14.8: An Ever-Changing Sea Level

It’s hard to imagine that the dry ground over which you drive and walk on a daily basis was once covered in hundreds of feet of seawater. Enormous changes in sea level across geologic time have dramatically reshaped coastlines. Sea level changes produced the shallow seas that gave rise to the world’s petroleum reserves and enabled (or prevented) human migration across Polynesia. Next time you are in downtown Los Angeles, look up at the US Bank Tower, the tallest building in California at 1,018 feet tall. That’s about 198 feet taller than the height of the sea level at its highest extent nearly 100 million years ago (Haq 2014).

Changes in sea level over geologic time are why we find whale bones in Southern California. Only a few million years ago, Southern California was submerged beneath the Pacific Ocean. Sedimentary rocks just a few miles from the Fullerton College campus contain shells of ancient marine organisms and other fossils. The Interpretive Center at Ralph B. Clark Regional Park in Fullerton hosts hundreds of marine and terrestrial fossils, including a gray whale skeleton and a giant sloth, all former residents of the Orange County as sea level rose and fell over geologic time. (See also Lozinsky 2010.)

While natural cycles contribute to sea level changes now and in the past, we focus here on sea level change as a result of human-caused warming of our planet. Sea level rise presents an immediate danger to people and structures along the coastlines of the world. Coastal-dwelling people will be forced to migrate elsewhere. Local, state, and federal governments will bear the cost of measures to adapt to or retreat from sea level rise, costs which are ultimately borne by the taxpayer. Sea level rise affects all of us, whether we live in the path of the rising sea or not.

A Closer Look at the Definition of Sea Level

At first glance, the height of the ocean—sea level—seems a simple enough affair. But upon closer inspection, we realize that a wide number of processes can cause the sea surface and sea level to change. Waves, tides, winds, atmospheric pressure gradients, ocean currents, Earth’s rotation, and even Earth’s gravitational field, which varies with the bumps and dips of the seafloor (such as seamounts, ridges, trenches), can affect sea level. The ocean is constantly in motion. At the same time, shifts in the height of the land caused by geologic forces—upward or downward—cause vertical motions in shorelines. As the ocean cools down or warms up due to climate change—causing seawater to contract or expand—and as ice caps and glaciers form or melt—changing the amount of water in the ocean—the volume of the ocean changes. At the peak of the last ice age about 20,000 years ago, sea level was lower by nearly 410–440 feet (125–134 m). All that extra seawater was frozen on land as glaciers (e.g., Lambeck et al. 2014). These many processes determine the sea level at any given moment and the rate at which sea level changes over time.

The first problem in estimating true sea level arises from Earth itself: it’s lumpy, way lumpy. As you know from Chapter 7, the seafloor boasts majestic mountains, deep and narrow trenches, and a whole host of other features that all impact sea level. And these variations in the shape (and chemical composition) of Earth’s crust cause differences in Earth’s gravitational field. Gravity, of course, is the restoring force for sea level. So variations in gravity cause variations in sea level.

Scientists define a theoretical surface known as the geoid (JEE-oyd; more fun to say than study, I’m afraid), the isosurface (equal magnitude surface) of Earth’s gravity. NOAA’s National Geodetic Survey defines the geoid as “the equipotential surface of the Earth’s gravity field which best fits global mean sea level.” The United States Geological Survey is a little less technical, describing the geoid as “the irregular-shaped ball” that serves as “an imaginary sea level surface.” Computer-generated images of Earth’s geoid resemble the kind of lopsided biscuits my mom used to bake (not the greatest cook but an incredible woman in every other way). The geoid works very well for measurements of sea level from space and computer models. But for establishing local variations in sea level, some oceanographers prefer the reference ellipsoid, essentially, “the surface of an ellipsoidal volume that approximates the geoid” (Gregory et al. 2019).

Thankfully, most people (including you and me) don’t ever have to think about the geoid or the reference ellipsoid. But it is important that you appreciate the technical difficulty of defining sea level. And important that you understand how scientists go to great lengths to establish criteria for determining the magnitude of changes in sea level and the potential for rising sea level to flood coastlines.

Defining Sea Level Change

Because a number of processes can cause sea level to change, it’s important to establish clear benchmarks to assess how much sea level changes at different places and times. A few more definitions help in this endeavor. According to Gregory et al. (2019), the sea surface can be defined as “the time-varying upper boundary of the ocean” whose height is calculated with respect to the reference ellipsoid. As with tide heights, the height of the sea surface can be positive or negative relative to the ellipsoid. From this definition, they establish mean sea level as “the time-mean of the sea surface”—the average of sea level, as it were—with a time period sufficient to average out the effects of waves and atmospheric pressure (among other things) that may cause short-term variations in sea level.

In general, we think of changes in sea level as caused by changes in the volume of the ocean (discussed below). Increases in the ocean’s volume cause sea level rise, while decreases cause sea level fall. However, sea level can change because of movements of Earth’s crust: coastlines can move up or down vertically in response to geologic and tectonic forces (the concept of isostasy referred to in Chapter 7). Because they’re most concerned about how changes in sea level will affect the integrity of beaches and structures at individual locations along coasts, oceanographers define the term relative sea level rise (also relative sea level change) as “the change in local mean sea level relative to the solid surface, i.e., seafloor or land.” When referring to the global increase in volume of the ocean—either through the thermal expansion of seawater as it warms or increases in the amount of water due to the melting of ice caps and glaciers into the ocean—oceanographers use the term global mean sea level rise (or global mean sea level change), defined as “the increase in the volume of the ocean divided by the ocean surface area” (Gregory et al. 2019). Most of the “sea level rise” that you hear about in the news refers to global mean sea level rise. For variations in sea level along a specific coastline, the term relative sea level rise is more appropriate.

Consideration is also given to the local effects of waves, tides, and storm surge—the change in sea level caused by synoptic-scale weather systems (occurring over hundreds of miles), such as hurricanes. Gregory et al. (2019) define extreme sea level rise as events with “an exceptionally high or low local sea-surface height” (emphasis mine).