How Stuff Works: How sea level works

Oct 20, 2009 (HowStuffWorks.com - McClatchy-Tribune News Service via COMTEX) -- MEVR | Quote | Chart | News | PowerRating -- We hear about sea level all the time. For example, the city of Denver is 5,281 feet above sea level. The peak of Mount Everest rises 29,029 feet above sea level.

The term "sea level" is also in the news right now, because global climate change may be raising the level of the sea. If that happens, and especially if it were to happen quickly, it would have a big effect on coastal cities. Cities in the United States like Los Angeles, New York, Boston and Miami, and their tens of millions of residents, would have major problems. Worldwide it would affect hundreds of millions of people.

But how do scientists measure sea level? Have you ever thought about that? It turns out that an accurate measurement of sea level is surprisingly difficult to pin down. You can see how hard it is if you go down to the beach and try to measure the level of the sea. If you were to stand on the ocean shore with a ruler, you would find it to be impossible to measure sea level _ the level changes by the second (waves), by the hour (tides) and by the week (planetary and solar orbit changes).

However, sea level is an important measurement for two main reasons. First, by having an accurate sea level measurement, it is possible to measure the height of everything on land accurately. Calculating the height of the aforementioned Mt. Everest is complicated by sea-level measurement inaccuracies. Second, by knowing sea level we can determine if the oceans actually are rising or falling over time. If scientific consensus about climate change is correct, then melting glaciers and ice caps, along with the expansion of the water itself from the warmth, should in fact lead to higher sea levels.

The problem with measuring the sea's level is that there are so many 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 did not 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 tides. Then you could measure sea level accurately.

If you did this, the level of the ocean's surface 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's shore to any point on land.

But the Earth is not in deep space _ it is in the middle of a chaotic solar system. There are all sorts of things changing the water level at any given moment, including: The tides, 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 down on the water or pulling it up; temperature differentials in the ocean, which change the density and volume of the water; rainfall and river water flowing into the ocean; and so on.

To get around all of these perturbations, scientists try using tide gauges. A tide gauge is a large (one foot or more in diameter), long pipe with a small hole below the water line. This pipe is often called a stilling well. Even though waves are changing the water level outside 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 can get a fairly accurate measurement of sea level.

You can see that getting an precise 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 measurements over extremely long time spans, to try to figure out what the sea level is and whether or not it is rising. The general consensus seems to be that the oceans rise about 2 millimeters per year right now.

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