The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) led by ESA and NASA aims at reconciling estimates of ice sheet mass balance from satellite gravimetry, altimetry and the mass budget method through community efforts. Building on the success of the two previous phases of IMBIE – during which satellite-based estimates of ice sheet mass balance were reconciled within their respective uncertainties and which showed a 6-fold increase in the rate of mass loss during the satellite era – IMBIE has now entered its third phase. The objectives of this new phase of IMBIE, supported by ESA CCI, are to (i) include data from new satellite missions including GRACE-FO and ICESAT-2, (ii) provide annual assessments of ice sheet mass balance, (iii) partition changes into dynamics and surface mass balance processes, (iv) produce regional assessments and (v) examine the remaining biases between the three geodetic techniques, all in order to provide more robust and regular estimates of ice sheet mass balance and their contribution to global mean sea level rise. In this paper, we report on the recent progress of IMBIE-3. Following the last IMBIE update produced for the IPCC’s sixth assessment report (AR6), for which we extended our time-series of mass change of Greenland and Antarctica until the end of 2020, we are now preparing our next annual update, which will cover the year 2021.

Melting of the Greenland and Antarctic ice sheets currently contributes more than one third of global sea-level rise. As Earth’s climate continues to warm throughout the 21st Century, ice loss is expected to increase further, with the potential to cause widespread social and economic disruption. To track the changes that are currently underway in the Polar regions requires detailed and systematic monitoring programmes. Given the vast and inaccessible nature of the ice sheets, this is only feasible from space. One technique that has proved particularly valuable in recent decades is that of satellite radar altimetry. Since the launch of ERS-1 thirty years ago, ESA satellites have provided a continuous record of ice sheet surface elevation change, and with it valuable information relating to the physical processes that drive ice mass imbalance.

Changes in surface elevation are a signature of multiple physical processes that drive ice sheet mass balance. Long term trends in surface elevation can indicate glacier dynamic imbalance, driven for example, by changes in ocean forcing. High frequency tidal and atmospheric pressure-induced oscillations in ice shelf height can be used to identify glacier grounding lines. Localised uplift and subsidence can be an indicator of the passage of water beneath ice sheets, whilst seasonal cycles in elevation can provide information relating to snowfall, surface ablation, and run-off.

With the ever-increasing volume and resolution of satellite topographic data, comes the greater potential to push the boundaries of the physical and glaciological processes that can be observed. Alongside this, comes the possibility to revisit historical instruments, such as ERS-1, ERS-2 and Envisat, to improve the fidelity and useability of these datasets; to exploit complementary sources of information, such as a new generation of super high resolution, meter-scale, Digital Elevation Models; and to employ advanced statistical techniques, to extract increasing information from the data acquired.

In this presentation, we bring together results from multiple ESA-funded studies, spanning ERS-1 through to the latest Sentinel-3 mission, to show how they are leading to fundamental advances in our understanding of ice sheet processes. Specifically, we describe how the achievements of the Cryo-TEMPO study, the Polar+ Surface Mass Balance study, the Polar+ 4D Greenland study, FDR4ALT and the S3 Land STM MPC come together to improve our understanding of the processes driving present day ice sheet mass imbalance. We will describe the progress made to improve the quality and useability of these measurements, from ERS-1 through to Sentinel-3, and present a number of case studies that show how this has delivered new insight into a diverse range of physical processes, including subglacial hydrology, ice sheet meteorology, grounding line mapping and long-term ice imbalance.

Multi-satellite data assessments have provided evidence for a six-fold increase in mass loss of the Antarctica Ice Sheet since 1992 (Shepherd et al., 2018). Driven mainly by an increase in ice discharge in the Amundsen Sea Embayment, these changes are likely to continue in future, and may indicate surpassing of the stability threshold for the West Antarctic Ice Sheet (Arthern and Williams, 2017). However, due to numerous unknown or poorly known parameters entering ice sheet simulations, the dynamic evolution of the Antarctic ice sheet remains the largest uncertainty in global sea-level projections. Remote sensing offers accurate observations of the ice sheet’s current dynamic state, including its mass balance components, crucial to limit projection ensembles to pathways satisfying present-day remote sensing observations.

Here we focus on isolating regional accelerations of mass change caused by ice dynamics and by surface-mass balance (SMB) in the GRACE/GRACE-FO data for 2002-2021. We quantify the ice-dynamic acceleration in Antarctica based on differencing GRACE/GRACE-FO and SMB, as represented by regional climate models and ERA-5 reanalysis data. We show that this indirect method presents an alternative to estimates based on determining dynamic acceleration from remote sensing of the surface-ice velocity, e.g. using InSAR. We find that with regard to the acceleration component, both the direct and indirect methods produce consistent estimates of dynamic acceleration, with similar uncertainties. Furthermore, we show that accelerations and interannual variations in GRACE/GRACE-FO data are largely driven by SMB variations related to largescale atmospheric circulation patterns. While the apparent acceleration of SMB shows large fluctuations depending on the time period considered, we show that the recovered ice dynamic acceleration is a stationary feature.

With the assumption that mass loss will continue at the present rate, we extrapolate the trends inferred from the satellite data to the year 2100. As a sensitivity experiment we in- and exclude the acceleration of dynamic discharge in the extrapolation. We show that a quadratic pathway consistent with today’s satellite observations produces 7.6 ± 2.9 cm of sea-level rise until 2100, in comparison with a linear extrapolation of the rates only of 2.9 ± 0.6 cm. We show that validation of larger projections ensembles with remote sensing observations is crucial to limit the spread of pathways in the numerical simulations, and, thus reduce the uncertainty in the projected sea-level contribution from Antarctica.

Arthern, R. J., and Williams, C. R. (2017). The sensitivity of West Antarctica to the submarine melting feedback. Geophys. Res. Lett. 44, 2352–2359. doi:10.1002/2017GL072514.
Shepherd, A., Ivins, E., Rignot, E., Smith, B., van den Broeke, M., Velicogna, I., et al. (2018). Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature 556, 219–222.

The internal temperature is a key parameter for the ice sheet dynamics. The actual temperature profile is a determinant of ice rheology, which controls ice deformation and flow, and sliding over the underlying bedrock. Importantly, the ice flow in turn affects its temperature profile through strain heating, which makes observed temperature profiles a powerful input for ice sheet model validation. Up to now temperature profile was available in few boreholes or from glaciological models. Recently, Macelloni et al. (2016) opened up new opportunities for probing ice temperature from space with the low-frequency passive sensors. Indeed, at L-band frequency, the very low absorption of ice and the low scattering by particles (grain size, bubbles in ice) allows a large penetration in the dry snow and ice (several hundreds of meters). Macelloni et al. (2019) performed the first retrieval of the ice sheet temperature in Antarctica by using the European Space Agency (ESA)’s Soil Moisture and Ocean Salinity (SMOS) L-band observations. They used the minimization of the difference between SMOS brightness temperature and microwave emission model simulations that include ice temperature emulator based on glaciological models. Here, in the framework of the ESA 4DAntarctica and SMOS Extension projects, we propose two main improvements.

First, a new method based on a Bayesian approach have been developed in order to improve the accuracy of the retrieved ice temperature and to provide an uncertainty estimation along the profiles. The Bayesian inference takes as free parameters: ice thickness, surface ice temperature, snow accumulation and geothermal heat flux (GHF). The parameter space investigation is achieved through a Markov Chain Monte Carlo (MCMC) method. Here, the differential evolution adaptive Metropolis (DREAM) algorithm is used for its performances. It runs multiple different Markov chains in parallel and uses a discrete proposal distribution to evolve the sampler to the posterior distribution (Laloy and Vrugt, 2012). For each SMOS brightness temperature observation, 1000 iterations are run on 5 parallel chains. The 2500 first iterations are discarded (aka. burn-in) and only the last 2500 are used for the final ice temperature profile estimation. The posterior probability distribution captures the most likely parameter set (i.e. a surface temperature, snow accumulation and GHF combination), and so, the most likely ice temperature profiles associated to this SMOS observation. It also provides the standard deviation which inform on the temperature uncertainty along the depth.

Moreover, Macelloni et al. (2019) used an ice temperature emulator based on a one-dimensional model (Robin 1955). As the Robin model neglects the horizontal advection, it can only be used in regions with very slow horizontal ice drift and this limits the retrieval to the Antarctic Plateau. In order to extend the analysis over Antarctica, a three-dimensional glaciological model (GRISLI, Quiquet et al., 2018) was used to generate temperature profiles as inputs for the Bayesian approach. In order to speed up the process, an emulator based on a deep neural network (DNN), was developed to reproduce GRISLI temperature field.

Most of the ice in the Antarctic ice sheet drains from the continent to the ocean through fast-flowing ice streams and glaciers. The high velocity of these features are thought to be maintained by the presence of water at the base of the ice sheet, which reduces friction. Subglacial water moving has been linked to transient glacier flow acceleration and enhanced melt at the grounding line. Therefore, the presence, location, and movement of water at the base of the ice sheet are likely significant controls on the mass balance of Antarctica.

The transport of subglacial water from the interior of Antarctica to the grounding line was once thought to be a steady state process. It is now known that subglacial water collects in hydrological sinks, which store and release water in episodic events. These features can be detected and quantified by satellite altimetry by searching for localized elevation change of the ice sheet’s surface. This behaviour is interpreted to be water moving in and out of ‘active’ subglacial lakes.

Quantifying the volume of water involved in these active events can inform on processes otherwise hidden from view, providing valuable information for simulating the subglacial environment. In particular, the period immediately following a lake drainage is dominated by recharge from the subglacial water draining into the lake, thus providing a mean to quantify the subglacial water fluxes. In some cases, subglacial lakes exist in hydrologically connected groups, providing rich information on such fluxes. One such group is the set of four subglacial lakes which exist beneath the Thwaites glacier. These lakes underwent two drainage events in 2013 and in 2017, with a clear period of recharge between the two events. Estimates of subglacial lake recharge rates were extracted and compared against modelled values. These observed rates of recharge were significantly greater than those produced by the modelled output, which implies subglacial melt production under the Thwaites glacier is underestimated.

Given the significance of subglacial water on the behaviour of ice sheets it is important to derive methods that can constrain and validate subglacial melting rates. Direct observations of the subglacial system are impossible due to the thickness of the ice sheet. Estimates of subglacial melt rates are constrained strictly by models, which take into account the impact of geothermal heat flux, vertical dissipation and frictional heat. With few in-situ observations across Antarctica it is currently not possible to validate the results produced from such models. Here, as part of the 4DAntarctic project, we use CryoSat-2 altimetry to produce time-dependent volume time-series for known active subglacial lakes across Antarctica. From these we can extract recharge rates, which act as a lower bound on subglacial melt production. We compare these values against rates of recharge derived by routing modelled subglacial melt across the Antarctic Ice Sheet bed. This provides us with a unique dataset to explore the sub-glacial environment, comparing direct observations of the subglacial network against the theoretical behaviour.

Ice sheets are a key component of the Earth system, impacting on global sea level, ocean circulation and bio-geochemical processes. Significant quantities of liquid water are being produced and transported at the ice sheet surface, base, and beneath its floating sections, and this water is in turn interacting with the ice sheet itself.

Surface meltwater drives ice sheet mass imbalance; for example enhanced melt accounts for 60% of ice loss from Greenland, and while in Antarctica the impacts of meltwater are proportionally much lower, its volume is largely unknown and projected to rise. The presence of surface melt water is also a trigger for ice shelf calving and collapse, for example at the Antarctic Peninsula where rising air and ocean temperatures have preceded numerous major collapse events in recent decades.

Meltwater is generated at the ice sheet base primarily by geothermal heating and friction associated with ice flow, and this feeds a vast network of lakes and rivers creating a unique bio-chemical environment. The presence of melt water between the ice sheet and bedrock also impacts on the flow of ice into the sea leading to regions of fast-flowing ice. Meltwater draining out of the subglacial system at the grounding line generates buoyant plumes that bring warm ocean bottom water into contact with the underside of floating ice shelves, causing them to melt. Meltwater plumes also lead to high nutrient concentrations within the oceans, contributing to vast areas of enhance primary productivity along the Antarctic coast.

Despite the key role that hydrology plays on the ice sheet environment, there is still no global hydrological budget for Antarctica. There is currently a lack of global data on supra- and sub-glacial hydrology, and no systems are in place for continuous monitoring of it or its impact on ice dynamics.

The overall aim of 4DAntarctica is to advance our understanding of the Antarctic Ice Sheet’s supra and sub-glacial hydrology, its evolution, and its role within the broader ice sheet and ocean systems.

We designed our programme of work to address the following specific objectives:

Creating and consolidating an unprecedented dataset composed of ice-sheet wide hydrology and lithospheric products, Earth Observation datasets, and state of the art ice-sheet and hydrology models
Improving our understanding of the physical interaction between electromagnetic radiation, the ice sheet, and liquid water
Developing techniques and algorithms to detect surface and basal melting from satellite observations in conjunction with numerical modelling
Applying these new techniques at local sites and across the continental ice sheet to monitor water dynamics and derive new hydrology datasets
Performing a scientific assessment of Antarctic Ice Sheet hydrology and of its role in the current changes the continent is experiencing
Proposing a future roadmap for enhanced observation of Antarctica’s hydrological cycle

To do so, the project will use a large range of Earth Observation missions (e.g. Sentinel-1, Sentinel-2, SMOS, CryoSat-2, GOCE, TanDEM-X, AMSR2, Landsat, Icesat-2) coupled with ice-sheet and hydrological models. By the end of this project, the programme of work presented here will lead to a dramatically improved quantification of meltwater in Antarctica, an improved understanding of fluxes across the continent and to the ocean, and an improved understanding of the impact of the hydrological cycle on ice sheet’s mass balance, its basal environment, and its vulnerability to climate change.