Sea level rise will be felt both through changes in mean sea level, and, perhaps more importantly, through changes in extreme sea level events. Even if there are no changes in extreme weather conditions (for example, increases in tropical cyclone intensity), sea level rise will result in extreme sea levels of a given value being exceeded more frequently. This change in the frequency of extreme events has already been observed at many locations. The increase in frequency of extreme events will depend on local conditions, but events that currently occur once every 100 years could occur as frequently as once every few years by 2100.
Global Mean Sea Level increased by 210mm between 1880 and 2009, and is continuing to rise at a fairly steady rate of just over 3mm/year. This rate of rise is undoubtedly contributing to the flooding problems of low-lying island states like Tuvalu, Kiribati and the Maldives.
This is exacerbated in some areas (e.g. Gippsland, Victoria and the Gulf coast of the U.S.) where large-scale land subsidence causes a rate of rise relative to the land which is substantially higher.
Correspondingly, some areas have seen less impact because they are rising. Australia is rising at about 0.3-0.4mm/year due to Glacial Isostatic Adjustment (GIA), but is still starting to feel the effects.
In addition, the continuing rise in mean level causes a corresponding increase in the frequency of extreme events, as detailed on the next page…..
The effect of rising mean sea levels will be felt most profoundly during extreme storm conditions when strong winds and falling pressure bring about a temporary and localised increase in sea level known as a storm surge. Storm surges occurring on higher mean sea levels will enable inundation and damaging waves to penetrate further inland increasing flooding, erosion and the subsequent detrimental impacts on built infrastructure and natural ecosystems. In the tropics storm surges are caused by tropical cyclones while elsewhere mid-latitude storms and their associated cold fronts are the main cause of storm surges.
State of the Climate 2012 also highlights the increase in global sea level and notes sea-level rise around Australia since 1993 is greater than, or equal to, the global average. Our observations show that sea-surface temperatures around Australia have increased faster than the global average.
Like mercury in a thermometer, ocean waters expand as they warm. This, along with melting glaciers and ice sheets in Greenland and Antarctica, drives sea levels higher over the long term. For the past 18 years, the U.S./French Jason-1, Jason-2 and Topex/Poseidon spacecraft have been monitoring the gradual rise of the world's ocean in response to global warming.
Sea level rise is one of the most readily recognizable manifestations of climate change, because it is directly observable without the aid of instrumentation, with very visible effects. Sea level rise is not as rapidly variable as many of the other indicators of climate change, such as temperature or precipitation. Rather it evolves relatively slowly and presents a clear expression of the integrated elements of our changing climate [ Statement of Dr. Waleed Abdalati, Chief Scientist, National Aeronautics and Space Administration before the Committee on Energy and Natural Resources United States Senate,19 April 2012]