How can they measure how much the ocean has risen?

Question

they say the ocean has risen x number of millimeter or what ever how do they measure that... wouldt the waves prevent such a measurement, and calculating it by the amount of ice that has melted wouldnt be accurate because some water does get evaporated.

Answer 1

it is better to know as to how nearer it has come?

Answer 2

It used to be done by datum. Points that were supposed to be neither rising or falling and comparing them against the average sea level. It wasn't very accurate. Today, from space. Satellites can measure to within fractions of a mm the altitudes of any point on the face of the earth. At some of the old datum points we now find they weren't stationary but were sinking.  There are mountains still growing. And the sea level, since we have been measuring it from space has only gone up an inch in the last 30 years.  Compared to NYC which has sunk 2.5 inches. In the early 1900's NYC was actually rising.  We can even measure the amount deserts rise and fall due to gravitational pull from the moon (same force that causes the tides in the oceans). It's also a force that acts on plate tectonics along with magnetic fields and core magma circulation.

Answer 3

well they take measurements at the shores......

see how much its risen/dropped

Answer 4

If you were to stand on the ocean shore and try to measure sea level with a ruler, you would find it to be impossible -- the level changes by the second (waves), by the hour (tides) and by the week (planetary and solar orbit changes). To get around this, scientists try using tide gauges. A tide gauge is a large (1 foot [30 cm] 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 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 many of the same problems. Scientists do the best they can, using 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 (although the last link below has an interesting discussion on that consensus...).

Answer 5

??

They take the average throughout the year.  And waves have nothing to do with it.