Better approach toward projecting, planning for rising sea levels on a warmer Earth

it. Sea-level rise projections should reflect these uncertainties.

“Recently, several groups have used alternative techniques to forecast maximum possible sea levels — known as upper bounds — that do not explicitly model ice dynamics. Upper bound estimates by the year 2100 projected using these techniques are up to six feet (three times higher than future sea level estimates from the Intergovernmental Panel on Climate Change (IPCC)). However, the physical basis underlying these projections and their likelihood of occurrence remain unclear.

“In our group, we think we can more consistently assess disparate sources of information. In two recent papers, we introduce a novel framework for projecting the mass balance of the Antarctic ice sheet that allows for the conversion of current and future uncertainties of ice-sheet dynamics into probability distributions that may be supplemented by expert judgments. The power of this framework arises from its ability to improve and compare projections in a transparent manner.

“Like watersheds on land, ice sheets discharge precipitation that falls over a wide drainage basin through relatively narrow outlets. Although ice flow is linked across basins, each basin may remain relatively independent over time periods less than a century. The framework described in these two papers projects mass balance separately for each drainage basin, while allowing for correlated trends driven by underlying physical processes occurring at larger spatial scales.

“The first paper, published in PNAS, introduces this ‘basin-by-basin’ framework and reveals that, even with limited information, a comprehensive probabilistic approach can provide insight that is missing from previous projections. We performed sensitivity analyses by changing the set of assumptions applied to each basin. For each set of assumptions, Monte Carlo simulations [computer algorithms based on random sampling] were used to generate 30,000 to 50,000 scenarios of mass changes originating from each basin and the continent as a whole.

“In previous scenario-based projections, the contribution of Antarctica to future sea-level rise is almost entirely derived from locations where present-day mass loss is concentrated. This is despite evidence that future discharge in other drainage basins — which comprise more than 96 percent of the ice sheet’s area — remains uncertain.

“By incorporating the entire ice sheet, the PNAS study demonstrated that uncertainty in ice discharge outside regions where scientists ‘expect’ ice loss might result in additional sea-level rise that must be considered in projections. In addition, we quantitatively show that the likelihood of upper bounds must be taken into account when assessing their magnitude and appropriate uncertainty reduction efforts.

“The second paper, published in Nature Climate Change, extended the framework to include Bayesian updating, which allows prior assumptions to be updated as new data are collected. We combined model-based basin-level projections with data-based extrapolations and previously reported continental-scale observations to forecast the Antarctic contribution to sea-level change.

“The paper projected a 95th percentile ice-mass loss equal to a 13-centimeter (5.1-inch) increase in sea level by 2100; other estimates provide upper bounds reaching up to 60 centimeters (roughly 23.5 inches), but with no quantification of probability. This paper suggests that most earlier projections either overestimated Antarctica’s possible contribution to sea-level rise; implied physical changes inconsistent with underlying methodological assumptions; or, assume an extremely low risk tolerance.

“Future work on this framework includes further addressing inconsistencies in different methodologies, which will continue to refine the range of upper-bound sea-level projections. Our group also intends to include the solid earth and gravitational response that modulates sea-level changes at the local level, allowing the generation of a global map of the local probability distribution of sea-level rise.”

Both papers were funded by the Princeton Environmental Institute’s Carbon Mitigation Initiative, and Princeton University’s Program in Science, Technology and Environmental Policy.

— Read more in Christopher M. Little et al., “Probabilistic framework for assessing the ice sheet contribution to sea level change,” Proceedings of the National Academy of Sciences (10 January 2013) (doi: 10.1073/pnas.1214457110); and Christopher M. Little et al., “Upper bounds on twenty-first-century Antarctic ice loss assessed using a probabilistic framework,” Nature Climate Change (17 March 2013) (doi:10.1038/nclimate1845)