Scientists used fiber optic sensing to get the most detailed measurements of ice properties ever taken on the Greenland ice sheet. Their results will be used to create more accurate models of the future movement of the world’s second largest ice cap, as the effects of climate change continue to accelerate.
The research team, led by the University of Cambridge, used a new technique in which laser pulses are transmitted through a fiber-optic cable to obtain highly detailed temperature measurements from the surface of the ice cap to the base, more than 1000 meters below.
Unlike previous studies, which measured temperature from separate sensors tens or even hundreds of meters apart, the new approach allows temperature to be measured along the entire length of a fiber-optic cable installed in a deep borehole. The result is a very detailed temperature profile, which controls the deformation of the fast ice and, ultimately, the rate at which the ice sheet is draining.
The temperature of the ice caps was thought to vary as a smooth gradient, with the hottest sections of the surface where the sun hits, and at the base where it is warmed by geothermal energy and friction as the ice sheet creaks. through the subglacial landscape towards the ocean. .
Rather, the new study found that the temperature distribution is much more heterogeneous, with areas of very localized deformation warming the ice further. This deformation is concentrated at the boundaries between ice of different ages and types. Although the exact cause of this deformation remains unknown, it may be due to dust in the ice from past volcanic eruptions or large fractures that penetrate several hundred meters below the ice surface. The results are reported in the journal Scientific progress.
The loss of mass from the Greenland ice sheet has increased six-fold since the 1980s and is now the main contributor to the global rise in sea level. About half of this mass loss comes from runoff. surface meltwater, while the other half is due to the discharge of ice directly into the ocean by fast-flowing glaciers that reach the sea.
In order to determine how ice moves and the thermodynamic processes at work in a glacier, accurate measurements of ice temperature are essential. Surface conditions can be detected by satellites or field observations in a relatively simple way. However, determining what is happening at the base of the kilometer-thick ice sheet is much more difficult to observe, and the lack of observations is a major cause of uncertainty in projections of the global elevation of the sea. sea level.
The RESPONDER project, funded by the European Research Council, addresses this problem by using hot water drilling technology to drill through Sermeq Kujalleq (Glacier store) and directly study the environment behind one of the most great glaciers of Greenland.
“We normally take measurements in the ice cap by attaching sensors to a cable that we descend into a drilled borehole, but the observations we’ve made so far don’t give us a full picture of what’s going on,” said co-author Dr Poul Christoffersen of the Scott Polar Research Institute who is leading the RESPONDER project. “The more accurate data we can collect, the clearer we can make that picture, which in turn will help us make more accurate predictions for the future of the ice sheet.”
“With typical detection methods, we can only attach a dozen sensors to the cable, so the measurements are widely spaced,” said first author Robert Law, a doctoral student at the Scott Polar Research Institute. “But by using a fiber optic cable instead, the entire cable essentially becomes a sensor, allowing us to get precise measurements from the surface to the base.”
To install the cable, scientists first had to drill through the glacier, a process led by Professor Bryn Hubbard and Dr Samuel Doyle of the University of Aberystwyth. After lowering the cable into the borehole, the team transmitted laser pulses through the cable and then recorded the light scattering distortions in the cable, which vary with the temperature of the surrounding ice. Engineers from Delft University of Technology in the Netherlands and geophysicists from the University of Leeds assisted in data collection and analysis.
“This technology represents a significant advance in our ability to record spatial variations in ice temperature over long distances and at very high resolution. With some further adaptations, the technique can also record other properties, such as deformation, at a similar resolution, ”Hubbard said.
“Overall, our readings paint a much more varied picture than what current theory and models predict,” Christoffersen said. “We found that the temperature was strongly influenced by the deformation of the ice in bands and at the boundaries between different types of ice. And it shows that there are limits in many models, including ours.
The researchers found three layers of ice in the glacier. The thickest layer is the stiff, cold ice that has formed over the past 10,000 years. Underneath, they found older ice from the last Ice Age, which is softer and more deformable due to the dust trapped in the ice. What surprised researchers the most, however, was a layer of hot ice more than 70 meters thick at the foot of the glacier. “We know of this type of hot ice coming from much warmer alpine environments, but here the glacier produces heat as it warps,” Law said.
“With these observations, we are starting to better understand why the Greenland ice sheet is losing mass so quickly and why ice discharge is such an important mechanism of ice loss,” Christoffersen said.
One of the main limitations of our understanding of climate change relates to the behavior of glaciers and ice caps. The new data will allow researchers to improve their models of how the Greenland ice sheet is moving now, how it might move in the future, and what that will mean for global sea level rise.
The research was partially funded by the European Union.