Beata Csatho

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.

Accumulation over the Greenland ice sheet from historical and recent records

Water accumulation, defined as precipitation minus evaporation, was estimated over all of Greenland as part of a program to understand changes in ice sheet mass and elevation. Over 360 historical and recent point accumulation estimates on the Greenland ice sheet were evaluated, and 276 estimates that were judged to be high quality were used to develop the accumulation map. The data set includes 99 points developed as part of four investigations of the past 5–15 years; these are judged to have the greatest accuracy. Using kriging, the average accumulation over Greenland is estimated to be ∼30 g cm−2 yr−1. For the interior part of the ice sheet above 1800 m elevation, where most of the data were acquired, the average accumulation is also estimated to be ∼30 g cm−2 yr−1. There are still many areas on the ice sheet, including northwest, southeast, and southern Greenland, where accumulation is highly uncertain, exceeding the mean ice sheet uncertainty at a point of ∼20–25%. In these regions, further sampling will be required to reduce uncertainty in both regional and ice-sheet-wide accumulation.

Response of Jakobshavn Isbrae, Greenland, to Holocene climate change

Rapid fluctuations in the velocity of Greenland Ice Sheet (GIS) outlet glaciers over the past decade have made it difficult to extrapolate ice-sheet change into the future. This significant short-term variability highlights the need for geologic records of preinstrumental GIS margin fluctuations in order to better predict future GIS response to climate change. Using 10 Be surface exposure ages and radiocarbon-dated lake sediments, we constructed a detailed chronology of ice-margin fluctuations over the past 10 k.y. for Jakobshavn Isbrae, Greenland9s largest outlet glacier. In addition, we present new estimates of corresponding local temperature changes using a continuous record of insect (Chironomidae) remains preserved in lake sediments. We find that following an early Holocene advance just prior to 8 ka, Jakobshavn Isbrae retreated rapidly at a rate of ∼100 m yr −1 , likely in response to increasing regional and local temperatures. Ice remained behind its present margin for ∼7 k.y. during a warm period in the middle Holocene with sustained temperatures ∼2 °C warmer than today, then the land-based margin advanced at least 2–4 km between A.D. 1500–1640 and A.D. 1850. The ice margin near Jakobshavn thus underwent large and rapid adjustments in response to relatively modest centennial-scale Holocene temperature changes, which may foreshadow GIS response to future warming.

Laser altimetry reveals complex pattern of Greenland Ice Sheet dynamics

Significance We present the first detailed reconstruction of surface elevation changes of the Greenland Ice Sheet from NASA’s laser altimetry data. Time series at nearly 100,000 locations allow the characterization of ice sheet changes at scales ranging from individual outlet glaciers to larger drainage basins and the entire ice sheet. Our record shows that continuing dynamic thinning provides a substantial contribution to Greenland mass loss. The large spatial and temporal variations of dynamic mass loss and widespread intermittent thinning indicate the complexity of ice sheet response to climate forcing, strongly enforcing the need for continued monitoring at high spatial resolution and for improving numerical ice sheet models. We present a new record of ice thickness change, reconstructed at nearly 100,000 sites on the Greenland Ice Sheet (GrIS) from laser altimetry measurements spanning the period 1993–2012, partitioned into changes due to surface mass balance (SMB) and ice dynamics. We estimate a mean annual GrIS mass loss of 243 ± 18 Gt⋅y−1, equivalent to 0.68 mm⋅y−1 sea level rise (SLR) for 2003–2009. Dynamic thinning contributed 48%, with the largest rates occurring in 2004–2006, followed by a gradual decrease balanced by accelerating SMB loss. The spatial pattern of dynamic mass loss changed over this time as dynamic thinning rapidly decreased in southeast Greenland but slowly increased in the southwest, north, and northeast regions. Most outlet glaciers have been thinning during the last two decades, interrupted by episodes of decreasing thinning or even thickening. Dynamics of the major outlet glaciers dominated the mass loss from larger drainage basins, and simultaneous changes over distances up to 500 km are detected, indicating climate control. However, the intricate spatiotemporal pattern of dynamic thickness change suggests that, regardless of the forcing responsible for initial glacier acceleration and thinning, the response of individual glaciers is modulated by local conditions. Recent projections of dynamic contributions from the entire GrIS to SLR have been based on the extrapolation of four major outlet glaciers. Considering the observed complexity, we question how well these four glaciers represent all of Greenland’s outlet glaciers.

Evaluation of recent precipitation studies for Greenland Ice Sheet

The retrieval of an accurate spatial and temporal record of contemporary Greenland precipitation is a uniquely challenging task because of the extreme variability in both atmospheric processes and the resulting precipitation distribution over relatively small spatial scales. A comparison of precipitation data sets composed of monthly mean values from recent studies shows a convergence on the general features of the long-term spatial patterns but substantial disagreement on the temporal variability both regionally and for all of Greenland. There is general agreement on a long-term Greenland average of about 35 cm yr−1 and on long-term values for regional scales, although differences for outlying data sets exceed 50% of the observed glaciological estimate for particular regions. A fundamental problem is the inadequate topographic representation of Greenland in the numerical analyses. Nearly all of the data sets are overly dry for high-elevation areas, as seen from comparisons with glaciological observations from Summit. The east-central region of Greenland is found to be particularly susceptible to the temporal discontinuities in data sets which employ operational analyses. In contrast, there is strong agreement among all methods on the temporal variability for the west-central region over a 15-year period. From the comparison it is concluded that none of the data sets is able to capture all of the regional-scale features. In general, however, the deficiencies of each data set are readily identifiable from comparison and evaluation in the context of circulation features. Agreement among the methods on particular regions and timescales gives increased confidence in drawing conclusions related to aspects of Greenlands precipitation climatology. In particular, an enhanced precipitation retrieval method is found to be less susceptible to data artifacts than other methods using operational analyses. In the north, anomalously high precipitation is associated with cyclonic development near the Fram Strait. For west-central Greenland the close agreement among methods is related to the dominant contribution of the mean circulation.

A New Methodology for Detecting Ice Sheet Surface Elevation Changes From Laser Altimetry Data

The integrated program Surface Elevation Reconstruction And Change detection (SERAC) was specifically designed and developed for detecting surface elevation and elevation changes from the Ice Cloud and land Elevation Satellite (ICESat). ICESat carried geoscience laser altimeter system (GLAS) with the primary goal of measuring elevation changes of the polar ice sheets to sufficient accuracy to assess their impact on global sea level. GLAS had three lasers that operated sequentially, with two to three campaigns per year. The footprint size was about 70 m and the point-to-point spacing between neighboring laser points reached 170 m. SERAC copes with different scenarios. Originally developed for calculating surface elevation changes of crossover areas, it was extended to along-track areas and the inclusion of non-ICESat laser data, such as Airborne Topographic Mapper (ATM), an airborne laser scanning system developed by NASA Wallops Flight Facility. The adjustment system of SERAC simultaneously computes the shape of surface patches containing laser points of the same time epoch, estimates surface elevation changes, and approximates the time series of elevation changes by a polynomial after removing the seasonal cycle. Results shown in the second part of the paper demonstrate the potential of SERAC for calculating detailed ice sheet elevation and volume change histories. Greenland Ice Sheet volume changes, calculated from a combined ICESat/ATM data set, show good agreement with previously published results and provide improved sampling in the rapidly thinning coastal regions of southern Greenland. Moreover, the polynomial approximation of the time series of surface elevation changes is taken to advantage in the last example of Northwest Greenland, illuminating the intricate thinning/thickening patterns that often vary considerably over short spatial scales.

Response of a marine‐terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago

Greenland’s largest outlet glacier, using 10 Be surface exposure ages and 14 C‐dated lake sediments. Our chronology of ice‐ margin change demonstrates that Jakobshavn Isbrae advanced to deposit moraines in response to abrupt cooling recorded in central Greenland ice cores ca. 8,200 and 9,300 years ago. While the rapid, dynamically aided retreat of many Greenland outlet glaciers in response to warming is well documented, these results indicate that marine‐terminating outlet glaciers are also able to respond quickly to cooling. We suggest that short lag times of high ice flux margins enable a greater magnitude response of marine‐terminating outlets to abrupt

Knowledge discovery in urban environments from fused multi-dimensional imagery

With all the exciting advances in sensor fusion and data interpretation technologies in recent years, including co-registration, 3-D surface reconstruction, object recognition, spatial reasoning, and more, high-quality detailed and precise segmentation of remote sensing spectral images remains a much needed key component in the comprehensive analysis and understanding of surfaces. Urban surfaces are no exception. In fact, urban surfaces can represent more challenge than many other types because of the very large variety of materials concentrated in relatively small areas. Segmentation (unsupervised clustering) or supervised classification based on spectral signatures from multi-and hyperspectral imagery, or based on other multidimensional signatures from stacked disparate (multi-source) imagery, provide delineation of materials with various compositional and physical properties in a scene. Such a cluster or classification map lends critical support to further reasoning for accurate identification of surface objects and conditions. It is, therefore, imperative to develop methods whose data exploitation power matches that of the discriminating power of the data acquisition instrument. We present a study of unsupervised segmentation, comparing the performances of ISODATA clustering and self-organized manifold learning on an urban image from a Daedalus multi-spectral scanner and on an AVIRIS hyperspectral image.

Thickening of the western part of the Greenland ice sheet

NASAs Program for Arctic Regional Climate Assessment (PARCA) includes measurements of ice velocity and ice thickness along the 2000 m elevation contour line in the western part of the ice sheet. Here we use these measurements together with published estimates of snow-accumulation rates to infer the mass balance, or rate of thickening/thinning, of the ice-sheet catchment area inland from the velocity traverse. Within the accuracy to which we know snow-accumulation rates, the entire area is in balance, but localized regions inland from Upernavik Isstrom and Jakobshavn Isbrae both appear to be thickening by about 10 cm a -1 .

Automated Labeling of Materials in Hyperspectral Imagery

We present a technique for automatically labeling segmented hyperspectral imagery with semantically meaningful material labels. The technique compares the mean signatures of each image segment to a spectral library of known materials, and material labels are assigned to image segments according to the most similar library entry. The similarity between spectral signatures is evaluated using our recently proposed CICRd similarity measure designed specifically for hyperspectral imagery. This measure considers both the continuum-intact reflectance spectrum and its continuum-removed representation. We provide a thorough assessment of this measure by comparison to several commonly used similarity measures on a well-studied low-altitude Airborne Visible/Infrared Imaging Spectrometer image of an urban area. We evaluate our results using both information-theoretic techniques and visual validation of the resulting spectral matches.