Roughly 80 per cent of Greenland's land surface is hidden under an ice sheet consisting of layers of compressed snow. It is accepted that approximately 100 billion tonnes of this ice are lost each year as the sheet progressively shrinks.
While most scientists agree that this process is accelerating, the reasons why have been poorly understood.
The traditional view has been that the ice sheet is shrinking simply because the world is getting warmer, but a new study, published in the journal Nature, indicates that a steady meltwater supply that results from gradual warming may in fact slow down glacier flow rather than speed it up.
On the other hand, sudden water input from storms could cause glaciers to speed up and spread, which in turn would lead to a faster melting process.
Christian Schoof, an assistant professor at the University of British Columbia's Department of Earth and Ocean Sciences is the study's author. He also holds a Canada Research Chair in Global Process Modeling.
“The conventional view,” says Professor Schoof, “has been that meltwater permeates the ice from the surface and pools under the base of the ice sheet. This water then serves as a lubricant between the glacier and the earth underneath it, allowing the glacier to shift to lower, warmer altitudes where more melt would occur.”
But Professor Schoof noted that during heavy rainfall, higher water pressure is required to force drainage along the base of the ice. He created computer models that account for the complex fluid dynamics occurring at the interface of glacier and bedrock. As a result he found that a steady supply of meltwater is well accommodated and drained through water channels that form under the glacier.
When sudden water input occurs, caused by short-term extremes such as massive rainstorms or the draining of a surface lake, this cannot easily be accommodated by existing channels. The water is allowed to pool and lubricate the bottom of the glaciers and accelerates ice loss.
Professor Schoof points out that this does not mitigate the issue of global warming, “… but it does mean that we need to expand our understanding of what's behind the massive ice loss we're worried about.”
The steady increase in temperature and short-term extreme weather conditions have both been attributed to global climate change. The European Environment Agency reports that ice loss from the Greenland ice sheet between 1993 and 2003 has contributed to an annual global sea-level rise of between 0.14 and 0.28 millimetres.
Professor Andrew Shepherd from the UK's University of Leeds is an expert on using satellites to study physical processes of the Earth's climate. He was not involved in the research, but he applauds the study as providing, “… an elegant solution to one of the two key ice sheet instability problems identified by the Intergovernmental Panel on Climate Change in their 2007 assessment report.”
“It now turns out,” said Professor Shepherd, “that contrary to popular belief, the Greenland ice sheet flow might not be accelerated by increased melting after all.”