Empirical constraints on ice-cliff calving

Abstract When calving cliffs at the margins of Earth’s ice sheets are taller than a critical height, runaway ice retreat may be triggered in a mechanism called the marine ice-cliff instability. However, this idea remains controversial, because tall cliffs for which this process applies have rarely been observed. Limited observations show that, consistent with the marine ice-cliff hypothesis, cliff heights rarely exceed ~100 m. But these observations neither constrain rates of retreat nor the potential for tall cliffs to fail in a self-sustaining process, resulting in poorly-constrained parameterizations of the instability. Here we address this limitation by examining ice cliffs across Earth’s major ice-sheet margins using laser altimetry and optical imagery. We show that cliffs taller than 100 m are rare but exist in diverse settings. Calving rates increase with cliff height and exceed existing parameterizations by up to a factor of 3, but we do not observe runaway retreat. Instead, we find that dynamic thinning combined with retreat into regions with small ice thickness gradients eventually suppresses ice-cliff growth. These results suggest that projections of future ice loss may significantly underestimate retreat by cliff-calving, but that the potential for instability will depend sensitively on quantities like ice thickness gradients, not solely on cliff heights as current parameterizations assume.