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 is the height of the ocean surface. Scientists measure sea level to figure out how much sea level rise is happening now because of global warming.
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. With all these ups and downs, how do scientists measure sea level?
To even out the differences in sea level caused by waves, scientists use instruments called tide gauges. These containers are installed in the shallow parts of the ocean. They block out the waves while measuring sea level. There are thousands of tide gauges around the world that are recording how high the ocean is all the time. The information they record is averaged. Satellites have also been used to measure sea level since 1992.
Today, sea level worldwide is rising. This is because global warming melts glaciers and causes seawater to expand, increasing the amount of water in the oceans. Sea level can also change over geologic time as plate tectonics changes the shape of the oceans and how much water they can hold.
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