Last week, I was lucky enough to check out Chasing Ice, the movie that chronicles photographer James Balog’s National Geographic assignment to photograph glacier changes. Balog’s project ballooned into what’s now known as “Extreme Ice Survey”, where repeat photography locations, video, and time-lapse cameras are set up around the world to capture how glaciers are responding to recent warming temperatures worldwide.
One of Chasing Ice’s most impressive scenes is of an incredible calving event (do yourself a favor and click that link!) on the Ilulissat Glacier (also known as Jakobshavn Isbræ) in Western Greenland. The event is the largest calving event ever captured on camera and is a dramatic exclamation point for a glacier that has retreated more in the last 10 years than in the previous 100.
For most glaciers, the relationship between air temperatures and retreat is straightforward. Warmer temperatures melt more ice than it accumulates as snow, and the glacier loses mass. This ice deficit manifests itself as glacier retreat over a time-scale known as the response rate. However, for glaciers that extend into lakes and oceans, calving (the process by which icebergs break off a glacier) plays a major role in determining how much mass is lost, and subsequently on the location of the terminus, making the relationship less straightforward.
Some of the earliest studies of calving glaciers occurred in Prince William Sound, Alaska, on the Columbia Glacier. The glacier is one of the fastest moving glaciers in the world, has spectacular calving events, and since 1982 has retreated 16 km. Some of the earliest research on the glacier predicted that once the front of the glacier began to float it would calve off and disintegrate, suggesting that ultimately, the location of the glacier terminus was determined by ice thickness and water depth. Since then, many observations have been made of floating glacier snouts that can be sustained for many seasons, but once the front of the glacier does begin to float, it becomes a lot more susceptible to calving.
Observations of calving glaciers suggest that the floatation of the terminus is often preceded by a long run of high melt or low snowfall years. If the glacier runs an ice deficit for many years, ice will still flow downslope, but there will be less of it. The fact that less ice will be supplied to the bottom of the glacier, as well as ice that has been melted from the bottom sections means that the glacier will thin substantially. This thinning will in turn make the glacier more buoyant.
Once a glacier begins to float, all bets are off on how resilient it will be. Some glaciers crack and disintegrate right away, while others, if they flow slowly enough, can survive the initial torque. However, once the glacier does begin to float, the glacial lake begins to have increasing influence on the glacier. One of the most fascinating aspects of the glacier/water interaction is that a floating glacier front can flow significantly faster than if it was grinding along the bottom of the lake (or bedrock). This is because while rock and gravels are rough, water offers very little resistance to glacier, and allows it to slide faster – essentially dragging the glacier into the water.
Grab some silly putty, and pull on one end while holding the other still. That’s what’s happening near the terminus! When the lake ‘pulls’ the terminus out to sea, it ends up stretching the glacier, which in turn thins it. When the glacier thins more, it becomes easier to trigger calving events, and every time there are calving events, the glacier speeds up. The ‘speed-up’ then stretches the glacier even thinner, and the cycle perpetuates itself until the glacier retreats into shallower water where it is no longer floating.
What this all suggests, is that once the glacier front is floating, it doesn’t matter much what happens to climate – there’s no going back until the glacier hits dry land. However, it also means that an initial climate induced thinning is enough to push the glacier ‘into the deep end’, past the ‘point of no return’.
I’ll leave this post with some work I have been doing on Bridge Glacier, a glacier in the Coast Mountains, about 170 km north of Vancouver, that spills into a lake. This animation is a series of Landsat images taken from early fall each year from 1972 to this past summer. Between 1972 and 1991 the glacier was grounded, and had a relatively stable terminus position. In the 80s, there was a stretch of hot, dry years that would have Bridge Glacier needing a loan from the bank to pay off its ice debts. From 1991 onwards, the glacier becomes increasingly unstable, begins to float and passes the ‘point of no return’ and retreats over 3.5 km. While that distance is impressive, what sticks out with me the most is how staggering the thinning is – it’s as if someone took all the air out of a balloon.
A lot of folks have been doing research on Columbia Glacier and Mendenhall Glacier in Alaska, as well as some calving glaciers in New Zealand and Patagonia, here’s a short selection of a few that look at how climate affects calving glacier life-cycles.
Boyce, E. S., Motyka, R. J., and Truer, M. 2007. Flotation and retreat of a
lake-calving terminus, Mendenhall Glacier, southeast Alaska, usa. Journal of
Glaciology, 53(181):211-224. (link)
Post, A., O’Neel, S., Motyka, R., and Streveler, G. 2011. A complex relationship
between calving glaciers and climate. Eos, Transactions American Geophysical
Union, 92(37):305. (link)