Sea level is the height of the sea. It is constantly changing, yet somehow we specify a location anywhere on Earth as being at a certain altitude or elevation.

As it turns out, defining and measuring sea level is unexpectedly difficult. There are influences from the moon and sun, the lumpiness of Earth and the law of gravity itself.

In spite of this, we can and do measure sea level. We try to find the best long-term average and apply it accordingly.

The weather can have a large effect on sea level at any given time as storms and wind whip up waves. The intensity and location of relatively stationary high-pressure centers and the constant motion of low-pressure storm systems both have an effect.

Of course, storm surge from hurricanes has done most of the damage in the United States and Caribbean islands. A similar situation prevails along the eastern and southern coasts of Asia.

We can measure tides over the entire 29-day cycle of the moon and use that to predict future tides, but it is not accurate without taking account of the sun, which has about one-half of the tidal influence of the moon.

The problem with the tides is that the solar and lunar tides are out of alignment and phase. The sun sometimes increases, sometimes decreases the net tide. Solar and lunar cycles are in phase only every 19 years, so accurate prediction requires measurements at least this long.

It then takes multiple decades and cycles to sort out the tides from other effects and find a reliable average.

This is only the simple stuff that eventually averages out with many measurements from many different locations. Other factors affect sea level over other time scales.

The shape and location of coastlines and offshore topography change over time. Adding to the difficulty is that sand and sediment are highly variable in location but are offshore and out of sight.

Trying to determine past sea levels and track changes is even more complex.

For example, the land can move up or down. During the Ice Age the added weight of 10,000 feet or more of ice caused Earth’s crust to sink into the mantle. Now with the ice melted, the land is rebounding up to 1 centimeter per year around northern latitude land masses.

As difficult as it is to come to an average sea level embedded in Earth’s whimsical geography, we can thank space technology for providing an accurate, precise and reliable way to measure sea height.

Satellites using radar bounce radio waves off the sea surface. Timing the return of the radio waves to the satellite measures the sea height to as little as 18 millimeters, a bit less than three-fourths of an inch.

Satellites produced some surprises as we learned that Earth’s gravity is not the same everywhere. Since different types of rocks have different densities, rocks deep in the mantle can affect the ocean surface.

Some spots are 300 feet higher or lower than would be the case if Earth were a uniform sphere.

We must understand all of these and other factors in order to predict the effect of a warming ocean. Melting ice redistributes mass, which affects how air and water spread out across the surface, which affects Earth’s rotation, which affects weather patterns.

Sea level exists as a concept for us. No measurement can define a “global” sea level. Such a thing does not exist. In reality it can only be what we define it to be at a given time.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.