Marine ice sheet instability amplifies and skews uncertainty in projections of future sea-level rise

Abstract

Sea-level rise may accelerate significantly if marine ice sheets become unstable. If such instability occurs, there would be considerable uncertainty in future sea-level rise projections due to imperfectly modeled ice sheet processes and unpredictable climate variability. In this study, we use mathematical and computational approaches to identify the ice sheet processes that drive uncertainty in sea-level projections. Using stochastic perturbation theory from statistical physics as a tool, we show mathematically that the marine ice sheet instability greatly amplifies and skews uncertainty in sea-level projections with worst-case scenarios of rapid sea-level rise being more likely than best-case scenarios of slower sea-level rise. We also perform large ensemble simulations with a state-of-the-art ice sheet model of Thwaites Glacier, a marine-terminating glacier in West Antarctica that is thought to be unstable. These ensemble simulations indicate that the uncertainty solely related to internal climate variability can be a large fraction of the total ice loss expected from Thwaites Glacier. We conclude that internal climate variability alone can be responsible for significant uncertainty in projections of sea-level rise and that large ensembles are a necessary tool for quantifying the upper bounds of this uncertainty.

Assessing Uncertainty in the Dynamical Ice Response to Ocean Warming in the Amundsen Sea Embayment, West Antarctica

“…The similarities between our results and those shown in Figure 3 of Gladstone et al (2012), despite the significant differences in model physics, indicates that topography, as well as the forcing, exerts a strong control on the temporal form of Pine Island Glacier grounding line retreat. Robel et al (2019) demonstrate that an ensemble becomes progressively more skewed toward greater retreat when the grounding line is located on a predominantly retrograde bedrock slope, because the rate of retreat in the extreme ensemble members diverges further away from the more moderate members, which is seen here in Figure 3. The skewness in the distribution toward the high end of sea level rise is fundamentally linked to the non-linearity in the rate of grounding line retreat (Robel et al, 2019).…”

Effect of Subshelf Melt Variability on Sea Level Rise Contribution From Thwaites Glacier, Antarctica

“…An additional reason for keeping the model simpler was to reduce model cost to allow many ensembles to be run. The recent study by Robel et al (2019) also investigating the impact of ocean variability on evolution of Thwaites Glacier included the same modeling limitations listed above, presumably for similar reasons. Future studies should explore what interactions these other effects may have with variability-induced delay and uncertainty.…”

“…In Figure a, we plot “fractional uncertainty,” defined by Robel et al () as 4 σ vol / μ vloss , where σ vol is the standard deviation of ice volume across the ensemble over the course of the simulation and μ vloss is the total ice loss at the end of the simulation averaged over the ensemble. Our ensembles exhibit a similar general shape, with slowly growing fractional uncertainty that increases substantially when the grounding line retreats through large overdeepened basin in the interior of Thwaites basin (at Years 300–500 in our simulations and Years 550–750 in their simulations) and then decreases.…”

“…The ice dynamic feedback we identify in this study appears to be strongest through overdeepened basins, and therefore, using a nonlinear basal friction law may result in longer delays under variable ocean forcing than we demonstrate here. Robel et al () demonstrate that use of a more nonlinear basal friction law will skew distributions toward greater mass loss under variable ocean forcing. However, their theory does not assess how the skew relates to the case of steady forcing, so it is unclear how it relates to our conclusions here.…”

“…The spread of the ensemble is also proportional to the timescale of the variability in the forcing, a relationship we also see. Robel et al (2019) use 500-member ensembles of an ice sheet model of Thwaites Glacier with many similarities to ours (fixed ice temperature, calving front, and basal friction; linear viscous basal friction law; basal friction optimized to present-day ice surface velocity; and 1-km mesh resolution) and a few notable differences (mass-conserving bed topography instead of BEDMAP2; two-dimensional shelfy-stream approximation instead of three-dimensional Blatter-Pattyn; a simpler depth-based melt-parameterization; different treatment of melt near the grounding line; and variability applied directly to the magnitude of melt rate at the grounding line instead of to ocean temperatures). They find that over the course of their 850-year simulation, ocean variability causes uncertainty of up to nearly 50% of the total mass loss at the end the simulation.…”