engleski

Exploring relationship between global temperatures and global sea levels

Global sea level has been rising over the last hundred or so years. The rise is expected to continue, and even to accelerate, in the future due to the global climate change. Some recent storm surge events, particularly those related to hurricanes, have revealed how sensitive the coastal population is to flooding. The future sea level rise will obviously exacerbate the sensitivity. The presentation starts with a brief overview of three different methods that are used to determine sea level change: reconstruction based on geological indicators, measurement with tide gauges and measurement with satellite altimeters. Sea level variability over hundreds of thousands of years and global sea level rise amounting to approximately 17 cm over the past century is described. It is pointed out that the recent rise depended on an increase of the ocean volume due to the absorption of heat and on an increase of the ocean mass caused by the melting of glaciers and ice sheets. It is also shown that the Mediterranean sea level rose more slowly for a while and that an acceleration was observed recently. Additionally, extreme flooding events in various places around the world are mentioned, in particular those in New Orleans (2005), Myanmar (2008), New York (2012) and Philippines (2013). The forecast of storm surges is briefly commented upon. Flooding events in the Adriatic are described as well ; it is pointed out that the Bakar tide gauge – the oldest one in Croatia – recorded four highest sea levels after the year 2008. When modeling mean sea level, one usually starts with the comparison of past sea levels obtained by a method to the observed values and then proceeds by applying the method to determine future sea levels. The method utilized may be either process-based or semi-empirical. The presentation proceeds with an overview of three variants of the semi-empirical method being used in analysis and projection of not only sea levels but also sea level trends. The variants differ in assuming that the response of sea level to the temperature forcing is equilibrium, inertial or a combination of the two. All variants enable temperatures, sea levels and/or sea level trends to be successfully regressed, albeit with controlling parameters that differ among the cases. The related response times vary considerably, with a realistic value (ca. 50 years) obtained only if both equilibrium and inertial dynamics are taken into account. It is stressed that a comparison of computed sea levels to those measured over the last century showed that the best agreement is provided by the purely inertial variant of the semi-empirical method. On the other hand, a comparison of computed sea level trends to the corresponding values determined from available measurements pointed to the equilibrium-cum- inertial variant of the semi-empirical method as the most successful one. Sea levels projected by using the three variants were found to be similar through the middle of the 21st century but also to radically diverge by the end of the 23rd century. Sea level trends considerably differed throughout the projection interval. It is therefore pointed out that one has to be careful while calibrating a method on the data having a specific spectral content and then using the method to prepare projections under the forcing having different spectral characteristics.

climate change ; global sea level rise ; temperature

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