LONDON (Thomson Reuters Foundation) - Many coastal areas of the United States could see floods that now happen about twice a century occur almost daily by the turn of the century, as ocean levels rise because of planetary warming and ice melt, scientists warned on Thursday.

By 2050 - just 30 years from now - almost 70% of 200 coastal areas studied could see such major floods at least once a year, said Sean Vitousek, a researcher with the U.S. Geological Survey (USGS) in California.

For low-lying cities such as Miami or Honolulu, current “nuisance” flooding - which temporarily blocks roads or backs up through stormwater drains - could soon become the norm, requiring residents to adapt or even leave, he said by phone.

Without efforts to curb climate-changing emissions or put in place new protective measures, flooding “will continue and get worse until it almost makes some areas impractical to live in because it’s happening so frequently”, Vitousek said.

“When what we consider now as the extreme happens at every high tide - that’s no joke,” the oceanographer and engineer told the Thomson Reuters Foundation.

Vitousek’s research, carried out with colleagues at the USGS, the University of Illinois at Chicago and the University of Hawaii, looked at historical records of sea level fluctuations from 200 tidal gauges around the U.S. coastline.

Those records were paired with models of expected sea level rise driven by climate change, to predict how often flooding was likely to occur in each location in coming decades.

The findings, published in the journal Scientific Reports, showed expected sea level rise of at least 1 metre (3 feet) by 2100 was likely to bring flooding on a near-daily basis in 93% of locations studied.

“You take high tide, add on an extra metre, and you’re exceeding thresholds at every high tide,” Vitousek said.

Existing flood defences and topography make some coastal areas less vulnerable to inundation, he said - but in many cases when protective structures were built “there wasn’t an awareness sea level was going up or an allowance made for it”, he added.

Data published by the World Resources Institute in February showed 80 airports worldwide, including 12 in North America, are likely to be underwater with 1 metre of sea level rise.

Some, from Boston to San Francisco, are responding by building new flood barriers, with San Francisco International Airport expected to spend more than $580 million to boost its seawall from three feet (one metre) to eight feet (2.4 metres), WRI noted.

To deal with the growing flood threat, rapidly slashing emissions to “flatten the curve” on sea level rise is key, Vitousek said.

But many places also will need to invest in efforts to adapt to higher sea levels, from rebuilding beaches to planting wave-calming mangroves or constructing sea walls.

In flood-threatened coastal areas, “there’s almost no way you can go forward in the future without these adaptation measures in place”, he said.

Such responses should be factored into planning and budgets soon, he and others advised.

Vitousek said the odds of flooding were rising “so fast”.

“For a lot of vulnerable locations, it can happen in the next decade or two - well before 2100 and in some cases well before 2050,” he said.