Observing Long-term Barystatic Sea Level Change with Satellite Gravimetry

As a leading indicator of global climate change, contemporary global mean sea level (GMSL) change is mainly driven by thermosteric (thermal expansion) and barystatic (ocean mass increase) contributions. GMSL change has been continuously measured by satellite altimetry since 1993, while thermosteric sea level change can be inferred from in-situ hydrographic measurements dating back to the 1970s. However, direct observations of barystatic sea level change were generally lacking until the launch of the Gravity Recovery and Climate Experiment (GRACE) in 2002. In the absence of GRACE, barystatic sea level estimation relies primarily on the so-called mass budget approach by summing individual surface mass change estimates (e.g. ice sheets, glaciers, and terrestrial water storage) obtained from different remote sensing or geophysical modelling techniques, providing an indirect observation due to the lack of global constraints. In this study, we use low-degree gravity fields obtained from satellite laser ranging (SLR), a traditional space geodetic technique over decades, to directly estimate barystatic sea level changes since 1993. To this end, we effectively address the issues of signal leakage and missing geocenter motion for SLR gravity fields using the forward modelling technique. Our SLR-based barystatic sea level estimates allow the direct observation-based assessment of the GMSL budget over the satellite altimetry era, and also provide an independent dataset for cross-validation and gap-filling between GRACE and its successor GRACE-FO. Using reprocessed altimetry data from NASA's Goddard Space Flight Center and updated thermosteric sea level ensembles, we reconcile the GMSL rise budget from 1993 to 2022. Our results show that the sum of thermosteric and SLR-based barystatic contributions (3.16 ± 0.64 mm/yr) agrees well with the altimetry-observed GMSL rate (3.22 ± 0.28 mm/yr), suggesting that the GMSL budget can be closed within uncertainties over the last three decades. Nevertheless, we observe increased budget residuals when using different altimetry datasets, especially in recent years, highlighting the ongoing challenges in accurately observing GMSL change and robustly closing the GMSL budget.