Sea-level rise has increased frequency of extreme coastal flooding worldwide, study finds

Human-caused sea-level rise has significantly increased the frequency of extreme coastal flooding worldwide, according to a new study led by a Tulane University researcher.

The research , published in the journal Nature Climate Change , found that coastal flooding events expected only once every 100 years are now, on average, about 12 times more likely to occur.

Climate change has made those events about four times more likely since 1900, said the study's lead author Sönke Dangendorf, the David and Jane Flowerree Associate Professor in River-Coastal Science and Engineering at Tulane University School of Science and Engineering .

Dangendorf and an international team analyzed long-term tide gauge records alongside climate model simulations to separate the influence of human activity, natural forces and local land movement.

"Extreme sea levels occur when high tides, storm surge and rising baseline sea levels combine. As sea levels rise, smaller storms can produce flooding that previously required more severe conditions," Dangendorf said. "At nearly half of the 130 sites analyzed in the study, a flood expected once every 100 years in 1900 now occurs at least once per decade."

In some locations, the increase is greater. At Sandy Hook, New Jersey, a 1-in-100-year event became about a 1-in-16-year event by 2005. In Wellington, New Zealand, a similar event became roughly a twice-per-year occurrence.

The findings have implications for coastal infrastructure and flood planning, as historical estimates of flood frequency may no longer reflect current conditions. New Orleans is widely recognized for having one of the most advanced flood protection systems in the world, developed in the years following Hurricane Katrina. The new findings reinforce how valuable such systems are - and why keeping them effective over time is essential.

"New Orleans has made enormous progress in reducing flood risk after Katrina," Dangendorf said. "Our results show that as environmental conditions continue to evolve, sustained maintenance and forward-looking planning are key to preserving that level of protection."

Local conditions can influence the magnitude of change. For example, inManila, land subsidence linked to groundwater use has increased the frequency of extreme flooding more than 300-fold. However, across most sites, the study found that human-driven climate change is the primary factor increasing flood frequency.

Natural forces contributed more to sea-level changes earlier in the 20th century, but the influence of human-caused warming has grown since the 1960s and now accounts for the largest share of rising sea levels and associated flood risk, Dangendorf said.

For communities like New Orleans, he said the research offers both a validation of past investments and a roadmap for sustaining resilience in the future.

"New Orleans is a global example of how to manage coastal risk," Dangendorf said. "Our work highlights that with continued attention and adaptation, that leadership can be maintained even as conditions change."