As the average temperature of the Earth's atmosphere rises, the atmosphere, in turn, warms up the land, ice and sea beneath it. The ice melts and flows, ultimately, into the oceans. The water expands, as do most substances as they warm. Both of these phenomena cause the sea level to rise.
Independent of climate change, a varietly of geologic forces can cause land to rise or sink. Depending on the direction, the change of land elevation can make the change in relative sea level smaller or greater.
The U.S. National Oceanic and Atmospheric Agency (NOAA) operates a series of sea-level gauges in coastal waters around the country. The station in Boston Harbor is located in the Fort Point Channel. With records going back to 1920, NOAA data indicate that, over the past century, the relative sea level has risen about 10 inches. Scientists estimate that about half of this is from rise in the absolute sea level, and half from land subsidence.
Recent data suggest that the rate of sea-level rise is increasing. Projections of sea-level rise for Boston range from 2 feet to as much as 6 feet by the end of the century, depending on how fast ice in Greenland and Antarctica melt.