The Ice Factor: New Info on Increased Sea Level Rise

This map of Kapa‘a shows flooding with 3 feet of sea level rise. Blue areas indicate water depth, with dark blue being the deepest. Green areas indicate low-lying areas not hydrologically connected to the ocean. NOAA/UH SeaGrant

A new study of the West Antarctic Ice Sheet published in the journal Nature suggests if greenhouse gas emissions remain unabated, global sea levels could be twice as high as projected.

The study by Robert M. DeConto and David Pollard presents evidence the collapse of the WAIS alone has the potential to contribute over a meter of sea level rise (SLR) by 2100 and more than 15 meters by 2500. Adding this figure to the thermal expansion of the oceans and the melting of other glaciers results in about 2 meters of SLR by 2100.

Scientists have always known the biggest wildcard in predicting SLR is how fast and how much the ice sheets will melt. Therefore, this news is not unexpected by the scientific community, but its implications for coastal communities have provided renewed urgency.

This map of Kapa‘a shows flooding with 6 feet of sea level rise. Blue areas indicate water depth, with dark blue being the deepest. Green areas indicate low-lying areas not hydrologically connected to the ocean. NOAA/UH SeaGrant

The Intergovernmental Panel on Climate Change, a scientific body that peer reviews all climate studies, published its fifth report in 2013, showing a range of SLR projections based on a range of greenhouse gas emission scenarios. The range is 10-36 inches by 2100.

These projections only factored in smaller scale ice melt in Greenland and Antarctica; larger scale ice cliff collapses were not accounted for because the physics of ice cliff collapse had not been well understood.

Dr. Chip Fletcher, of the University of Hawai‘i School of Ocean and Earth Science and Technology, said this is the first time a study has defined the math around ice sheet collapse, and additional research is needed to define other ice collapse processes — which could add to the numbers.

The Wilkins Ice Shelf in West Antarctica experienced several disintegration events in 2008. Credit: NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER

Asked about the differences between Greenland and Antarctic ice sheets, Fletcher said, “Both ice sheets are very complex, one is not simpler than another, and both have idiosyncratic features that are not well understood.”

Greenland is mainly comprised of outlet glaciers that are land bound and susceptible to rapid air temperature warming and meltwater lubrication. The WAIS has these and also has floating ice shelves that melt by a pattern of warm ocean water underneath and warm air above. What DeConto and Pollard modeled was the landward retreat of ice into progressively deeper water-filled valleys. The loss of this previously buttressed inland ice greatly accelerates the rate of SLR.

In Hawai‘i, one foot of SLR by 2050 and three feet by 2100 has been considered a rule of thumb. If the study is accurate, we could see a six-foot scenario by 2100 or earlier if emissions continue unabated. SLR is expected to continue over many centuries.

Ruby Pap

Fletcher said the research paints a picture of circumstances under business-as-usual greenhouse emissions. However, there are signs the world is taking steps to curb emissions, such as the 2015 COP 21 global climate agreement, which set a target of limiting global temperature increases to 2 degrees Celsius above pre-industrial levels. China and the U.S. signed this agreement.

“I think with China turning away from coal it won’t be as bad as business as usual. So the word from DeConto is bad but the word from China is good — they offset each other and we are right back at the same rule of thumb. But stay tuned, the picture is ever-changing,” Fletcher said.

• Ruby Pap is a Coastal Land Use Extension Agent at University of Hawai‘i Sea Grant College Program. She can be reached at rpap@hawaii.edu.