CM – Fiber optics used to measure the temperature of the Greenland ice sheet


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May 14, 2021

from the University of Cambridge

Scientists have used fiber optic scanning to get the most detailed measurements of ice properties ever made 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 sheet as the effects of climate change continue to accelerate.

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The research team, led by Cambridge University, used a new technique that involves transmitting laser pulses in a fiber optic cable to get highly detailed temperature readings from the surface of the ice sheet to the base, more than 1,000 meters below.

Im Unlike previous studies, which measured the temperature by separate sensors tens or even hundreds of meters apart, the new approach allows temperature to be measured over the entire length of a fiber optic cable installed in a deep borehole. The result is a very detailed temperature profile that controls how fast the ice deforms and how fast the ice sheet flows.

It was assumed that the temperature of the ice sheet varied as a uniform gradient, with the warmest sections on the surface, where the sun hits, and at the base where it is heated by geothermal energy and friction as the ice sheet drags over the subglacial landscape towards the ocean.

Instead, the new study found that the temperature distribution is far more heterogeneous and areas with highly localized deformation continue to heat the ice. This deformation focuses on the boundaries between ice of different ages and types. Although the exact cause of this deformation is unknown, it may be due to dust in the ice from previous volcanic eruptions or large fractures that penetrate hundreds of meters below the surface of the ice. The results are published in the journal Science Advances.

The mass loss from the Greenland ice sheet has increased sixfold since the 1980s and is now the largest contributor to global sea level rise. About half of this mass loss is due to the runoff of surface meltwater, while the other half is driven by the drainage of ice directly into the ocean by fast-flowing glaciers reaching the ocean.

To determine how the ice is moving moving and which thermodynamic processes take place within a glacier, precise ice temperature measurements are essential. Conditions on the surface can be detected relatively easily by satellites or field observations. However, it is far more difficult to determine what is happening at the base of the kilometer-thick ice sheet, and a lack of observations is a major contributor to the uncertainty in predictions of global sea level rise.

The RESPONDER project, funded by the European Research Council, is concerned dealt with this problem using hot water drilling technology to drill through Sermeq Kujalleq (Store Glacier) and study the environment at the foot of one of Greenland’s largest glaciers.

« We normally take measurements inside the ice sheet using sensors on one Install cables that we lower into a drilled borehole. However, the observations we have made so far have not given us a complete picture of what is happening, « said co-author Dr. Poul Christoffersen from the Scott Polar Research Institute, who leads the RESPONDER project. « The more accurate the data we can collect, the clearer we can create this image, which in turn helps us make more accurate predictions about the future of the ice sheet. » Connect a dozen sensors to the cable so that the measurements are very far apart, « said first author Robert Law, Ph.D. Candidate at the Scott Polar Research Institute. “By using a fiber optic cable, essentially the entire cable becomes a sensor, so we can get precise measurements from the surface to the base.”

To install the cable, the scientists first had to drill through the glacier, a process by Professor Bryn Hubbard and Dr. Samuel Doyle of Aberystwyth University. After lowering the cable into the borehole, the team sent laser pulses in the cable and then recorded the distortions of the light scattering in the cable, which vary depending on the temperature of the surrounding ice. Engineers from Delft University of Technology in the Netherlands and geophysicists from the University of Leeds helped with the data acquisition and analysis.

« This technology is a great advance in our ability to record spatial variations in ice temperature over long distances and with very high resolution. With a few more adjustments, the technique can also record other properties such as deformations at a similarly high resolution, « he said Hubbard.

 » Overall, our readings paint a picture that is far more diverse than current theories and models predict, « said Christoffersen . « We found that temperature is greatly affected by the deformation of the ice in bands and at the boundaries between different types of ice. This shows that many models, including our own, have limitations. » The researchers found three layers of ice in the glacier. The thickest layer consists of cold and stiff ice that has formed over the past 10,000 years. Below they found older ice from the last Ice Age, which is softer and more malleable due to the dust trapped in the ice. What surprised the researchers most, however, was a more than 70 meters thick layer of warm ice at the bottom of the glacier. « We know this type of warm ice from much warmer alpine environments, but here the glacier creates the heat by deforming itself, » said Law.

« With these observations, we begin to better understand why the Greenland ice sheet is loses mass so quickly and why ice shedding is such an important mechanism for ice loss, « said Christoffersen.

One of the main limitations in our understanding of climate change is related to the behavior of glaciers and ice sheets. The new data will allow researchers to improve their models of how the Greenland ice sheet is currently moving, how it might move in the future, and what this will mean for global sea level rise.

The research was carried out in part by funded by the European Union.

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