SAN FRANCISCO - Driven by global warming, the ocean is expected to rise nearly 5 feet along California's coastline by the end of the century, hitting San Francisco Bay the hardest of all, according to a state study released Wednesday.

Runways along the bay at SFO could be under water by the end of this century. (Michael Macor / The Chronicle) Nearly half a million people and $100 billion in property, two-thirds of it concentrated around the bay, are at risk of major flooding, researchers found in the most comprehensive study to date of how climate change will alter the state's coastal areas.

Rising seas, storms and extreme high tides are expected to send saltwater into low-lying areas, flooding freeways, the Oakland and San Francisco airports, hospitals, power plants, schools and sewage plants. Thousands of structures at risk are the homes of low- and middle-income people, the study said.

Vast wetlands that nourish fish and birds and act as a buffer against flooding will be inundated and could turn into dead pools. Constructing seawalls and levees, if needed, could cost $14 billion plus an annual maintenance cost of $1.4 billion, the study said.

The study shows a greater sea-level rise for California than previous studies because it takes into account recent changes in glaciers and ice sheets in Antarctica and Greenland.

Worldwide forecasts

Scientists worldwide forecast that sea levels will rise for centuries even if greenhouse gas emissions are halted immediately, and California cities and counties must learn to deal with that inevitability just as they plan for earthquakes, the study advises.

Regional planners are recommending that some new construction be halted, other properties protected and still others abandoned.

The study was conducted by the internationally known Pacific Institute, a nonprofit research group in Oakland, and was paid for by the California Energy Commission, Caltrans and the state Ocean Protection Council.

With California leading the nation in regulating greenhouse gas emissions, Gov. Arnold Schwarzenegger in 2005 ordered state agencies to form a Climate Action Team to research and plan for global warming. Three dozen studies are expected this year, on air quality and health, frequency of wildfires, the use of energy and fresh water supplies.

"No other state has done this kind of assessment of coastal risk," said Peter Gleick, president and founder of the Pacific Institute and a leading water expert. The new assessment, he said, puts the state "far ahead in our ability to both identify possible impacts and implement effective policies to prevent them."

Although large sections of the Pacific Coast are not vulnerable to flooding, sea-level rise is expected to accelerate erosion, resulting in a loss of 41 square miles of the coast and affecting 14,000 people, the study said.

Flooding projections

Researchers at the U.S. Geological Survey prepared maps of San Francisco Bay showing projected inundation, though they don't include the Sacramento-San Joaquin River Delta. Calculations for inundation don't take into account existing seawalls and levees along the Peninsula and at Oakland International Airport.

Large portions of the Bay Area are at risk because European settlers in the 1800s filled shoreline marshes to build towns and cities.

Will Travis, executive director of the Bay Conservation and Development Commission, said the Pacific Institute study struck him because two-thirds of the projected property damage was in low-lying areas around San Francisco Bay. Cities and counties haven't planned for the rise, he said, and his agency is trying to build awareness.

"We as a region have to get out in front of the state and nation in dealing with the problem. The study shows that low- and moderate-income people will be dealing with it. We have the equivalent of New Orleans' Lower Ninth Ward."

Lessons from New Orleans after Hurricane Katrina are not to build below sea level, he said. But parts of northern Silicon Valley - where pumping groundwater in past decades has caused land to subside - are already below sea level, he said, leaving Google, Sun, Intel and other large company complexes vulnerable to inundation.

Earthquake lessons

Just as Californians learned about seismic safety in response to earthquakes, "we have to learn to build in areas that will someday be below sea level," he said. It's particularly difficult, he said, because there is "no certainty which areas would be below sea level." His agency is co-sponsoring an international design competition to come up with designs for sea-level rise.

As the atmosphere and oceans warm, ice sheets and glaciers melt, swelling the volume of oceans. Oceans already have started to rise. Over the past century, San Francisco waterfront tidal gauges show a rise of 8 inches.

The new projection of a 4.6-foot, or 55-inch, rise is higher than the 23-inch estimate of the United Nations Intergovernmental Panel on Climate Change, the body that compiles findings of international scientists.

In its last calculations, the panel didn't include melt from the Greenland and Antarctic ice sheets, which have accelerated over the past decade. Since then, scientists have begun to forecast higher rises.

For the regional study, Pacific Institute scientists used the 4.6-foot sea-level rise based on forecasts by a Scripps Institution of Oceanography team led by oceanographer Daniel Cayan, which draws on sophisticated models, satellite sensors and a broad range of data.

Concentrations of greenhouse gases have been increasing in the atmosphere since the Industrial Revolution, and the bulk of climate scientists agree that the gases are trapping the sun's radiation emanating from Earth, warming the planet.

Infrastructure in danger along the bay, coast

Some of the infrastructure at risk along the 1,000-mile- long shore of San Francisco Bay and the 1,200-mile-long California coast, according to a new study on the rising sea level:

3,500 miles of roads and highways

330 hazardous waste sites, including several in Alameda, Santa Clara, San Mateo and Los Angeles counties

280 miles of railway

140 schools

34 police and fire stations

30 coastal power plants

29 sewage-treatment plants, including 22 on the bay and seven on the Pacific Coast

2 Bay Area airports:

San Francisco and Oakland international

To learn more: Read the study at links.sfgate.com/ZGJX.

Source: Pacific Institute