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Human-driven sea-level rise has increased frequency of extreme coastal flooding, study says

Human-driven sea level rise has increased how often extreme coastal flooding occurs around the world, according to a study released Wednesday.

Experts say the findings are crucial for coastal infrastructure and flood planning as the planet warms.

Extreme flooding events are caused by a combination of storm surges, high tides and unusual sea level rise alongside natural climate patterns and human influence. Hurricane Ian in 2022, which caused significant flooding, is an example of a hurricane that scientists said was strengthened by climate change. Coastal flooding threatens hundreds of millions of people living in low-lying coastal areas across the globe each year, causes billions of dollars in damage and can be deadly.

Extreme sea level events that were once historically rare — those with a 1% chance of happening in a given year — are now, on average, about 12 times more likely, according to a new study published in the journal Nature Climate Change on Wednesday. Those events have become about four times more likely due to human-driven climate change, the research shows.

The researchers examined the frequency of extreme sea level events — which are the drivers of coastal flooding — by considering long-term records from tide gauges at more than 100 sites alongside climate modeling. The study looked at the increase from 1900 to 2005. It was limited through 2005 due to the availability of models that attribute instances to human-driven climate change. The authors acknowledged the results were conservative, given that human contributions to changes in coastal extremes have only increased since then.

The experts separated the influence of human activity, natural forces and local land movement. While sea level changes earlier in the 20th century could largely be attributed to natural forces, since the 1960s, it's human-caused warming that's the primary factor behind rising sea levels, scientists say.

A separate study published in the journal Science Advances on Wednesday also supports the notion that human-caused climate change is responsible for extreme water levels. It found climate change at the root of 58% of the days with extreme water levels during a period from 2000 to 2018. Climate change has also, on average, nearly tripled the number of days exceeding extreme water level thresholds since the 1970s, according to that study.

"Essentially every coastal flood today has human fingerprints on it through climate change," said Ben Strauss, chief scientist at Climate Central and a co-author of the Science Advances study. "Without the extra bit of sea level rise caused by global heating, most of these events would not have reached the status of flood."

The research in Nature Climate Change didn't fully examine individual human factors, said Sönke Dangendorf, the lead author, but he noted greenhouse gases — the result of burning fossil fuels such as oil, gas and coal — are the most significant.

"In particular since the 1970s, it's by far the dominating factor, and this is of course not good news at the moment," said Dangendorf, also an associate professor at Tulane University. "The risk is evolving and with the evolving risk we need to do more for adaptation."

Jeff Williams, a retired United States Geological Survey oceanographer who wasn't part of either study, said the research shows that planners need to take into account the increased risks. They should also consider what it will cost to increase coastal protection and determine who pays for it, he said.

The current protections for New Orleans, for instance, "will likely not be adequate beyond the next couple decades," Williams said.

Nations across the globe are increasingly using renewable energies like solar and wind. Last year, clean power generation exceeded overall global electricity demand growth, and the share of renewables hit more than one-third of the world's electricity mix for the first time. Even in the United States, where the Trump administration has boosted fossil fuels, solar is growing as coal power declines. As such, scientists have recently said the world is no longer on track for the worst case scenario for warming — but it's also not on track to the best case, either.

Dangendorf, the Tulane author, said: "The impacts, even of a relatively little sea level rise, can be pretty impactful on our coasts."

"There is a silver lining because we have control about how much we emit, right?" he said. "So we can stop that development, at least to some degree."

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Alexa St. John is an Associated Press climate reporter. Follow her on X: @alexa_stjohn. Reach her at [email protected].

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