Extreme coastal floods that once reached communities only rarely are becoming far more common as human-driven climate change raises sea levels, according to new research published in Nature Climate Change.
The findings matter for coastal planning, flood protection, insurance, infrastructure investment, and public safety.
Scientists say communities need to account for higher flood frequency as warming pushes seas upward and makes extreme water levels more likely.
Major coastal floods occur when high tides and storm surges combine with seas that are already rising. Natural climate patterns and other human influences also affect local water levels, but long-term sea-level rise now plays a larger role in coastal flood exposure.
Climate change has also strengthened storms such as Hurricane Ian, which caused severe flooding in 2022. Flooding already threatens hundreds of mn people in low-lying coastal regions each year, while causing billions of dollars in damage and sometimes killing residents.
Floods that historically had a 1% annual chance of striking a coastline are now about 12 times more likely on average, according to the study. The researchers found that human-driven climate change has made those events about four times more likely.
The study examined extreme sea-level events, which often trigger coastal flooding. Researchers used long-term tide gauge records from more than 100 locations, combined with climate modelling, to assess changes between 1900 and 2005.
The analysis stopped in 2005 because later model coverage was not strong enough to isolate human-driven climate change with the same confidence. The researchers said the results probably understate current risk because human influence on coastal extremes has grown since then.
The team separated changes caused by human activity, natural forces, and shifts in land levels. Earlier in the 20th century, natural forces explained more of the sea-level changes. Since the 1960s, human-caused warming has become the main driver.
A separate study published in Science Advances reached a similar conclusion. It found that climate change contributed to about 58% of days with major floods between 2000 and 2018.
That study also found that climate change has nearly tripled the number of days when seas exceed extreme flood levels since the 1970s. Together, the two studies show how sea-level rise turns formerly rare coastal floods into more routine events.
“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.
Sönke Dangendorf, lead author of the Nature Climate Change study and associate professor at Tulane University, said the research did not fully isolate every human factor. Still, he said greenhouse gases from burning oil, gas, and coal are the most important contributor.
“Since the 1970s, it’s by far the dominating factor, and this is of course not good news,” Dangendorf said. He said the threat is increasing and coastal communities need stronger preparation.
Jeff Williams, a retired United States Geological Survey oceanographer who was not involved in either study, said planners need to factor rising threats into coastal protection plans. He also said authorities need to assess the cost of stronger defences and decide who pays for them.
Current flood protections for New Orleans will probably not remain adequate beyond the next couple of decades. The scale of adaptation required in cities already protected by major flood-control systems.
Jeff Williams, a retired United States Geological Survey
The research comes as countries expand renewable power generation from solar and wind. Clean power generation last year exceeded overall global electricity demand growth, while renewables reached more than one-third of the world’s electricity mix for the first time.
In the United States, solar power continues to grow while coal generation declines, even as federal policy has again favoured fossil fuels. Scientists now say the world is no longer tracking the worst-case warming path, but it is also not moving fast enough toward the safest outcome.
“The impacts, even of a relatively little sea level rise, can be pretty impactful on our coasts,” Dangendorf said.
According to Beinsure analysts, the insurance and infrastructure message is direct. Coastal flood models need to treat today’s rare-event assumptions with caution, because past return periods no longer describe current risk.
Higher sea levels turn ordinary storms into more expensive flood events, raising pressure on insurers, reinsurers, public budgets, and coastal property markets.
