What Is Sea Level?

How Is Sea Level and Elevation Above Sea Level Measured?

Great Barrier Reef in the Pacific Ocean
Rising sea levels could flood low-lying islands and reefs, such as the Great Barrier Reef. Peter Adams/Photolibrary/Getty Images

We often hear reports that sea level is rising due to global warming but what is sea level and how is sea level measured?  When it is stated that "sea level is rising," this usually refers to "mean sea level," which is the average sea level around the earth based on numerous measurements over a long period of time.  The elevation of mountain peaks are measured as the height of the peak of the mountain above mean sea level.

Local Sea Level Varies

However, just like the surface of the land on our planet Earth, the surface of the oceans is not level either.  The sea level on the West Coast of North America is usually about 8 inches higher than the sea level on the East Coast of North America.  The surface of the ocean and its seas varies from place to place and from minute to minute based on many different factors.  Local sea level can fluctuate because of high or low air pressure, storms, high and low tides, and snow melt, rainfall and river flow into the oceans (as part of the ongoing hydrologic cycle).  

Mean Sea Level

The standard "mean sea level" around the world is usually based on 19 years of data that average out hourly readings of the seal level around the world.  Because mean sea level is averaged around the world, using a GPS even near the ocean can result in confusing elevation data (i.e. you might be on a beach but your GPS or mapping app indicates an elevation of 100 feet or more).

 Again, the height of the local ocean can vary from the global average.  

Changing Sea Levels

There are three primary reasons why sea level changes: 

1) The first is the sinking or uplift of landmasses.  Islands and continents can rise and fall due to tectonics or due to the melting or growing of glaciers and ice sheets.

 

2) The second is the increase or decrease in the total amount of water in the oceans.  This is primarily caused by the increase or decrease in the quantity of global ice on the Earth's landmasses.  During the biggest Pleistocene glaciations about 20,000 years ago, mean sea level was about 400 feet (120 meters) lower than mean sea level today.  If all of the Earth's ice sheets and glaciers were to melt, sea level could be up to 265 feet (80 meters) above current mean sea level.

3) Finally, temperature causes water to expand or contract, thus increasing or decreasing the volume of the ocean.  

Impacts of Sea Level Rise and Fall

When sea level rises, river valleys become inundated with seawater and become estuaries or bays.  Low-lying plains and islands are flooded and disappear under the sea.  These are the primary concerns about climate change and rising mean sea level, which appears to be rising by about one-tenth of an inch (2 mm) each year.  If climate change results in higher global temperatures, then glaciers and ice sheets (especially in Antarctica and Greenland) could melt, dramatically increasing sea levels. With warmer temperatures, there would be expansion of the water in the ocean, further contributing to a rise in mean sea level.

 Sea level rise is also known as submergence, since land above current mean sea level is drowned or submerged.

When the Earth enters a period of glaciation and sea levels drop, bays, gulfs, and estuaries dry up and become low-lying land.  This is known as emergence, when new land appears and the coastline is increased.    

 

For further information, visit the NOAA Sea Level Trends website.