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
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.
Shop Windows to the Universe Science Store!
The Summer 2010 issue of The Earth Scientist
, available in our online store
, includes articles on rivers and snow, classroom planetariums, satellites and oceanography, hands-on astronomy, and global warming.
You might also be interested in:
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
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
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
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
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
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