ESA and NASA satellites deliver first joint picture of Greenland Ice Sheet melting

Category: Ice sheets

ESA and NASA satellites deliver first joint picture of Greenland Ice Sheet melting

Posted on by Nicola Bortun

Story published by The European Space Agency

Global warming is driving the rapid melting of the Greenland Ice Sheet, contributing to global sea level rise and disrupting weather patterns worldwide. Because of this, precise measurements of its changing shape are of critical importance for adapting to climate change.

Now, scientists from the UK Centre for Polar Observation and Modelling have delivered the first measurements of the Greenland Ice Sheet’s changing shape using data from ESA’s CryoSat and NASA’s ICESat-2 ice missions.

Although both satellites carry altimeters as their primary sensor, they make use of different technologies to collect their measurements. CryoSat uses a radar system to determine Earth’s surface height, while ICESat-2 uses a laser system for the same task.

Although radar signals can pass through clouds, they also penetrate the ice sheet surface and have to be adjusted to compensate for this effect. Laser signals, on the other hand, reflect from the actual surface but cannot record when clouds are present. The missions are therefore highly complementary, and combining their measurements has been a holy grail for polar science.

A new study from scientists at the CPOM and published today in Geophysical Research Letters, shows that CryoSat and ICESat-2 measurements of Greenland Ice Sheet elevation change agree to within 3% of the changes taking place.

This confirms that both satellites can be combined to produce a more reliable estimate of ice loss than either could achieve alone. It also means that if one mission were to fail, the other could be relied upon to maintain our record of polar ice change.

Between 2010 and 2023, the Greenland Ice Sheet thinned by 1.2 m on average. However, much larger changes occurred across the ice sheet’s ablation zone where summer melting exceeds winter snowfall; there, the average thinning amounted to 6.4 m.

Image credit: Professor Andrew Shepherd (Northumbria University)

The most extreme thinning occurred at the ice sheets outlet glaciers. At Sermeq Kujalleq in west central Greenland (also known as Jakobshavn Isbræ), the peak thinning was 67 m, and Zachariae Isstrøm in the northeast the peak thinning was 75 m.

Altogether, the ice sheet shrank by 2347 cubic kilometres across the 13-year survey period – similar to the amount of water stored in Africa’s Lake Victoria. The biggest changes occurred in 2012 and 2019, when the ice sheet shrank by more than 400 cubic kilometres because of extreme melting in those years.

Greenland’s ice melting also has profound effects on global ocean circulation and weather patterns. These changes have far-reaching impacts on ecosystems and communities worldwide. The availability of accurate, up-to-date data on ice sheet changes will be critical in helping us to prepare for and adapt to the impacts of climate change.

“We are very excited to have discovered that CryoSat and ICESat-2 are in such close agreement,” says lead author and CPOM researcher Nitin Ravinder. “Their complementary nature provides a strong motivation to combine the data sets to produce improved estimates of ice sheet volume and mass changes. As ice sheet mass loss is a key contributor to global sea level rise, this is incredibly useful for the scientific community and policymakers.”

The study made use of four years of measurements from both missions, including those collected during the Cryo2ice campaign, a pioneering ESA-NASA partnership initiated in 2020. By adjusting CryoSat’s orbit to synchronise with ICESat-2, ESA enabled the near-simultaneous collection of radar and laser data over the same regions.

This alignment allows scientists to measure snow depth from space, offering unprecedented accuracy in tracking sea and land ice thickness.

Credit: ESA

Tommaso Parrinello, CryoSat Mission Manager at ESA, expressed optimism about the campaign’s impact: “CryoSat has provided an invaluable platform for understanding our planet’s ice coverage over the past 14 years, but by aligning our data with ICESat-2, we’ve opened new avenues for precision and insight.

CryoSat Image Credit: European Space Agency

“This collaboration represents an exciting step forward, not just in terms of technology but in how we can better serve scientists and policymakers who rely on our data to understand and mitigate climate impacts.”

“It is great to see that the data from ‘sister missions’ are providing a consistent picture of the changes going on in Greenland,” says Thorsten Markus, project scientist for the ICESat-2 mission at NASA.

“Understanding the similarities and differences between radar and lidar ice sheet height measurements allow us to fully exploit the complementary nature of those satellite missions. Studies like this are critical to put a comprehensive time series of the ICESat, CryoSat-2, ICESat-2, and, in the future, CRISTAL missions together.”

ESA’s CryoSat continues to be instrumental in our understanding of climate related changes in polar ice, working alongside NASA’s ICESat-2 to provide robust, accurate data on ice sheet changes. Together, these missions represent a significant step forward in monitoring polar ice loss and preparing for its global consequences.

For more information, contact the lead author: Nitin Ravinder (n.ravinder@northumbria.ac.uk)

Recent research offers ‘multi-century, multi-model projections of the Antarctic Ice Sheet evolution’

Posted on by Nicola Bortun

Ice sheet models (scientific simulations which aim to predict future behaviour of ice sheets) often disagree on the timing and magnitude of sea level rise up until 2300. For example, projections of Antarctica’s contribution to sea level rise beyond 2100 remain highly uncertain due to processes such as Ice Sheet and Ice Cliff instability which could cause Antarctic melting to contribute more rapidly to sea level rise.

The Coupled Model Intercomparison Project Phase 6 (CMIP6) is an international effort using different climate models to better understand how the Earth’s climate system responds to various factors. ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6), co-led by CPOM, is the part of CMIP6 project that examines the ice sheets, aiming to improve predictions of their future contribution to sea level rise, which is critical to assess the impact of melting ice on sea level rise, oceans currents, and weather patterns. By pulling together a range of different models, ISMIP6 supports the scientific community by producing more accurate, robust, multi-century projections of sea level rise and quantifying their associated uncertainties.

The most recent report using the ISMIP6 Model ensemble was released on Wednesday 4 September) in the AGU Earth’s Future journal. The new study titled Evolution of the Antarctic Ice Sheet Over the Next Three Centuries From an ISMIP6 Model Ensemble (Seroussi et al). investigates the behaviour of the Antarctic Ice Sheet until 2300 using an ensemble of 16 ice-flow models and forcing from global climate models. This is the first multi-century, multi-model projections of the Antarctic Ice Sheet evolution and shows that ice flow models are relatively consistent in predicting a limited Antarctic sea-level rise up until 2100. However, beyond the end of this century, Antarctica’s ice losses increase rapidly thereafter with the choice of ice flow model and different potential influential factors such as carbon emissions (known as climate forcing scenarios) becoming sources of uncertainty.

This model ensemble shows that, under high carbon emission scenarios, some simulations show high levels of ice retreat after 2100 with potential sea level rise of up to 1.7m in 2200 and 4.4 m by 2300. In particular, key regions in West Antarctica, including the Bungenstock Ice Rise, the Siple Coast and the Amundsen Sea sector are predicted to undergo rapid retreat. Results saw strong variations between models on the onset of retreat but good agreement on the pattern of retreat.

In addition to the choice of ice sheet model, this study also highlights the importance of the emission scenario, as ice losses under both low and high emissions remain similar during the 21st century, the two scenarios produce significantly varied results post 2100. This highlights the importance of reducing emissions for the future stability of the Antarctic Ice Sheet as well as the importance of further work on developing and improving accurate and robust models of ice shelf retreat and potential collapse beyond 2100 so that policy makers and scientists can make decisions today which will protect the future of Antarctica beyond tomorrow.

Find out more about the 6th phase of the Coupled Model Intercomparison Project (CMIP6)

New £8.4M investment continues support for long-term polar science, co-led by the British Antarctic Survey (BAS) and UK’s Centre for Polar Observation and Modelling (CPOM)

Posted on by Nicola Bortun

£8.4M has been awarded to the British Antarctic Survey and the Centre for Polar Observation and Modelling to deliver the next 5 years of their long-term polar science activities. The UK Polar Research Expertise for Science and Society (PRESCIENT) programme provides UK national capability (science, such as ongoing datasets and models, which underpins wider scientific research) to understand the impacts of environmental stressors, such as rising global temperatures on polar marine ecosystems. PRESCIENT will also measure and predict polar ice sheet contributions to global sea level rise and extend and improve measurements of changes to polar sea ice.

Announced today the funding is part of £101 million investment by the Natural Environment Research Council, part of UK Research and Innovation, in the UK’s network of leading environmental science research centres to support large-scale environmental observations, modelling and analysis, and research capabilities through innovations in platforms, sensors and data science. These data are crucial for managing natural resources, biodiversity, human health and building our understanding of and resilience to environmental hazards and climate change. It underpins science across the UK’s environmental research sector and supports critical scientific advice to government.

PRESCIENT will also aid the BAS transition to low carbon science delivery, by progressing delivery of airborne science using remotely piloted autonomous systems (RPAS), while delivering independent scientific advice and support to a range of stakeholders in government, business, and wider society, ensuring that our scientific activities and expertise is available to support solutions.

National capability is research funding which, unlike shorter term projects, can span decades and provides ongoing support for large-scale, complex scientific projects of national significance, informing strategic needs and decision-making of the country. Using techniques such as satellite altimetry to study ice motion and the polar oceans, CPOM incorporate the results into models used across the polar research community. CPOM’s data sets and models have been developed and maintained for almost a quarter of a century, and the long-term maintenance of this capability helps provide robust understanding and insights of the cryosphere.

CPOM also contribute to a range of interdisciplinary multi-centre National Capability research projects including CANARI, BIOPOLE, and TerraFIRMA, which have been running since 2022, offering satellite derived estimates of aspects of the cryosphere (such as ice thickness, floe size and sea height), as well as developing advanced simulations. The longevity of our datasets, and the accuracy of our models mean we have a broader view of past and possible future changes. By contributing to projects such as the previous multi-centre National Capability project UKESM (UK Earth System Model), integrating ice sheet model and advanced sea ice physics into the system, we can produce robust projections of ice sheet instability and Arctic sea ice loss, thereby informing sea level rise predictions. Our PRESCIENT programme with BAS continues this work into 2029.

This funding has been awarded from NERC’s National Capability Single Centre Science initiative, one of the UK’s largest environmental science investment programmes.

Read more on the British Antarctic Survey (BAS) website.

European satellite ERS-2 returns to earth after almost 30 years

Posted on by Nicola Bortun

(First published: 22 Feb 2024)

This week we watched along with many across the globe, as pioneer European satellite ERS-2 finally made its journey back to earth after almost 30 years monitoring earth from the sky.

For many current and former CPOM scientists, this was an emotional moment, as the data from this satellite has made (and continues to make) an integral contribution to understanding the cryosphere. In fact, CPOM Director Professor Andrew Shepherd used ERS-2 data for his first paper 23 years ago ‘Inland Thinning of Pine Island Glacier, West Antarctica’ which used satellite altimetry and interferometry to show that the glacier ‘had thinned by up to 1.6 meters per year between 1992 and 1999’.

Part of the European Space Agency’s (ESA) earth observation programme, the revolutionary satellite was launched in April 1995 into a sun-synchronous polar orbit at an altitude of around 800 km, and was one of the most powerful and sophisticated satellites of its time.

Due to its three-axis stabilization it was able to point directly towards our planet and could observe most areas of the earth, using SAR (Synthetic Aperture Radar) to view land surfaces, polar ice and oceans, measuring ocean-surface temperature, sea winds and sea level changes via Radar Altimetry. On top of this it could also monitor ozone levels.

The data this satellite collected has been crucial in monitoring land surface changes, warming oceans, natural disasters, and importantly for the Centre for Polar Observation and Modelling – monitoring diminishing polar ice and sea-level rise. ERS-2 (and it’s twin ERS-1 launched a few years prior) paved the way for other programmes including the EU Space program’s Copernicus Sentinels and ESA’s CryoSat Earth Explore Mission, both of which continue to provide vital data for CPOM’s research. In fact, we are using ERS-2 data to extend our datasets further back in time, in order to create a fuller view of the evolution of the cryosphere over the last 30 years.

It was retired in 2011 and has been de-orbiting since then. Now it’s 16-year journey home is complete, broken and burned up in the atmosphere with the remaining parts landing safely in the ocean yesterday but even though the physical entity is gone, the data it produced, having been used for thousands of scientific papers, continues to provide information for scientists at ESA, CPOM and beyond.

Image credit: ESA-SJM Photography esa.int/ESA_Multimedia/Images/2024/02/Saying_goodbye_to_ERS-2

Examining the Antarctic and Greenland Ice Sheets and their impact on future sea level rise new research out now

Posted on by Nicola Bortun

New research states that future rises in sea level could be better estimated by gaining a clearer understanding of the Antarctic and Greenland ice sheets.

Global climate change and its impact on sea levels is a pressing issue and trying to accurately predict just how much they will rise in future is subject to ongoing analysis. The changing nature of ice sheets is a vital factor in the projection of future sea level rise.

Led by the University of Lincoln and involving Dr Sammie Buzzard from the Centre for Polar Observation and Modelling (Northumbria University), the paper was published today in the Nature journal, Nature Reviews Earth & Environment.

An important aspect of the review highlights that short-term fluctuations in climate could have an amplification feedback effect, meaning that ice sheets are more sensitive to climate change than previously understood.

The research was sponsored by the World Climate Research Programme’s Climate and Cryosphere project, the International Arctic Science Committee, and the Scientific Committee on Antarctic Research.

The paper is available to read online: Nature Reviews Earth & Environment 

CPOM study evaluates satellite methods for estimating supraglacial lake depth published in The Cryosphere

Posted on by Nicola Bortun

During the melt season (typically from May to September) on the Greenland ice sheet, water collects in depressions on the surface of the ice, creating supraglacial lakes. If these lakes have enough water and the right conditions, they can crack open (hydrofracture) which allows water to flow from the ice surface down to the bedrock underneath, where it acts like a lubricant. These lakes on the Greenland ice sheet are incredibly important, but identifying exactly how deep they are using satellite data is difficult.

This research compares different ways of measuring the depth of these supraglacial lakes, using tools including a radiative transfer equation (RTE), ArcticDEM digital elevation models, and ICESat-2 photon refraction. The team of researchers led by CPOM PhD Researcher Laura Melling (Lancaster University) applied these methods to five lakes in southwest Greenland.

The paper examines the uncertainty in these estimates, which affects our understanding of the total lake volume and how that, in turn, can interfere with predictions about how fast the ice is moving. This work demonstrates how combining information from multiple different satellite sources can improve our ability to track meltwater on top of the Greenland Ice Sheet.

Figure 1: The locations of the five supraglacial lakes in relation to the study region. Contour lines calculated from the ArcticDEM 100 m mosaic are visible on the base map as dashed grey lines. The inset map indicates the location of the study area within Greenland. Panels (1)–(5) show Lake 1 to Lake 5 in detail, where the background is a true-colour image acquired on the date shown in Table A1 for each lake. The manually delineated lake outline is given in red, and the ICESat-2 transect is given in orange. The ICESat-2 ground tracks were cropped to the lake edges. The background images in panels (1)–(5) are the Sentinel-2 tiles detailed in Table A1. The base map data are courtesy of Earthstar Geographics via Esri.
Credit: Melling et al., Feb 2024, https://doi.org/10.5194/tc-18-543-2024

Authors include: CPOM PhD Researcher Laura Melling (Lancaster University), CPOM Associate Investigator Amber Leeson, CPOM Principal Investigator Malcolm McMillan (Lancaster University), CPOM Senior Research Associate Jennifer Maddalena (Lancaster University), Jade Bowling (Lancaster University), CPOM PhD Researcher Emily Glen (Lancaster University), Louise Sandberg Sørensen (Technical University of Denmark), Mai Winstrup (Technical University of Denmark), and Rasmus Lørup Arildsen (Technical University of Denmark).

Read the full paper here.

CPOM publishes paper ‘Multipeak retracking of radar altmetry waveforms over ice sheets’ in Remote Sensing of Environment

Posted on by Nicola Bortun

CPOM publishes paper Multipeak retracking of radar altimetry waveforms over ice sheets in Remote Sensing of Environment, introducing a new processing approach, aimed at overcoming challenges associated with using radar altimetry to measure ice sheet changes over rugged coastal topography in Antarctica and Greenland.

Image: Geographic distribution of waveform peaks across (a) Greenland and (b) Antarctica for Sentinel-3A cycle 44, with the modal value of the number of peaks shown for each 2 2 km grid cell. Taken from the paper.

This new MultiPeak processing approach which is applied to data from the Copernicus Sentinel-3 mission, significantly improves the accuracy and quantity of elevation retrievals and and has the potential to enhance ‘the capability of SAR altimeters to track ice sheet imbalance. This is an important step forward in developing more sophisticated monitoring approaches for tracking ice loss in the most complex regions which contribute to global seal level rise. The paper is co-authored by CPOM Senior Research Associate Dr Qi Huang (Lancaster University), CPOM Associate Investigator Professor Mal McMillan (Lancaster University), CPOM Systems, Data, Product Manager Alan Muir (UCL), CPOM Affiliate Joe Phillips (Lancaster University) and CPOM Research Fellow Dr Thomas Slater (Northumbria University).

Polar ice sheet melting records have toppled during the past decade

Posted on by Nicola Bortun

Press release: 20 April 2023, Northumbria University – Polar ice sheet melting records have toppled during the past decade | Northumbria University, Newcastle

The seven worst years for polar ice sheets melting and losing ice have occurred during the past decade, according to new research, with 2019 being the worst year on record.

The melting ice sheets now account for a quarter of all sea level rise a fivefold increase since the 1990’s according to IMBIE, an international team of researchers who have combined 50 satellite surveys of Antarctica and Greenland taken between 1992 and 2020.

Their findings are published today in the journal Earth System Science Data.

Global heating is melting the polar ice sheets, driving up sea levels and coastal flooding around our planet. Ice losses from Greenland and Antarctica can now be reliably measured from space by tracking changes in their volume, gravitational pull, or ice flow.  NASA and the European Space Agency (ESA) and awarded funding to the Ice Sheet Mass Balance Intercomparison Exercise (IMBIE) in 2011 to compile the satellite record of polar ice sheet melting. Data collected by the team is widely used by leading organisations, including by the Intergovernmental Panel on Climate Change (IPCC).

In their latest assessment, the IMBIE Team which is led by Northumbria University’s Centre for Polar Observations and Modelling have combined 50 satellite surveys of Antarctica and Greenland to determine their rate of ice melting. They have found that Earth’s polar ice sheets lost 7,560 billion tonnes of ice between 1992 and 2020 equivalent to an ice cube that would be 20 kilometres in height.

The polar ice sheets have together lost ice in every year of the satellite record, and the seven highest melting years have occurred in the past decade.

The satellite records show that 2019 was the record melting year when the ice sheets lost a staggering 612 billion tonnes of ice.

This loss was driven by an Arctic summer heatwave, which led to record melting from Greenland peaking at 444 billion tonnes that year. Antarctica lost 168 billion tonnes of ice the sixth highest on record due to the continued speedup of glaciers in West Antarctica and record melting from the Antarctic Peninsula. The East Antarctic Ice Sheet remained close to a state of balance, as it has throughout the satellite era.

Melting of the polar ice sheets has caused a 21 mm rise in global sea level since 1992, almost two thirds (13.5 mm) of which has originated from Greenland and one third (7.4 mm) from Antarctica.

In the early 1990s, ice sheet melting accounted for only a small fraction (5.6 %) of sea level rise. However, there has been a fivefold increase in melting since then, and they are now responsible for more than a quarter (25.6 %) of all sea level rise. If the ice sheets continue to lose mass at this pace, the IPCC predicts that they will contribute between 148 and 272 mm to global mean-sea level by the end of the century.

Professor Andrew Shepherd, Head of the Department of Geography and Environmental Sciences at Northumbria University and founder of IMBIE, said:

“After a decade of work we are finally at the stage where we can continuously update our assessments of ice sheet mass balance as there are enough satellites in space monitoring them, which means that people can make use of our findings immediately.

Dr In’s Otosaka from the University of Leeds, who led the study, said:

“Ice losses from Greenland and Antarctica have rapidly increased over the satellite record and are now a major contributor to sea level rise. Continuously monitoring the ice sheets is critical to predict their future behaviour in a warming world and adapt for the associated risks that coastal communities around the world will face.

This is now the third assessment of ice loss produced by the IMBIE team, due to continued cooperation between the space agencies and the scientific community. The first and second assessments were published in 2012 and 2018/19.

Over the past few years, ESA and NASA have made a dedicated effort to launch new satellite missions capable of monitoring the polar regions. The IMBIE project has taken advantage of these to produce more regular updates, and, for the first time, it is now possible to chart polar ice sheet losses every year.

This third assessment from the IMBIE Team, funded by the ESA and NASA, involved a team of 68 polar scientists from 41 international organisations using measurements from 17 satellite missions, including for the first time from the GRACE-FO gravity mission. Importantly, it brings the records of ice loss from Antarctica and Greenland in line, using the same methods and covering the same period in time. The assessment will now be updated annually to make sure that the scientific community has the very latest estimates of polar ice losses.

Dr Diego Fernandez, Head of Research and Development at ESA, said:

This is another milestone in the IMBIE initiative and represent an example of how scientists can coordinate efforts to assess the evolution of ice sheets from space offering unique and timely information on the magnitude and onset of changes.

The new annual assessments represent a step forward in the way IMBIE will help to monitor these critical regions, where variations have reached a scale where abrupt changes can no longer be excluded.

The study, Mass balance of the Greenland and Antarctic ice sheets from 1992 to 2020, is published in the journal Earth System Science Data, and the new dataset is publicly available on the British Antarctic Survey website.

£47m to address critical environmental challenges facing the UK

Posted on by Nicola Bortun

The UKRI have awarded £47M to NERC research centres to address six critical environmental science challenges facing the UK, including climate change mitigation strategies, coastal flooding & erosion, and extreme weather.

CPOM is to collaborate on 3 projects BIOPOLE, CANARI & TerraFIRMA.

Biogeochemical processes and ecosystem function in a changing polar system (BIOPOLE), £9 million

Led by the British Antarctic Survey, in collaboration with:

  • British Geological Survey
  • Centre for Polar Observation and Modelling
  • National Oceanography Centre
  • UK Centre for Ecology & Hydrology.

Project partners include:

  • Alfred Wegener Institute, Germany
  • Helmholtz Centre for Polar and Marine Research, Germany
  • University of Alaska Fairbanks, USA
  • University of Alberta, Canada
  • University of Bristol, UK
  • University Centre in Svalbard, Norway.

Climate change is proceeding faster at the poles than any other region, resulting in sea ice loss and glacial melting.

There is a clear urgency in understanding the full implications of these changes for the polar regions themselves and for the wider Earth system.

BIOPOLE will provide a step change in the knowledge and predictive capability concerning how polar ecosystems regulate the chemical balance of the world’s oceans and, through it, their effect on global fish stocks and carbon storage.

Climate change in the Arctic-North Atlantic region and impact on the UK (CANARI), £12 million

Led by the National Centre for Atmospheric Science, in collaboration with:

  • British Antarctic Survey
  • British Geological Survey
  • Centre for Polar Observation and Modelling
  • National Centre for Earth Observation
  • National Oceanography Centre
  • UK Centre for Ecology & Hydrology.

The project partner is the Met Office Hadley Centre, UK.

The weather and climate of the UK is shaped by the large-scale circulation of the atmosphere and ocean in the North Atlantic.

This project will advance understanding of the impacts on the UK arising from climate variability and change in the Arctic-North Atlantic region. It will focus on extreme weather and the potential for rapid, disruptive change.

This will enable the UK to play an internationally leading role in addressing the challenges of understanding regional climate change and provide detailed information about impacts on the UK.

Future impacts risks and mitigation actions (TerraFIRMA), £9.5 million

Led by the National Centre for Atmospheric Science, in collaboration with:

  • British Antarctic Survey
  • British Geological Survey
  • Centre for Polar Observation and Modelling
  • National Centre for Earth Observation
  • National Oceanography Centre
  • Plymouth Marine Laboratory
  • UK Centre for Ecology & Hydrology.

Project partner is the Met Office Hadley Centre, UK.

This project will provide reliable guidance on the risks and impacts of future climate change. It will assess a range of mitigation strategies:

  • impacts on allowable carbon budgets and pathways to net zero
  • wider environmental, economic and societal impacts, for example, sustainable development goals
  • co-benefits, for example, air quality.

The full details can be found at: £47m to address critical environmental challenges facing the UK UKRI

World’s ice is melting faster than ever

Posted on by Nicola Bortun

ESA World’s ice is melting faster than ever

As global temperatures increase, the melting of the massive ice sheets that blanket Antarctica and Greenland has accelerated, making a significant contribution to sea-level rise. In total, Earth is losing around a trillion tonnes of ice each year which is not being replenished.

CPOM director, Andrew Shepherd of the University of Leeds is a leading climate scientist working with ESA and NASA. Join Andrew as he discusses how long-term satellite observations from ESA’s Climate Change Initiative are key in monitoring changes in ice sheets over decades.