A new technical report by the National Oceanic and Atmospheric Administration suggests that climate change-induced sea level rise over the course of this century, especially in Hawaii, may be far worse than predicted in the Intergovernmental Panel on Climate Change scenario that has been serving as a guide for a number of local efforts to address climate change impacts.

As a result, the state’s Sea Level Rise Vulnerability Assessment and Adaptation Report, due to the Legislature by year’s end, may be more useful as a guide for shorter-range planning for non-critical structures that can be moved or replaced relatively easily.

The local scientists and planners developing the SLR report, required by Act 83 of the 2014 Legislature, have based their inundation scenarios for coastal areas throughout the state on the IPCC’s “worst of the worst-case scenarios,” according to Dr. Chip Fletcher, University of Hawaii associate dean for the School of Ocean and Earth Science and Technology.

High surf erodes the highway’s edge on the North Shore of Oahu. Courtesy: Teresa Dawson

In that scenario, sea level rises about half a foot by 2030, a foot by 2050, 2 feet by 2075, and 3.2 feet (or roughly 1 meter) by 2100.

The NOAA report, however, suggests that the current melting rate of alpine glaciers and glaciers in Greenland and Antarctica, as well as the rate of thermal ocean expansion, may cause sea levels globally to rise an average of 0.3 m (about a foot) in the low-consequence/high-probability scenario but up to an average of 2.5 meters (about 9 feet) in its extreme-consequence/low-probability scenario by 2100. Static-equilibrium effects will cause some regions around the globe to experience even higher sea levels, the report states, and the tropics is one of them.

“Hawaii is sitting in the worst region of all,” Fletcher said.

He and others working on the state’s SLR report had believed when they started that a 1-meter rise in sea level was an extreme scenario, which he said is appropriate for long-range planning of long-lived, expensive, critical structures or infrastructure such as a nuclear power plant or a hospital in the coastal zone. But under NOAA’s new projections, Hawaii is expected to see a 1.3-meter rise in sea level by 2100 under its intermediate case, he said. Under its most extreme, but least probable case, the state would see a 3.3-meter (nearly 11-foot) rise.

In light of NOAA’s new scenarios, Tetra Tech’s draft predictions for the SLR report are now far less speculative and much more reliable than they were before. Under a 3.2-foot rise in sea level, Tetra Tech as of press time had estimated that inundation impacts on Oahu alone could cost $11.8 billion, impact 9,400 acres and 3,800 structures, and displace 13,300 residents. The firm’s planner Kitty Courtney stressed at an SLR workshop that the economic impact reflects the potential cost if nothing is done to mitigate impacts.

‘Planning Envelope’

The NOAA report, titled “Global and Regional SLR Scenarios for the United States,” is the result of work begun in August 2015 for the Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Task Force, a joint task force of the National Ocean Council and the U.S. Global Change Research Program.

Using the best available science, the task force is charged with developing future relative sea levels, associated coastal flood hazard scenarios, and tools to “serve as a starting point for on-the-ground coastal preparedness planning and risk management processes, including compliance with the new Federal Flood Risk Management Standard,” the report states.

The report describes six global mean sea level (GMSL) rise scenarios: Low, Intermediate-Low, Intermediate, Intermediate-High, High and Extreme, ranging from most likely to least likely to occur.

In setting the upper bounds of its SLR projections for 2100, the scientists who produced the report assessed the latest literature on “scientifically supported upper-end GMSL projections, including recent observational and modeling literature related to the potential for rapid ice melt in Greenland and Antarctica.”

“The projections and results presented in several peer-reviewed publications provide evidence to support a physically plausible GMSL rise in the range of 2.0 meters to 2.7 (meters), and recent results regarding Antarctic ice-sheet instability indicate that such outcomes may be more likely than previously thought,” the report states.

Despite the low probability that sea levels will actually rise to 2.7 meters by the end of the century, the report’s authors warn against planners discounting this.

“For decisions involving long planning horizons and with a limited adaptive management capacity, the high degree of uncertainty in late-21st century GMSL rise looms large. Failure to adequately account for low-probability, high-consequence outcomes significantly increases future risks and exposure,” the report states. “For many decisions, it is essential to assess worst-case scenarios, not only those assessed as the scientifically ‘likely’ to happen.”

The report recommends that to assess a system’s overall risk and determine long-term adaptation strategies, planners should define a “scientifically plausible upper-bound (which might be thought of as a worst-case or extreme scenario) as the amount of sea level rise that, while low probability, cannot be ruled out over the time horizon being considered.”

For shorter-term planning, such as for adaptation strategies within the next 20 years, the report suggests that planners define a “central estimate or mid-range scenario (given assumptions about greenhouse gas emissions and other major drivers).”

“This scenario and the upper-bound scenario can together be thought of as providing a general planning envelope,” the report states.

Local Impacts

Although NOAA’s intermediate SLR scenario clearly anticipates a rise of more than 1 meter, the state’s report isn’t likely to include a robust analysis of a rise higher than that.

Fletcher, however, made it clear that NOAA’s higher-consequence scenarios would devastate certain coastal areas of the state. Under NOAA’s high scenario, he said, inundation would rise to the point where it would permanently drown Ewa Beach on Oahu’s south shore, which is home to tens of thousands of residents.

Tetra Tech’s Courtney, who also spoke at the workshop, presented several preliminary maps and charts indicating that even an increase in sea level of 1 to 3 feet could cause significant and widespread damage, especially when combined with increased erosion, annual high wave flooding, and a 1 percent annual chance of a coastal flood (also known as a 100-year flood).

A couple of the maps she displayed showed the numerous spots along Oahu’s coastal highway, including areas on the windward side and along Honolulu’s impending rail transit alignment, that would be vulnerable to inundation due to sea level rise. Another map highlighted the more than two dozen schools, hospitals and clinics, police and fire stations, and wastewater treatment plants within the Honolulu area that would be flooded by a 100-year flood under a 3.2-foot rise in sea level. And yet another showed that that flood area would extend a mile or more inland from the current FEMA VE zone boundary, where landowners are required to have flood insurance.

With regard to the potential impacts on the Honolulu rail transit system, Courtney noted, “Transit-oriented development is probably what we do really need to do … but on the other hand, we gotta make sure we’re taking into consideration some of these long-term impacts of sea level rise.”

Referring to some of her maps showing projected inundation on Oahu’s west and north shores, Courtney said that beaches are going to be lost and many of them are state parks or recreation areas. She also noted that increased erosion will also likely unearth or damage historic cultural sites, such as those at Kawela Bay.

“What do we need to do to protect a beach? … How do we continue to have beaches in the state?” she asked.

So far, no inundation charts for any of the outer islands have been presented. When the report is complete, Courtney indicated that the most thorough inundation and economic impact assessments in the report will be for the islands of Kauai, Maui and Oahu, for which there is a rich amount of historical data. “For Molokai and Lanai, we have some limitations in historical records for coastal erosion and annual high wave flooding,” she said.

An assessment for the Big Island will also be included.

Reprinted with permission from the current issue of Environment Hawaii, a non-profit news publication founded in 1990. All issues published in the last five years are available free to Environment Hawaii subscribers at www.environment-hawaii.org. Non-subscribers must pay $10 for a two-day pass. All issues older than that are free to the public.

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