The Ice Sheet’s Hidden Drainpipes: Why Moulin Shape Changes Water Pressure Under Greenland

Takeaway: Under Greenland’s ice, the shape of a moulin near its waterline can decide whether a day’s meltwater hits the glacier bed like a pulse or a cushion. Greenland’s summer meltwater does not simply run off the ice sheet like rain down a street. Much of it pours into deep vertical shafts called moulins, then rushes to the glacier bed, where water pressure can influence how ice slides toward the sea. This episode matters because the hidden plumbing under ice helps shape sea-level forecasts, field safety, and how scientists connect a warmer atmosphere to changes in glacier motion. We unpack a modeling study that asks a surprisingly concrete question: does the shape of a moulin matter? Many glacier models treat moulins like simple cylinders, but field explorers have found shafts that widen, pinch, curve, and open into chambers. Trunz and colleagues test idealized moulin shapes—cylinders, cones, hourglasses, diamonds, bottles, and goblets—to see how each one stores and releases meltwater. The key result is not that every twist in the ice matters equally. What matters most is the moulin’s size and shape near the range where the water level rises and falls each day. That “dynamic storage” can either amplify or dampen pulses of water pressure reaching the bed. Along the way, we explain moulins, subglacial channels, water pressure, why glacier beds can behave like wet brakes or slippery floors, and why better observations inside these icy shafts are still hard to get. We also talk through what this kind of simplified model can and cannot tell us about a real ice sheet full of connected drainage pathways. Paper featured: Trunz, C., Covington, M. D., Poinar, K., Andrews, L. C., Mejia, J., & Gulley, J. (2022). Modeling the influence of moulin shape on subglacial hydrology. Journal of Geophysical Research: Earth Surface, 127, e2022JF006674. https://doi.org/10.1029/2022JF006674 Disclosure: This Waterlines episode package is written for production with AI-generated host voices.