Oct 252013
 

Increased exposure to coastal flooding along the Los Angeles coastline due to sea level rise
Global warming is producing rising sea levels worldwide. But how rapidly is sea level increasing off southern California and how will it affect us? Global sea levels are projected to rise as much as nine inches by 2030, one and a half feet by 2050 and four and a half feet by 2100. Most of this rise is expected to result from the melting of the Greenland and Antarctic Ice Sheets, which store the equivalent of nearly 200 feet of sea level. The onset of deglaciation which began 20,000 years ago slowed to a stop 2000 years ago, then resumed at modern rates sometime between 1840 and 1920. Since 2006, that rate has accelerated.

So far, sea level in Southern California has risen at a slower pace than the global level. However local sea-level increases are expected to surpass the global mean rise, increasing by one foot in 20 years, two feet by 2050 and as much as five and a half feet by the end of the century.

According to a 2012 report by by the National Research Council, that’s because much of California is sinking from the effects of an ice sheet that has long since disappeared. During the last ice age, an ice sheet depressed northernmost Washington, creating uplift around it. Since the ice melted, the flexure in the continental plate began to slowly release, causing uplift in northernmost Washington and subsidence in the rest of Washington, Oregon and California.

For the coast north of Cape Mendocino, tectonics offset that subsidence. Ocean plates are descending below North America at the Cascadia Subduction Zone, causing regional uplift along much of the Washington, Oregon, and northernmost California coast. Global Positioning System (GPS) measurements show this area is rising about 1.5–3.0 mm per year (or 5.9–11.8 inches each century).

However, south of Cape Mendocino in the San Andreas Fault Zone, tectonic plates move horizontally, creating little vertical motion. GPS measurements (which also measure compaction of wetland sediments, and/or fluid withdrawal or recharge), indicate this area is sinking at an average rate of about 1 mm per year (or 3.9 inches each century), though GPS-measured rates vary widely across locations. Records from 12 West Coast tide gauges concur ─ most gauges north of Cape Mendocino show that relative sea level has been falling over the past 6–10 decades, and most of the gauges to the south show that relative sea level has been rising.

Regional sea-level rise also varies due to local factors affecting the dynamic height of the sea, such as wind, air pressure, and surface-heating influence of climate patterns such as the El Niño/La Niña–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These result in changes in ocean circulation on seasonal and multidecadal timescales that involve redistributing mass by altering the temperature and salinity of the upper ocean.

These climate patterns may also explain why California has not yet experienced the same sea-level rise as measured around the globe. Satellite altimetry, tide gauge, and ocean temperature measurements show a long-term increase in sea level off the U.S. west coast. According to Dr. Peter Bromirski’s 2011 study, trends along the West Coast ─ estimated from tide gauge measurements and confirmed by satellite altimetry since 1992 ─ signify relatively flat sea levels since about 1980. The study points to the Pacific Decadal Oscillation (PDO), which alternates between positive and negative phases. Presently in the positive phase, wind-driven ocean currents draw water away from the west coast and pull colder, denser water up from the depths (upwelling), depressing the sea level for the eastern Pacific. If the PDO shifts, as Bromirski suggests it may be doing, the associated wind patterns would shift, reducing upwelling, which could accelerate sea-level rise back to global rates or beyond.

Satellite altimetry records assessed by the Intergovernmental Panel on Climate Change (IPCC) also showed that sea level fell about 0–6 mm each year from 1993 to 2003 along the west coast. The IPCC suggested that the largest fraction of this short-term variation was caused by ENSO. But note that ENSO and the PDO do not act independently of each other. ENSO may play a significant role in decadal and longer sea-level variability. While ENSO can influence the PDO, the PDO can modulate tropical Pacific circulation as well as ENSO.

Higher baseline sea levels could add to storm levels, making extremes more common, leading to more coastal flooding and erosion, inundation, wetland loss, structural damage, and salinity intrusion into coastal aquifers. Add a large El Niño event, and coastal sea levels could rise an additional four to 12 inches for several winter months.

The largest waves have been getting higher and winds have been getting stronger in the northeastern Pacific, according to several observational studies. But wave and wind records go back only about 35 years, and to some extent reflect large El Niños and PDO fluctuations. If proven to be a long-term trend, the frequency and magnitude of extremely high coastal wave events will increase. But if not, sea-level rise will still magnify the impact of storm surges and high waves on the coast. What is currently defined as a 100-year flood today will occur much more frequently as sea level rises and the number of people exposed to risks from 100-year floods will increase substantially. We do have evidence from tidal gauges that tidal ranges are trending up in Southern California, mostly in La Jolla. Coincidentally, the occurrence of high sea level storm extremes has increased at La Jolla has increased 30-fold since 1933. Although Southern California so far has been spared a significant increase in sea level, expectations are that sea level will rise here faster in the coming decades.

Originally published in Catalina Marine Society’s OceanBights, p. 12

References

Peter D. Bromirski, Arthur J. Miller, Reinhard E. Flick, and Guillermo Auad, Dynamical suppression of sea level rise along the Pacific coast of North America: Indications for imminent acceleration

Committee on Sea Level Rise in California, Oregon, and Washington;Board on Earth Sciences and Resources; Ocean Studies Board; Division on Earth and Life Studies; National Research Council, Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future Committee

Images

1. Increased exposure to coastal flooding along the Los Angeles coastline due to sea level rise: Joe Abraham, adapted from map produced by the The Pacific Institute

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