Hawaii has about 15 years to prepare before sea-level rise leads to a rapid increase in high-tide floods.

That’s what Philip Thompson, director of the University of Hawaii Sea Level Center, hopes residents take away from a recent report from UH Manoa and NASA. Thompson and researchers from across the country analyzed decades of sea level data from 89 harbors in the U.S., including six in Hawaii.

They compared that to known tidal cycles and sea level rise projections from the National Oceanic and Atmospheric Administration, discovering that by the mid-2030s Hawaii will experience an “inflection point” where the number of flood days will increase rapidly.

“It’s important to not get complacent during the times when things are not changing very rapidly and be prepared for those periods,” Thompson said. “We can now point to specific points in time based on what we did in the paper, when things will change very quickly.”

He was inspired to look into how high-tide flooding will be impacted by sea level rise after a cluster of floods impacted Honolulu in the summer of 2017. That was a year in which tidal cycles, which are affected by Earth’s proximity to the moon, intersected with Hawaii’s seasonal high tides.

“The summer of 2017 was kind of like a window into the future,” Thompson said. “That year was particularly bad, but over time we expect to see more and more of these flood clusters happening.”

Until 100 years ago, the global sea level had been consistent for more than 3,000 years. Since people started burning significant amounts of fossil fuels, the sea level has risen about 7 inches. Half of that rise has occurred since 1993, according to data from NASA.

Tidal cycles and ocean events, like El Niño and ocean eddies, can cause high-tide flooding on their own. But when the sea level is higher, floods will happen more often. The analysis from UH and NASA showed that Hilo, for example, could see a season with 50 high-tide floods by 2040.

“The effects should be fairly similar around the state and the amount of impact that you experience in any given place is really dependent on factors like: what’s the infrastructure look like? Are there areas of the road that are really close to the water at a relatively low level?” he said.

In addition to analyzing the impact on coastal roads and low-lying communities, Thompson said Hawaii’s drainage infrastructure needs to be well-maintained and phasing out cesspools should be a high priority.

“The more frequent we have these higher tides, the more likely it is to have seepage from those cesspools into our groundwater,” he said. “It’s a public health issue.”

Many infrastructure decisions are based on maps from the Federal Emergency Management Agency, which accounts for what will flood during rare natural disasters like hurricanes. Thompson hopes that decision-makers will start preparing their communities for these smaller, high-tide floods before they become common.

“Their impact is accumulated over many small events,” he said. “Can we think about that when we’re developing and redeveloping areas as opposed to just relying on FEMA flood maps?”

The study looked at coastal communities across the U.S. and found that many will start to experience recurring high-tide floods in the coming decades. When an ocean event, like El Niño, and tidal cycles converge in a single year, the city will have an “extreme” season with many flooding “clusters.” But if the sea level continues to rise at the same rate, these extreme seasons will later become the norm.

“It’s important that we make decisions based on sound science,” Thompson said. “I’m hoping that this type of work can really help to make the state of Hawaii a more resilient place in the face of climate change over the next decades.”