Sea levels in Puget Sound are likely to rise half a foot by midcentury, according to a new analysis that not only factors in global warming...

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Sea levels in Puget Sound are likely to rise half a foot by midcentury, according to a new analysis that not only factors in global warming, but also local weather patterns and geology.

The latter explains why sea levels on the state’s northwestern coast are projected to stay the same over the next several decades despite a warming climate: The Olympic Peninsula is being lifted as tectonic plates collide below the sea.

But under some of the scenarios considered in the study, climate change could intensify the winter winds that push water toward Washington’s shorelines, aggravating sea-level rise.

“We tried to look at the factors that are important locally,” said Philip Mote, a research scientist at the University of Washington’s Climate Impacts Group and one of the study’s authors. “This is the first time this has been done for our region.”

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Under the study’s worst-case scenario, which Mote stresses is highly unlikely, sea levels in Puget Sound could rise more than 4 feet by 2100. That would require a confluence of bad news: runaway greenhouse gas emissions; accelerated melting of glaciers on Greenland and Antarctica; and an exaggerated climate response to rising levels of carbon dioxide in the atmosphere.

“It’s kind of like having your house destroyed by a flood, your car smashed by falling trees, and you forgot to mail your insurance check,” Mote said. “The worst-case scenario almost never happens, but it’s good to know what it is for planning purposes.”

More probable is a sea-level rise of 14 inches in Puget Sound by the start of the next century, the study concludes.

But that’s enough to have profound impacts, said co-author Hugh Shipman, a coastal geologist for the Washington Department of Ecology.

Even a rise of 6 inches would raise the odds of flooding with every major storm. Higher water would accelerate erosion, hamper stormwater drainage systems, and damage coastal structures like piers and docks, he said.

“There’s a lot of homes in Puget Sound built on sand spits that will see a lot more erosion, a lot more storm damage,” Shipman said.

The new analysis uses as its starting point the most recent report from the Intergovernmental Panel on Climate Change (IPCC), a U.N.-sponsored group made up of the world’s top climate scientists and policy experts. The group declared last year that global warming is “unequivocal” and that there’s little doubt carbon dioxide and other emissions from cars and industries are largely to blame.

Global sea levels are expected to rise between 7 inches and 2 feet by the end of the century, depending largely on the rate at which greenhouse gases are pumped into the atmosphere. Sea levels could climb even more if the vast amounts of ice on Greenland and Antarctica melt more rapidly than expected.

Another major contributor to sea-level rise is the expansion of water as it warms.

The Washington scientists used the same range of emissions scenarios and computer models as the international group but added local details about geologic movement, wind patterns and geography.

“We’re trying to bring all this data together to provide a comprehensive picture of what our coastlines might expect to see … and help local governments and private homeowners make decisions,” said co-author Spencer Reeder, the ecology department’s lead policy strategist for climate change.

Earlier projections had hinted that sea-level rise might be more severe around Olympia, as a result of the ground sinking. But more recent GPS measurements don’t show any dramatic slumping in South Puget Sound, Reeder said.

The central and southern portions of Washington’s Pacific Coast are being uplifted slightly. The new study projects sea levels there to rise about 5 inches by 2050 and nearly a foot by 2100.

There’s still too much uncertainty to provide forecasts tailored to specific communities, Reeder said.

But even if all greenhouse-gas emissions were shut off tomorrow, the IPCC points out that the oceans will continue warming and expanding for decades.

“A rise in sea level is pretty much locked in,” Reeder said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com