Warm weather conditions have always been considered contributors to melting ice sheets, but researchers have found that the rain can also accelerate ice loss, especially in the late summer and autumn.

In a study published in the journal Nature Geoscience, Samuel Doyle and colleagues gathered measurements for water pressure at ice sheets, ice motion and discharge from the river and combined them with records of heavy rains across the western portion of the Greenland ice sheet, capturing large-scale effects due to unusually warm but wet weather from late August to early September in 2011.

According to their findings, the cyclonic weather system caused a lot of surface runoff, overwhelming the ice sheet's drainage system, which led to a dramatic rise in ice flow throughout the ice sheet's western section and and 87 miles into its interior.

Principal investigator Alun Hubbard likened the situation to a city's sewage system overwhelmed temporarily by heavy rains. Ice sheet plumbing also involves a network of channels, pipes and cavities and these fill up with water as surface runoff finds its way to the ice sheet.

"[This leads] to flooding and high water pressures, which literally hydraulically lifts the ice sheet up off its bed ... sending it on its way," he explained.

Doyle adds that the timing of cyclonic events is also crucial. As the added rain occurred as the melt season ended, it coincided with the ice sheet's drainage at a point that it has started to close down. In other words, there was just too much water for the drainage system of the ice sheet to properly cope.

The accelerated ice motion was detected by all available GPS and satellite tracking stations, and the response was obvious in glaciers caving into waters and terminating on dry land.

The influence heavy rainfall events have were not previously considered as part of assessing flow and melt responses in ice sheets. The study's finding is significant because cyclonic conditions have been predicted to rise in number in the future. As the rains increase, then so will mass lost from the Greenland ice sheet.

At the moment, the Greenland ice sheet accounts for more than 0.7 mm of sea-level increase around the world per year, at least two times the rate that melt from the Antarctic ice sheet contributes.

Photo: Sascha Kohlmann | Flickr