Empirical constraints on ice cliff calving.

When calving cliffs at the margins of Earth's ice sheets exceed a critical height, runaway retreat may be initiated in a process called the marine ice cliff instability. However, sufficiently tall ice cliffs have rarely been observed. To address this gap, we measured the characteristics of 15,330 ice cliffs across Earth's ice sheet margins using laser altimetry. We show that cliff height is highly correlated with mélange thickness, water depth, and crevassing. We found that tall cliffs calve more rapidly than parameterizations assume, but dynamic thinning and retreat into regions with low ice thickness gradients prevent runaway retreat. These observations suggest that model projections may underestimate subannual rates of cliff calving, but that the potential for instability will depend sensitively on additional quantities, not cliff height alone.