Relative sea level
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The relative sea level is the mean sea level related to a local reference land level.
Relative sea level change
The relative sea level changes as a consequence of Sea level rise. It changes also due to vertical motions of the land level.
Global warming is a major cause of sea-level rise. The global mean sea level rises as a consequence of:
- Increase of the ocean water mass, due to melting of land ice (in particular the polar ice caps) and due to decrease of groundwater and surface water storage on land;
- Expansion of the ocean water volume due to decrease of water density with increasing temperature.
The influence of global warming on sea-level rise varies along the world's coastlines as a consequence of :
- Residual changes in the strength and distribution of ocean currents;
- Residual changes in atmospheric pressure distribution;
- Residual local changes in seawater salinity;
- Changes in the earth gravitational field related to melting of polar ice caps.
Land level change occurs as a consequence of:
- Isostatic rebound: adjustment of the earth crust due to melting of polar ice caps, causing a rise of the formerly covered areas and a sink of adjacent areas that were not covered;
- Tectonic activity;
- Land subsidence due to soil compaction caused by groundwater extraction, oil/gas mining and/or drainage of organic soils.
Relative sea level change is the net result of these different factors. This implies in most cases a relative rise of the mean sea level, which in some regions may even be substantially stronger than the rise of the global mean sea level. However, in some other regions with strong land uplift the relative sea level is still falling.
Response of the coastal system to sea level changes
The response of the coast to sea level changes can be classified as:
- regressive coast, seaward coastline shift in the case of a falling relative sea level;
- transgressive coast, landward shoreline shift in the case of a rising relative sea level;
- prograding coast, seaward coastline shift when net sediment supply to the coast dominates the impact of relative sea-level rise;
- retrograding coast, landward shoreline shift when net sediment supply to the coast is insufficient to compensate for relative sea-level rise.
- Sea level rise
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- Greek case studies: The implications of the expected sea level rise on the low lying areas of continental Greece in the next century
- Slangen A.B.A., Katsman C.A., van der Wal R.S.W., Vermeersen L.L.A. and Riva R.E.M. 2012. Towards regional projections of twenty-first century sea-level change using IPCC SRES scenarios. Clim. Dyn. 38 (5): 1191-1209, doi:10.1007/s00382-011-1057-6.
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