* 480,000 people would live in expanded flood zone

* $100 billion in property at risk from rising sea

* Erosion expected to take additional toll on coast

* New sea walls, levees could minimize threat

By Steve Gorman

LOS ANGELES, March 11 (Reuters) - California's farms and cities may be left high and dry by prolonged drought, but climate change is expected to leave much of the state's fabled shoreline awash in excess seawater before too long.

Nearly 500,000 people and $100 billion worth of property in coastal California are at risk of severe flooding from rising sea levels this century unless new safeguards are put in place, researchers reported on Wednesday.

With global warming expected to lift ocean levels along the California shore by 1 to 1.4 meters (1 to 1.4 yards) before the year 2100, large tracts of the picturesque Pacific coast also will be lost to accelerated erosion, their study found.

The report suggests that the heightened flood risk could be minimized by investing about $14 billion in a system of newly built or upgraded sea walls, levees and offshore breakwaters to reinforce some 1,100 miles (1,770 kilometers) of coast.

But such coastal "armoring" structures come with their own cost, the loss of beaches.

The state-funded report from the Pacific Institute, an environmental think tank, marks the first sweeping assessment of how California's entire 2,000-mile (3,200 km) shoreline, including San Francisco Bay, and the millions who live along it may be affected by higher sea levels.

The extent of elevated oceans assumed in the study were projected by California state researchers working from medium- to high-greenhouse gas emissions scenarios of the U.N.'s Intergovernmental Panel of Climate Change.

But Heather Cooley, co-author of the report, said it does not reflect the worst-case rise in sea levels that might occur. Like most climate models, the study excludes the potential effects of melting ice sheets in Greenland and Antarctica because of uncertainty over how they will play out, she said.

Instead, the higher sea levels anticipated in the study are attributed mostly to thermal expansion -- the phenomenon by which water increases in volume as it warms.

AIRPORTS AND BIG SUR IN HARM'S WAY

Likely flood casualties include both the San Francisco and Oakland international airports, as well as 3,500 miles (5,630 km) of roads and 280 miles (450 km) of railways, 140 schools, 30 power plants and 29 wastewater treatment facilities, the study found.

But some of the aesthetic beauty and recreational values associated with the California shore, one of the state's prime natural assets, are at risk, including the famed central coastal cliffs of Big Sur. In all, 41 square miles (66 km) of coast will be lost to erosion, according to the study.

"Changes to California coasts are inevitable," Cooley said. "We need to evaluate and assess what our values are and which qualities of the coast we want to maintain."

Flood damage envisioned by the study would result from storm surges occurring with greater frequency and intensity in low-lying areas once a safe distance from the shore. Areas that already lie within an existing coastal flood plain would face even greater risk.

About 260,000 people live in flood-prone areas around San Francisco Bay and other low-lying coastal communities up and down the state. That number would grow to 480,000 if sea levels were to rise 1.4 meters (1.4 yards) without any mitigating actions being taken, the report found.

Populations in San Mateo County, south of San Francisco, including the Silicon Valley towns of Palo Alto, Mountain View and Sunnyvale, are considered especially vulnerable, along with parts of Orange County south of Los Angeles.

The study estimates that nearly $100 billion worth of property, in year 2000 dollars, would be put in harm's way from flooding -- two-thirds of it concentrated around San Francisco Bay. The majority of that is residential.

Poor individuals and those of racial or ethnic minorities account for a disproportionate number of residents at risk, even though many of the projected new flood zones lie in relatively affluent areas.

"A lot of people have wondered whether California will be the next (Hurricane) Katrina, and we do have a lot of those conditions in place," said study co-author Matthew Heberger, speaking of the hurricane that flooded New Orleans in 2005. (Editing by Philip Barbara)

AlertNet news is provided by