A new study has found that, globally, coastal floods previously expected once every 100 years are now about 12 times more likely due to human-caused sea-level rise.
The findings, published in the journal Nature Climate Change, show that coastal flooding events have become four times more likely since 1900.
"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," lead author Sönke Dangendorf, associate professor in river-coastal science and engineering at Tulane University, US, said.
Researchers analysed long-term tide gauge records alongside climate model simulations to separate the influence of human activity, natural forces and local land movement.
"At nearly half of the 130 sites analysed in the study, a flood expected once every 100 years in 1900 now occurs at least once per decade," Dangendorf said.
The increase was found to be greater for locations including Sandy Hook in the US state of New Jersey, where a one-in-100-year event became about a one-in-16-year event by 2005.
In New Zealand's Wellington, a one-in-100-year event became roughly a twice-per-year occurrence, the researchers found.
Local conditions such as sinking land may further influence the magnitude of change, they added.
For example, in Manila, land subsidence linked to groundwater use has increased the frequency of extreme flooding more than 300-fold.
However, across most sites studied, human-driven climate change was found to be the primary factor increasing flood frequency.
Dangendorf said 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.
"Globally, the median (typical) frequency of an historical 1-in-100-year extreme sea-level event has increased (nearly) 12-fold, with human-driven radiative forcing (climate change) alone quadrupling the likelihood of such events," the authors wrote.
As historical estimates of flood frequency may no longer reflect current conditions, the study's findings have implications for coastal infrastructure and flood planning, they said.
"These findings provide direct, observation-based evidence that climate change has already reshaped coastal flood hazard, underscoring the urgency of integrating attribution science into coastal adaptation, risk management and policy frameworks," the team wrote.