It has become one of today's iconic symbols. The melting of the Greenland Ice Sheet has made headlines around the world. While not disputing the fact that Greenland is losing ice, scientists tell a slightly different story from the headline writers. The Greenland Ice Sheet is not so much melting as being transported ever more rapidly into the sea, and while few scientists doubt that this is happening as a result of climate change, many of the mechanisms involved are only now being uncovered.
In recent years considerable attention has been focused on the effect which climate change is having on the large continental ice sheets of Greenland and Antarctica. So far the East Antarctic ice sheet, which contains the vast majority of the Earth's ice, has shown little sign of change, but the smaller Greenland and West Antarctic ice sheets are both displaying significantly altered behaviour patterns. Each of these ice sheets contains enough water to raise global sea levels by around seven metres, so any significant melting of these reserves is a cause for concern.
Though the primary driver for the melting of the continental ice sheets is climate change, many of the mechanisms involved are complex and in many cases are not yet fully understood. In a landmark study published in Science in 2006, Rignot and Kanagaratnam estimated total mass loss from the Greenland ice sheet. Their conclusion was that two thirds of the ice being lost from the Greenland ice sheet was occurring because of increased flow rates of the major outlet glaciers. These glaciers carry large volumes of ice from the interior to the coast. This ice then breaks off the terminus of the glacier into the ocean, where it forms icebergs, a process known as calving. Only one third of ice loss was found to be occurring directly because of increased surface melt in the summer.
The role of the outlet glaciers is therefore the key to understanding the future response of the Greenland ice sheet to climate change. The largest outlet glacier in Greenland is Jakobshavn Isbrae. This glacier drains a staggering 100,000 square kilometres of the ice sheet. It is also one of the fastest moving glaciers on Earth. In the mid 1990s it was moving at an average of around six kilometres per year. By the year 2005, the average surface velocity of this glacier had increased to over twelve kilometres per year. Similar increases in velocity have been measured for most of Greenland's outlet glaciers. This means that the annual loss of ice from the Greenland ice sheet has doubled over a fifteen year period.
Clearly if this trend continues, it will have a major impact on sea levels. In recent years scientists have made significant progress in understanding why Greenland's outlet glaciers are speeding up so dramatically. In part it is due to increased melting at the surface. The meltwater gathers in ponds on the surface of the ice. Often weaknesses in the ice lead to sudden and catastrophic draining of these meltwater ponds. Much of this meltwater manages to find its way down to the base of the ice sheet, often through more than one kilometre of ice. Increased water at the base of the outlet glaciers provides additional lubrication, which lessens the friction between the glacier and its bed, leading to faster flow rates.
The most significant effect however is believed to be due to warmer ocean temperatures. In another paper published in Science in 2006, Bindschadler suggested that because of increased stratification in the mid layers of the ocean, warmer saltier water is now penetrating far into the fjords in which the outlet glaciers terminate. This warmer water pools around the base of the outlet glaciers, causing them to melt from beneath. The direct consequence of this melting is that it causes the grounding line of the glacier retreat. The grounding line is the point where the base of the glacier loses contact with its bed as it enters the ocean. Beyond this point the terminus of the glacier is actually floating. As the grounding line retreats, resistance to flow at the bed of the glacier becomes lessened, and so the glacier speeds up. Additionally it is believed that in the case of Jakobshavn Isbrae this influx of warmer water also contributed to the break up of a floating ice shelf at the glacier terminus. This ice shelf helped to constrain the glacier flow by providing backwards pressure against the glacier. Following the break up of this ice shelf a rapid increase in the glacier flow rate was observed.
This highlights the global scale of the problem. Increased ice loss in Greenland appears to be a direct consequence of warming in the mid ocean layers, resulting from increasing stratification of ocean water. As such this problem can only be tackled by reducing the uptake of heat by the ocean. Unfortunately declining sea ice levels in the Arctic mean that the ocean is now absorbing significantly more solar energy each year. Serious attempts to reduce carbon emissions may eventually have some effect, but due to the considerable inertia in the system it is likely that in the short term at least, mass loss from the Greenland ice sheet will continue to increase.
While it is unlikely that we are witnessing the collapse of the Greenland ice sheet at this stage, it is likely that the coming decades will see significant rises in sea levels, as increased rates of ice loss occur. To reverse this pattern on any meaningful timescale will take major cuts to greenhouse emissions, and even this may not be sufficient. The fear is now that we have now set in motion a series of events which are now beyond our control. Talk of any action which will stop or reverse the loss of ice from Greenland is just that; talk. We cannot turn back the clock and stop the loss of ice from the Greenland ice sheet. Our best hope is that it will reach a new equilibrium within a few years.
Rignot, E. and P. Kanagaratnam, Changes in the Velocity Structure of the Greenland Ice Sheet. Science, 2006. 311(5763): p. 986-990.
Bindschadler, R., CLIMATE CHANGE: Hitting the Ice Sheets Where It Hurts, in Science. 2006. p. 1720-1721.