New insights into uncertainties in Antarctic elevation change estimates by comparing radar and laser altimetry

Abstract. Satellite radar altimetry has provided continuous observations of Antarctic Ice Sheet (AIS) surface elevation change since 1992. However, uncertainties in radar-derived elevation estimates remain substantial, primarily due to the influence of local surface topography and time-variable signal penetration into snow and firn. The launch of the ICESat-2 laser altimetry mission in late 2018 established a new benchmark for high-accuracy surface elevation measurements, enabling inter-comparison with radar altimetry results and improved assessment of associated uncertainties. In this study, we use the ICESat-2 measurements to evaluate radar altimetry-derived elevation change estimates from CryoSat-2 over the 6 905 000 km² large and relatively flat interior of the AIS, where topography-related errors are small. We apply a suite of radar-specific correction methods to the CryoSat-2 measurements, including multiple retracking algorithms and empirical corrections for the time-variable surface and volume scattering of the radar signal. We analyse a 5.5-year overlap period between ICESat-2 and CryoSat-2 (April 2019-October 2024) to assess how the different correction methods influence the CryoSat-2 surface elevation change estimates and their uncertainties.

ICESat-2 observations indicate a thickening of 97 ± 4 km³ yr^-1, coinciding with several events of excess snowfall during 2019-2024. All CryoSat-2 solutions yield systematically lower thickening trends, with the smallest bias (0.6 ± 1.0 cm yr⁻¹ or 42 km³ yr^-1) obtained using the AWI-ICENet1 convolutional neural network retracker. The remaining trend differences correlate with the ICESat-2 trend signal itself. We discuss possible causes of these systematic differences, one of which is the hypothesis that temporal variations in radar signal penetration associated to temporal variations in snow properties continue to induce systematic errors in inferred surface elevation changes. If the mean trend difference here were representative of the entire grounded AIS (12 352 700 km²), it would correspond to an underestimation of AIS volume and mass trends by approximately 74 km³ yr⁻¹ and 28 Gt yr⁻¹, respectively. These results underscore the challenges of using radar altimetry to resolve subtle, long-term trends related to surface mass balance changes, while also demonstrating the potential of combined laser-radar altimetry analysis to reduce uncertainties in AIS volume and mass balance estimates.