Much of the Snohomish and Stillaguamish river deltas could be awash in water and the coastline along western Snohomish County could shift inland slightly by the end of this century, according to a new University of Washington study.

Climate change modeling by scientists at the university suggests the sea level in Puget Sound could rise by 4 feet by the year 2100, according to the study released on Thursday.

The scientists put together a “worst case” model to help emergency responders and planners prepare for the potential impacts of climate change. A more moderate — and likely — scenario suggests sea levels will rise by about 2 feet by 2100.

“Some people may want a worse case scenario for planning,” said Philip Mote, a UW research scientist. “This is the worst-case scenario. We can’t rule out higher rates of sea-level rise, but given what we know now, they seem improbable.”

Local emergency planners intend to review the new projections, especially along the Snohomish River, which is affected by the regular ebb and flow of tides all the way up to Snohomish.

“We’ll take a look at it and see if we can incorporate it into our own decision making process,” said Steve Thomsen, the county’s public works director.

County officials will pass their findings along to the diking districts that manage levees along the Snohomish River. There’s a possibility those levees would have to be raised, Thomsen said.

Local emergency responders say they worry that higher sea levels could be a problem when flooding occurs at high tide.

Snohomish County officials this year plan to figure out what they need to do to prepare for the effects of climate change.

“The executive and the County Council set aside some money for a climate change vulnerability assessment,” said Christopher Schwarzen, spokesman for County Executive Aaron Reardon. “We would use the UW report as one of our baselines of information to then go out and do this assessment.”

The report was put together by the UW’s climate impact group and the state Department of Ecology.

Looking at the near future — 2050 — the report suggests that sea levels could climb between 6 and 22 inches by 2050, and between 14 and 50 inches by 2100.

Local leaders will have to sort out finer details of the effects of a rising sea, said Sascha Petersen, a costal research scientist at the UW Pacific Northwest Climate Impacts Group.

In Snohomish County, they might want to estimate how the shoreline would recede, whose property would be inundated and how much damage would be caused — and what it would cost.

The study found that “the middle of the road” climate change model, which is used by most around the world, suggests that sea levels will rise 23 inches by 2100, Mote said.

The “most likely scenario” doesn’t account for the possibility of things going as badly as possible, he said.

The UW scientists got to their “50 inch” number by assuming that nothing is done to reduce greenhouse gas emissions, which are blamed for trapping heat in the atmosphere. That, climate experts project, is contributing to a fast rise in global temperatures.

The researchers also added in other factors that a commonly used international climate model left out, such as the continued melting of glaciers in Greenland and Antarctica and the geologic uplift of plate tectonics displacing ocean water, Mote said.

And it’s the worst-case scenario that emergency planners worry about.

“If you have a high-value project and a low risk tolerance, then you want to plan for the worst-case scenario,” he said.

Reporter Lukas Velush: 425-339-3449.