Coastal Processes, Hazards, and Society

Thermosteric Sea Level Change - Thermal Expansion and Sea Level Rise


Thermosteric Sea Level Change - Thermal Expansion and Sea Level Rise

Another substantial mechanism for changing sea level is related to thermal expansion/contraction properties of water molecules themselves. In our high school science classes, we all learned that as the temperature of different substances increases, the molecules within those substances become more "excited". These excited molecules that bump into each other more frequently take up more space, so the warmer substance will expand in volume and will have a lower density. The behavior of water molecules follows this same pattern. When liquid saltwater warms up, its density (mass per unit volume) decreases as the volume increases. As temperatures of the ocean increases, the volume of seawater increases and can produce a higher sea level. Conversely, as seawater cools down, the density increases as the volume decreases. This produces lower sea levels.

graph of 0-700m global ocean heat content 1960-2020. Shows an increase from 1980 to 2020
This figure shows thermosteric (i.e., due to ocean temperature change) contributions to sea level rise from the late 1950s. Thermosteric estimates are based on measurements of warming of deep water areas and surface warm water areas of the ocean. Expansion coefficients have been applied by NOAA scientists in the Ocean Climate Laboratory Image to make these estimates.

Geoscientists and physical oceanographers are developing mathematical models to explain and predict the impact of even small changes in ocean temperature on sea level. In the image above, you will notice that different ocean layers contribute to rise at different rates. Some scientists believe that the deep ocean layers, as thick and deep as they are, will volumetrically produce even higher sea levels if they warm in the absence of polar glaciers. Better empirical modeling will continue to be refined so that we will have a better sense of the impact that this phenomenon has on overall sea level change.

Seemingly small temperature changes (even as small as 0.1 degrees Celsius), when extrapolated over the entire globe, can produce a significant sea level rise effect when considered over time. On an annual basis, the impact might not seem like a lot (just a few mm./yr. on average), but over a decade or two, this adds up to a substantial change. As such, most scientists believe that recent sea level change may be strongly tied to increased warming of the atmosphere, which in turn warms the ocean. Given this fact, many scientists are alarmed by the additive impact of melting of glaciers, which ultimately act as the cooling mechanism for the deep sea. If glaciers are not present, the ocean's ability to overturn will be impaired and, it is argued, this can cause more rapid hyper-warming of the ocean's waters leading to even higher sea levels. This is an example of a positive feedback mechanism.