Hurricane Ian is bringing historic storm surge. It holds lessons for California's coast

Hurricane Ian has made landfall in the Gulf Coast of Florida as a Category 4 storm. Along with roaring winds, it brings life-threatening storm surge — sea water plowed forward by hurricane winds.

“Essentially what the hurricane is doing is bulldozing that water and sending it towards the coastline,” said Chronicle newsroom meteorologist Gerry Díaz. As the water nears the shore, it produces a rise in sea level that can lead to dangerous and damaging coastal flooding.

The National Hurricane Center warned on Wednesday morning that the storm surge in combination with the tide could cause water moving inland from the shoreline to reach unprecedented levels of 12 to 18 feet above ground along some stretches of Florida’s west coast.

“They have gone a very long time without suffering significant hurricane impacts,” said Will Downs, an atmospheric scientist at the University of Miami Rosenstiel School of Marine, Atmospheric and Earth Science.

In the century since the area last experienced a major hurricane, the population has soared to 3 million and climate change has intensified the impact that storms have on coastal flooding. Sea level rise due to global warming also impacts coastal areas of California, on the opposite side of the country, where some 68% of the state’s population resides .

While California has only seen one hurricane make landfall in recorded history , other storms that contribute to flooding and coastal run-up are forecast to worsen in a warming world .

“We just built up in these areas that are, by nature, extremely vulnerable,” said Patrick Barnard, a research geologist with the U.S. Geological Survey who specializes in coastal climate change impacts.

A report published earlier this year, led by multiple federal agencies, stated that the change in the coming years could be substantial. Within the next 30 years, relative sea level along U.S. coastlines is expected to rise an average of 10 to 12 inches. This is about as much as it rose over a century from 1920 to 2020.

This change is due to global warming, which is adding water into the ocean by melting glaciers and ice sheets, and heating seawater up, increasing the volume of ocean water and raising sea levels.

“Even if the waves in the storm stayed the same, the higher the sea level is, the further it's going to move up the shoreline,” said Gary Griggs, a coastal geologist at UC Santa Cruz. “Every inch of sea level rise is going to make it a little bit worse.”

The rates aren’t the same around the entire country. The change in relative sea level along California coasts is projected to be as much as 8 inches in the next three decades, on average.

These projections are sobering, said Ben Hamlington, a research scientist at NASA Jet Propulsion Laboratory who leads the NASA Sea Level Change Team.

“If they do come to pass, the impacts will be significant,” Hamlington said.

This is especially true because so many communities have developed in dynamic coastal settings. Rising sea levels raise the stakes even more because they are the foundation that tides and storm surges build on top of.

“When you have storms, like the ones we see in Florida, or the tides and waves and El Niños we see here in California, then those impacts become that much worse — they get amplified,” Hamlington said.

Extreme atmospheric river storms, for example, are projected to become more common in California as the planet continues to warm, emerging research finds. El Niño events can also dramatically raise sea levels off the California coast. In 1983, this rise caused big problems.

“We had an El Niño which elevated the water level in San Francisco almost two feet above predicted,” Griggs said. “Then we had about eight or nine storms that came in, at high tides.”

The result? Widespread flooding damage to houses, parks, roads and sewer lines, Griggs said.

By the end of the century, relative sea level could rise 2 feet compared to 2020. But failing to curb future greenhouse gas emissions could raise this number to between 3.5 and 7 feet. While there is uncertainty with these projections, reducing emissions and limiting warming could rein in future flooding impacts.

“There's plenty that can be done now, which will pay dividends down the road,” Barnard said.

Jack Lee (he/him) is a San Francisco Chronicle staff writer. Email: jack.lee@sfchronicle.com