The glaciers in the peripheral regions of Greenland and Antarctica have significantly increased their flow rate in recent decades. They transport more ice into the adjacent oceans and thus contribute to rising sea levels. Scientists around the world are using computer models to predict how climate change will affect glaciers and sea levels in the future. The more precisely the models can reproduce the physical processes of the glacier’s flowing, the more reliable the results of the computer simulations will be.
However, there are some as yet unanswered questions. For example, it is not yet known exactly how the unevenness of the rock bed affects the friction of the ice layers above it and, with that, the flow of the glacier. Bo Persson from the Peter Grünberg Institute at Jülich has now contributed to a fundamental understanding of the sliding process. He used a theory of contact mechanics, which he developed himself and has already applied successfully. At the same time, he also took into account the work of other scientists from the 1970s who had, for example, dealt with the phenomenon of regelation: Due to its special chemical structure, ice can melt when the pressure on it increases and freeze again when the pressure decreases.
What is particularly significant in the new mathematical model: “It shows that cavities form when glaciers glide over the ground at a typical speed,” says Persson. “For glaciers 1,000 metres and higher, such as the polar caps, the temperature of the ice on the glacier floor is at the melting point. The reason for this is the geothermal heat from the earth’s interior, which cannot be dissipated upwards – the ice is too thick. The cavities thus fill with water. This not only forms a kind of lubricating film between the glacier and the ground, which reduces friction. It presses against the layers of ice above and thus carries part of the glacier mass along with it: the glacier slides faster.” The sliding speed calculated by the researcher by means of his model corresponds to the values observed in nature. This is an important indication that Persson’s model reflects reality well. In principle, the interface between ice and rocky ground is difficult to access for research because the glacier lies above it.
Persson came across the glaciologists’ questions at a workshop held by the Alfred Wegener Institute, the Helmholtz Centre for Polar and Marine Research. He has been dealing with friction and related phenomena for more than twenty years, but initially with completely different objects: it had to do with the interactions between tyres and roads. Later, Persson researched, among other things, the physics of lubricating rubber seals and the friction on ice. Only recently has he theoretically described the sliding friction of a finger on haptic touchscreens. It is always essential to take into account the roughness of the respective surfaces on different length scales, from a thousandth of a millimetre to a metre, when two objects come into contact.
In glaciers, for example, the roughness of the bedrock is responsible for the phenomenon of regelation: the alternating melting and freezing of the ice is due to local pressure fluctuations, which in turn are caused by the unevenness of the ground. Persson explains the phenomenon by means of a bump: “The moving glacier pushes the ice against this bump from one side. At higher pressure, however, a lower temperature than usual is sufficient to break up the crystal structure of the ice and melt it.” On the other side of the bump, the pressure of the glacier is reduced and the melting point of the ice increased.