Understanding Sea Level Rise

One of the impacts of climate change is sea level rise, which threatens coastal areas with increased erosion and flooding, and estuarine systems with loss of habitat.

This photo of the Stinson Beach off-ramp at high tide is part of the California King Tides Project to document sea level rise.

Why Is Sea Level Rising?

There are two main factors contributing to rising sea levels across the globe, both caused by a warming global climate. Thermal expansion, or the increase in volume of the oceans as sea water temperatures rise, is responsible for about half of the global rise in sea levels. Melting land-based ice, from glaciers and ice caps, contribute the other half. Additionally, differences in coastal land elevation changes from isostatic rebound, tectonic plate movement, and subsidence from extraction of groundwater play a role in the ultimate sea levels seen on a coastline.

How Much, How Fast?

Historical Data

Tide gauges show that global sea level has risen along our coastline about 7 inches during the 20th century, and recent satellite data shows that the rate of sea-level rise is accelerating.

Due to the factors mentioned above, sea level relative to coastal elevation is rising more rapidly along the California coast than in the north along Oregon and Washington.

Estimated, observed, and projected global sea-level rise from 1800 to 2100, from Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future (2012).

Locally, bathymetry and tidal action can create sea levels that are very different between two nearby areas of coastline, as is seen in the northern and southern parts of the San Francisco Bay. This website from San Francisco Baykeeper shows historical data with several projections for the region. Note how El Nino years show a marked increase in sea level rise.

CalAdapt lets you explore maps of potential inundation areas for different degrees of sea-level rise along the coast of California and the San Francisco Bay.

Projecting the Future

Regional projections for California, Oregon, and Washington show a sharp distinction at Cape Mendocino in northern California. South of that point, sea-level rise is expected to be very close to global projections. However, projections are lower north of Cape Mendocino because the land is being pushed upward as the ocean plate moves under the continental plate. An earthquake magnitude 8 or larger, which occurs in the region every few hundred to 1,000 years, would cause the land to drop and sea level to suddenly rise. Storms can change sea level dramatically on a local scale, and large waves from storms combined with the interaction between runoff and high tides are a concern as some research predicts increasingly intense and more frequent storm events.

There are four reports commonly referenced that provide estimates of future projected sea level rise for California. These are:

You can easily see what these reports predict for California's coastline by using this Sea Level Rise Projection tool from the OCOF project.

In their publication Climate change projections of sea level extremes along the California coast, Cayan et al. 2008 combine information on predicted tides, weather forcing, and El Nino-related variability to estimate the height of extreme events on coastal California up to the year 2100.

Impacts of Rising Seas and Tides

A number of studies have tried to evaluate the potential impacts of sea level rise on California. The National Research Council released a report and educational video in 2012 explaining the factors that cause regional variation in sea level rise along the west coast of the United States, including climate patterns such as the El NiƱo, effects from the melting of ice sheets, and geologic processes, such as plate tectonics. For coastal areas. Revell et al. 2011 use coastal geology to predict possible erosion distances, and Heberger et al. 2011 evaluate the potential economic and population impacts of coastal flooding. The Heberger et al. study also makes available their GIS datasets. Hanak and Moreno 2012 describe California coastal vulnerabilities and management options, including coastal armoring, beach nourishment, and living coastline management practices.

The potential impacts of sea level rise in San Francisco Bay and the Delta region have also attracted considerable attention. Knowles 2009 using a hydrodynamic model of the San Francisco estuary has investigated potential inundation areas for the Bay and Delta out to the year 2100. Cloern et al. 2011 give projections for a set of indicators for the region under climate change, including water and air temperature, salinity, sediment loading, and extreme water levels. This work has emerged from an hydrodynamic modeling effort at the USGS called CASCaDE. The tidal marshes in San Francisco Bay are another focus of interest. PRBO has developed an interactive map exploring the impact of sea level rise on tidal marsh habitat and bird distributions, and Stralberg et al. 2011 evaluate the potential for natural marsh sustainability under a high sea level rise scenario.

Learn More

These short videos explain sea level rise, why it is happening, how fast it is happening, the various projections of the future, and the types of impacts it is expected to have along the west coast and San Francisco Bay:
Presentation given in November, 2012 by Tom Suchanek, Climate Change Coordinator, Western Ecological Research Center, USGS.
Educational video from the The National Research Council.

Related resources:
Search the Commons: Sea level rise
Resources for Evaluating the Impacts of Sea Level Rise
Sea Level Rise Analysis Tools

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