New study doubles estimates of land vulnerable to sea level rise

Turns out future sea level rise in Hawaii may be a bigger problem than previously thought.

A team of researchers from the University of Hawaii at Manoa and the state Department of Land and Natural Resources found that island land affected by sea level rise may be double previous projections.

The study, published Thursday in the Nature journal, Scientific Reports, indicates that even some low-elevation areas a mile or two inland are vulnerable in a world of rising oceans caused by climate change.

Lead researcher Tiffany Anderson, a faculty member at the Department of Earth Sciences at the UH-Manoa School of Ocean and Earth Science and Technology, said, “A lot of areas will be affected. Losing beaches and access to them is really tragic for Hawaii and its culture.”

Early estimates about how a rising ocean would affect the islands were based on a “bathtub” approach in which a static sea surface is projected onto an unchanging terrain.

Charles “Chip” Fletcher, UH professor of geology and co-author of the the study, said this latest study is more realistic, factoring in additional processes, including chronic erosion and wave run-up, that can lead to more far-reaching and widespread flooding.

The bathtub approach alone ignores 35 to 54 percent of the land area exposed to one or more of the hazards, depending on location and sea level, Fletcher said.

The study also describes the discovery of a “critical point” at which marine flooding accelerates and reaches farther landward.

On Oahu, the critical point falls between 2 and 3 feet of sea level rise. At 2 feet of sea level rise, 6 square miles of the island will be affected by some kind of ocean flooding. At 3 feet of sea level rise, the affected area more than doubles to 12.5 square miles.

Especially vulnerable areas include Oahu’s low-lying south shore from Waikiki to the Daniel K. Inouye International Airport, and Ewa Beach.

Fletcher, associate dean of the UH-Manoa School of Ocean and Earth Science and Technology, said sea levels are expected to rise anywhere from 2 to 8 feet by the end of the century, depending on the behavior of the melting Antarctic ice sheet.

At the current rate of acceleration, he said, rising seas will reach more than 2 feet above present by the end of the century, which agrees with modeling by the Intergovernmental Panel on Climate Change in 2013. But that report makes it clear that rapid melting of Antarctica or Greenland is not considered, and since then several studies have identified a rising probability that glaciers in West Antarctica are engaged in rapid and irreversible retreat.

Earlier this summer a report found that Antarctic ice is melting three times faster than only 10 years ago. If these trends continue and worsen, 6 to 8 feet of sea level rise becomes a possibility by the end of the century.

Fletcher said coastal Oahu will increasingly experience the same kind of nuisance flooding that already plagues the streets of Mapunapuna and the coastal areas of South Florida, where millions of people are at risk.

The most vulnerable areas will experience flooding at high tide on a daily basis within only two or three decades. Summer high tides will cause the greatest amount of inundation, but other times of the year will see flooding as well. Coastal communities throughout the state will experience widespread coastal flooding from the run up of waves at least once to a few times each summer.

The Hawaii study took two years to complete with help from the UH-Manoa High Performance Computer Cluster. The computer was fed the topography of the land and sea floor, a model of mathematical equations, and it made calculations of the waves and erosion along the shoreline.

On Oahu, Maui and Kauai, calculations were made every 65 feet, with the number of calculations reaching into the thousands. Statewide calculations were made from more than 10,000 shoreline locations under four scenarios of future sea level rise.

The modeling presented in this study was conducted to support the Hawai‘i Sea Level Rise Vulnerability and Adaptation Report, which was completed in December to help with government planning.

Adopted by the Hawaii Climate Commission, the report projects climate change-driven sea level rise of 3.2 feet over the next 30 to 70 years.

The report forecasts a future of coastal flooding, erosion and property damage affecting hotels, businesses, malls, schools and community centers, as well as the displacement of thousands of people from their homes.

According to the report, potential impacts of 3.2 feet of sea level rise on Oahu alone include the loss of $12.9 billion in structures and land, 3,800 structures, including hotels in Waikiki, the displacement of 13,300 residents and the loss of 17.7 miles of major roads.

The latest study, titled “Modeling multiple sea level rise stresses reveals up to twice the land at risk compared to strictly passive flooding methods” is online and free to everyone.

“This study is important because we want to head into the future prepared and knowledgeable and ready to develop policies that are empirically based,” Fletcher said.

The study was supported by the DLNR, H.K.L. Castle Foundation, the U.S. Geological Service Pacific Islands Climate Adaptation Science Center, Honolulu Board of Water Supply, Honolulu Office of Climate Change Sustainability and Resiliency, Hawaii Community Foundation, County of Kauai and the National Oceanographic and Atmospheric Administration.

Anderson and team are currently incorporating rainfall into the computer model to determine how sea level-related flooding might be exacerbated during high-tide rainfall events.

To see how your neighborhood might be affected, go to pacioos.hawaii.edu/shoreline/slr-hawaii, which is an online mapping tool and companion to the Hawai‘i Sea Level Rise Vulnerability and Adaptation Report.

WATERWORLD

The potential impact of a 3.2-foot rise in sea level on Oahu would include:

3,800

Structures lost including hotels in Waikiki

$12.9 billion

Estimate value of structures and land lost

13,300

Number of residents displaced

17.7

Miles of major roads lost