By Amina Khan / Los Angeles Times

The effects of rising oceans on coastal flooding may be even worse than we thought. Scientists have found that a mere 10 to 20 centimeters, or about 4 to 8 inches, of sea-level rise — which is expected by 2050 — will more than double the frequency of serious flooding events in many parts of the globe, including along the U.S. West Coast.

The findings, described in Scientific Reports, highlight the environmental and economic impacts of sea-level rise on coastal areas, and the need to properly predict and prepare for these effects.

As global warming marches onward and land-ice reserves continue to melt into the seas — thanks in large part to human-produced greenhouse gases — oceans are continuing their upward creep.

Researchers have long made global-scale estimates of sea-level rise and analyzed what effects the ocean’s ascent will have on coastal erosion, on the environment and on human communities. Those estimates have taken into account storm surge and tidal fluctuations, among other variables. But they haven’t included a crucial factor: waves.

“Most of these tide gauges are within harbors or in protected areas, so they don’t record any water level associated with waves,” said Sean Vitousek, a coastal scientist at the University of Illinois at Chicago.

Waves might seem like small potatoes compared with high tides, but they can have a big impact, Vitousek said.

“Waves often generate a pretty significant portion of the actual flood levels,” he explained. “For instance, if you think about just tides and storm surge, then in some areas, waves can add an additional 50 to 100 percent of that existing water level.”

That’s particularly true in California. Much of the flooding in the state is dominated by wave-driven events — which is why El Nino years with extremely large waves can have such profound effects on coastal erosion. A recent paper by one of Vitousek’s co-authors showed that the 2015-16 El Nino season caused unprecedented levels of erosion across much of the West Coast.

Even on islands in the Pacific and other equatorial regions where waves are smaller and the tides bring only a few centimeters of change, waves can have an outsize impact — in part because humans may build closer to the water line in those areas and be unprepared for the changes that come with sea-level rise.

For this paper, Vitousek and his colleagues combined wave, tidal and storm surge models with their data on sea-level projections. The results showed that 10 to 20 centimeters of sea-level rise happening no later than 2050 could have major impacts in many parts of the globe, including around India, the Indian Ocean and the tropical Atlantic along the west coast of Africa.

“Often these areas are fairly low-lying, have a lot of development, and those areas would also be fairly impacted by future sea-level rise,” he said. “As you go to higher latitudes, you’ll still get these effects, but not quite as much, because the waves are larger, the tide is larger and so sea-level rise doesn’t represent the same percent or relative contribution to those areas.”

The effects will be most pronounced in the tropics, the researchers found. Areas such as the Marshall Islands in the central Pacific are particularly vulnerable.

“You’ll still see impacts at higher latitudes — the California coast, the Pacific Northwest — but you probably won’t see the same dramatic effects that you would in the tropics,” Vitousek said. “It’s really going to happen everywhere, (but) it’ll happen faster in the tropics.”

The scientists are sharing this information with state and local agencies to try to figure out where the most vulnerable areas may be — and what should be done to prepare them. For the moment, Vitousek said, one of the easiest solutions may be more beach nourishment, even though it’s a temporary solution at best.

The scientists hope to calculate how much economic damage could be incurred by the effects of sea-level rise. In the meantime, they’re continuing to study its impact on the California coastline in ever more granular detail — and that in-depth understanding may help scientists continue to improve their models for the world.

“I think it calls to attention the future consequences of sea level rise,” said William Sweet, an oceanographer with the National Oceanic and Atmospheric Administration in Silver Spring, Md., who was not involved in the work.

Sweet recently led a NOAA report showing that sea-level rise will progress faster in some places and slower in others — variations that would be key in understanding coastal flooding effects in different areas. The Scientific Reports paper did not take this into account.

“In reality it’s not going to be a uniform rise,” he said. “The change in land elevation itself, changes in circulation, change in gravity in the future as the ice caps change are all going to cause a very non-uniform rise in sea levels.”

Future, increasingly detailed assessments of the impacts of sea-level rise, he added, will need to take that variability into account.