Global sea level has been rising at an increasing rate since the 20th century. Analysis of a global network of tide gauge records shows that sea level has been rising at the rate of about 0.6 inches per decade since 1900. Since 1992, satellite altimeters indicate that the rate of rise has increased to 1.2 inches per decade—a significantly larger rate than at any other time over the last 2000 years. In the next several decades, continued sea level rise and land subsidence will cause tidal flood frequencies to rapidly increase due to typical storm surges and high tides in many coastal regions.
The two major causes of global sea-level rise are thermal expansion caused by warming of the oceans (since water expands as it warms) and the loss of land-based ice, such as glaciers and ice sheets, due to increased melting. The oceans are absorbing more than 90% of the increased atmospheric heat associated with emissions from human activity.
With continued ocean and atmospheric warming, sea levels will likely rise for many centuries at rates higher than that of the current century. In the United States, almost 40% of the population lives in relatively high-population-density coastal areas, where sea level plays a role in flooding, shoreline erosion, and hazards from storms. Globally, eight of the world's 10 largest cities are near a coast, according to the U.N. Atlas of the Oceans.
Sea level rise at specific locations may be more or less than the global average due to local factors such as land subsidence from natural processes and withdrawal of groundwater and fossil fuels, changes in regional ocean currents, and whether the land is still rebounding from the compressive weight of Ice Age glaciers. In urban settings, rising seas threaten infrastructure necessary for local jobs and regional industries. Roads, bridges, subways, water supplies, oil and gas wells, power plants, sewage treatment plants, landfills—virtually all human infrastructure—is at risk from sea level rise.
Higher sea levels mean that deadly and destructive storm surges push farther inland than they once did, which also means more frequent nuisance flooding. Disruptive and expensive, nuisance flooding is estimated to be from 300 percent to 900 percent more frequent within U.S. coastal communities than it was just 50 years ago.