USGS Contributions to the Climate Change Science Program

Global sea level and the Earth's climate are closely linked. As the climate has warmed following the "Little Ice Age" in the 19th century, sea level has been rising about 1 to 2 millimeters per year due to the reduction in volume of ice caps, ice fields, and mountain glaciers in addition to the thermal expansion of ocean water. If present trends continue, including an increase in global temperatures caused by increased greenhouse-gas emissions, many of the world's mountain glaciers, will disappear. For example, at the current rate of melting, all glaciers will be gone from Glacier National Park, Montana, by the middle of the 21st century. During cold-climate intervals, sea level falls because of a shift in the global hydrologic cycle: water is evaporated from the oceans and stored on the continents as large ice sheets and expanded ice caps, ice fields, and mountain glaciers. Global sea level was about 125 meters below today's sea level at the last glacial maximum about 20,000 years ago. Sea levels during several previous interglacials were about 3 to as much as 20 meters higher than current sea level. The evidence comes from two different but complementary types of studies. One line of evidence is provided by old shoreline features. Wave-cut terraces and beach deposits from regions as distinct as the Caribbean and the North Slope of Alaska suggest higher sea levels during past interglacial times. A second line of evidence comes from sediments cored from below the existing Greenland and West Antarctic ice sheets. The fossils and chemical signals in the sediment cores indicate that both major ice sheets were greatly reduced from their current size or even completely melted one or more times in the recent geologic past. The USGS role in sea-level research is national in scope and ranges from remote sensing and geologic mapping of wetlands to studies of coastal erosion and evidence of older shorelines in the geologic record.

Red shows areas along the Gulf Coast and East Coast of the United States that would be flooded by a 10-meter rise in sea level. Population figures for 1996 (U.S. Bureau of the Census, unpublished data, 1998) indicate that a 10-meter rise in sea level would flood approximately 25 percent of the Nation's population.

Annual mean relative sea level recorded at the Solomons Island, Maryland, tide gauge 1937-97 (from National Oceanic and Atmospheric Administration).

Extreme high tide, March 1, 1998, Hoopers Island, Eastern Shore, Chesapeake Bay, Maryland. In the Chesapeake Bay, the USGS is conducting research to reconstruct the detailed pattern of relative sea-level change during the last 6,000 to 8,000 years.