Ice-sheet instabilities widen possibilities of future sea-level rise

Glaciers flowing into the sea will somewhere lose contact with the bedrock underneath and start to float on the ocean. That location, called the grounding line, depends sensitively on the ice thickness; so too does the rate of ice flow. And if the bedrock slopes inward, away from the ocean, positive feedback between those dependencies produces the so-called marine ice-sheet instability: An inward fluctuation of the grounding line triggers an accelerating—and potentially unstoppable—retreat of the ice sheet that raises global sea levels precipitously (see Physics Today, July 2014, page 10 ).

Alexander Robel (Georgia Tech) and colleagues have developed a framework to understand how the dynamics associated with the marine ice-sheet instability amplify the uncertainty in predictions of future sea-level rise. To do so, they combined two complementary approaches: stochastic perturbation theory applied to a minimal model incorporating the essential dynamics, and large ensemble simulations with a state-of-the-art model of Antarctica’s Thwaites Glacier. Their findings underline a feature of Earth’s changing climate: Even as external forcing factors such as the concentration of greenhouse gases rise steadily, internal fluctuations can lead to an amplified, nonlinear response.

The accompanying figure encapsulates a key result. It shows the widening spread in outcomes for one parameter, grounding-line retreat, from 10 000 simulations each of different internal climate variations in the minimal model. For comparison, the green band shows results for a bed sloping toward the sea, which doesn’t allow for the marine ice-sheet instability. The figure also illustrates a conclusion the researchers could make with certainty: The instability skews the distribution of possible outcomes, making faster retreat and greater sea-level rise more likely. (A. A. Robel, H. Seroussi, G. H. Roe, Proc. Natl. Acad. Sci. USA, 2019, doi:10.1073/pnas.1904822116 .)