The plumbing of ice sheets

One of the largest uncertainties in forecasts of the melting ice caps is the role played by water at the base of the ice in lubricating ice flow. Recent observations of the Greenland Ice Sheet have shown that summer meltwater penetrating from the ice surface substantially affects the rate at which the ice slides. Faster sliding would quicken the rate at which ice is lost from the ice sheet, but the observations to date are inconclusive about the overall effect of the meltwater: some indicate that speed-up does indeed occur, whilst others suggest more melting actually leads to a slow-down of the ice.

I will present efforts to understand this behaviour using mathematical models. I will discuss the current hypotheses for what is happening to the drainage system beneath the ice. In the simplest description, water flows through a sequence of connected cavities formed as the ice slides over small-scale bumps in the bed. The water flow generates heat, melting the ice and causing the formation of river-like channels. This evolution of the drainage system significantly alters its capacity to drain the water from the crevasses, and ultimately controls the rate at which the ice slides. I will show calculations of ice flow for different amounts of melting to investigate the impact of recent climatic changes on the Greenland Ice Sheet.