Measurements suggest that sea level has risen worldwide approximately 15-20 cm (6-8 inches) in the last century. Part of that rise has been attributed to the historic warming of the atmosphere and the oceans. Approximately 2-5 cm (1-2 inches) of the rise resulted from the melting of mountain glaciers. Another 2-7 cm has resulted from the expansion of ocean water that resulted from warmer ocean temperatures. A number of factors unrelated to greenhouse gases may also be responsible for part of the historic rise in sea level, including the pumping of ground water, and melting of the polar ice sheets in response to the global warming that has taken place since the last ice age. Nevertheless, scientists are currently unable to account for all of the measured sea level rise of the last century.
Along most of the U.S. coast, sea level has been rising 2.5-3.0 mm/yr (10-12 inches per century). Because tide gauge stations measure sea level relative to the land, which includes changes in the elevations of both water levels and the land, tide gauges measure "relative sea level rise," not the absolute change in sea level. The rate of sea level rise in the United States varies from about 1 cm per year (three feet per century) along the Louisiana Coast, to a drop of several millimeters per year (a few inches per decade) in parts of Alaska. The rapid rate in Louisiana resulted from the settling of newly created land formed by the sediments that washed down the Mississippi River. In Galveston, the removal of groundwater led the land above the water table to sink.
In areas that were covered by the Wisconsin Ice Sheet during the last Ice Age, by contrast, the land is rising because of the removal of the weight of the ice, which had previously compressed the land downward. As a result, the sea is dropping relative to the Alaska Coast, and sea level is rising more slowly than the global average along the Washington and Oregon coasts. Even though the ice sheet advanced as far south as New York, the New England Coast is not rising: These coastal areas had relatively little ice cover. Moreover, because the sea has risen 100 meters since the last ice age, ocean water now exerts a downward force on parts of the continental shelf that had been above sea level.
The mid-Atlantic is experiencing a greater rate of relative sea level rise because the entire region is sinking. During the ice age, the mass of the glaciers caused an upward bulge around the edges, similar to the bulge that occurs around one's thumb when one squeezes a rubber ball. Just as removing one's thumb causes the bulge to disappear, the removal of the glaciers from the northern part of the continent has caused adjacent areas to subside. Thus, the southern Atlantic coastal areas, which were farther from the glaciers, are not subsiding as much at the mid-Atlantic, if at all.
Sea level is rising more slowly in other parts of the world. Relative sea level is falling in most of Scandinavia, leading ports to become progressively shallower in some areas. Outside of the United States, Europe, and Japan, the coastal areas have too few tide gauges to be sure about regional sea level rise. Nevertheless, Bangkok has rapid subsidence due to groundwater pumping. River deltas tend to subside, causing unusually high rates of sea level rise in Bangladesh, similar to the subsidence in Louisiana. Australia, New Zealand, and Pacific atoll nations appear to have average rates of sea level rise, suggesting that they are neither sinking nor uplifting.