'Hidden plumbing' helps slow Greenland ice flow: Hotter summers may actually slow down flow of glaciers

A letter published in Nature on 27 January explains how increased melting in warmer years causes the internal drainage system of the ice sheet to 'adapt' and accommodate more melt-water, without speeding up the flow of ice toward the oceans. The findings have important implications for future assessments of global sea level rise.

The Greenland ice sheet covers roughly 80% of the surface of the island and contains enough water to raise sea levels by 7 metres if it were to melt completely. Rising temperatures in the Arctic in recent years have caused the ice sheet to shrink, prompting fears that it may be close to a 'tipping point' of no return.

Some of the ice loss has been attributed to the speed-up of glaciers due to increased surface melting. Each summer, warmer temperatures cause ice at the surface of the sheet to melt. This water then runs down a series of channels to the base of the glacier where it acts as a lubricant, allowing the ice sheet to flow rapidly across the bedrock toward the sea.

Summertime acceleration of ice flow has proved difficult for scientists to model, leading to uncertainties in projections of future sea level rise.

"It had been thought that more surface melting would cause the ice sheet to speed up and retreat faster, but our study suggests that the opposite could in fact be true," said Professor Andrew Shepherd from the University of Leeds School of Earth and Environment, who led the study.

"If that's the case, increases in surface melting expected over the 21st century may have no affect on the rate of ice loss through flow. However, this doesn't mean that the ice sheet is safe from climate change, because the impact of ocean-driven melting remains uncertain."

The researchers used satellite observations of six landlocked glaciers in south-west Greenland, acquired by the European Space Agency, to study how ice flow develops in years of markedly different melting.

Although the initial speed-up of ice was similar in all years, slowdown occurred sooner in the warmest ones. The authors suggest that in these years the abundance of melt-water triggers an early switch in the plumbing at the base of the ice, causing a pressure drop that leads to reduced ice speeds.

This behaviour is similar to that of mountain glaciers, where the summertime speed-up of ice reduces once melt-water can drain efficiently.

Study co-author Dr Edward Hanna from the University of Sheffield added: "This work also underlines the usefulness of modern gridded climate datasets and melt-model simulations for exploring seasonal and year-to-year variations in Greenland ice sheet dynamics and their relationship with the global climate system."

The study was funded by the Natural Environment Research Council's National Centre for Earth Observation, the Philip Leverhulme Trust, and by the European Commission Ice2Sea project.