Sea Level and Climate Change Sea Level by Mike Johnson Office of Global Programs, NOAA Scientists think that average sea level has risen between 10 and 25 cm over the past 100 years. Uncertainties in future estimates are understandable, since the future is unknown. First of all, the ocean surface is always on the move.
Waves go up and down.
Tides go up and down. Even with the best instruments, it is difficult to measure to millimeter accuracy when you have constantly moving waves and tides of meters in height. But we can do that. For the past 100 years or so, scientists have been monitoring mean sea level by recording tide gauge measurements in harbors around the world, and averaging the wave and tide readings in each harbor over long time periods to obtain mean sea level at that location. A hundred years of data is good.