The West Coast will see an ocean several inches higher in coming decades, with most of California expected to get sea levels a half foot higher by 2030, according a report released Friday.
The study by the National Research Council gives planners their best look yet at how melting ice sheets and warming oceans associated with climate change will raise sea levels along the country's Pacific coast. It is generally consistent with earlier global projections, but takes a closer look at California, Oregon and Washington.
Although the six inches expected for California by 2030 seem minor, the report estimated that sea levels there will be three feet higher by 2100. About 72 percent of the state's coast is covered by sandy cliffs, and the rest include beaches, sand dunes, bays and estuaries.
"Rising seas increase the risk of coastal flooding, storm surge inundation, coastal erosion and shoreline retreat, and wetland loss," the report said. "The cities and infrastructure that line many coasts are already vulnerable to damage from storms, which is likely to increase as sea level continues to rise and inundate areas further inland."
Northern California, Oregon and Washington can expect a less dramatic increase — about four inches by 2030 and two feet by 2100 — because seismic activity is causing land to rise north of the San Andreas Fault, offsetting increasing sea levels, and drop south of it. The fault runs out to sea at Cape Mendocino.
The most immediate threat over the next few decades will come from periodic ocean-warming El Nino events, said Gary Griggs, director of the Institute for Marine Sciences at the University of California at Santa Cruz, who was one of the scientists assembled by the council to produce the report.
"During those events, sea level is elevated as much as a foot above normal and then we've got typically larger waves coming in with the high tides," particularly in the Northwest, he said.
If a major earthquake occurs beneath the Pacific Ocean off Oregon, in what is known as the Cascadia subduction zone, that could cause the land to drop, allowing sea level to rise another three feet, the report said. Such a major temblor occurred 300 years ago.
The report was commissioned by states and federal agencies looking for detailed information so they can plan for an accelerated rate of erosion along beaches, bluffs and sand dunes that are already crumbling into the sea.
"A lot of the data we had before was worldwide data or has the caveat, 'Can't be used for planning purposes,'" said Susan Hansch, chief deputy director of the California Coastal Commission. "It all comes down to the better data you have, the better decisions you can make."
Sea levels rise for two reasons due to global warming.
Warmer water expands, which can cause as many as 23 inches of sea level rise by 2100, according to the Nobel Prize winning Intergovernmental Panel on Climate Change.
Then, warmer temperatures cause ice sheets in Greenland and west Antarctica to melt slowly, adding another foot or more to sea levels by 2100, scientists said. But those estimates are for the planet as a whole. Some places will see higher seas and others will get less dramatic increases.
Globally, sea levels have risen about eight inches over the last century, but the rate has been increasing significantly, said Griggs.
The report summarized published projections, such as the IPCC report of 2007, and updated it with computer modeling, as well as an analysis of tidal gauge readings and satellite measurements along specific sites on the West Coast.