Measuring Sea Level Change NSF DRL

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?

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