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From changes in stream flows to acidifying oceans and widespread forest die-offs, the Pacific Northwest is already experiencing signs of a changing climate, according to the most comprehensive analysis yet of impacts in the United States.

The third National Climate Assessment, released Tuesday, warns that no part of the country is immune, and that the effects of climate change will become increasingly disruptive in the coming decades.

“Climate change, once considered an issue for the distant future, has moved firmly into the present,” the report says. “Corn producers in Iowa, oyster growers in Washington state and maple syrup producers in Vermont are all observing climate-related changes that are outside of our recent experience.”

The national assessment is mandated by Congress and published roughly every four years. But this marks the first time it has zeroed in on local impacts, with sections on nine geographic regions, including the Northwest.

Nationwide, average temperatures have increased by 1.3 to 1.9 degrees Fahrenheit since 1970, with another 2- to 4-degree increase expected before the end of the century. The past decade was the hottest on record in the U.S., and 2012 was the hottest year.

Across much of the country, the warming has led to more intense rainstorms, fewer cold snaps, prolonged allergy seasons, and shifts in bird migrations and the types of plants that grow in gardens, says the report, which was written by more than 250 experts and reviewed by the National Academy of Science.

Winter storms have grown in frequency and intensity and have shifted northward since the 1950s, it says.

“Taken together, the evidence tells an unambiguous story: the planet is warming, and over the last half century, this warming has been driven primarily by human activity,” the report says.

Release of the report gives President Obama an opportunity to ground his campaign against climate change in science and numbers, endeavoring to blunt the arguments of those who question the idea and human contributions to such changes. This summer, the administration plans to propose regulations restricting emissions from existing coal-fired power plants.

Some fossil-energy groups, conservative think tanks and Republican senators immediately assailed the report as “alarmist.” Senate Republican leader Mitch McConnell of Kentucky said Obama was likely to “use the platform to renew his call for a national energy tax. And I’m sure he’ll get loud cheers from liberal elites — from the kind of people who leave a giant carbon footprint and then lecture everybody else about low-flow toilets.”

The state-by-state, region-by-region impacts could help move the climate-change debate forward, White House officials said in a conference call. Calling it “actionable science,” White House adviser John Podesta said the report would give people information on observed climate changes in their parts of the nation.

A March Gallup Poll found most Americans are convinced the climate is changing, but only about one in three considers those changes a serious threat to their way of life.

Unlike many parts of the country, the effects of climate change in the Pacific Northwest have been well studied for nearly two decades.

But the new report reflects additional data and analysis, said Amy Snover, director of the Climate Impacts Group at the University of Washington and a lead author of the regional section.

“In this case, we really tried to look at … the major changes that will have the most widespread consequences for people, the economy, the environment and our infrastructure,” she said.

The three issues that rose to the top were shifting precipitation, effects on the region’s forests, and coastal impacts — from sea-level rise to the increasing corrosivity of ocean water caused by dissolved carbon dioxide.

Sea Change

Ocean acidification threatens to scramble marine life on a scale almost too big to fathom. Read and watch the Seattle Times special report "Sea Change: The Pacific's Perilous Turn." Read more.

Since 1950, average spring snowpack in the Cascades has declined about 20 percent, said co-author Philip Mote, director of the Oregon Climate Change Research Institute.

By 2050, the annual snowmelt is expected to occur three to four weeks earlier, while summer stream flows in much of the region are forecast to drop significantly.

That’s going to cause problems for salmon, power supplies and farmers, Mote pointed out. “All of the demands come to a head in late summer, when the water supply is lowest.”

While average, global sea levels have risen about 8 inches over the past century, the Northwest has been buffered, thanks to its geologic setting. The offshore collision of tectonic plates has been pushing the coastline up for the past 314 years — since the last Cascadia megaquake and tsunami.

When the next quake strikes, the coast will drop as much as 6 feet in an instant. But even if the quake doesn’t occur before the end of this century, computer models predict sea levels along the coast will still rise about 2 feet, due to melting glaciers and ice fields, and the expansion of seawater as it gets warmer.

Portions of Seattle’s waterfront will be underwater at high tide if sea levels rise by 13 inches — and some models project increases of as much as 50 inches.

The forests for which the Northwest is famed are also in for big changes.

Warmer and drier conditions are already blamed for insect infestations and an increase in the number and ferocity of wildfires across the West since the 1970s.

By 2080, the amount of forestland that burns every year in the Northwest is expected to quadruple, to 2 million acres.

“We already have seen tremendous die-offs from insect damage and fire, and over time it’s just going to increase and lead to really dramatic changes in our forest landscape,” Snover said.

But she and her colleagues found less to worry about when it comes to Northwest agriculture. In some cases, climate change will lead to longer growing seasons, Mote said. And even though water might be in shorter supply, farmers should be able to adapt and plan for the changes.

Since the first national assessment, governments, businesses and tribes in the Northwest have taken the threat seriously, Snover said.

Seattle is factoring climate change into its estimates of electrical supply. Bellingham has taken sea-level rise into account in plans for port redevelopment.

Mote, a veteran of many climate studies, was at the White House on Tuesday.

“In some ways, this is stuff that we’ve known for 20 years,” he said. “We still have a lot of opportunities to both reduce emissions and reduce impacts, but the clock is ticking.”

Material from The Associated Press and McClatchy news services is included in this report.

Sandi Doughton at: 206-464-2491 or