Tech I don't know where the heck you got a number like 3 to 6 feet of global sea level rise over the next few decades but that is off by few orders of magnitude...
http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsProbability.html
1. Global warming is most likely to raise sea level 15 cm by the year 2050 and 34 cm by the year 2100. There is also a 10 percent chance that climate change will contribute 30 cm by 2050 and 65 cm by 2100. These estimates do not include sea level rise caused by factors other than greenhouse warming.
2. There is a 1 percent chance that global warming will raise sea level 1 meter in the next 100 years and 4 meters in the next 200 years. By the year 2200, there is also a 10 percent chance of a 2-meter contribution, and a 1-in-40 chance of a 3-meter contribution. Such a large rise in sea level could occur either if Antarctic ocean temperatures warm 5°C and Antarctic ice streams respond more rapidly than most glaciologists expect, or if Greenland temperatures warm by more than 10°C. Neither of these scenarios is likely.
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6. Along most coasts, factors other than anthropogenic climate change will cause the sea to rise more than the rise resulting from climate change alone. These factors include compaction and subsidence of land, groundwater depletion, and natural climate variations. If these factors do not change, global sea level is likely to rise 45 cm by the year 2100, with a 1 percent chance of a 112 cm rise.
Along the coast of New York, which typifies the United States, sea level is likely to rise 26 cm by 2050 and 55 cm by 2100. There is also a 1 percent chance of a 55 cm rise by 2050 and a 120 cm rise by 2100.
55 cm is 20.5 inches...less than 2 feet in the next 100 years.
Not only that, but it isn't like the sea will rise those 20.5 inches all at once. It will occur slowly and the coastline will change with it. A sea level rise of 3-6 feet over a few decades would be Bilblical in its effect.