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Marshall

Hi, I’m Marshall Brain with today’s question.

How do they measure sea level?

An accurate measurement of sea level is very hard to pin down. It seems like it would be easy, but it is actually quite complicated. It’s an important measurement though, for two reasons. First, by having an accurate sea level measurement, it is possible to measure the height of everything on land accurately. Second, by knowing sea level, we can determine if the oceans are rising or falling over time. The concern is that global warming, and other weather changes caused by man might be leading to an overall rise in sea level. If so, coastal cities are in big trouble.

The problem with measuring the level of the sea is that there are so many different things that perturb it. If you could take planet earth, and move it out into deep space, so that the sun, moon and other planets didn’t affect it, and there were no temperature variations worldwide, then everything would settle down like a still pond. Rain and wind would stop, and so would the rivers. Then, you could measure the sea level accurately. If you did this, the level of the ocean’s water projected across the entire planet would be called the geoid. On land, you can think of the geoid as the level that ocean water would have if you were to dig a canal from the ocean shore to any point on land, but the earth is not in deep space, it’s in the middle of a chaotic solar system.

There are all sorts of things changing the water level at any given point including the tides, which are caused by the moon, large and small waves caused by wind and the tides, high and low pressure areas in the atmosphere, which change the surface level of the ocean by pushing or pulling on it, temperature changes in the ocean itself, which change the density and volume of the ocean water, rainfall and river water flowing into the ocean, and so on.

If you were to stand on the ocean shore, and to measure the sea level with a ruler, you would find it to be impossible. The level changes by the second from the waves, by the hour from the tides, and by the week from planetary and solar orbit changes. To get around all of these different variations, scientists try using something called tide gauges. A tide gauge is a large, like one foot in diameter, and long, like 10, 20 or 30-feet long pipe, with a small hole below the water line. This pipe is often called a stilling well. Even though the waves are changing the water level outside of the gauge constantly, they have little effect inside the gauge. The sea level can be read relatively accurately inside this pipe. If read on a regular basis, over a time span of years and then averaged; you could get a good measurement of sea level. You can see that getting an accurate reading, for example, down to the millimeter level is extremely difficult. Satellites are now used as well, but they suffer from many of the same problems. Scientists do the best they can, using extremely long time spans to try and figure out what the sea level actually is, and whether or not it is actually rising. The general consensus seems to be that the oceans rise about 2 mm per year right now.

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