There is strong evidence that global sea level is now rising at an increased rate and will continue to rise during this century.
While studies show that sea levels changed little from AD 0 until 1900, sea levels began to climb in the 20th century.
The two major causes of global sea-level rise are thermal expansion caused by the warming of the oceans (since water expands as it warms) and the loss of land-based ice (such as glaciers and polar ice caps) due to increased melting.
Records and research show that sea level has been steadily rising at a rate of 1 to 2.5 millimeters (0.04 to 0.1 inches) per year since 1900.
This rate may be increasing. Since 1992, new methods of satellite altimetry (the measurement of elevation or altitude) indicate a rate of rise of 3 millimeters (0.12 inches) per year.
This is a significantly larger rate than the sea-level rise averaged over the last several thousand years.
For more information:
Sea Levels Online, Center for Operational Oceanographic Products and Services
Global Warming: Frequently Asked Questions, NOAA National Climatic Data Center
Warming of the World Ocean, NOAA 200th Anniversary Web Site
Sea level rise, NOAA Science on a Sphere®