The Main Cause of a Rise in Sea Level

As of 2011, sea levels are rising along most of the U.S. coastline. Over the previous 100 years, sea levels rose 5 to 6 inches above the global average, particularly along the mid-Atlantic and Gulf coasts. A rise in sea level can have a variety of harmful effects on adjacent inland areas and ecosystems.

Global Warming

• Greenhouse gases accumulating in the atmosphere trap heat and raise the global temperature. A panel from the U.S. National Research Council stated in 2006 that Earth was the hottest it had been in the last 400 years, or perhaps even the last 2,000 years. Global temperature increased by 0.5 to 1 degree Fahrenheit during the 20th century, causing thermal expansion of the oceans, and also melting glaciers and ice sheets. In 2007, the International Panel on Climate Change predicted that global warming would cause sea levels to rise by as much as 2 feet by 2100. However, this figure is likely to be conservative, because it does not take accelerating ice loss from melting glaciers and polar ice sheets into account.

Other Causes

• Other factors, such as coastal subsidence and natural warming cycles, can also contribute to higher sea levels. These changes generally occur over a long time scale, and operate independently of global warming. The combination of these natural factors with sea level increases resulting from global warming may intensify the hazards of sea level rise.

Effects

• Rising seas can erode beaches, intensify flooding, overwhelm wetlands and lowlands, and deposit salt in rivers and underground water supplies. Coastal wetlands provide habitat for threatened species, and also protect local areas from flooding. As old wetlands erode, new ones form, but they are much smaller, and developed areas usually impede their size and reach. Rising sea levels also increase flooding from storms, because higher water leads to higher storm surges. At the same time, beach erosion reduces storm protection for coastlines.

Responses

• The U.S. Army Corps of Engineers and coastal states have taken a variety of measures in response to rising sea levels, including placing more sand on beaches to offset erosion. In addition, many private property owners have elevated their structures in order to be eligible for lower flood insurance rates. States like North Carolina have passed laws to ensure that dunes, wetlands and beaches are able to migrate inland as seas rise. In some cases, these human responses intensify the effect of rising sea levels on the local environment. As an example, erecting sea walls eliminates bay beaches, which provide habitat for numerous organisms, and are also important for recreation, fishing and transportation.