Plate Nº 88 · recorded November 14, 2026
Earth & ClimateReported finding
Sea-Level Models Miss Up to Two-Thirds of Oʻahu's Threatened Land
Traditional flood-only projections miss up to two-thirds of Oʻahu's threatened land and 85–100% of at-risk infrastructure, a UH Mānoa multi-hazard study finds.
By Marcus Bennett4 min read713 words
In brief
- Single-hazard models overlook up to two-thirds of threatened land area and 85–100% of at-risk infrastructure on Oʻahu
- A tipping point falls between 2073 and 2102, at 0.9–1.2 m of sea-level rise
- At the threshold, buildings affected nearly double from about 5,900 to 11,100
- Kamehameha Highway in Kaʻaʻawa becomes impassable at just 0.3 m of rise, around midcentury
- Published in Environmental Research: Climate (2026), DOI 10.1088/2752-5295/aeab02
By Marcus Bennett
Oʻahu's coastal infrastructure threat is up to two-thirds larger than traditional sea-level rise models suggest, according to a new University of Hawaiʻi at Mānoa study published in Environmental Research: Climate. Single-hazard projections — those counting only passively rising water — overlook 85% to 100% of the critical infrastructure, including buildings, roadways and cesspools, likely to be affected.
The researchers, based at the UH Mānoa Climate Resilience Collaborative (CRC), overlaid three physical models onto detailed infrastructure maps: passive flooding from rising water, shoreline erosion, and annual high-wave events. They mapped the results against buildings, parcels, roads, schools, hospitals, parks, wastewater systems and the resident population, using high-resolution interagency climate projections.
Lead author Anna Mikkelsen, a graduate student in the Department of Earth Sciences at the School of Ocean and Earth Science and Technology (SOEST), said: "This tells us that it is critical to use multiple hazard models, and that areas with little data, such as other Pacific islands, that rely on simpler passive flood model estimates may be underestimating the risk to their coastal infrastructure."
When does the tipping point arrive?
Coastal impacts will not rise at a steady, linear rate. After decades of chronic nuisance flooding, Oʻahu crosses a rapid tipping point between 2073 and 2102, depending on global emissions trajectories. The threshold sits somewhere between 0.9 and 1.2 meters (3 to 4 feet) of sea-level rise.
At that threshold, scattered local problems become widespread infrastructure failure:
- Buildings affected nearly double, from about 5,900 to 11,100
- Residents affected more than double, from about 27,800 to 58,100
- Roads affected rise from 92 km (57 miles) to 210 km (130 miles)
- Sewer mains affected climb from 61 km (38 miles) to 151 km (94 miles)
Senior author Chip Fletcher, director of CRC and dean of SOEST, put it plainly: "Sea-level rise will not come to O'ahu as a slow, harmless rise in the water level." Waves and erosion already damage homes, roads and wastewater systems, he noted, and agencies should plan now "while there is still time to choose how we adapt instead of reacting to each crisis as it comes."
Which hazards hit which parts of the island?
Because the team modeled multiple flooding mechanisms against multiple infrastructure types, they could separate regional threats. Honolulu faces mostly passive and wave-driven flooding across its heavily built environment. The west side suffers intermittent, sudden damage from recurring annual high-wave events. On the North Shore, coastal erosion drives most impacts.
Roads and wastewater systems carry the greatest risk. Mikkelsen cited two concrete cases: in Kaʻaʻawa, a 200-meter (660-foot) stretch of Kamehameha Highway — the only road on the east side — becomes impassable from erosion at just 0.3 meter (about 1 foot) of sea-level rise, expected around midcentury. On the west side, Farrington Highway, which carries up to about 40,000 vehicles a day with no alternate public route, starts developing chronically impassable sections at 0.9 meter (3 feet) of rise.
Wastewater threats extend beyond roads. Cesspools and septic systems already leach wastewater into the nearshore environment, degrading coral reefs and water quality and raising the risk of waterborne illness. "These pathways would increase as sea levels rise because the groundwater also rises, and these systems may become flooded or unable to function properly," Mikkelsen said. The study also found centralized sewer systems at growing risk of flooding this century.
What does this change for planners?
The study updates the 2017 Hawaiʻi SLR Vulnerability and Adaptation Report, which UH helped produce, and supplies community-level data that county and state agencies can fold into long-range planning. Its findings argue for tailored, place-based adaptation — coastal drainage retrofits in urban Honolulu, highway connectivity fixes and decentralized wastewater solutions for rural communities.
All hazard modeling from the open-access study is publicly available through the SOEST Climate Viewer, so practitioners elsewhere can test the multi-hazard approach against their own coastlines.
The full paper is Anna B. Mikkelsen et al., "Sea-level rise and coastal hazards expose critical infrastructure vulnerabilities across Oʻahu," Environmental Research: Climate (2026), DOI: 10.1088/2752-5295/aeab02.
Communities that plan with single-hazard maps are, in effect, planning for a smaller island.
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News editor covering regulation, policy and their impact on general science news at SciBeat.
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