The most widely feared consequence of global warming appears to be sea level rise (SLR). Environmental advocacy groups are polluting the airwaves and internet with lurid images of flooding of Bangladesh and Pacific islands, and raising the specter of hundreds of millions of environmental refugees. Even sober scientists, while not endorsing such obvious scare stories, predict an acceleration of the ongoing global rise, which a system of tidal gauges places at about 18 cm (7 inches) for the past century, Other scientists stoutly maintain that there has been no acceleration -- even during the strong global warming of 1920-1940.
Recall that Obama even predicted a deceleration of SLR when he accepted his party's nomination in 2008: "This was the moment when the rise of the oceans began to slow, and our planet began to heal." So far, the existing data can be used to support all three sides. Some tidal-gauge data tend to show deceleration starting in 1960 (Holgate).
With estimates of past SLR all over the place, how does one proceed? The principal data have come from gauges, which measure not only tides but storms and everything else. And from these measurements one extracts a steady rise in local sea level. There are about two dozen stations in the world with long-enough records dating back to the early 1900s, which have been used by the international tidal gauge network, located in Liverpool, England.