Rising Sea Levels Put Coastlines in Crisis

The prevailing American perception of global warming summons visions of vast fractured swathes of drought-ravaged savannas, or of polar bears in peril, stranded on Arctic ice floes and set adrift in seas of glacial melt.

But climate change is not a crisis solely reserved for the planet’s distant wild places. As sea levels rise and storms grow progressively stronger, coastlines all over the world will erode at ever-increasing rates, endangering coastal communities, infrastructures, and ecosystems.

Beaches naturally fluctuate between periods of growth and decay. Erosion is simply the process by which sand and other sediment are removed from shorelines by waves, winds, and tidal currents. Gentler waves move sand shoreward in the summer, while in the winter, turbulent seas transport sand offshore and into deeper waters. This cycle allows beaches to maintain a quasi-stable position that geologists refer to as dynamic equilibrium.

The slow warming of the oceans results in the expansion of sea water. This thermal expansion, along with water contributed by melting glaciers and ice sheets, has led to the rise in sea levels we witness today.

Higher seas aggravate the natural equilibrium of gently sloping beaches and drive them landward. In addition to providing critical habitat for shore birds, invertebrates, and marine mammals, sandy beaches act as natural buffers between oceans and inland communities. As these beaches decay, they leave coastlines vulnerable to wave damage, inundation, and accelerated erosion.

All along California’s mountainous coasts, waning beaches allow high tides to gnaw into sea cliffs and bluffs, rendering them susceptible to collapse and threatening seaside developments. In 2011, for example, a 40-foot stretch of California’s famous Highway 1 caved in just north of Big Sur, resulting in a $2.5 million recovery project. The San Francisco and Monterey Bay areas continue to struggle with severe chronic erosion, and in Pacifica, collapsing cliffs have forced residents to permanently abandon their homes. With an estimated 950 of California’s 1,120 miles of coastlines actively eroding, the state’s shoreline communities face greater risks than most everywhere else in the US.

The warming climate will also mean a dramatic increase in the severity and frequency of storms. A review of global rainfall data conducted at the University of Adelaide projects that for every degree increase in global temperature, there will be a 7 percent increase in extreme rainfall intensity. As temperatures continue to rise, this could have significant ramifications for storm systems.

While increased rainfall may help to nourish beaches by carrying sediments down to the coast from watersheds and estuaries, the intensified onslaught of waves associated with extreme storms can wear beaches down to cobbles and underlying bedrock.

Rob Thieler, PhD, a research geologist for the US Geological Survey’s Coastal and Marine Geology Program, explains the potential impacts that increased violent storm activity would have on US shorelines.

“After a storm,” he says, “there’s a period of rebuilding in which sand that was pulled into deeper waters is moved back ashore. Where beaches are more frequently pounded by storm events, that natural recovery process can be inhibited.”

In the absence of the barrier provided by healthy beaches, heavy storms can have devastating effects on shoreline communities and their infrastructures.

With nearly 40 percent of the nation’s entire population living along coasts, one of the most urgent dilemmas facing coastal states is how to protect residents from such damage by mitigating erosion.

As geologists attempt to adapt to the changing sea level–which many expect to rise over two feet in the next century–their options seem few and unfavorable.

Shoreline armoring, a process in which sea walls are installed to harden uplands against wave damage, ultimately leads to further beach loss by cutting off the natural sediment supply and causing the fronting beach to disappear.

Beach nourishment provides a slightly more sustainable approach by dredging sand from coastal inlets and redistributing it along waning beaches. But this method is an ephemeral fix that costs millions of dollars, and in some cases, it endangers the integrity of habitats that host aquatic life.

According to Chip Fletcher, PhD, a professor of geology and geophysics at the University of Hawaii at Manoa, the most viable option may be to relocate coastal communities farther inland, enabling the natural degradation of beachfront properties.

“In many shorelines,” he says, “the land immediately adjacent to a beach is rich with sand. Erosion would release that sand, allowing the beach environment to stay healthy and robust, even as it migrates with the rising sea level.”

The issue then becomes one of economics.

Fletcher proposes a buy-out program funded by public dollars — whether township, county, state, or federal — that would incentivize homeowners to willingly relinquish their properties. It’s a large sum to muster, but he thinks the key might lie in carbon control.

“A tax could be levied on companies that produce carbon dioxide and other pollutive greenhouse gases,” he suggests. “That money, in turn, would be allocated toward purchasing coastal lands for the benefit of the entire community.”

As a federal research scientist, Thieler maintains a policy-neutral position, but he notes that Fletcher’s approach seems to be compatible with the realities facing geologists today.

Equipped with limited fiscal resources and potentially dwindling sand supply, it will become increasingly difficult for society to combat the accelerating rates of erosion inflicted by climate change.

Moving forward, the most effective mitigation measures will be those that address the causes of erosion rather than the symptoms. Until we can manage to reduce our impact on the factors that influence global warming, the future of the American coastlines will continue to hang in the balance.

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