Head: Dr. Tilo Schöne

Water level monitoring is a central research topic of our group. We observe and analyze the changes, focusing on the oceans, coastal regions and the inland water cycle. The processes can be very fast, as in the case of tsunamis, or very slow, as in the case of sea level rise or the drying up of the Aral Sea. To analyze these complex phenomena, we use a variety of geodetic methods, such as radar altimetry, tide gauges, buoys, or GNSS measurements.

From the Ocean to the Coast

Sea level variations occur at different temporal scales in the Earth's history in connection with warm and cold phases of our planet while over the last few decades, the dominant signal has been the anthropogenic increase. In order to understand the changes that have occurred in recent decades, satellite measurements from various radar altimeter missions, both completed and still active, are being evaluated, improved and harmonized. In addition, global and regional sea level phenomena and their variability, as well as their impact on coastal areas, are being studied.

The current mean global sea level rise derived from radar altimetry over the past 32 years (at latitudes of ±60°) is about 3.5 mm/year. It may be significantly less or more in some regions. In addition, the Earth's surface is sinking locally in some coastal regions (subsidence), caused, for example, by intensive groundwater extraction ("Sinking Jakarta") or natural gas extraction. These effects locally produce a much higher observed "relative" sea level rise. As part of our monitoring program of the risks of local sea level change, we and our partners operate three GNSS-controlled tide gauges in Indonesia (Surabaya, Semarang, Jakarta). The coastal areas of Semarang and Jakarta are experiencing annual subsidence of up to 10 cm, posing a significant natural hazard to the local population.

The long-term rise in sea level is overlaid with variability that ranges from years to decades. Linking atmospheric and oceanic variability leads to an enhanced understanding and more precise long-term trend determination. With optimized radar altimeter data, the variability of the ocean surface can be observed in proximity to coastlines. For instance, our research include the estimation of long-term sea level trends along the North Sea coast and the investigation of storm surges. The 2-D water level measurements of the recently launched SWOT mission facilitate the analysis of small-scale ocean dynamics in proximity to the coast. One example of our research is the inflow of salty North Sea water into the Baltic Sea in December 2023.

Continental Water Budget

Another research area in the frame of the German GreenCentralAsia initiative is the water availability in Central Asia. Water resources consist mainly of precipitation (rain or snow) and melting glaciers. Radar altimetry is applied to observe and analyze changes in inland water masses, for example in reservoirs or rivers. This enables early detection of water scarcity and provides local decision-makers with options for action. The data and information generated is made available to the public via data portals. This research area is complemented by the development and operation of a hydro-meteorological network in Central Asia and the operation of observatories as part of the Global Change Observatory Central Asia. Based on our expertise in the development and operation of Remotely Operated Multi-Parameter Stations (ROMPS), the network contributes to the international exchange of data, the recording of meteorological and hydro-meteorological parameters, the monitoring of glacier changes and the derivation of geodynamic parameters. In addition, the data and results are used to validate satellite observations in the framework of the "Issyk Kul Observatory", which is operated jointly with CAIAG (Bishkek, Kyrgyzstan), and to harmonize radar altimetry.

Our geodetic monitoring infrastructure contributes to the activities of the IAG and GGOS and provides us with the basis for deriving long-term, short-term and rapid changes in various Earth system parameters.

Schöne, T., Illigner, J., Zubovich, A., Zech, C., Stolarczuk, N., Sharshebaev, A., & Borisov, M. (2024). Performance Analyses of Sentinel-3A and Sentinel-3B Over Lake Issyk Kul (Kyrgyzstan). In International Association of Geodesy Symposia. Berlin, Heidelberg: Springer. doi:10.1007/1345_2024_268.

Mann, T., Schöne, T., Kench, P., Lambeck, K., Ashe, E., Kneer, D., Beetham, E., Illigner, J., Rovere, A., Marfai, M. A., & Westphal, H. (2023). Fossil Java Sea corals record Laurentide ice sheet disappearance. Geology, 51(7), 631-636. doi:10.1130/G51038.1

Esselborn, S., Schöne, T., Illigner, J., Weiß, R., Artz, T., & Huang, X. (2022). Validation of Recent Altimeter Missions at Non-Dedicated Tide Gauge Stations in the Southeastern North Sea. Remote Sensing, 14(1): 236. doi:10.3390/rs14010236.

Weisse, R., Dailidienė, I., Hünicke, B., Kahma, K., Madsen, K., Omstedt, A., Parnell, K., Schöne, T., Soomere, T., Zhang, W., & Zorita, E. (2021). Sea level dynamics and coastal erosion in the Baltic Sea region. Earth System Dynamics, 12(3), 871-898. doi:10.5194/esd-12-871-2021.

Siriwardane-de Zoysa, R., Schöne, T., Herbeck, J., Illigner, J., Haghighi, M., Simarmata, H., Porio, E., Rovere, A., & Hornidge, A.-K. (2021). The 'wickedness' of governing land subsidence: Policy perspectives from urban Southeast Asia. Plos One, 16(6): e0250208. doi:10.1371/journal.pone.0250208.