How Sea Level Rise Made Idalia’s Storm Surge Worse

The Washington Post

15:52 JST, September 2, 2023

Rising oceans substantially worsened the devastating storm surge that Hurricane Idalia flung at the Florida coast on Wednesday, according to scientific experts and data analyzed by The Washington Post.

An unusual, dramatic surge in sea levels in the Gulf of Mexico, which began around 2010, could have added nearly five inches to the height of the waters that raced ashore, the figures suggest. More gradual sea level rise between 1939 and 2010 added about four more inches.

In many locations, this overall rise of about nine inches in sea level could have made the difference between flooding and staying dry, experts said.

“In this region, the eastern Gulf Coast, sea level rise has been faster since 2009, 2010,” said Jianjun Yin, a climate scientist at the University of Arizona who published a study in the Journal of Climate this year on this recent acceleration and how it has affected hurricanes. “So with this faster sea level rise, the storm surge could get higher and higher.”

The highest water level recorded with Idalia – a preliminary value of 6.89 feet above the average highest daily tide recorded from 1983 to 2001 – would, if confirmed, be the highest reading that the tide gauge at Cedar Key, a small island community battered by Idalia’s surge, has ever measured. It eclipses the prior record, recorded in 2016, by just under a foot.

Cedar Key, where at least a third of all buildings are believed to have sustained damage during Idalia, is also home to the best long-term record of sea level in Florida’s Big Bend region. Idalia made landfall Wednesday morning about 60 miles north of Cedar Key along Florida’s coast.

The Post calculated the sea level trend at the Cedar Key tide gauge, divided into two periods to emphasize the recent acceleration – a method also used in Yin’s study. The analysis began in 1939 because of substantial data missing before that date. The results suggest that the sea level has risen by around nine inches since 1939 – with about half of the rise occurring in the past 13 years.

Idalia brought the latest examples of new high-water marks being set in the gulf during hurricanes in recent years. According to Yin’s study, Hurricane Ian set a record at Fort Myers, Fla., last year, as did Hurricane Michael at Apalachicola, Fla., in 2018.

The sea level in any particular location rises and falls because of changing seasons, global weather patterns such as El Niño and the tides. It rises dramatically during hurricanes. All of these fluctuations represent departures from a mean sea level, a background state that scientists can calculate. It is this mean sea level that has risen at Cedar Key, just as it has all along the U.S. coastline and beyond as the planet warms.

According to scientists, this means that when a major storm occurs in the Gulf of Mexico, it is now occurring atop a higher sea level, and its storm surge can reach farther inland.

“There is strong evidence that sea levels are higher on Florida’s west coast than they would have been 100 years ago, in large part due to human-caused climate change,” said Daniel Gilford, a climate scientist with Climate Central.

Yin said his own data for sea level rise at Cedar Key is consistent with what The Post found, showing much more rapid change after 2009 or 2010. Yin added that the contribution of sea level rise to Idalia’s storm surge could actually be higher than these numbers suggest, because of additional amplifying effects he believes can occur as seas rise.

The National Oceanic and Atmospheric Administration’s official record of sea level rise at Cedar Key also shows large change, equivalent to nearly 10 inches since 1914. The agency goes further back in time but does not emphasize the recent acceleration.

Not all sea level rise is a result of climate change. Some is due to the sinking of land rather than the rising of the ocean, for instance. But in Cedar Key, unlike in much of Louisiana and Texas, such subsidence is not a major factor. Experts at the University of Nevada at Reno, who use GPS data to track the Earth’s movements, put that sinking at less than a millimeter per year in this location.

Scientists are debating the precise causes of the rapid recent sea level rise along the Gulf Coast. While Yin’s study makes the case that it is tied to a slowing Atlantic Ocean circulation – a predicted effect of climate change – other research suggests that much of it may reflect a combination of climate change and natural variability in the ocean.

Either way, the sharp rise of the ocean in this region is a preview of the kind of rapid rates of change that scientists say we can expect later this century if climate change continues to worsen.

“There’s no doubt in my mind that sea level rise has exacerbated the flooding that we saw,” said Ben Kirtman, who directs the NOAA Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.

But that’s just one way climate change is worsening hurricanes, Kirtman said, adding that Idalia was also probably more intense to begin with because of warmer waters along its path.

While a higher mean sea level means higher hurricane storm surges, simply calculating how much the sea level has risen underestimates some of the effect, Kirtman said. That’s because “you cross thresholds,” he said.

“If your house is at eight feet, you’re on Cedar Key and on stilts, the house might not flood if the tidal flooding is at seven feet,” Kirtman said. “But eight feet is a big disaster. So the additive effect of nine inches is a huge impact in terms of localized damage.”

This threshold effect may be even more noticeable in places that saw a lower storm surge than Cedar Key, such as St. Petersburg and Tampa. At the St. Petersburg tide gauge, preliminary data suggests the storm surge reached 3.79 feet above the average highest daily tide from 1983 to 2001.

That’s precisely the kind of location where sea level rise would have made a big impact, said Philip Orton, a coastal scientist at the Stevens Institute of Technology.

“There would have been far less flooding in Tampa Bay and around Tampa Bay’s perimeter without the effect of sea level rise,” Orton said.