This habilitation thesis presents the findings of the sea level change studies conducted at the Institute of Geodesy of the Technischen Universität Darmstadt betweeen 2001 and 2013.

Sea level is an important indicator of climate change. It has been traditionally measured by coastal tide gauges and by satellite altimetry since 1993. Tide gauge measurements indicate a coastal average sea level rise of 1-2 millimeters per year over the 20th century. Over the last two decades the average sea level rise increased to 3.3±0.7 millimeters per year, consistently measured by tide gauges and satellite altimetry. The 2013 Intergovernmental Panel on ClimateChange (IPCC AR5) predicts a global mean rise of 50 ± 20 cm by 2100 for a medium warming scenario for the interval 2081-2100.

Sea level rise is not uniform and some regions will be more affected than others. It can possibly exacerbate the effects of other factors, such as flooding and ground subsidence. Because of its potential impact on coastal regions, rising sea level is one of the major threatsof climate warming. Changes in each component of the climate system, ocean, land and ice sheets, affects sea level. The two primary contributors of sea level rise, thermal expansion due to ocean warming and melting of continental glaciers and ice sheets, have been identifiedbut large uncertainties remain. Locally non-climatic components, as subsidence, can causerelative sea level rise much larger than the global average mean sea level rise.

The global and highly accurate analysis of sea level variations is made possible by spacebasedtechniques. Their main innovation is the use of the same accurate and global reference frame ensuring long-term, precise monitoring and integration in a Global Geodetic ObservingSystem, which is crucial for many practical applications. This thesis focuses on the use of geodetic techniques. Its aim is a comprehensive analysis of the regional sea level variability and of its causes with particular attention to the coastalzone. The three main scientific objectives are: improvement of multi-mission satellite altimetry records, quantification of global and regional sea level change and attribution of sea level rise. Firstly the altimeter data from different missions are unified, improved in the coastal zoneand validated with in-situ and model data. Secondly global and regional estimations of sea level variability from altimetry and tide gauge data are made. The third part of the work is dedicated to the analysis of the reason for sea level change. Here satellite altimetry andgravity missions data are combined with model data to detect the causes of this variation. The analysis includes the separation of mass and volume sea level change and the closing of the water budget.

This work shows the challenges of merging satellite data of different types for the understanding of physical processes in sea basins. It also deals with the challenges of new satellite altimetry missions in the coastal zone, where altimetry provides a consistent link to tide gauge stations co-located with Global Navigation Satellite System observations. It finally discusses the importance of highly accurate sea level variability and trends for modeling coastal processes and for long-term predictions.