Revisiting the role of elastic deformation on GNSS observations of vertical land motion at tide-gauge locations

Sea-level observations from tide gauges are commonly corrected for vertical land motion (VLM) using data from permanent GNSS stations. While these corrections are necessary to fully understand global, regional, and local sea-level changes, one critical assumption that is frequently made is that VLM trends derived over the short GNSS records are representative for the full tide-gauge record length. While this assumption is generally valid for motions associated with Glacial Isostatic Adjustment (GIA) or long-term sediment compaction processes, for elastic deformation due to ongoing mass redistribution in the earth system it is not. Instead of using geophysical models, we compute elastic deformation directly from the 15-year GRACE record. We compute the vertical land motion signal in GNSS observations, corrected for this deformation. We find that elastic deformation translates into trends of more than 1 mm/yr for many GNSS stations, including many stations close to tide gauges. This deformation is not only caused by changes in the cryosphere, but to a large extent by changes in land hydrology. This land hydrology record shows considerable decadal variability on global and regional scales, and since most GNSS records used to correct tide-gauge observations are substantially shorter than 15 years, VLM inferred over the short record is often not representative for the 15-year trend, and as such, not representative for the full tide-gauge record. This elastic deformation explains a substantial part of the observed VLM signals, not only the well-known uplift signals on both ice sheets, but also the subsidence in Australia, as well as regional patterns visible in South America. On regional scales, this correction results in more consistent sea-level observations at tide-gauge locations, especially along the sparsely-observed South Atlantic Ocean.