WEBVTT - How do we measure sea level?

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<v Speaker 1>Welcome to brain stuff front House, stuff works dot Com

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<v Speaker 1>where smart Happens. Hi a Marshall Brain with today's question,

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<v Speaker 1>how do they measure sea level? An accurate measurement of

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<v Speaker 1>sea level is very hard to pin down. It seems

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<v Speaker 1>like it would be easy, but it's actually quite complicated.

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<v Speaker 1>It's an important measurement though, for two reasons. First, by

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<v Speaker 1>having an accurate sea level measurement, it's possible to measure

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<v Speaker 1>the height of everything on land accurately. Second, by knowing

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<v Speaker 1>sea level, we can determine if the oceans are rising

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<v Speaker 1>or falling over time. The concern is that global warming

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<v Speaker 1>and other weather changes caused by man might be leading

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<v Speaker 1>to an overall rise in sea level. If so, coastal

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<v Speaker 1>cities are in big trouble. The problem with measuring the

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<v Speaker 1>level of the sea is that there are so many

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<v Speaker 1>different things that perturb it. If you could take planet

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<v Speaker 1>Earth and move it out into deep space so that

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<v Speaker 1>the Sun, the Moon, and the other planets didn't affect it,

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<v Speaker 1>and there were no temperature variations worldwide, then everything would

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<v Speaker 1>settle down like a still pond. Rain and wind would stop,

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<v Speaker 1>and so would the rivers. Then you could measure the

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<v Speaker 1>sea level accurately. If you did this, the level of

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<v Speaker 1>the ocean's water projected across the entire planet would be

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<v Speaker 1>called the g oid. On land, you can think of

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<v Speaker 1>the geoid as the level that ocean water would have

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<v Speaker 1>if you were to dig a canal from the ocean

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<v Speaker 1>shore to any point on land. But the Earth is

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<v Speaker 1>not in deep space. It's in the middle of a

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<v Speaker 1>chaotic solar system. There are all sorts of things changing

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<v Speaker 1>the water level at any given point, including the tides

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<v Speaker 1>which are caused by the moon, large and small waves

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<v Speaker 1>caused by wind and the tides, high and low pressure

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<v Speaker 1>areas in the atmosphere, which changed the surface level of

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<v Speaker 1>the ocean, and by pushing or pulling on it, temperature

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<v Speaker 1>changes in the ocean itself which changed the density and

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<v Speaker 1>volume of the ocean's water, rainfall and river water flowing

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<v Speaker 1>into the ocean, and so on. If you were to

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<v Speaker 1>stand on the ocean shore and try to measure the

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<v Speaker 1>sea level with a ruler, he would find it to

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<v Speaker 1>be impossible. The level changes by the second from the waves,

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<v Speaker 1>by the hour from the tides, and by the weak

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<v Speaker 1>from planetary and solar orbit changes. To get around all

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<v Speaker 1>these different variations scientists try using something called tide gauges.

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<v Speaker 1>A tide gauge is a large like one foot in

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<v Speaker 1>diameter and long like ten or twenty or thirty feet

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<v Speaker 1>long pipe with a small hole below the water line.

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<v Speaker 1>This pipe is often called a stilling well. Even though

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<v Speaker 1>the waves are changing the water level outside the gauge constantly,

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<v Speaker 1>they have little effect. Inside the gauge, the sea level

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<v Speaker 1>can be read relatively accurate lee inside this pipe. If

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<v Speaker 1>read on a regular basis over a time span of

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<v Speaker 1>years and then average, you can get a good measurement

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<v Speaker 1>of sea level. You can see that getting an accurate reading,

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<v Speaker 1>for example, down to the millimeter level, is extremely difficult.

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<v Speaker 1>Satellites are now used as well, but they suffer from

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<v Speaker 1>many of the same problems. Scientists do the best they can,

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<v Speaker 1>using extremely long time spans to try to figure out

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<v Speaker 1>what this sea level actually is and whether or not

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<v Speaker 1>it is actually rising. The general consensus seems to be

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<v Speaker 1>that the ocean's rise about two millimeters per year right now.

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