14.10: What Causes Sea Level Change

Fundamentally, sea level rises or falls as the volume of the ocean changes. That’s the simple view. However, on any given shoreline, sea level may change due to changes in the elevation of the land. As we have seen, sea level rise depends on any of a number of factors at any one location.

The global rise in sea level results primarily from two principal processes that increase the volume of the ocean: (1) thermosteric sea level rise; and (2) barystatic sea level rise. Thermosteric sea level rise results from the thermal expansion of seawater when heated. Increases in the temperature of seawater cause its volume to expand. Barystatic sea level rise comes from the addition of water to ocean basins—an increase in ocean mass—from melting land ice (e.g., ice caps and glaciers), changes in land storage of water (e.g., water formerly stored behind dams), and the atmosphere (e.g., precipitation; Gregory et al. 2019).

Local sea level rise also experiences thermosteric and barystatic sea level rise, but they’re also subject to vertical land movements, defined as “the change in height of the seafloor or land surface” (Gregory et al. 2019). Upward vertical motions are called uplift (or emergence), while downward vertical motions are called subsidence (or submergence). Any number of natural and manmade processes may cause uplift or subsidence. What’s most important are the effects of vertical land motions on relative sea level rise. Where a coast is uplifted, relative sea level rise may be slower than if the land exhibits no vertical motion. On the other hand, a subsiding coastline may experience an accelerated relative sea level rise because the land is losing height while sea level is increasing.

Understanding the individual contributions of thermal expansion, ocean mass gains, and vertical land movements on sea level rise is important for predicting future changes and impacts. For example, the rate of thermal expansion of seawater increases with increasing temperature. Thus, as the ocean warms, thermal expansion will accelerate (e.g., Widlansky et al. 2020). Similarly, increases in the rate of melting of glaciers and ice caps will accelerate contributions to ocean water mass (e.g., The IMBIE Team 2020). Indeed, thermal expansion and increased ice melt explain the acceleration in global mean sea level rise that has been observed since the 1970s (Frederikse et al. 2020).

Most of the differences in relative sea level along US coastlines and around the world can be explained by vertical land motions. Tectonic compression (pushing land together) may cause uplift of a coastline—essentially reducing the rate of sea level rise—while tectonic extension (pulling land apart) may cause subsidence—making the rate of relative sea level rise faster. At the same time, many coastlines (along the United States and around the world) are still adjusting to the retreat of glaciers at the end of the last ice age some 16,000 years ago. When present, glaciers depress land masses. Once removed, the land masses return to their former position. This movement of land masses in response to the presence or absence of glaciers is known as glacial isostatic adjustment (NOAA 2023b). There’s also a kind of edge effect with glaciers: the land along their southern terminus—the unglaciated land at the edge of a glacier—experiences an uplift as the land under the glacier sinks. This uplift—known as a forebulge—disappears when the glacier retreats. Forebulge collapse—the subsidence of land following retreat of a glacier—explains a large part of the subsidence that is happening along the north and central US East Coast. Glacial isostatic adjustments may last tens of thousands of years (Whitehouse 2018).

Human activities also contribute to subsidence. Extraction of underground resources—especially groundwater and hydrocarbons—can cause land to subside. In the 1940s, so much water, gas, and oil had been pumped out from beneath Long Beach, California, that 20 square miles of land sank. Some spots subsided nearly 30 feet (Waldie 2015). Long Beach became popularly known as the “Sinking City” in the ’50s. Fortunately, legislation and monitoring have brought the city’s subsidence under control.

The same cannot be said for Louisiana, which may now be called the “Sinking State.” A recent study along coastal Louisiana revealed land subsidence rates of 0.35 inches (9 mm) per year—not including sea level rise—due to natural and human activities (Nienhuis et al. 2017). Disruptions of natural processes of sediment transport that supply and trap sand—including dams, surface hardening, and sand mining—and any number of other human activities may also contribute to subsidence.

The degree to which any of these processes contribute to local sea level rise will vary across different parts of the world. As noted by the Intergovernmental Panel on Climate Change (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate, “responses to sea level rise are local and hence always based on relative sea level experienced at a particular location. . . . Extreme sea level events at the coast that are rare today will become more frequent in the future. . . . One important response for preparing for future sea level rise is to improve observational systems” (e.g., Oppenheimer et al. 2019).