Ocean Volume & Sea Level

Thermal Expansion of 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 temperature of different substances increase, the molecules within those substances become more "excited". In order to accommodate the extra movement of these excited molecules that bump into each other more frequently, the substance will expand in volume and will produce a lower density. The behavior of water molecules follows this same pattern. When liquid salt water warms up, its density (mass per unit volume) decreases as the volume increases.

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Figure 4:21: 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 both deep water areas as well as warm water areas of the ocean. Expansion co-efficients have been applied by NOAA scientists in the Ocean Climate Laboratory Image to make these estimates. Note that this is only one of the contributions to sea level oscillation, and there are other factors that both add to or detract from these values to produce the observed change in global eustatic sea levels.

Credit: NOAA/NESDIS/NODC Ocean Climate Laboratory

NASA scientists and technology have shown that this effect can be localized or even global. As demonstrated in their SEA LEVEL VIEWER, the ocean surface has different elevations which are, in part, due to temperatures of the water in given areas. If you watch the short discussion on Hurricane Katrina you will notice that warm waters and elevated sea levels in the Gulf of Mexico were observed prior to the arrival of the storm. The presence of these warm, elevated sea levels were concerning for scientists who forecasted significant impacts for gulf coast communities which were borne out as demonstrated in your exploration above.

Specific observations and facts to know about short-term sea level change:

As temperatures of the ocean increases, the volume of seawater increases and can produce a higher sea level. This effect can be relatively local if the water mass is relatively constrained, or can be global if significant portions of the ocean's water mass is impacted by warming. Conversely, as seawater cools down, the density increases as the volume decreases. This produces lower sea levels.

Geoscientists and physical oceanographers are developing empirically-derived (from direct experiments and real observations) 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, theoretically. 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 about the impact that this phenomena has on overall sea level change.

Although no single model has yet been accepted, it is a proven fact that any change in ocean temperature can produce changes in ocean volume. The question is how much volume change occurs, and what is the effect globally? As with sediment deposition, seemingly small temperature changes (even as small as 0.1 degrees Celsius), when extrapolated over the entire globe, can produce a fairly 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/year on average), but over a decade or two or even over a lifetime, 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, some 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. It is clear that there is work to do on this topic, but see chapter 10.6 from IPCC's Fourth Assessment Report: Climate Change 2007.