If global warming plays out according to some of the most dire predictions, welcome to Lake Napa.

A 20-foot rise in sea level, resulting from the melting of Greenland's ice cap, would turn the Napa River into a permanent arm of San Francisco Bay, enveloping half of downtown, all of Auto Row and much of Old Town.

Even a lesser three-foot increase in the world's oceans would wreak havoc, sabotaging the assumptions behind the half-finished $311 million flood control project which is supposed to protect the heart of the city in a major flood.

Predictions of rising sea levels are the stuff of nightmares. The ocean at the Golden Gate Bridge rose eight inches during the 20th century. International climate scientists most recently predicted a three-foot rise during the 21st century.

But if global warming takes hold, three feet would be just the beginning.

For perspective, The Register called on officials at the Napa County Flood Control District, whose job is to keep water out of downtown and the neighborhoods.

Heather Stanton, flood district manager, said she is well acquainted with the threat of global warming and predictions that low-lying land could be inundated by swollen oceans. "I've been watching the melting ice caps for years. I've found it frightening," she said.

"The polar bears are drowning. How much more evidence do you need it's here today," she said.

Some people need more. Critics of the most recent studies, and the media focus on the prospect of warming, say the threat is being exaggerated.

While experts debate the future of the planet, Stanton said her professional responsibilities are clear: finish the flood control project. Even if the oceans do rise, "this project will serve the community for a long time to come," she said.

A modest rise might not be a problem, she said. The flood project is designed to handle a so-called 100-year flood with a couple of feet to spare.

Even if worst disaster scenarios play out, the effects will be felt gradually, Stanton said. The flood project should protect homes and businesses for decades to come, she said.

Richard Thomasser, the district's watershed manager, sat in on the discussion of global warming. He teased Stanton about how her south Napa home would fare. "At 20 feet, Heather, you're beach front," he said.

In contrast, his Browns Valley home, elevation 190 feet, would be high and dry, he said.

Maybe engineering solutions -- higher levees or a barrier at the Golden Gate Bridge -- can save Napa, Thomasser said. "It's not going to happen overnight. I'd assume agencies like us would come together for a solution. The whole Bay Area would have to be protected," he said.

Global warming or no global warming, Michael DeSimoni, who owns two riverfront blocks south of Third Street, plans to start construction this summer of a mixed-use retail-office-condo development.

DeSimoni does not think his project is threatened by rising oceans. "It may be a hoax," he said of reports of manmade climate change. "I think Mother Nature is in charge. The rest is all an assumption."

Next to DeSimoni's property sits the historic Napa Mill, with a flood wall under construction that tops out at 18 feet above sea level. Owner Harry Price isn't panicking.

"If you do that too much, it stops you from making progress right now," Price said.

Price is convinced that global warming is real. As awareness grows, he hopes that governments, businesses and individuals will cut back on their production of carbon dioxide and other gases that trap heat in the atmosphere. The worst-case scenarios are not inevitable, he said.

Skip Keyser, a commercial Realtor, said businesses interested in renting in downtown have more immediate worries than long-term global warming. They want to know when the flood control project will be finished to protect against winter rains.

Keyser also believes that the waters will inevitably rise if humans don't change their ways. "Who do we have to blame? We have ourselves to blame," he said.

While polar melting would flood low-lying property the world over, Keyser noted that his personal abode near Yountville, elevation 78 feet, is safe for now.

While the threat of rising oceans is hypothetical for most people, Chris Malan and John Stephens of the Living Rivers Council have begun asking Napa officials to act now and stop commercial and residential development of Gasser Foundation property behind South Napa Marketplace, adjacent to the river.

"We should not be putting people in harm's way," Malan said. Why allow development on land that could be under water within decades, she said.

"A big cloud is looming on the horizon," said Stephens, who thinks a 20-foot rise in sea levels will happen sooner than most scientists are predicting. "It will inundate the most valuable property in the city of Napa and our infrastructure."

Joe Peatman, the executive director of the Gasser Foundation, thinks the Living Rivers Council is overreacting. "There is a lot of validity attached to global warming, but I don't think there is a lot of validity attached to 20 feet," he said.

As proof that Gasser treats the subject seriously, Peatman said, the foundation would be launching a Sustainable Napa Valley Initiative to promote energy conservation among nonprofit organizations. The goal is to reduce the production of the greenhouse gases that contribute to climate change, he said.

Juliana Inman, an architect and Napa City Council member, voted in favor of the Gasser master plan, which will include a 2,000-seat theater and up to 500 homes.

Global warming commands her attention, yet she isn't ready to pull the plug on continued construction along the river, Inman said. "Do you say no more building in Napa County unless it's above the 45-foot mark? I don't think we're ready to go there yet," she said.

"What if 30-foot water is coming? Where do we move Napa to?" Inman said. "We will have to make an orderly retreat upriver."

Unfortunately, there isn't scientific or political consensus about how to handle the threat of significant climate change and rising oceans, Inman said. Rather than react to alarmist "second-hand or third-hand" citizen testimony, government officials need more solid guidelines, she said.

As things now stand, "CEQA doesn't require us to take that deep a look into the future," she said of the California Environmental Quality Act which is used to evaluate new projects.

Keith Rogal, who plans to develop the 152-acre former Napa Pipe property with up to 4,000 housing units, said the site would be raised several feet during development, enough to handle a small rise in sea level.

Fundamentally, the issue is much bigger than what could happen in Napa, Rogal said. "Globally, we really can't afford a 20-foot rise," he said. If Napa Pipe is someday under water, so is the Netherlands.

Rogal doesn't believe the worst will happen. Once nations wake up to the threat of global warming, people will radically change their lifestyles.

"You don't wait until the ocean rises and kills millions of people and wipes out a large part of the Earth's land mass," Rogal said. "Aggressively, all of us have to play our part."