Vertical Land Motions at Coastlines from Space Geodesy to Understand Sea Level Change

Abstract:

Vertical land motions play an important role in understanding how sea levels have changed over the past century, or how future sea levels may impact coastal areas. To be useful in long term sea level studies, vertical land motion should be determined with standard errors one order of magnitude less than the contemporary climate signals of 1-3 mm/year observed in sea level records. This metrology demand constitutes a challenge in geodesy. Here, we review the most successful instrumental methods that have been used to determine vertical displacements at the Earth’s surface, so that the above-mentioned sea level issues can be addressed adequately in terms of accuracy. A special focus will be given to the use of the Global Positioning System (GPS) and to the combination of satellite radar altimetry minus tide gauge data. We will update previous data analyses and confront the updated results from both methods. Then, using a joined set of relatively robust vertical velocities from nearly 500 sites along the world coastlines, we will examine to what extent the assumption of coastal vertical land motion cancelling out, on average, with or without glacial isostatic adjustment (GIA) corrections, was valid in past global sea level rise studies, and whether subsidence processes are predominant at coastlines.