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An obvious increase of extreme weather events such as heatwaves, floods and tropical storms during the last decades has given raise to worldwide concerns and indicates most likely the potential impacts of a global warming in the future. Besides these temporarily restricted events other impacts which emerge only gradually, such as rising sea levels are at least as serious.

Due to the high concentration of natural and socio-economic values located especially around coastal zones, rising sea levels raise significant concern. More than 1.2 billion people, representing 23 percent of the world’s population live on coastal plains at densities about three times higher than the global mean. (1)
The population densities also increase seaward, with the highest densities occurring below the 20m elevation. 11 of the world’s 15 largest cities are located on the coast or estuaries and urbanisation still will be an important trend in the future with about 20 large cities with more than 8 million people projected to spring up by 2010. (2,3)
Therefore, human exposure to sea level rise is significant, growing and of special concern. Increasing storm damage, coastal flooding, salt water contamination of fresh water supplies, flooding of coastal wetlands and barrier islands could all be negative external effects of even a small amount of sea-level rise.

Over the 20th century sea levels are estimated to have risen 10 to 20 cm. Although this rise during the 20th century was faster than during previous centuries, it is probably more related to the end of the “Little Ice Age” than to human induced changes. (4)
However the balance of scientific evidence now suggests that anthropogenic emissions of greenhouse gases are having a significant effect on the earth’s climate and among many consequences are expected to accelerate the sea-level rise globally.
The latest IPCC Report (2001) expected the sea-level to rise by 9 to 88 cm on average compared to 1990 levels by the end of the 21st century, mainly due to the greenhouse gases we have already emitted to date and likely future emissions. The state of the art model used to project these estimates assumed the contribution of the ice loss from Greenland to be in the range of 2-7cm above 1990 by 2100, with the Antarctic taking up ice to the same extent and therefore thermal expansion and melting continental ice caps to contribute the most to rising sea-levels in the future. But the IPCC Report (2001) also suggested that continuing greenhouse gas emissions could trigger polar ice-cap melting beyond 2100, with a potential sea-level rise of 5m within the next millennium and even more dramatically that “a local warming over Greenland is likely to be one to three times the global average and that a local warming of larger than 3°C , if sustained for millennia, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea-level rise of about 7m." (5)
This tipping-point could be reached by a global warming of about 1.2 Celsius degrees above pre-industrial levels, which reduces the “window of opportunity” significantly and makes sudden mitigation and adaptation measures even more urgent, since the present temperatures have risen already 0.8 degrees above the pre-industrial level.

1 GOMMES, R. ET AL (1997): Potential impacts of sea-level rise on populations and agriculture, FAO, Rome.
2 SMALL, C. AND R.J. NICHOLLS (2003): A Global Analysis of Human Settlement in Coastal Zones. In: Journal of Coastal Research, Vol. 19 (3), 584–599.
3 NICHOLLS, R.J. (1995): Coastal Megacities and Climate Change. In: Geojournal, Vol. 37(3), 369-379.
4 CHURCH, J.A. ET AL (2001): Changes in Sea Level. In: Houghton, J.T. et al: Climate Change 2001. The Scientific Basis. Cambridge, 639-693.
5 IPCC, ED., CLIMATE CHANGE 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, Cambridge, 2001.