Future sea level rise poses serious threats to the viability of coastal communities but is challenging to project using deterministic modeling approaches.

Now, in a study published May 20 by Proceedings of the National Academy of Sciences (PNAS), an international team of experts provide research on ways to predict sea level rise utilizing Structured Expert Judgment, or SEJ. 

Using this technique, the team of scientists asked 22 ice sheet experts to estimate plausible ranges for future sea level rise due to the projected melting of each of the Greenland, West Antarctic, and East Antarctic ice sheets under low and high future global temperature rise scenarios.

The projections yielded a small but meaningful probability of total global sea level rise exceeding two meters by the year 2100 under the high temperature scenario, which is well above the ‘likely’ upper limit presented in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).

The findings suggest that coastal communities should not rule out the possibility of 21st-century sea level rise in excess of two meters when developing adaptation strategies.

“Past IPCC assessments haven’t placed a number on the chances of high-impact, low-probability occurrences, like those short of a one-in-six roll of the dice,” said co-author Michael Oppenheimer, Albert G. Milbank Professor of Geosciences and International Affairs and the Princeton Environmental Institute at Princeton University’s Woodrow Wilson School of Public and International Affairs. “According to the participating experts, if emissions continue on a business-as-usual path, the probability of the ice sheets adding a meter or more to sea level in this century falls in that category, and that’s a risk we shouldn’t take.” 

The SEJ process provided an opportunity for experts to discuss their scientific rationales for the quantitative judgments they make on uncertainties relating to future ice sheet contributions to sea level. This unique approach also served to identify some poorly understood but potentially critical processes, such as “marine ice cliff instability,” which may act in future as significant tipping points in ice sheet response to temperature rise.

Although projections of future sea level rise cannot be validated, the ability of experts to quantify uncertainty from their field of expertise can be, and has been, statistically validated. A key innovation in this work is the modelling of randomly determined dependences between the processes of accumulation, runoff, and discharge at each ice sheet — techniques, say the research team, that they hope can be further developed and applied in other areas of climate research.

In addition to Oppenheimer, the authors are Jonathan Bamber, University of Bristol; Robert Kopp, Rutgers University; Willy Aspinall, University of Bristol, and Roger Cooke, Resources for the Future (RFF).

The paper, “Ice Sheet Contributions to Future Sea-Level Rise from Structured Expert Judgement,” first appeared online May 20 in PNAS.