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Speaker 1: Welcome to brain Stuff from house stuff works dot com

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Speaker 1: where smart happens. Hi a Marshall brain with today's question,

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Speaker 1: how do they measure sea level? An accurate measurement of

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Speaker 1: sea level is very hard to pin down. It seems

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Speaker 1: like it would be easy, but it's actually quite complicated.

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

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Speaker 1: having an accurate sea level measurement, it's possible to measure

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Speaker 1: the height of everything on land accurately. Second, by knowing

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Speaker 1: sea level, we can determine if the oceans are rising

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Speaker 1: or falling over time. The concern is that global warming

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Speaker 1: and other weather changes caused by man might be leading

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Speaker 1: to an overall rise in sea level. If so, coastal

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Speaker 1: cities are in big trouble. The problem with measuring the

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Speaker 1: level of the sea is that there are so many

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Speaker 1: different things that perturb it. If you could take planet

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Speaker 1: Earth and move it out into deep space so that

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Speaker 1: the Sun, the Moon, and the other planets didn't affect it,

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Speaker 1: and there were no temperature variations worldwide, then everything would

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Speaker 1: settle down like a still pond. Rain and wind would stop,

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Speaker 1: and so would the rivers. Then you could measure the

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Speaker 1: sea level accurately. If you did this, the level of

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Speaker 1: the ocean's water projected across the entire planet would be

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Speaker 1: called the g oid. On land, you can think of

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Speaker 1: the geoid as the level that ocean water would have

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Speaker 1: if you were to dig a canal from the ocean

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Speaker 1: shore to any point on land. But the Earth is

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Speaker 1: not in deep space. It's in the middle of a

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Speaker 1: chaotic solar system. There are all sorts of things changing

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Speaker 1: the water level at any given point, including the tides

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Speaker 1: which are caused by the Moon, large and small waves

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Speaker 1: caused by wind and the tides. High and low pressure

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Speaker 1: areas in the atmosphere would change the surface level of

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Speaker 1: the ocean by pushing or pulling on it, Temperature changes

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Speaker 1: in the ocean itself, which changed the density and volume

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Speaker 1: of the ocean's water, rainfall and river water flowing into

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Speaker 1: the ocean, and so on. If you were to stand

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Speaker 1: on the ocean shore and try to measure the sea

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Speaker 1: level with a ruler, you would find it to be impossible.

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Speaker 1: The level changes by the second from the waves, by

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Speaker 1: the hour from the tides, and by the weak from

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Speaker 1: planetary and solar orbit changes. To get around all these

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Speaker 1: different variations, scientists try using something called tide gauges. A

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Speaker 1: tie gauge is a large like one foot in diameter

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Speaker 1: and long like ten or twenty or thirty feet long

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Speaker 1: pipe with a small hole below the water line. This

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Speaker 1: pipe is often called a stilling well. Even though the

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Speaker 1: waves are changing the water level outside the gauge constantly,

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Speaker 1: they have little effect inside the gauge. The sea level

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Speaker 1: can be read relatively accurately inside this pipe. If read

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Speaker 1: on a regular basis over a time span of years

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Speaker 1: and then average, you can get a good measurement of

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Speaker 1: sea level. You can see that getting an accurate reading,

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Speaker 1: for example, down to the millimeter level, is extremely difficult.

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Speaker 1: Satellites are now used as well, but they suffer from

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Speaker 1: many of the same problems. Scientists do the best they can,

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Speaker 1: using extremely long time spans to try to figure out

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Speaker 1: what this sea level actually is and whether or not

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Speaker 1: it is actually rising. The general consensus seems to be

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Speaker 1: that the ocean's rise about two millimeters per year. Right now,

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Speaker 1: Do you have any ideas or suggestions for this podcast?

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Speaker 1: If so, please send me an email at podcast at

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Speaker 1: how stuff works dot com. For more on this and

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Speaker 1: thousands of other topics, go to how stuff works dot

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