Nature Climate Change | Climate Feedback

AGU 2009: Getting to the bottom of glacial melting

Harvey Leifert

It is pretty well established that Greenland glaciers are sliding into the ocean at an accelerating pace, due to greater volumes of meltwater lubricating their bases, which lie on bedrock. Well no, not exactly, say three researchers who have conducted extensive studies at Uummannaq, on Greenland’s west coast.

“Good stories die hard,” commented Ian Howat of Ohio State University, briefing the media at the American Geophysical Union’s Fall Meeting in advance of his session. He said the idea had been around at least since 2002, citing a comment that year by NASA glaciologist Jay Zwally, who wrote that warmer summer temperatures increased surface meltwater production and water flow to the base, and that basal sliding provides a mechanism for rapid response of the ice sheets to climate change. By “rapid,” Zwally probably meant decades to centuries, said Howat, but he was interpreted as saying weeks, months, or years.

In fact, Howat said, we are only beginning to understand the complexity of subsurface processes in Greenland’s massive glaciers, but the basal lubrication hypothesis has already been proved wrong. If basal meltwater caused glacial speedup, Howat said, we would probably see regular seasonal and even daily cycles of movement, and this is not the case.

Meltwater, seeping through cracks from surface lakes down through glaciers, does play a role, Howat said, but it is much more complicated than had been thought. The real key to the large-scale glacial accelerations of recent years is the glacial front, where it meets the ocean.

Using inexpensive GPS devices deployed on glaciers, Howat and his team have been measuring the 21st century meaning of “a glacial pace.” They have found that most of the time, there is no correlation between glacial meltwater at the base on an ice sheet and ice loss along the coast.

To better calculate the volume of water melting through a glacier, John Adler of the University of Colorado and colleagues are measuring surface lakes from the air. These lakes can suddenly vanish as they drain into the glacier. The water creates an intricate network of passages, called moulins, that regulate the flow and determine where water collects between ice and bedrock at various times of year.

This water certainly contributes to the movement of glaciers, said Howat, perhaps 100 metres per year, but along the coast, glaciers are flowing into the sea at rates as high as 10 kilometres per year, and the meltwater cannot explain that. Further, there is no apparent correlation between between peak meltwater in summer and the sudden acceleration in outlet glaciers. Some glaciers even accelerate in the dead of winter, when there is no significant meltwater, he noted.

Howat suggested that sudden increases in ocean temperature might be the explanation, affecting the friction that holds glacial outlets firm against the walls of the fjords through which they flow.

There is still much to be learned about how Greenland’s glaciers behave. Alberto Behar of NASA’s Jet Propulsion Laboratory described a series of attempts to investigate “what lies below,” by sending instruments deep into moulins. So far, all have failed. Behar, who worked on the Mars exploration rovers, told reporters that each year’s efforts have helped refine what is needed for a successful subglacial probe, and he remains hopeful of finding the right combination of materials and instruments.

Rubber duckies may provide part of the answer. Behar dropped 100 of them into moulins two years ago, each carrying an e-mail address for its finder to report its location when found. So far, however, not a single rubber ducky has reported in.