Forget 3 feet, Hawaii should be planning for 6 feet of sea level rise, new research says

Melting Greenland ice sheet. Credit: NOAA

Hawai`i may be planning for only half the sea level rise that is possible within the lives of todayʻs newborns.

A new study from British, American, Dutch and German climate researchers argues that sea levels could be more than 6 feet higher than now in 80 years, wiping out many of the worldʻs most important cities and coastlines.

Many of those coastal areas would be inundated far sooner than that. That includes much of Honolulu. 

Most of the worst-case planning in Hawai`i assumes a 3-foot or one-meter rise, but this study suggests we should be planning for twice that.

One planning organization in the Islands, Honoluluʻs city Climate Change Commission, is on board with the 6-foot recommendation by 2100. Commisson member Victoria Keener said 3 feet is possible much earlier, by mid-century.

The authors of the new paper are cautious about their numbers because the science is very complex. But they say that if the worldʻs great ice reservoirs—the immense Greenland and Antarctic ice sheets—melt as many climate models suggest, the world will be a very, very different place.

Imagine all cargo needing to be offloaded from ships at anchor, because all the harbor facilities are submerged. Imagine the most valuable property in Hawai`i underwater. Imagine no coastal road access—meaning many Hawaiian communities could be reached only by boat.

Part of the problem is that many researchers have been focused on keeping warming to 2 degrees Centigrade, but warming of as much as 5 degrees by the end of the century now seems possible. That is because fossil fuel use is rising instead of falling, global carbon dioxide levels are rising at faster pace rather than stabilizing, and the global ice sheets canʻt help but respond by melting.

The authors of this daunting report are some of the worldʻs premier researchers. They include Jonathan L. Bambera of the UKʻs University of Bristol School of Geographic Science, Michael Oppenheimer of Princeton University, Robert E. Kopp of Rutgers University, Willy P. Aspinall of the University of Bristol School of Earth Sciences, Roger M. Cooke of the Dutch Delft University of Technology Department of Mathematics. The paper was edited by Stefan Rahmstorf, of Germanyʻs Potsdam Institute for Climate Impact Research. They consulted with climate scientists in America and Europe.

It is entitled, " Ice sheet contributions to future sea-level rise from structured expert judgment," and was published this week in PNAS, the Proceedings of the National Academy of Sciences.

The authors are cautious, but they are clear that the world should stop focusing on the lower range of sea level rise estimates. Many nations are already planning and building structures to protect their shorelines from near-term sea level rise.

"Adaptation measures accounting for the changing hazard, including building or raising permanent or movable structures such as surge barriers and sea walls, enhancing nature-based defenses such as wetlands, and selective retreat of populations and facilities from areas threatened by episodic flooding or permanent inundation, are being planned or implemented in several countries," the authors write.

But it might not be enough.

"Our findings support the use of scenarios of 21st century global total (sea level rise) exceeding 2 m for planning purposes," they write. That translates to more than 6 feet, at the upper level of the estimates.

It doesnʻt stop there. Temperatures at those levels will progressively melt ice sheets, and the world in 2200 could see as much as 7.5 meters in sea level rise above todayʻs levels, the authors say. That represents a 25-foot rise.

Many of the best estimates of sea level change before now have not included the impacts of the ice sheets, because they are so extraordinarily difficult to model. They have not gotten easier to model, but both the Antarctic and Greenland ice sheets have been losing mass at an increasing rate, and the authors say melting ice sheets have now exceeded mountain glaciers in their contributions to sea levels.

In response, there has been "a focused effort by the glaciological community to refine process understanding and improve process representation in numerical ice sheet models." As scientists learn more, they also learn how much they still donʻt know, and the uncertainty about the future rises, the authors said.

The most common estimate of 1 meter of sea level rise by 2100 comes from the 5th International Panel on Climate Change report from 2013. Numerous studies since then have indicated that its estimates are significantly too conservative, and that temperatures and sea levels are rising and are expected to continue rising far faster than that study estimated.

Is there enough water on Earth for sea levels to rise as much as they suggest? There is. NOAA and others have calculated that if all the planetʻs ice melted, there is enough water and ice to increase sea levels by more than 200 feet. National Geographic mapped what that might look like. https://www.nationalgeographic.com/magazine/2013/09/rising-seas-ice-melt-new-shoreline-maps/

But much of the polar ice would remain frozen, so such catastropic sea levels are unlikely.

Still a possible 6-plus feet by 2100 is catastrophic enough for a coastal state like Hawai`i, and it is a level that most planning has not considered.

The Hawai‘i Climate Change Mitigation and Adaptation Commission last year recommended government and other agencies plan for 3.2 feet or 1 meter of ocean rise.

The Hawai`i Sea Level Rise Vulnerability and Adaptation Report of December 2017 also targets that level, but indicates it could come far earlier than 2100: "this magnitude of sea level rise could occur as early as year 2060 under more recently published highest-end scenarios." Thatʻs only 40 years out.

Some of the most aggressive estimates have come from the City of Honoluluʻs Climate Change Commission, which last year adopted numbers closest to those suggested by the new paper. That recommendation, adopted by the City, recommends the 6-foot number be used in planning for projects with long lifespans. Its sea level rise guidance is here.

You can view the impacts of 3 feet of rise a this site from the University of Hawai`i School of Ocean and Earth Science and Technology: 

There is extensive information on sea level rise in the Islands at the Pacific Islands Ocean Observing System website: 

© Jan TenBruggencate 2019