveforms from two observation epochs by shifting them vertically. We evaluate the accuracy of the pulse correlation method using laser altimeter data acquired over the NASA Wallops Flight Facility, VA, a region where no elevation change is expected within the time period of the surveys, and at Assateague Island, MD, a highly dynamic barrier island where several meters of erosion and deposition have been observed. Results show that use of the pulse correlation method generates elevation change estimates similar in magnitude to those obtained by simply differencing coincident laser altimeter elevation measurements (dubbed the spot comparison method). Along the beach at Assateague Island, MD, similar patterns of accretion and deposition are detected using both the pulse correlation and spot comparison methods, although some horizontal resolution is lost using the pulse correlation method because of the wide footprint spacing of the waveform-recording laser altimeter used in the study. In the test case presented here, increasing the size of the laser footprint from 25 to 60 m caused the magnitude of the vertical change signal to be underestimated, confirming that the resolution of the measurement technique and the scale of the deformation features should be considered when planning survey missions. The use of this method can improve the accuracy of surface change estimates made using laser altimeter waveforms, especially beneath vegetation, by eliminating the subjective interpretation of waveforms used to extract a single elevation measurement. ©

Elevation change of the southern Greenland ice sheet from 1978 to 1988: Interpretation

The spatial pattern of elevation change estimates derived from satellite radar altimeter data for the period from 1978 to 1988 for the southern Greenland ice sheet is examined. As reported previously, the results from 12 ice cores widely distributed in the study area indicate that much of the spatial variability in the elevation change estimates can be explained by temporal variations in accumulation. Most notably, the areas of largest thickening and thinning, east and west of the ice divide around 66°N, recently experienced substantial decadal fluctuations in accumulation sufficient to explain the observed elevation change rates that span the range of ±24 cm yr−1. Elevation change estimates in the NW of the study area around the 2000‐m elevation contour were found to be inconsistent with both short‐ and long‐term changes in accumulation fluctuations. Examination of stratigraphy in several shallow ice cores in the NW indicates that significant melting took place in 1977 and 1978, and this was followed by many years of very little melt activity. This likely resulted in a significant downward bias in the 1978–1988 elevation change estimates in the NW. Comparison of measured surface velocities with those calculated assuming steady state balance show very good agreement overall, but there are many instances in the southeast where the steady state velocities are significantly smaller than measured surface velocities. We cannot rule out the possibility that increased ice flow in lower‐elevation outlet glaciers has migrated farther upstream thereby causing negative long‐term mass imbalance in southeast Greenland.

Greenland ice sheet elevation change since 1978 from radar and laser altimetry1

Repeat satellite radar altimetry gives estimates of surface elevation change for the southern half of the Greenland ice sheet between 1978 and 1988, and repeat aircraft laser altimetry gives equivalent estimates for 1993–1998. Over large areas away from the coast, the two data sets show the ice sheet to be in balance, but with localized zones of significant thickening or thinning. Rates of surface elevation change derived by comparing localized 1980 ground surveys with 1993–1994 laser surveys show excellent agreement with the 1978–1988 radar results, suggesting that the radar altimeter provides reliable estimates of elevation change when data are averaged over areas of ice sheet with low surface slopes. Regions where there are significant differences between the 1978–1988 and 1993–1998 results can be explained either by changes in near‐surface snow conditions that affect the radar reflecting horizon, or by temporal variability in snow accumulation rates.

Temporal change in the extinction coefficient of snow on the Greenland ice sheet from an analysis of Seasat and Geosat altimeter data

The extinction coefficient of snow k/sub e/ along the central portion of the Greenland ice sheet is mapped using data from the Seasat (1978) and Geosat (1985-1989) altimeters. The extinction coefficient is obtained by fitting altimeter waveforms with a surface/volume scattering model. The authors find that in the lower latitudes the Seasat and Geosat extinction coefficients are very nearly the same, while in a specific higher latitude region of the ice sheet the Seasat k/sub e/ values exceed the Geosat values by over 100%. By analyzing 18 months of the Geosat data, the author quantified the variability inherent in the extinction coefficient measurements. The results show that the observed temporal variation in the extinction coefficient from 1978 to 1985 is three times larger than the measured variability. This indicates that the average grain size of the near surface snow in this region may have decreased during the time span between the two altimeter datasets. The temporal change in extinction coefficient found in this study demonstrates the important contributions that time-series analysis of satellite datasets can make to the study of the polar ice sheets. In addition, these results have important implications for the study of long-term elevation change over the ice sheets using altimeter data. The author's study demonstrates that significant biases could be introduced into ice-sheet elevation change estimates because of temporal variations in the surface conditions of the ice sheet. Future investigations of ice-sheet mass balance using altimetry data should be aware of this possibility.