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 to 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 6 inches expected for California by 2030 seem minor, the report estimated that sea levels there will be an average of 3 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.
Seaside cliffs will be cut back about 30 yards over the next 100 years, and sand dunes will be driven back even more, said Robert A. Dalrymple, a professor of civil engineering at Johns Hopkins University and chairman of the group that wrote the report. After about 50 years, coastal wetlands will eventually be overwhelmed without new sources of sand or room to move inland.
That could be problematic in Northern California, though, since dams hold back about a third of the sand that once washed into the sea there from the Klamath River, the report noted.
Northern California, Oregon and Washington can expect a less dramatic increase _ about 4 inches by 2030 and 2 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.
Oregon has the advantage of tough basalt formations on much of the coast, but long stretches of Washington are low-lying sandy beaches.
“Anything close to the seas is vulnerable,” Dalrymple said.
Sea levels rise for two reasons due to global warming. Warmer water expands, which can cause as many as 23 inches of sea level to rise by 2100, according to the Nobel Prize-winning Intergovernmental Panel on Climate Change. Warmer temperatures also cause ice sheets in Greenland and west Antarctica to melt, adding another foot or more to sea levels by 2100, scientists said.
Those estimates, however, were for the planet as a whole. Some places will see higher seas, while others will get less dramatic increases.
Globally, sea levels have risen about 8 inches over the last century, but the rate is increasing significantly, said Gary Griggs, one of the scientists assembled to produce the report.
The most immediate threat over the next few decades will come from periodic ocean-warming El Nino events, said Griggs, who also is the director of the Institute for Marine Sciences at the University of California at Santa Cruz.
“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.
Storms during El Ninos in 2009 and 2010 ate away 40 feet from cliffs on the ocean side of San Francisco, leaving paved parking lots hanging over the void, said Ben Grant of the San Francisco Planning and Urban Research Association. The organization is promoting a strategy that would allow the ocean to have its way in some areas, shield others with rocks, and dump new sand to maintain beaches.
The report noted that some computer models suggest storms will be stronger as global warming progresses. But Dalrymple said there was no clear consensus in scientific literature, and data from buoys don’t go back far enough to conclude that wave sizes were continuing to grow in the Northwest.
If a major earthquake occurs beneath the Pacific Ocean off Oregon and Washington, in what is known as the Cascadia subduction zone, that would cause the land to drop, allowing sea level to rise another 3 to 6 feet immediately, the report said. Such a major temblor occurred 300 years ago, but becomes more likely as time passes.
In Long Beach, Wash., a town on a sandy spit at the mouth of the Columbia River, residents are more concerned about the prospects of a tsunami from such an earthquake, Mayor Bob Andrew said.
A dune separating the town from the ocean has been growing in recent years, making sea level rise less of a concern, he said.
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. In Oregon, Greg Sieglitz, a monitoring program manager at the state Watershed Enhancement Board, said they helped sponsor the study to help them evaluate land purchases of coastal wetlands.
The report differed slightly from projections currently used by California officials, with the newer study estimating lower sea levels in 2100. The study summarized published projections and updated it with an analysis of tidal gauge readings and satellite measurements along specific sites on the West Coast.
Susan Hansch, chief deputy director of the California Coastal Commission, said that “a lot of the data we had before was worldwide data or has the caveat, `Can’t be used for planning purposes.”
“It all comes down to the better data you have, the better decisions you can make,” she said.