Geologic records show an average increase of about 2 inches every 100 years from the most recent ice age until the beginning of the past century.
Then, from 1900 to 2000, tide gauges around the world showed sea level rise accelerating to 4 to 8 inches.
At the Golden Gate Bridge, the Pacific Ocean crept 7 inches higher during the century, the result of continuing glacial melting and thermal expansion of the oceans.
Although the rise in sea level has tapered off somewhat since the 1980s, models suggest sea level rise of 4 to 36 inches by 2100.
Those figures could change when a major report from an international team of climate scientists is released next week. On Thursday, Reuters reported that the range was likely to narrow in the new report.
No one knows how far or how fast that will happen. The change is likely to be gradual and could be modest, especially if emissions are curtailed. But scientists are still deeply concerned because of the possibility of a catastrophic surprise, such as huge ice sheets sliding into the oceans.
In a recent study published in the journal Science, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany wrote that since 1990, global sea level rise has followed the upper limits of what climate modelers predicted.
Rahmstorf argues that those computer-run models may underestimate the speed at which the sea level will rise because they do not adequately take into account some of the complex mechanisms affecting glaciers.
State officials are taking notice.
In one of the first efforts of its kind in California, state officials in the fall began planning to address the threats that rising seas pose to the Bay Area. One of their first steps was to compile maps that show what would happen if the sea level rose 3 feet -- the upper limit for what might occur by 2100, according to climate models used by the state.
The maps show a dramatic level of inundation: Portions of Richmond, and virtually all of San Francisco and Oakland international airports, for example, would be under water.
"There are some areas that are extremely vulnerable," said Leslie Lacko, a coastal planner for the San Francisco Bay Conservation and Development Commission.
Threat to infrastructure
According to a study done in 1990, a 3-foot rise in sea level would threaten $48 billion in real estate, roads and pipes around San Francisco Bay.
"We have a lot of infrastructure that is very vulnerable to a little bit of sea level rise," said the study's author, Peter Gleick, president of the Pacific Institute in Oakland.
At risk are airports, highways, buildings and other key public works projects such as the East Bay Municipal Utility District sewer plant in Oakland and the railroad tracks near Benicia.
"They're all at sea level. We're going to have to spend a lot of money to protect them," Gleick said.
In addition to the flooding in low-lying areas caused by rising seas, storms would worsen, especially those that come on high tides.
Take, for example, a powerful storm in February 1998 that struck during a high tide that was about 2 feet higher than normal because of natural anomalies.
Waves splashed over the San Francisco Embarcadero, as much as 4 feet of water washed over U.S. Highway 101 in Marin County and hundreds of people fled their homes around the Bay, according to the U.S. Geological Survey, which has examined how rising sea level from El Niño "wreaks havoc" on the Bay Area.
The storm occurred during a massive El Niño event, which was not linked to global warming, but it dramatically illustrated how higher sea level and high tides can combine to intensify the effects of storms.
"If we have that same El Niño, and that same high tide in 20 years, the damage would be much more extreme," said Amy Luers, the California climate manager for the advocacy group Union of Concerned Scientists.
"We have developed our society to manage our existing weather and existing extremes," Luers added. "What global warming is doing is making those more challenging."
Fires, severe weather and floods resulting from climate change are also getting attention from the insurance industry.
"This is a growing issue. There is growing awareness," said Sam Sorich, president of the Association of California Insurance Companies, whose 300 member companies hold about 40 percent of the homeowners insurance policies in the state.
Insurance rates are going up around the country in part because of natural hazards that could be exacerbated by warming, according to Even Mills, a staff scientist at Lawrence Berkeley Laboratory who studies climate change's effects on insurance.
And, he added, as sea levels continue to rise, more homeowners will want flood insurance.
"The demand will go up, I am sure," he said.
So far, however, California faces fewer problems with coastal flooding and storm surge than states along the Gulf Coast and the Atlantic, where, for example, Allstate Insurance Co. recently decided it would no longer issue new homeowner policies in some states because of the increased threat of hurricanes.
The greater threat of hurricanes is partly because ocean conditions are, naturally, in a phase of greater hurricane frequency. But climate scientists say warmer oceans will increase the intensity of hurricanes.
"I haven't seen any actions that are conspicuous (among insurance companies in California) like we've seen in the Gulf states and the East Coast," Mills said.
'Alarming, but it's slow'
Teams of scientists and modelers anticipate a global rise in sea level ranging from 4 to 36 inches during the next 100 years, depending on the climate's sensitivity to greenhouse gases and whether emissions continue to increase, level off or decline in the coming decades.
Even if sea level rises 12 to 24 inches, it would amount to a threat.
"It's alarming, but it's slow," said John Andrew, chief of special planning for the state Department of Water Resources.
An 18-inch rise in sea level would pose massive problems for the state's water delivery system. With a higher sea level, salt water from the ocean would reach deeper into the Delta and could intrude into drinking water intakes that pump fresh water to 23 million Californians, including 500,000 in Contra Costa County.
"A foot and a half would be huge," Andrew said.
Even a 12-inch rise in sea level would have a big effect in the Delta, where peak high tides that come once a century would instead come once a decade. Many of the levees in the Delta, which are designed for 100-year events, would be seriously compromised and could crumble or break under the pressure.
Compounding the risk in the Delta, big storms often bring heightened seas and high rivers. The Delta could face onslaughts of water from both sides.
A study published in 2005 estimated a two-in-three chance of catastrophic levee failures in the Delta by 2050, partly because of added pressure on levees from a rise in sea level.
The paper, by UC Davis geologist Jeffrey Mount and UC Berkeley professor Robert Twiss, said that sea level rise may be a bigger problem for Delta levees than was previously understood. Not only does a higher sea level increase the pressure on the levees, but water rising higher on the levees exposes more of the levee to that pressure and exponentially increases the likelihood of failure.
Rising seas, eroding coast
"Sea level rise will increase the extent and frequency of flooding in the coastal zone," said Mark Johnsson, a geologist at the California Coastal Commission.
Large developed areas in the Orange County city of Huntington Beach, for example, are below sea level and protected by aging public works levees that, in Johnsson's estimation, are "not great."
Storms riding on higher seas also will increase the rate of erosion on beaches and cliffs up and down the coast, threatening houses from San Diego to the Bay Area and raising the cost of sand-replenishment projects on Southern California beaches.
In some places such as Monterey and La Jolla, the bluffs are sturdy, Johnsson said. But other real estate with prime ocean views is more vulnerable to erosion, including Solana Beach in San Diego County, Sand City and Marina in Monterey County and Pacifica just south of San Francisco.
Just how much more vulnerable cliff-side houses in those communities will be with rising seas is uncertain, but "it's going to increase the rate of coastal bluff erosion," he said.
Like the San Francisco Bay Conservation and Development Commission, the state Coastal Commission is in the early stages of developing and considering how California might deal with the consequences of a rise in sea level along the rest of the coast.
The commission could require bluff-top houses to be set farther back, for example.
The issue could pit regulators motivated to protect the long-term health of beaches and wetlands against property owners' desires for stunning vistas.
Property owners often would rather build a sea wall to prevent erosion than give up their views.
But coastal regulators say those walls jeopardize the future of beaches. Still, they have little legal authority to reject applications from property owners who want to build them, Johnsson said.
The problem with sea walls, Johnsson said, is that they disrupt natural processes that ensure that beaches are maintained, even as the bluffs erode. Sea walls also increase erosion at their edges.
Without a sea wall, a rising sea will swallow a beach, but at the same time its waves cut away the bluffs to create new beaches
With a sea wall, however, the back of the beach is fixed in place. There is nowhere for new beaches or wetlands to become established. Eventually, the rising sea will come up against the wall, typically a concrete or steel structure meant to harden the cliffs to erosion.
"You lose the beach," Johnsson said. "Sea walls protect property, but they inevitably destroy beaches in the long run in this environment of rising sea level."
Lacko, of the Bay Conservation and Development Commission, had a similar criticism for sea walls that might be built in the Bay.
Although they are an obvious solution to the flood threat around the Bay, sea walls would destroy wetlands and turn a living bay into nothing more than a "giant reflecting pool," Lacko said.
Susanne Moser, a geographer at the National Center for Atmospheric Research in Boulder, Colo., put it this way: "Do we really want an America behind sea walls?"
Betsy Mason contributed to this story. Mike Taugher covers natural resources. Reach him at 925-943-8257 or firstname.lastname@example.org.