Penn StateNASA

Absolute Versus Relative Sea Level Change

In fact, changes in the height of the ocean as a result of melting ice or warming seas (absolute or eustatic sea level change) only tell part of the story. The level of the ocean can also change because the underlying land is rising or falling with respect to the ocean surface. Such relative sea level change usually affects a local or regional area, and in numerous cases, is actually outpacing the rate of sea level change.

Relative sea level changes can be caused by plate tectonic forces. For example, the Great Japan earthquake on March 11, 2011 caused part of the island of Honshu to rise up by nearly three meters. The sea floor can also drop when huge amounts of sediment are deposited by rivers and deltas. The weight of the sediment depresses the underlying crust often at a faster rate than the sediment is being deposited. This process is happening along the east coast of the US today in places along the coasts of Maryland, North Carolina and Georgia, and the Gulf Coast especially in the Mississippi delta region. In parts of Florida where large amounts of water have been pumped out of aquifers for water supply, the land is also subsiding rapidly.