thermal

Rapid Sampling of Ice-Sheet Temperatures

Posted by William Colgan on September 10, 2018
Applied Glaciology, New Research / No Comments

We are starting a new two-year project to design, build and deploy a new type of ice-drill to measure temperatures at the ice-bed interface of the Greenland Ice Sheet. Why? Because we are unsure whether the bed is frozen or thawed beneath about one third of the ice sheet. As the rate at which ice flows is dependent on ice temperature – and basal ice temperature in particular – this translates into uncertainty in simulations of how ice sheet form and flow will evolve over time.

We suspect that climate change is likely driving an expansion of the thawed-bedded portion of the ice sheet — eroding the frozen-bedded portion — over time. But in the last sixty years, direct temperature measurements of the ice-bed interface have only been made at six inland ice-sheet locations. These scarce, but tremendously valuable, basal ice temperatures have been measured at the sites of ice core deep-drilling projects. These deep-drilling projects take months or even years to create a 30 cm wide borehole to the ice-sheet bed from which to retrieve delicate ice core.

Figure 1 – Schematic of the HOTROD melt-tip and cross-section of the umbilical cord. The umbilical cord will both power the melt-tip as well as contain embedded ice-temperature sensors.

This project will design, build and deploy a drill for rapid sampling of ice-sheet basal temperatures. HOTROD will use an approximately 5KW electric melt-tip to open 3 cm wide access boreholes to depths of 500 m within days. The HOTROD umbilical cord will not only power the melt-tip, but also have embedded temperature sensors that — with the melt-tip — make a one-way trip to the ice-sheet bed. The heart of the melt-tip will be recently designed heating elements intended for rapid heating of energy-efficient domestic hot water supplies.

In 1971, thermal drilling was used to recover the top 372 m of ice core at Dye-3. The Dye-3 deep ice core was subsequently completed to 2037 m with electro-mechanical drilling in 1981. Thermal drilling technology was last used in Greenland in 1974, to recover a 403 m ice core at Crete, Greenland1. While there’s been numerous hot-water drilling projects since then, the working memory of thermal drilling is fading. The goal of this project is to successfully deploy a melt-tip thermal drill to measure a 500 m deep ice-sheet temperature profile with less than ten days of drilling. Initial field-testing activities will begin in 2019.

Figure 2 – The Dye-3 ice-drilling trench. In comparison to the multi-year logistical footprints of deep ice-coring projects, the HOTROD melt-tip drill will require trace logistics.

We hope that the advent of rapid melt-tip drilling will be a disruptive technology within the sphere of ice-sheet research now dominated by conventional electro-mechanical and hot-water drilling systems. A concerted effort to sample more temperatures at the ice-bed interface may potentially shift our understanding of ice-sheet basal temperatures and even ice-sheet sensitivity to climate change. This project is funded by Villum Experiment, a programme of Villum Foundation that funds science and engineering projects that challenge the norm and have the potential to transform traditional approaches2.

1Langway, C. 2008. The History of Early Polar Ice Cores. Cold Regions Research and Engineering Laboratory. Technical Report 08-1.

2Villum Foundation. 2018. 53 bold ideas receive funding from VILLUM FONDEN.

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Greenland Ice Sheet “Thermal-Viscous Collapse”

Posted by William Colgan on July 17, 2015
Climate Change, New Research / No Comments

We have a new study in the AGU open access journal Earth’s Future this month, which introduces the notion of thermal-viscous collapse of the Greenland ice sheet1. While people tend to think of ice as a solid, it is actually a non-Newtonian fluid, because it deforms and flows over longer time-scales. Of the many strange material properties of ice, the non-linear temperature dependence of its viscosity is especially notable; ice at 0 °C deforms almost ten times more than ice at -10 °C at the same stress. This temperature-dependent viscosity makes ice flow very sensitive to ice temperature. We know that the extra meltwater now being produced at the surface of the Greenland ice sheet, relative to 50 or 100 years ago, contains tremendous latent heat energy. So, in the study, we set out to see if the latent heat in future extra meltwater might have a significant impact on future ice sheet form and flow.

We first developed a conceptual model of what we called “thermal-viscous collapse”, which we define as the enhanced ice flow resulting from warming ice temperatures and subsequently softer ice viscosities. We decided there were three key processes necessary for initiating a thermal-viscous collapse: (1) sufficient energy available in future meltwater runoff, (2) routing of that extra meltwater to the ice-bed interface, and (3) efficient transfer of latent energy from meltwater to the ice. Drawing on previous model projections and observational process studies, and admittedly an injection of explicit speculation, we concluded that it is plausible to warm the deepest 15 % of the Greenland ice sheet, where the majority of deformation occurs, from characteristic Holocene temperatures to the melting-point in the next four centuries.

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Figure 1 – Three key elements of thermal-viscous Greenland ice sheet collapse: (1) Sufficient energy available in projected Greenland meltwater runoff, (2) Routing of a fraction of meltwater to the interior ice-bed interface, and (3) Efficient energy transfer from meltwater to ice. This cross-sectional profile reflects mean observed Greenland ice surface and bedrock elevations between 74.1 and 76.4°N. Dashed lines illustrate stylized marine and land glacier termini.

We then used a simple (first-order Navier-Stokes) model of ice flow to simulate the effect of this warming and softening on the ice sheet over the next five centuries. We used a Monte Carlo approach, whereby we ran fifty simulations in which multiple key parameters were varied within their associated uncertainty. As may be expected, warming the deepest 15 % of the ice sheet by 8.8 °C, from characteristic Holocene temperatures to the melting-point, had a significant influence on ice sheet form and flow. Due to softer ice viscosities, the mean ice sheet surface velocity increased three fold, from 43 ± 4 m/yr to 126 ± 17 m/yr, resulting in an ice dynamic drawdown of the ice sheet, causing a 5 ± 2 % ice sheet volume reduction within 500 years. This is equivalent to a global mean sea-level rise contribution of 33 ± 18 cm (or just over one US foot). Of course, the vast majority of the sea level rise associated with thermal-viscous collapse would occur over subsequent millennia.

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Figure 2 – Probability density time series of ensemble spread of 50 simulations in prescribed ice temperature (a), mean surface ice velocity (b), and ice volume (c), over a 200-year spin-up to transient equilibrium, and the subsequent 500-year combined transient forcing and spin-down period.

Perhaps a caveat or two: Just like simulating a marine instability induced collapse of the West Antarctic ice sheet, our simulation of a thermal-viscous collapse of the Greenland ice sheet is an entirely hypothetical end-member scenario. It is admittedly difficult to interpret end-member assessments when their probability of occurrence is unknown. In our case, we did not attempt to constrain the probability of a thermal-viscous collapse of the Greenland ice sheet, we merely demonstrated that initiating a thermal-viscous collapse appears plausible within four centuries, and assessed the associated sea-level rise contribution. Additionally, it may be debatable whether the combination of crevasses and reverse drainage can indeed route meltwater throughout the ice sheet interior, but I suppose that is a debate worth having!

Reference

1Colgan, W., A. Sommers, H. Rajaram, W. Abdalati, and J. Frahm. 2015. Considering thermal-viscous collapse of the Greenland ice sheet. Earth’s Future. 3. doi:10.1002/2015EF000301.

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