Scientists predict that climate change translates into rising sea levels, but how sea levels will change at the local scale is not well understood. One reason is because of vertical land motion: the coastal change in land height that affects observations of sea-level rise. Most sea level studies use one group of models based on glacial isostatic adjustment (GIA) to account for vertical land movement. Models based on GIA have known biases over regional scales, however, and capture only one component of vertical land motion. They neglect various other processes that exist over different spatial and temporal scales. These types of models are used in the studies that form the foundation of the syntheses presented by the Intergovernmental Panel on Climate Change. Direct observation of vertical land motion using GPS is therefore vital. Despite many advances in this technique, systematic errors remain unaccounted for, limiting its accuracy. The EU-funded GPS4SEALEVELS (Advancing understanding of sea level change through improved vertical land movement estimates) initiative addressed the accuracy of sea-level change estimates derived from tide gauges and satellite altimetry. In particular, it addressed the accuracy of GPS vertical land motion corrections at tide gauges and their impact on sea level change estimates. To do this, researchers developed a new GPS solution that includes more than 700 GPS stations globally distributed with data from 1995 onwards. The vertical land motion estimates obtained from this new solution were compared against several alternative solutions available. Vertical land motion errors resulting from reference frame errors, observation modelling errors and relative local vertical land motion between GPS and tide gauges were thoroughly assessed. The main result of this research is that the impact of GPS velocity errors on global sea-level change estimates is still beyond the objective of 0.1 mm/yr. At regional level, the impact increases due to the spatial correlation of the vertical land motion errors. GPS4SEALEVELS research will have a major impact on the quality of results produced by the sea level community. It will also inform the work of researchers in a broad range of other geoscience fields.
Vertical land motion, sea-level rise, GPS4SEALEVELS, GPS time series