Uncertainties in Antarctic elevation change estimates by comparing radar and laser altimetry

Since 1992, surface elevation change estimates of the Antarctic Ice Sheet (AIS) have been derived from satellite radar altimetry. However, large uncertainties remain due to local topography and the time-variable signal penetration into snow and firn. The unprecedented accuracy of measurements from the ICESat-2 satellite laser altimetry mission, launched in 2018, now enables inter-comparison with radar altimetry results. The primary goal of this study is to improve understanding of the uncertainties in AIS volume and mass balance estimates by quantifying how results from ICESat-2 and the CryoSat-2 radar altimetry mission diverge under different processing regimes. To do so, we analyse coincident ICESat-2 and CryoSat-2 measurements over the 6.9 million km² area of the relatively flat and large AIS interior, where topography-related errors are small. We apply a suite of state-of-the-art 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. From April 2019 to October 2024, ICESat-2 observations show a thickening of 97 ± 4 km³ yr⁻¹, coincident with excess snowfall in this period. CryoSat-2 solutions indicate systematically lower thickening rates than ICESat-2. The smallest bias (0.6 ± 1.0 cm yr⁻¹ or 42 km³ yr⁻¹) between the results from the two missions is found when using the AWI-ICENet1 convolutional neural network retracker. One of our hypotheses is that the systematic radar-laser differences might be due to residual errors related to the time-variable radar penetration, particularly affected by the heavy snowfall events in recent years. While further work is needed to test this hypothesis, our study demonstrates both the challenges of resolving subtle, long-term surface mass balance trends using radar altimetry and the value of joint laser-radar analyses for improving AIS volume and mass balance estimates.