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Climate Change and Sea Level Rise: A Challenge to Science and Society

Hans-Peter Plag

Nevada Geodetic Laboratory, University of Nevada, Reno, USA

Local Sea Level (LSL) rise is one of the major anticipated impacts of future global warming with potentially devastating consequences, particularly in many low-lying and often subsiding and densely populated coastal areas. Faced with a trade-off between imposing the very high costs of coastal protection and adaptation upon today's national economies and leaving the costs of potential major disasters to future generations, governments and decision makers are in need of scientific support for the development of mitigation and adaptation strategies for the coastal zone. of assessments of future coastal LSL rise as input for policy making for coastal zone management. Low-frequency to secular changes in LSL are the result of a number of location-dependent processes including ocean temperature and salinity changes, ocean and atmospheric circulation changes, mass exchange of the oceans with other reservoirs in the water cycle, and vertical land motion. LSL changes in response to mass exchange with land-based ice sheets, glaciers and water storage are spatially variable due to vertical land motion induced by the shifting loads and gravitational effects resulting from both the relocation of surface water mass and the deformation of the solid Earth under the load. As a consequence, close to a melting ice mass LSL will fall significantly and far away increase more than the global average. The so-called sea level equation is an integral equation, which expresses LSL as a function of present and past mass changes in ice sheets, glaciers, land water storage, and the resulting mass redistribution in the oceans. Mass exchange between oceans and the ice sheets, glaciers, and land water storage has the potential to change coastal LSL in many geographical regions. However, predictions of mass-induced LSL changes exhibit significant inter-model differences, which introduce a large uncertainty in the prediction of LSL variations caused by changes in ice sheets, glaciers, and land water storage. While the sea level equation has been tested extensively in postglacial rebound studies for the viscous (post-mass change) contribution, a thorough validation of the elastic (co-mass change) contribution has yet to be done. Accurate observations of concurrent LSL changes, vertical land motion, and gravity changes required for such a test were missing until very recently. For the validation, new observations of LSL changes, vertical land motion, and gravity changes close to rapidly changing ice sheets and glaciers in Greenland, Svalbard, and other regions, as well as satellite altimetry observations of sea surface height changes and satellite gravity mission observations of mass changes in the hydrosphere are now available.

The complexity of the Earth system and its inherent unpredictability make it difficult to predict Global Sea Level (GSL) rise and, even more so, LSL over the next 100 to 200 years. Humans have re-engineered the planet and changed major features of the Earth surface and the atmosphere, thus ruling out extrapolation of past and current changes into the future as a reasonable approach. The risk of rapid changes in ocean circulation and ice sheet mass balance introduces the possibility of unexpected changes. Therefore, science is challenged with understanding and constraining the full range of plausible future LSL trajectories and with providing useful support for informed decisions. Currently, the range of plausible future sea level trajectories turns out to be too large to be a useful basis for the planning of mitigation and adaptation. In the face of largely unpredictable future sea level changes, monitoring of the relevant processes and development of a forecasting service on realistic time scales is crucial as decision support. Forecasting and "early warning" for LSL rise would have to aim at decadal time scales, giving coastal managers sufficient time to react if the onset of rapid changes would require an immediate response. The social, environmental, and economic risks associated with potentially large and rapid LSL changes are enormous. Therefore, in the light of the current uncertainties and the unpredictable nature of major forcing processes for LSL changes, the focus of scientific decision support may have to shift from projections of LSL trajectories on century time scales to the development of models and monitoring systems for a forecasting service on decadal time scales. The scientific requirements for such a service are improved monitoring of the relevant forcing processes and the development of models with predictive capabilities on decadal time scales.