Speaker 1: Welcome to brain stuff front House, stuff works dot Com
Speaker 1: where smart Happens. Hi a Marshall Brain with today's question,
Speaker 1: how do they measure sea level? An accurate measurement of
Speaker 1: sea level is very hard to pin down. It seems
Speaker 1: like it would be easy, but it's actually quite complicated.
Speaker 1: It's an important measurement though, for two reasons. First, by
Speaker 1: having an accurate sea level measurement, it's possible to measure
Speaker 1: the height of everything on land accurately. Second, by knowing
Speaker 1: sea level, we can determine if the oceans are rising
Speaker 1: or falling over time. The concern is that global warming
Speaker 1: and other weather changes caused by man might be leading
Speaker 1: to an overall rise in sea level. If so, coastal
Speaker 1: cities are in big trouble. The problem with measuring the
Speaker 1: level of the sea is that there are so many
Speaker 1: different things that perturb it. If you could take planet
Speaker 1: Earth and move it out into deep space so that
Speaker 1: the Sun, the Moon, and the other planets didn't affect it,
Speaker 1: and there were no temperature variations worldwide, then everything would
Speaker 1: settle down like a still pond. Rain and wind would stop,
Speaker 1: and so would the rivers. Then you could measure the
Speaker 1: sea level accurately. If you did this, the level of
Speaker 1: the ocean's water projected across the entire planet would be
Speaker 1: called the g oid. On land, you can think of
Speaker 1: the geoid as the level that ocean water would have
Speaker 1: if you were to dig a canal from the ocean
Speaker 1: shore to any point on land. But the Earth is
Speaker 1: not in deep space. It's in the middle of a
Speaker 1: chaotic solar system. There are all sorts of things changing
Speaker 1: the water level at any given point, including the tides
Speaker 1: which are caused by the moon, large and small waves
Speaker 1: caused by wind and the tides, high and low pressure
Speaker 1: areas in the atmosphere, which changed the surface level of
Speaker 1: the ocean, and by pushing or pulling on it, temperature
Speaker 1: changes in the ocean itself which changed the density and
Speaker 1: volume of the ocean's water, rainfall and river water flowing
Speaker 1: into the ocean, and so on. If you were to
Speaker 1: stand on the ocean shore and try to measure the
Speaker 1: sea level with a ruler, he would find it to
Speaker 1: be impossible. The level changes by the second from the waves,
Speaker 1: by the hour from the tides, and by the weak
Speaker 1: from planetary and solar orbit changes. To get around all
Speaker 1: these different variations scientists try using something called tide gauges.
Speaker 1: A tide gauge is a large like one foot in
Speaker 1: diameter and long like ten or twenty or thirty feet
Speaker 1: long pipe with a small hole below the water line.
Speaker 1: This pipe is often called a stilling well. Even though
Speaker 1: the waves are changing the water level outside the gauge constantly,
Speaker 1: they have little effect. Inside the gauge, the sea level
Speaker 1: can be read relatively accurate lee inside this pipe. If
Speaker 1: read on a regular basis over a time span of
Speaker 1: years and then average, you can get a good measurement
Speaker 1: of sea level. You can see that getting an accurate reading,
Speaker 1: for example, down to the millimeter level, is extremely difficult.
Speaker 1: Satellites are now used as well, but they suffer from
Speaker 1: many of the same problems. Scientists do the best they can,
Speaker 1: using extremely long time spans to try to figure out
Speaker 1: what this sea level actually is and whether or not
Speaker 1: it is actually rising. The general consensus seems to be
Speaker 1: that the ocean's rise about two millimeters per year right now.
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