Tuesday :: Jan 9, 2007

The Crack of Doom?

by Christina Hulbe

So says the The Scotsman, along with a number of news outlets and blogs, about the calving of the remnant  Ayles Ice Shelf along the northern coast of Ellesmere Island in the Canadian high Arctic. The event took place in August of 2005 but was only recently discovered in a survey of archived satellite data.

Was the Ayles Ice Shelf calving event important?  Yes. Was it as dramatic as the news and blog stories suggest?  Probably not.  While this was certainly a warming-related event, the action began half a century or more ago.  We have arrived just in time for the finale.

Ice shelves are large, landfast platforms of floating ice that form primarily where grounded (resting on bedrock) glacier ice flows to the coast and goes afloat at the ocean surface.  Ice shelves typically spread to fill coastal embayments. The bay walls, islands, and shoals in the sea floor provide resistance to the gravity-driven horizontal spreading (a very viscous fluid flow) of the ice and in that way help support the shelf.  In small systems, sea ice at the seaward front may also play a stabilizing role.  

Icebergs are large, freely-floating chunks of ice that calve (break off) at the seaward front of an ice shelf. Iceberg calving is a normal process for all ice shelves and glaciers that terminate in water. The usual pattern is frequent small events and infrequent large events.  

Less commonly, an ice shelf may be composed of thick, multi-year sea ice that holds fast to the coast.  Sea ice is freely-floating ice that forms by freezing at the ocean surface. Composite shelves with both glacier-fed and sea ice provinces are also possible.  The ice shelves of Ellesmere Island are a few 10's of meters thick but in Antarctica ice shelves range from 200 meters to nearly 1 kilometer in thickness.

Ice shelves are thought to be sensitive indicators of climate change because they exist at sea level and are thus in contact with relatively warm sea water and and experience warmer mean annual air temperatures than glacier ice at higher elevations. In Antarctica, ice shelves are not found in areas where the mean annual temperature exceeds -5 degrees Celsius.  Correlation is not causality, of course, and the exact relationship between climate and shelf viability is not agreed upon (this is a topic of some of my own research).

Most of Earth's existing ice shelves are found around Antarctica.  The shelves of Ellesmere Island are among the very small number found in the Arctic.  For comparison, ice shelves cover a surface area of about 1.5 million square kilometers around Antarctica and 1300 square kilometers around Ellesmere Island.  While much smaller than those in the Antarctic, Arctic ice shelves are equally interesting and have a climate story of their own to tell.

The Ellesmere ice shelves are recent features, having formed after the northern hemisphere's postglacial climate optimum, between 8,000 and 6,000 years ago.  A narwhal tusk and stranded driftwood found on the currently ice-bound shore of the Wootton Peninsula indicate that the Ellesmere coast was ice-free at about 7,000 years ago and blocked by sea ice about 3,000 years later.  The glacier ice components of the Ellesmere shelves appear to be somewhat younger.  

When Robert Peary first explored Ellesmere Island in 1906, he found a contiguous expanse of ice shelf along its northern coast.  That ice has been retreating ever since, at a rate that appears to have picked up in the latter part of the 20th Century.  If you want to read more about these unique systems, the best single source I know is the Ellesmere Island chapter of the US Geological Survey Satellite Image Atlas of Glaciers of the World - North America.

What strikes me about this event is not the loss of the Ayles remnant itself but what its loss appears to say about a much larger player in the global climate system: Arctic sea ice.  Multi-year sea ice abutting the fronts of the Ellesmere ice shelves has been demonstrated to be an important factor in shelf mechanics so loss of the shelf remnant is probably connected to changes in the sea ice.  

Some of the Arctic's thickest sea ice is found along Ellesmere Island's northern coast in both summer and winter (a result of Arctic ocean circulation).  Sea ice thickness is a notoriously difficult quantity to measure and while we have good satellite-based time series of sea ice extent going back to the early 1970's, we have a relatively sparse ice thickness record.  What is known comes from observations made with upward-looking sonar (from submarines).  The short story is that there appears to have been an overall decline of more than 40% since the mid-20th Century but the data are sparse in both space and time, we don't really know if this is a trend or not.  The prolonged retreat of the Ellesmere Island ice shelves is corroborative evidence to support the idea that the decline in sea ice thickness is indeed a sustained trend.

So if you are fond of mine safety analogies, don't think of the Ayles Ice Shelf as the canary, think of it as one of the fleas jumping off the corpse.

I'd like to take the opportunity in this first post to thank eriposte and everybody else at The Left Coaster for inviting me to write about climate science topics. As a long-time reader, I value The Left Coaster as a resource for both information and insight into the important issues of our time.

Christina Hulbe :: 12:07 AM :: Comments (22) :: Digg It!