What are the key controls on ice flow?

Welcome to Week 5 of Climate Change: Challenges and Solutions! I hope you’re enjoying the content – there’s loads of great stuff still to come including our live session on 23rd March 2017, 15:00 UTC!

This week, one of the most striking elements was the video of the calving event from Helheim Glacier in July 2010. Factors such as air temperature, water temperature, glacier speed and the underlying topography all affect calving events and these are natural processes. A glacier that has equal inputs (accumulation) to outputs (ablation) is said to be in a steady state. But the mass balance of glaciers in Greenland is becoming increasingly negative – ablation is becoming greater than accumulation. So what are the key controls on ice flow that can affect an entire ice sheet, rather than an individual calving zone?

helheim%20glacier%20summer%202010
Cameras from Swansea University recorded the enormous calving event at Helheim Glacier.

Water!

This may seem contrary to expectation, but the Greenland Ice Sheet is actually very wet. The surface is covered in supraglacial lakes and channels (see image below) from melting that have a large effect on ice velocity. When they drain through moulins (holes in the ice that allow the water to reach the bed), the water can then provide a lubricating surface for the glacier to flow on top of which can accelerate flow. This has been observed at many glaciers along the west coast of Greenland (e.g. Fitzpatrick et al, 2014, Zwally et al, 2002 and Das et al, 2008) and can be attributed to events of fast flow. A key problem of understanding this mechanism is logistical; how does one trace the water beneath 1km of ice? Also, gaining the frequency of satellite images that are needed to know about the effects of drainage on velocity is troublesome – lakes can drain within 2 hours, while many satellites that orbit these regions only acquire images every 2 weeks or more.

This is less of a control in Antarctica as the ice sheet is much colder and drier than Greenland. Also, as the ice tends to be frozen to the bed (cold-based ice), this means that water that reaches the bed might just stay there in subglacial lakes rather than flowing out to the margins in subglacial streams. This could be why Antarctica has 400+ subglacial lakes whereas Greenland (what we know so far) only has 3.

 

Ice Thickness

The thickness of ice can have a direct influence over the way it flows. There are two key components to the way ice flows; how it deforms and what the driving stress is. Driving stress is a function of ice density, gravity, ice thickness and surface slope – this is Glen’s Flow Law. This means that ice that is thick and on a steep gradient has a higher driving stress (and velocity) than ice that is thin and on a shallow gradient. Ice flow may be resisted by forces such as basal drag (against the bed) and lateral drag (against the sides of a glacier).

Underlying Bed Topography

This seems intuitive; water will always choose the easiest path to flow, so ice should too. And this does appear to be the case – areas that are deeper tend to have faster flow; for example the image below shows the deep bed beneath Jakobshavn Isbrae. This is Greenland’s largest outlet glacier and has been accelerating in velocity since the late 1990s. The trigger mechanism for accelerating velocity may not have been due to bed topography (more likely warm ocean temperatures and the loss of buttressing from its floating tongue) but its continued speed up may be related to the topography sloping downwards. Underlying bed topography is more likely to be the cause of long-term changes in ice velocity than short-term, seasonal changes (given the topography remains the same between summer and winter).

velocity
Higher velocities are given in blue and slower velocities in red. Note how the fastest flowing areas of Jakobshavn Isbrae also correspond with the deepest areas of the subglacial topography.

Glacier Terminus

The way a glacier responds to external influences massively depends on whether its terminus is land-based or ends in water. This is because the presence of water introduces a whole new bundle of controls that could be relevant (perhaps the most influential is water temperature) and new mechanisms for ablation, such as calving (the breaking away of large parts of the glacier to become icebergs).

 

Other Controls

There are many many controls on long-term and seasonal ice flow. Many of these are still being investigated; for example the hydrology of a glacier is a topic of contemporary research. Temperature is a fairly self-explanatory control and this relates to ice deformation – warmer ice can deform more easily and therefore flow faster. Crevassing at the end of glaciers can cause large calving events in water terminating glaciers (and this is considered to be a control on the Helheim calving event from the video).

I hope you’ve enjoyed learning about the key controls on ice flow; now consider how many of these may relate to Climate Change and the warming of the Greenland Ice Sheet. Will a greater amount of water draining to the bed result in faster ice flow or slower – and why? Will this change throughout the summer? Let me know @LTaylor1995.

Liam

 

 

 

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