A probabilistic framework for quantifying the role of anthropogenic climate change in marine-terminating glacier retreats

Many marine-terminating outlet glaciers have retreated rapidly in recent decades, but these changes have not been formally attributed to anthropogenic climate change. A key challenge for such an attribution assessment is that if glacier termini are sufficiently perturbed from bathymetric highs, ice-dynamic feedbacks can cause rapid retreat even without further climate forcing. In the presence of internal climate variability, attribution thus depends on understanding whether (or how frequently) these rapid retreats could be triggered by climatic noise alone. Our simulations with idealized glaciers show that in a noisy climate, rapid retreat is a stochastic phenomenon. We therefore propose a probabilistic approach to attribution and present a framework for analysis that uses ensembles of many simulations with independent realizations of random climate variability. Synthetic experiments show that century-scale climate trends substantially increase the likelihood of rapid glacier retreat. This effect depends on the timescales over which ice dynamics integrate forcing. For a population of synthetic glaciers with different topographies, we find that external trends increase the number of large retreats triggered within the population, offering a metric for regional attribution. Our analyses suggest that formal attribution studies are tractable and should be further pursued to clarify the human role in recent ice-sheet change. We emphasize that early-industrial-era constraints on glacier and climate state are likely to be crucial for such studies.

Automated summary

Many marine-terminating outlet glaciers have been rapidly retreating in recent years, but it is still unclear whether this is due to anthropogenic climate change or natural climate variability. A study using simulations of idealized glaciers suggests that rapid retreat is a stochastic phenomenon in a noisy climate. They propose a probabilistic approach to attribution and provide a framework for analysis using ensembles of simulations with random climate variability. The study shows that century-scale climate trends increase the likelihood of rapid glacier retreat. This research emphasizes the importance of further pursuing formal attribution studies to understand the human role in recent ice-sheet changes.