Measuring Sea Level Change

NSF DRL-1316782

What is sea level?

Or should we say what is mean sea level?

Mean Sea Level

Before we answer the question “What is mean sea level?” let’s take a closer look at how sea level changes.

• Daily – Sea level rises and

falls – Called ocean tides

• Seasonally– In Tampa Bay,

water level tends to be higher in the summer and lower in the winter

Sea Level Variations

Sea Level Variations: Multi-year changes

Sea level also varies in terms of years

e.g., during El Niño years, the sea level along the US Pacific coast tends to be higher than during La Niña years.

When we say global sea level rise, what do we really mean?

Mean sea level -- The average of the sea level of a large region, or even the entire earth.How do we “get” the mean sea level?– Before we can get the “mean sea

level”, how do we get “just” the sea level?

– We measure sea level using scientific instruments• Measuring is not very difficult. • It has been done for several

hundreds of years.

History & Technology of measuring sea level

In the past, we used a stilling well.As the water level rises and falls• the float rises and falls. • mechanical device (at the

top) “draws” the sea level on a roll of paper

• Small holes near the bottom are designed to “filter” out the wave motion

History & Technology of measuring sea level

The water level is measured with sound (called acoustic sensor), which is like sonar.The drum recorder is replaced by a computer.

Today, we use much more sophisticated equipment but the principal concept remains similar.

A modern water level measurement station

Housing to keep the computer and equipment out of the weather

Sensor to measure wind speed and direction

Solar panel to supply power

Satellite antenna to send the data to the internet

Computer to process and store data

Acoustic sensor to measure water level

Stilling well to “filter” out wave motion

The previous slides illustrate how we measure water level at the coast, then how do we measure water level out in the ocean?• We could not measure

water level in the deep ocean until about 20 years ago.

Now satellites can accurately measure elevation using satellite altimetry.

What causes sea level change?

What causes sea level change?

Climate change (warming) will result in the following:• Thermal expansion – Water expands when

becomes warmer • For example, a 0.03

expansion results in a sea level rise of 1.2 m (or 4 ft).

• Continental ice melting adds water to the ocean