Background

Fluctuations in sea level have occurred on various temporal scales throughout the Earth's history in connection with warm and cold phases of our planet. In recent decades, an anthropogenic rise has been the dominant signal. To evaluate the changes in recent decades, the satellite measurements of various completed and still active radar altimeter missions are analysed, improved and harmonised. In addition, the phenomena of global and regional sea level and its variability are being analysed, as well as the effects of the sea level change on low-lying coasts.

The actual mean value of the global sea level rise, which was derived using radar altimetry from the past 32 years (at the latitude band of ±60°), is approx. 3.5 mm/year. But regionally, it can have significantly smaller or larger values. In addition, in some coastal regions the Earth's surface locally sinks considerably (Subsidence), which is caused, for example, by excessive groundwater ("Sinking Jakarta") or natural gas extraction. These effects then generate a multiple of "relative" sea level rise locally. In our monitoring programme on the hazards of local sea level changes, we operate three GNSS-controlled tide gauges in Indonesia together with partners (Surabaya, Semarang, Jakarta). The coastal area of Semarang and Jakarta subside by up to 10 cm per year and, therefore, constitute a significant natural hazard for the population.

The long-term rise is superimposed by sea level variability with time scales of years to decades, which is related to climate fluctuations. In order to determine the long-term trend more precisely, we link the measurements with atmospheric and oceanic variability and analyse the relevant processes. With the help of harmonised radar altimeter data, the variability of the sea level can be observed right up to the coasts. For example, we are working on the improved estimation of long-term sea level trends on the North Sea coast and on the investigation of extreme events such as storm surges. The 2-D water level measurements from the new SWOT mission also allow us to analyse the small-scale ocean dynamics near the coast. One example of this research is the inflow event of salty North Sea water into the Baltic Sea in December 2023.

Key scientific questions

• How has the global and regional sea level changed since 1990?
• What hazard result from the combination of sea level rise and subsidence for coastal populations?

Related projects

• COLSATI | Coastal sea level dynamics monitored by satellite altimetry and tide gauges
• Ready for SWOT | Variability of water levels at the coasts and inland as measured by SWOT
• Sinking Jakarta | Hazard Potential of sea level rise and subsidence in Southeast Asia
• ADS | Harmonised Radar Altimeter Data