Improved Tracking of Ice Surface Elevation Change using CryoSat-2 Radar and ICESat-2 Laser Altimetry

Satellite radar and laser altimeters have been used to detect fluctuations in the surface elevation of ice sheets, ice caps and glaciers, since the early 1990s and early 2000s, respectively. The two methods are complementary in terms of their weather dependency and precision over rugged terrain – radar offers all-weather capability while laser offers high precision. They also have different limitations originating from their orbital geometries and their electromagnetic interactions with ice surfaces. However, the extent of these differences in elevation changes has not been fully explored over different types of land ice. In this thesis, I have developed methods to process contemporaneous measurements from CryoSat-2 radar and ICESat-2 laser altimeters and have evaluated their complementarity to improve the retrieval of elevation changes over glaciers and ice sheets.

Firstly, I assessed the differences between CryoSat-2 and ICESat-2 measurements acquired over the entire Greenland Ice Sheet, which has a total area of 1.7 million km2 and an ablation zone of 200,000 km2. I derived interannual trends and seasonal elevation changes using both missions and showed that they agreed to within a small fraction (6 %) of the observed signals. Between 2018 and 2022, CryoSat-2 and ICESat-2 recorded thinning rates of 11.4 ± 0.8 cm/yr and 11.7 ± 1.3 cm/yr, respectively, across the whole ice sheet. Furthermore, CryoSat-2 and ICESat-2 recorded seasonal amplitudes of 62.9 ± 26.5 cm and 59.4 ± 24.4 cm, respectively, within the ablation zone of the ice sheet. Given the excellent agreement, I combined elevation and volume changes over Greenland during their common period and found that the ice sheet lost volume at a rate of 196 ± 37 km3/yr between 2010 and 2022, accompanied by a large interannual variability of 129 km3/yr.

Next, I assessed the capability of CryoSat-2 and ICESat-2 to track fluctuations over a much smaller ice surface by deriving elevation and volume changes at Skeiðarárjökull, a 1500 km2 outlet glacier of the Vatnajökull Ice Cap in southern Iceland. Between 2018 and 2023, CryoSat-2 and ICESat-2 observed surface lowering at rates of 0.98 ± 0.19 m/yr and 0.84 ± 0.14 m/yr across the whole glacier, respectively. Both missions resolved the spatial pattern of thinning consistently, with an R2 of 0.85 and agreeing to within a difference of 0.33 m/yr. The low-elevation areas of the glacier experienced a sustained thinning of 1.91 ± 0.06 m/yr between 2011 and 2023. Across the high-elevation areas, the surface thickened at a rate of 0.27 ± 0.1 m/yr until 2019 and thinned at a rate of 0.52 ± 0.14 m/yr afterwards. I examined volume changes from CryoSat-2, ICESat-2, and sequential digital elevation models and found them to agree to within 0.2 km3/yr — 11 % of the observed changes. Given the excellent agreement between the three techniques, I combined them to estimate that Skeiðarárjökull lost 34.3 ± 2.1 km3 of its volume between 2000 and 2023.

Lastly, I explored the capacity of CryoSat-2 and ICESat-2 to detect ice dynamic imbalance across Western Palmer Land in the Antarctic Peninsula. To do this, I used a model to account for fluctuations in firn thickness in the altimeter records of surface elevation change. Between 2010 and 2025, the firn height thickened at a rate of 0.17 m/yr. The firn correction improved the temporal agreement — R2 between CryoSat-2 and ICESat-2 increased from 0.84 to 0.95 across the whole basin, with differences within 0.11 m. Between 2010 and 2015, Western Palmer Land experienced modest dynamic thickening, with its volume increasing at a rate of 1.5 ± 3.7 km3/yr. However, between 2015 and 2020, the region underwent rapid volume losses at a rate of 19.8 ± 2.3 km3/yr, which increased further to 26.1 ± 3.4 km3/yr between 2020 and 2025. The increase in volume losses between 2010 and 2025 has been more rapid than the increase in ice discharge between 1992 and 2015. The accelerated dynamic imbalance occurred predominantly along the southern margins of Western Palmer Land, where the ice is grounded on bedrock below sea level and is exposed to melting driven by oceanic thermal forcing. The area of dynamical imbalance increased threefold between 2010 and 2025, spreading over 70 km inland at some glaciers.