Ice Sheets and Ice Sheet Mass Balance 3

Fluctuations in the mass of ice stored in Antarctica and Greenland are of considerable societal importance. The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) is a joint-initiative of ESA and NASA aimed at producing a single estimate of the global sea level contribution to polar ice sheet losses. Within IMBIE, estimates of ice sheet mass balance are developed from a variety of satellite geodetic techniques using a common spatial and temporal reference frame and a common appreciation of the contributions due to external signals. The project brings together the laboratories and space agencies that have been instrumental in developing independent estimates of ice sheet mass balance to date.

In its first phase, IMBIE involved 27 science teams, and delivered a first community assessment of ice sheet mass imbalance to replace 40 individual estimates. The project established that (i) there is good agreement between the three main satellite-based techniques for estimating ice sheet mass balance, (ii) combining satellite data sets leads to significant improvement in certainty, (iii) the polar ice sheets contributed 11 ± 4 mm to global sea levels between 1992 and 2012, and (iv) that combined ice losses from Antarctica and Greenland have increased over time, rising from 10% of the global trend in the early 1990’s to 30% in the late 2000’s.

Demand for an updated assessment has grown, and there are now new satellite missions, new geophysical corrections, new techniques, and new teams producing data. The period of overlap between independent satellite techniques has increased from 5 to 12 years, and the full period of satellite data over which an assessment can be performed has increased from 19 to 40 years. It is also clear that multiple satellite techniques are required to confidently separate mass changes associated with snowfall and ice dynamical imbalance – information that is of critical importance for climate modelling.

This presentation outlines the approach for the second phase of IMBIE, including the project organisation, the work programme and schedule, the main science goals, and its current status, and reviews the recent and historical contributions that the Antarctic and Greenland ice sheets have made to global sea level rise

Constraining past ice mass changes, identifying their cause(s) and determining rigorous error estimates, is important for closing the sea level budget and as an input for and test of numerical models. For the Antarctic ice sheet, considerable uncertainty remains between different methods and groups. Estimates obtained from altimetry, gravimetry, and mass-budget methods can yield conflicting results with error estimates that do not always overlap, while the, commonly adopted, use of different forward models to isolate and remove the effects of glacio-isostatic adjustment (GIA) and surface mass balance (SMB) processes introduces another source of uncertainty which is hard to quantify. To address both these issues, we present a statistical modelling approach to the problem. We combine the observational data, including satellite altimetry, GRACE, GPS and InSAR, and use the different degrees of spatial and temporal smoothness to constrain the underlying geophysical processes. This is achieved via a spatio-temporal Bayesian hierarchical model, employing dimensionality reduction methods to allow the solution to remain tractable in the presence of the large number (> 10ˆ7) of observations involved. The resulting trend estimates are only dependent on length and smoothness properties obtained from numerical models, but are otherwise entirely data-driven. As a consequence, the solutions provide a valuable independent test of the forward models. Here, we present the annually-resolved spatial fields for i) dynamic ice loss, ii) SMB anomaly, iii) firn compaction and iv) (the time invariant) GIA, using a combination of GRACE, ICESat, ENVISat, CryoSat 2 and GPS vertical uplift rates, for 2003-2013. The elastic flexure of the crust is also determined simultaneously. We focus here primarily on the mass trends rather than solid earth effects which are presented elsewhere in this conference.  We obtain a mean rate of -82±23 Gt/yr for the 11 year period with a sustained negative mean trend of dynamic imbalance to which West Antarctica is the largest contributor, mainly triggered by high thinning rates of glaciers draining into the Amundsen Sea Embayment. The Antarctic Peninsula has experienced a dramatic increase in mass loss in the last decade following the destabilization of the Southern Antarctic Peninsula. The total mass loss is partly compensated by a significant mass gain in East Antarctica due to a positive trend of SMB anomalies.

The Antarctic ice shelves provide buttressing and stability to the inland grounded ice and play, therefore, an important role in controlling inland ice dynamics and mass imbalance. As a result, the determination of ice shelf thickness is necessary to understand changes in stability caused by external forcing. Knowledge of ice shelf thickness at the continental scale is essential for buttressing parameterisations in ice sheet models, mass balance calculations using the mass budget method and parameterisations in sub-shelf ocean models.

Assuming hydrostatic equilibrium for the floating ice, thickness can be inferred from surface elevation measurements provided by satellite radar altimetry. Whilst this methodology has been employed previously sensor limitations have led to a lack of data coverage and relatively poor accuracy around the grounding zone, where a break in slope exists and is a region that is particularly sensitive to changes in sub shelf melting.

Here we determine ice shelf thickness using four years (2011-2014) of altimetry measurements from ESA’s Cryosat-2 instrument, with its dual antennae interferometric mode of operation allowing for the alleviation of the issues affecting previous data sets. The small across track spacing of the satellite has led to large improvements in spatial coverage, with ~92% of the ice shelves being surveyed. Substantial improvements in data coverage is seen across all ice shelves in comparison to the previous dataset, with improvements greater than 50% seen over the Totten and Venable Ice Shelves. Additionally, large improvements are seen south of 81.5° for the Ross and Filchner-Ronne Ice Shelves.

The resultant surface digital elevation model is validated against NASA ICEsat laser altimetry data, showing improvements in accuracy, particularly within the grounding zone. Across the whole continent, mean bias in the data is less than 1m, with a fourfold decrease in standard deviation compared to the previous dataset. The largest reductions are seen in the grounding zone, with a 22m mean absolute reduction in standard deviation compared to the previous data product.

Derived thickness estimates are validated using multiple Radio Echo Sounding (RES) campaigns across the continent. Extensive high spatial density RES measurements across the Amery ice shelf, provided by the Australian National Antarctic Research Expedition (ANARE), allow for a thorough accuracy assessment of our product. Mean absolute error between the datasets is 3.3%, rising to 4.6% in the grounding zone. This represents a two to three times increase in accuracy compared to previous HE derived ice thicknesses at the grounding line. 

This dataset will facilitate improved error estimations in mass budget calculations, as well as ice sheet and sub-ice shelf ocean models. Additionally, our product provides important advances to the data used in the BEDMAP 2 product. The data product will be made made publically available to the community.

Recent studies in West Antarctica have aimed to elucidate the drivers of ‘dynamic thinning’, a process by which oceanic and/or atmospheric forcing instigates accelerated thinning of coastal ice. These studies have been especially concentrated in the Amundsen Sea sector, where the largest losses of ice have occurred over the past decade, and from where there are now considerable observational records of glacial, ice-shelf and ocean change to constrain ice-ocean-atmosphere interactions. There are, however, other parts of West Antarctica, notably the sector draining into the Bellingshausen Sea, where dynamic thinning is also occurring. In this sector, the pace and timing of dynamic thinning, and whether it is responding to the same or different atmospheric or oceanic stimuli to forcing in the Amundsen Sea sector, have remained poorly resolved.

Using grounding line migration as a proxy for dynamic thinning, we present changes to grounding line position along the Bellingshausen Sea facing coastline of West Antarctica as observed over multiple epochs from 1975 with optical and radar satellite imagery. Using Landsat MSS/TM/ETM+/OLI imagery, we derive advance and retreat quantifications along the coastline for 5 to 10-year periods from 1975-2015. For parts of the coastline, we validate the trends inferred from the optical imagery with ERS-1/ERS-2 double-differenced interferometric synthetic aperture radar (InSAR) derived grounding line positions for years 1992, 1994, 1996 and 2011.

Analysis of grounding line positions reveals: i) significant grounding line retreat at the Ferrigno Ice Stream, which unlike glaciers and ice streams throughout the remainder of the coastline is not buttressed by substantial downstream ice shelves; and, ii) additional noteworthy retreat at the glaciers and ice streams flowing into the southern Stange Ice Shelf. These findings correspond with observations of recent rapid glacial thinning across both regions (see supplementary abstract document). However, we find: iii) insignificant grounding line retreat along the margin of the Venable Ice Shelf between 1985 and 2015, despite observations of significant surface thinning over the last decade.

Overall, our findings underscore the requirement for close observation of this important coastline in coming years, not only to elucidate further the changing nature of dynamic thinning along this sector, but also to assess the wider stability of the West Antarctic Ice Sheet (WAIS). Such observations will be possible through, for example, the enhanced remote sensing capabilities afforded by the ESA Sentinel-1, Sentinel-2, CryoSat-2 and NASA Landsat 8 platforms. 

The combination of GRACE and altimetry data may yield a high resolution mass balance time series of the Greenlandice sheet, highlighting the varying individual mass loss behaviour of major glaciers. By including the Canadian arctic ice caps in the estimation, a more reliable estimate of the mass loss of both Greenlandand the Canadian ice caps may be obtained, minimizing the leakage errors otherwise unavoidable by GRACE. Actually, the absolute value of the Greenlandice sheet mass loss is highly dependent on methods and how the effects of Arctic Canadian ice caps are separated in the GRACE signal; here altimetry data and firn models may provide important constraints.

In the presentation we outline the basis of the joint estimation method, including corrections for firn density and compaction, and use CryoSat and EnviSat data together with GRACE data to provide composite time series of mass loss of the ice caps and ice sheet basins for the period 2003-15. This period shows a marked increase of ice sheet melt, especially in NW and NE Greenland, but also show large variability, with the melt anomaly year of 2012 showing a record mass loss, followed by 2013 with essentially no Greenland mass loss at all, and lately return to the “normal” mass loss rate. 

While GRACE sees these later year anomalies quite clearly, the CryoSat data are affected strongly by the icesheet-wide 2012 melt event, where change in CryoSat radar penetration in the firn results in an erroneous mass gain estimate over central parts of theGreenlandice sheet. The overall estimate of mass loss trend forGreenlandduring the CryoSat period yields values around -280 GT/yr, with the Canadian ice cap mass loss estimated at around -40 GT/yr and -25 GT/yr for the Baffin and Ellesmere regions, respectively. The used estimation methods can readily be extended to other altimetry mission data, such as Sentinel-3 and AltiKa, and could also potentially be used to further integrate other EO parameters such as ice velocity changes of the major outlet glaciers, as produced e.g. in the ESA Greenland Ice Sheet CCI project.