Sea level is the base level for measuring elevation and depth on Earth.

Because the ocean is one continuous body of water, its surface tends to seek the same level throughout the world. However, winds, currents, river discharges and variations in gravity and temperature prevent the sea surface from being truly level.

So the surface of the ocean can be used as a base for measuring elevations, the concept of "local mean sea level" has been developed. In the United States and its territories, local mean sea level is determined by taking hourly measurements of sea levels over a period of 19 years at various locations, and then averaging all of the measurements.

The 19-year period is called a Metonic cycle. It enables scientists to account for the long-term variations in the moon's orbit. It also averages out the effects of local weather and oceanographic conditions.

Sea level is measured in relation to the adjacent land. Just like the ocean, the elevation of land may rise and fall over time. For example, the tremendous weight of a glacier on land pushes the land down, closer to sea level. That same land bounces back (a process called postglacial rebound) if the ice retreats, or melts, and its weight is removed.

Local mean sea-level measurements are a combination of sea-level variations and movement of the land.

Changes in Sea Level
Sea level may vary with changes in climate. During past ice ages, sea level was much lower because the climate was colder and more water was frozen in glaciers and ice sheets. At the peak of the most recent ice age, about 18,000 years ago, sea level was perhaps 100 meters (300 feet) lower than it is today.

Global warming, the current period of climate change on Earth, is causing glaciers and ice sheets to melt. Melting ice sheets cause an elevation in sea level. This phenomenon is called sea-level rise.

Sea-level rise threatens low-lying areas around the world. Island nations, such as Maldives and Comoros, are particularly at risk. Coastal cities, such as America's New York City, New York, and Mumbai, India, must also prepare for higher sea levels.