When many people think of global warming, their first concern is the possible rise of sea levels. With a large number of the world's cities in coastal areas, this is a very real problem. There are two major causes of rising sea levels. First, extra water is produced when the ice melts. A second cause is the natural expansion of sea water as it becomes warmer. Nimbus 5 and 7 satellite data, analyzed over the last 15 years, shows that the range of sea ice around both poles continues to shrink, as it melts.[Scientific American, p.20, Jan 1989] In 'The Challenge of Global Warming', Abrahamson suggests that even with the level of greenhouse gases present today, the earth may warm enough in the next 50 years or so to completely melt the sea ice in the Arctic Ocean!
While estimates of global warming vary by a factor of 2 for a given level of greenhouse gases, sea level estimates by different researchers show a much wider variation. The estimation problem goes back to our lack of understanding of global ocean dynamics, with added complications of understanding how various forms of ice are deposited, melted, and redistributed. It would, of course, help the sea-level researchers if they knew exactly how much warming to expect, and when to expect it. But estimates are estimates, and the one shown below, made by the EPA in 1983 is probably reasonable.[Projecting Future Sea Level Rise;Hoffman,Keyes,Titus,GPO,1983]
It is also important to realize that the elevations of various costal land areas are rising and sinking due to geological factors. Thus the sea level in Sitka, Alaska appears to be falling, while in Galveston, Texas, local sea level is rising much faster than the global average. ***
