Search

ArticlePublished: 10 December 2019This is an unedited manuscript that has been accepted for publication. Nature Research are providing this early version of the manuscript as a service to our customers. The manuscript will undergo copyediting, typesetting and a proof review before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers apply.Mass balance of the Greenland Ice Sheet from 1992 to 2018The IMBIE TeamNature (2019)Cite this article6914 Accesses1410 AltmetricMetricsdetailsAbstractIn recent decades, the Greenland Ice Sheet has been a major contributor to global sea-level rise1,2, and it is expected to be so in the future3. Although increases in glacier flow4–6 and surface melting7–9 have been driven by oceanic10–12 and atmospheric13,14 warming, the degree and trajectory of today’s imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet’s volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. Although the ice sheet was close to a state of balance in the 1990s, annual losses have risen since then, peaking at 335 ± 62 billion tonnes per year in 2011. In all, Greenland lost 3,800 ± 339 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.6 ± 0.9 millimetres. Using three regional climate models, we show that reduced surface mass balance has driven 1,971 ± 555 billion tonnes (52%) of the ice loss owing to increased meltwater runoff. The remaining 1,827 ± 538 billion tonnes (48%) of ice loss was due to increased glacier discharge, which rose from 41 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. Between 2013 and 2017, the total rate of ice loss slowed to 217 ± 32 billion tonnes per year, on average, as atmospheric circulation favoured cooler conditions15 and as ocean temperatures fell at the terminus of Jakobshavn Isbræ16. Cumulative ice losses from Greenland as a whole have been close to the IPCC’s predicted rates for their high-end climate warming scenario17, which forecast an additional 50 to 120 millimetres of global sea-level rise by 2100 when compared to their central estimate.

Nonparametric estimators of multivariate functions typically converge more slowly as dimension increases. We show that one can evade this curse of dimensionality by assuming a simplified vine copula model for the dependence between variables and compare several methods in simulations. We further extend the applicability of this finding in two ways: We introduce an approach to estimate regression functions based on the copula density and discuss a generic technique to make nonparametric function estimators applicable to discrete data. Nichtparametrische Schätzer von multivariaten Funktionen konvergieren für gewöhnlich langsamer, wenn die Anzahl der Variablen zunimmt. Wir zeigen, dass man diesem Fluch der Dimensionen entkommen kann, indem man ein vereinfachtes Vine-Copula Modell für die Abhängigkeit zwischen den Variablen annimmt, und untersuchen darauf basierende Schätzmethoden in einer Simulationsstudie. Im Anschluss wird die Anwendbarkeit dieser Eigenschaft in zweierlei Hinsicht erweitert: Wir zeigen, wie man allgemeine Regressionsprobleme mit copula-basierten Schätzungsgleichungen lösen kann, und diskutieren eine generische Methode, um nichtparametrische Schätzer auf Daten mit diskreten Variablen anwendbar zu machen.

Ice velocity is one of the products associated with the Ice Sheets Essential Climate Variable. This paper describes the intercomparison and validation of ice-velocity measurements carried out by several international research groups within the European Space Agency Greenland Ice Sheet Climate Change Initiative project, based on space-borne Synthetic Aperture Radar (SAR) data. The goal of this activity was to survey the best SAR-based measurement and error characterization approaches currently in practice. To this end, four experiments were carried out, related to different processing techniques and scenarios, namely differential SAR interferometry, multi aperture SAR interferometry and offset-tracking of incoherent as well as of partially-coherent data. For each task, participants were provided with common datasets covering areas located on the Greenland ice-sheet margin and asked to provide mean velocity maps, quality characterization and a description of processing algorithms and parameters. The results were then intercompared and validated against GPS data, revealing in several cases significant differences in terms of coverage and accuracy. The algorithmic steps and parameters influencing the coverage, accuracy and spatial resolution of the measurements are discussed in detail for each technique, as well as the consistency between quality parameters and validation results. This allows several recommendations to be formulated, in particular concerning procedures which can reduce the impact of analyst decisions, and which are often found to be the cause of sub-optimal algorithm performance. Keywords: ice velocity; Synthetic Aperture Radar; Greenland ice sheet; Climate Change Initiative

We analysed volume change and mass balance of outlet glaciers on the northern Antarctic Peninsula over the periods 2011 to 2013 and 2013 to 2016, using high-resolution topographic data from the bistatic interferometric radar satellite mission TanDEM-X. Complementary to the geodetic method that applies DEM differencing, we computed the net mass balance of the main outlet glaciers using the mass budget method, accounting for the difference between the surface mass balance (SMB) and the discharge of ice into an ocean or ice shelf. The SMB values are based on output of the regional climate model RACMO version 2.3p2. To study glacier flow and retrieve ice discharge we generated time series of ice velocity from data from different satellite radar sensors, with radar images of the satellites TerraSAR-X and TanDEM-X as the main source. The study area comprises tributaries to the Larsen A, Larsen Inlet and Prince Gustav Channel embayments (region A), the glaciers calving into the Larsen B embayment (region B) and the glaciers draining into the remnant part of the Larsen B ice shelf in Scar Inlet (region C). The glaciers of region A, where the buttressing ice shelf disintegrated in 1995, and of region B (ice shelf break-up in 2002) show continuing losses in ice mass, with significant reduction of losses after 2013. The mass balance numbers for the grounded glacier area of region A are −3.98 ± 0.33 Gt a−1 from 2011 to 2013 and −2.38 ± 0.18 Gt a−1 from 2013 to 2016. The corresponding numbers for region B are −5.75 ± 0.45 and −2.32 ± 0.25 Gt a−1. The mass balance in region C during the two periods was slightly negative, at −0.54 ± 0.38 Gt a−1 and −0.58 ± 0.25 Gt a−1. The main share in the overall mass losses of the region was contributed by two glaciers: Drygalski Glacier contributing 61 % to the mass deficit of region A, and Hektoria and Green glaciers accounting for 67 % to the mass deficit of region B. Hektoria and Green glaciers accelerated significantly in 2010–2011, triggering elevation losses up to 19.5 m a−1 on the lower terminus during the period 2011 to 2013 and resulting in a mass balance of −3.88 Gt a−1. Slowdown of calving velocities and reduced calving fluxes in 2013 to 2016 coincided with years in which ice mélange and sea ice cover persisted in proglacial fjords and bays during summer.

The next generation, post-Sentinel-1, ESA’s C-band synthetic aperture radar (SAR) system is conceived to provide simultaneously high azimuth resolution and wide swath width (HRWS). There are different ways in which the imaging capabilities of the HRWS SAR system can be exploited, which translate to different operation modes. The more attractive are the wide swath modes, operating in ScanSAR, with 400 km swath width and a resolution of 5m x 5m for single/dual-polarization and, maybe even more noteworthy, the fully-polarimetric 280 km swath width at 5m x 5m single-look resolution. These modes represent a factor four improvement in terms of azimuth resolution with respect to Sentinel-1. Considering also the extended swath or the quad-pol capabilities, the information rate will increase by close to and order of magnitude. Indeed, wide unambiguous swath coverage and high azimuth resolution pose contradicting requirements on the design of spaceborne SAR systems. Nevertheless, recent studies have shown that by applying Digital Beam Forming (DBF) techniques, such as Scan-on-Receive (SCORE), and Multiple Azimuth Phase centers (MAPS), it becomes possible to overcome these fundamental limitations of conventional SAR systems. The use of MAPS in azimuth enables the decoupling of the high azimuth resolution and wide-swath SAR coverage. It employs a multichannel receiver in combination with mutually displaced multiple aperture elements and the azimuth resolution results determined by the length of the individual sub-aperture elements. At the same time, employing multiple channels in elevation, according to the SCORE technique, allows to collect radar echoes from a wide image swath despite using a receiver aperture with large vertical extension. The trade-off between antenna gain and swath width can thus be relaxed. In this framework, DLR has reviewed the capabilities of the HRWS SAR system in light of the associated requirements provided by ESA and of the science requirements associated to operational GMES applications. Indeed, many and potentially new applications can benefit from the HRWS SAR operational modes. Moreover, a HRWS application performance toolkit has been designed and implemented to compare product-level performance for different operating modes and mission scenarios. The established applications defined within the GMES services and selected for the HRWS performance study are: Deformation monitoring Regional land cover Ocean applications (wind and currents retrieval and oil spill monitoring) Land ice (wet snow mapping and ice drift) Sea ice (iceberg detection and ice motion) Iceberg detection Security (vessel detection ) Thus, th resulting HRWS toolkit includes for every application analytical expressions or numerical models and, if these are not available, real SAR images as well as numerical algorithms and some explicit simulations of the data and of the inversion process are employed. The tool uses as input the HRWS SAR instrument performance for the different applicable modes and produces as output results comparable with the existing C-band SAR missions. Due to its wider swath, high resolution and multipolarimetric capabilities, the performance for the HRWS SAR system show a substantial improvement when compared to those of Sentinel-1A, for most of the applications and operational scenarios. In the final paper a short description of the employed product-level performance models together with the main results will be provided. Furthermore, an analysis based on the different applications performance and on their relative relevance will give a single operational mode for the best compromise.