VANCOUVER - Hundreds of millions of people who are living in low-lying coastal areas around the globe have two options when it comes to protecting themselves from rising sea levels, says a British Columbia-based scientist.

John Clague, a professor at Simon Fraser University, said Sunday that people can either "defend" their communities or "retreat" from the threat of sea levels that are expected to rise over the next century.

Clague was one of four researchers who addressed the issue during an annual gathering of the American Association for the Advancement of Science in Vancouver.

So massive is the issue that Margaret Davidson, a director with the U.S. National Oceanic and Atmospheric Administration, said as many as 500 million people could be affected worldwide, and the economic impacts have yet to be determined.

"It's a bit like the options open to the military during a war," said Clague. "We can defend or we can retreat. Both are not very palatable options."

While sea-level fluctuations are natural and have remained relatively constant during the past 5,000 years, the situation has begun to change over the last 200 years, he said.

During the last century, sea levels were rising by about two millimetres a year, but more recently they have increased to about three millimetres a year, he said.

"Why is that happening? Well, probably obvious to you, glaciers are melting and oceans are warming, and a warmer ocean occupies more space so the sea rises as a consequence of that," said Clague.

Three millimetres a year may not sound like much, Clague added, but it adds up, and over a century could mean a minimum 30-centimetre rise in sea levels — although he thinks the jump will be more like one metre.

Combined with other environmental factors, like storm surges, high tides and erosion, an increase of just 30 centimetres can cause severe problems for low-lying communities, he said.

Meanwhile, Davidson said some aboriginal communities in Alaska have already been forced to move inland because of problems related to flooding and coastal erosion.

She said the issue impacts anybody living on an island or a low-lying region, whether they be residents of Alaska, Louisiana or the south Pacific nation of Tuvalu.

"It has been estimated that it's close to 500-million people worldwide that we're going to have to figure what we do with over the next 80 years," she said.

None of the scientists could say exactly how much of an impact rising sea levels would have on the global economy, but they all agreed it would be massive.

Denise Reed, a professor at the University of New Orleans, said fixing her city's levies cost more than $14 billion.

"Depending on the nature of the place and depending upon what we're trying to protect, the protection strategy can be really expensive," she said.

But cost hasn't stopped some communities from planning ahead

David Flanders, a research scientist at the University of British Columbia, said he has been working with the Metro Vancouver community of Delta, B.C., on several strategies.

Delta residents can hold the line against rising sea levels by building higher dikes, or by implementing unique architectural solutions, like building homes on stilts, he said.

The community can also create habitat or barrier islands in inter-tidal zones that reduce the impact of winds and waves during storms, said Flanders.

Or, residents can begin planning for a long-term retreat, an actual movement of the community, he added.

Whatever residents of Delta or any other low-lying coastal community decide, Clague said he thinks there's still time to plan for the problem, which will also include grappling with funding issues.

"Where the money's going to come from?" he asked. "I don't think anybody knows because it's a large amount of money and, you know, it's outside the ability, I think, of most communities to deal with this problem."

The science symposium wraps up in Vancouver Monday.