The ice cover of Greenland is the second largest expanse of ice in the world, and hence, plays a crucial and determinative role in the dynamics of global warming and how it affects the world. Thus, any changes in the volume or location of the ice sheets and glaciers would have a drastic global effect. Due to the current trend of climate change spurred on by global warming, the melting of these ice reserve is a critical issue that will determine the climatic events of the future. Any sudden melting or shift could result in a significant rise of sea levels, causing a majority of islands and coastal cities to be flooded and/or submerged. This would greatly reduce the area of inhabitable as well as agricultural land, leading to overcrowding and a shortage of food supply. In light of these implications, any inexplicable change in the ice cover needs to be thoroughly examined and investigated, in order to provide a strategic plan of action to counteract the effect of the change.
In 2006, two supra glacial lakes on the ice sheets of Greenland mysteriously disappeared. These lakes were the result of accumulation of melted ice over a period of several decades, and hence, contained billions of gallons of water. Therefore, their sudden disappearance was quite alarming. On investigation, scientists discovered that the lakes had vanished due to rapid drainage of the water over the course of a couple of hours. However, the exact reason for this drainage was still undetermined.
Two years later, a study proposed a tentative cause for the drainage, but was still unable to fully explain it. It was only recently, in 2015, that the exact process was finally elucidated and understood.
How Do Greenland’s Lakes Disappear?
◆ Most supra glacial lakes found in the ice sheets of Greenland drain gradually in the presence of nearby permanent crevasses called moulins, which are basically vertical shafts spanning from the glacier surface all the way down to the bedrock. The water from the lakes enters these moulins, and is drained slowly towards the bedrock, but this is a very slow process, and has not been observed to completely drain a lake. However, recently, a few supra glacial lakes have been noted to drain rapidly within a few hours. The North Lake was the first such disappearing lake to be observed (2006). Its cumulative volume of approximately 12 billion gallons of water was drained in a total time of just 90 minutes. This rapid drainage in such a short time suggests that, the water gushed out of the lake at speeds higher than those of the Niagara falls in the US.
◆ This observation was inconsistent with the general understanding of the function of moulins. Hence, researchers proposed that an additional factor must have contributed to the disappearance of the lake. With this in mind, a team of scientists studied the glacier using GPS (global positioning system) stations. These stations not only recorded the movement of the glacier, but also recorded data regarding the surface of the ice during the draining event. Analysis of the data revealed that, during draining, a large hydro fracture (fracture induced by water pressure) occurred beneath the basin of the lake, thereby expanding the moulin, and encouraging rapid draining. But this did not provide any clues as to what triggered the fracture.
◆ In order to investigate the trigger, 16 GPS stations were then arranged concentrically around the lake, and the data gleaned from them finally elucidated the whole phenomenon. According to the data, for a initial period of approximately 12 hours, water seeped down to the bedrock via the moulins. The accumulation of this water at the bedrock and below the glacier exerted an upward force on the glacier, causing it to rise and starts fracturing. This fracturing expanded the moulins, and resulted in complete and speedy drainage of the entire volume of water.
◆ Based on the data, the researchers surmised that, when the amount of water at the bedrock is low, the weight of the ice itself promotes further compaction of the glacier. But when the water level passes a critical amount, it develops tension, and pushes the lower levels of the glacier upwards in a rudimentary dome shape. This induces the formation of cracks along the column of the glacier, resulting in widened moulins and disappearing lakes.
◆ Additionally, the excessive water at the bedrock also facilitates faster movement of the glacier, as the water acts as a buffer between the ice mass and the bedrock surface, thereby reducing friction. The faster movement of glaciers could lead to them entering the seas, where after melting, they will increase the water levels drastically.
This phenomenon needs to be further investigated to determine its distribution and occurrence across all the major glacial surfaces of the world. This is because, as the environment gets warmer, more supra glacial lakes will be formed in such regions, which could in turn create hydro fractures, and contribute to faster glacial melting and movement, eventually causing a significant rise in ocean water levels.