How do they measure sea level?

Sea level is an important measurement, but it's also a complicated one. Find out why in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from house stuff works dot com
where smart happens. Hi a 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

(00:22):
like it would be easy, but it's actually quite complicated.
It's an important measurement though, for two reasons. First, by
having an accurate sea level measurement, it's 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

(00:45):
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

(01:06):
the Sun, the Moon, and the 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

(01:28):
called the g oid. 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

(01:50):
which are caused by the Moon, large and small waves
caused by wind and the tides. High and low pressure
areas in the atmosphere would change the surface level of
the ocean by pushing or pulling on it, Temperature changes
in the ocean itself, which changed the density and volume
of the ocean's water, rainfall and river water flowing into

(02:12):
the ocean, and so on. If you were to stand
on the ocean shore and try 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 weak from
planetary and solar orbit changes. To get around all these
different variations, scientists try using something called tide gauges. A

(02:37):
tie gauge is a large like one foot in diameter
and long like ten or twenty or thirty 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 the gauge constantly,
they have little effect inside the gauge. The sea level

(03:00):
can be read relatively accurately inside this pipe. If read
on a regular basis over a time span of years
and then average, you can 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

(03:21):
many of the same problems. Scientists do the best they can,
using extremely long time spans to try to figure out
what this sea level actually is and whether or not
it is actually rising. The general consensus seems to be
that the ocean's rise about two millimeters per year. Right now,
Do you have any ideas or suggestions for this podcast?

(03:44):
If so, please send me an email at podcast at
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