Sea level data has been collected continuously since 1854 at this tide gauge house in San Francisco, California, US.
Click on image for full size
Courtesy of NOAA and photographer Captain Albert E. Theberge

Sea Level

Measuring sea level, the height of the ocean surface, allows scientists to calculate whether sea level is changing over time and how much sea level rise is happening now because of global warming.

But measuring sea level is not easy because the sea is not level. If you tried to draw a flat line at the top of the ocean to mark the sea level you would find that in some places there was water above the line and on other places there was water below the line. This is because of tides and waves. The ocean surface can also bulge upward because of the low atmospheric pressure of a storm.

To even out the differences in sea level caused by waves, scientists use tide gauges. These are containers that block out the waves while measuring sea level. If the information is averaged over a year, then variations like tides are evened out too. This average is called Mean Sea Level.

Satellites are also used to measure sea level. Sea surface height measurements have been recorded from satellites since 1992 by projects of NASA and the French Space Agency.

Some changes in sea level are worldwide, and others happen in one region of the world.

A change in sea level in a region can happen if the level of the ocean has changed with respect to the land. This is called a relative change in sea level. For example, the land that New Orleans, Louisiana (US) is built upon is sinking lower each year, a process called subsidence. Because the land is sinking, the sea level appears to be rising.

When sea level changes worldwide it is called a eustatic change in sea level. This is happening today as global warming melts glaciers and causes seawater to expand , increasing the volume of water in the oceans. In New Orleans, effects of the relative rise in sea level are worsened by eustatic sea level rise due to global warming. Eustatic sea level can also change over geologic time as plate tectonics changes the shape of the oceans and how much water they can hold.

Last modified July 22, 2008 by Lisa Gardiner.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Climate Change and Sea Level Rise

The coast of Bangladesh is home to millions of people, however rising sea level caused by global warming is expected to change that. This South Asian country is one of many low coastal areas worldwide...more


Have you ever walked along a beach at low tide? Everyone likes to look for uncovered sea shells or the small creatures in leftover puddles of water during low tide. One thing is for sure, low tide can...more

Global Warming: Scientists Say Earth Is Heating Up

Earth’s climate is warming. During the 20th Century Earth’s average temperature rose 0.6° Celsius (1.1°F). Scientists are finding that the change in temperature has been causing other aspects of our planet...more

Glaciers and Ice Sheets

For a glacier to develop, the amount of snow that falls must be more than the amount of snow that melts each year. This means that glaciers are only found in places where a large amount of snow falls each...more

Plate Tectonics

The main force that shapes our planet's surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics. This picture shows how the rigid outer layer of the Earth,...more

Global Ice Age Climate Patterns Influenced by Bering Strait

Sometimes, a small change in the Earth can lead to a big change in climate. A new study shows that changes in the Bering Strait might have affected ocean currents and climate worldwide thousands of years...more

Paleozoic "Sediment Curve" Provides New Tool for Tracking Sea-floor Sediment Movements

Geologists have developed a new sediment curve which shows where sediment-on-the-move is deposited during the development of sedimentary rocks. The sediment curve covers the entire Paleozoic Era. Bilal...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA