By Matt Owens November 14, 2013
Above: Pine Island Glacier (PIG), Antarctica - a massive iceberg calves from its floating ice shelf. This time-lapse animation runs from the end of October through November 14, 2013. The iceberg is almost 30 km wide. Animation by Matt Owens.
As this dramatic event unfolds, an even more dramatic loss of Antarctic ice continues out of sight. Over the past several decades, Antarctica's ice has been shrinking - losing mass. And that ice loss has been accelerating. Just for Antarctica alone, the rate is about 20 to 30 billion tonnes per year (net loss), as of the latest measurements (Morrow et al. 2013). And Greenland is losing ice mass even faster.
This is not just a natural cycle. Sea level is rising quickly, now at a rate of 32 centimeters per century (for the period from 1994 to 2013). And this rate is substantially higher than the essentially zero rate of rise experienced since the end of the last ice age.
In an interview yesterday, climate scientist and glaciologist Richard Alley explained the big picture behind this new city-sized iceberg in Pine Island Bay: "we're seeing thinning - probably a good bit of thinning - of the non-floating ice. And it's fairly clearly been linked to warmer waters getting up under these floating extensions." These warmer ocean waters are caused by the energy imbalance our fossil fuel pollution is creating in the atmosphere, aka global warming.
The new iceberg calved from what's known as the Pine Island Glacier floating ice shelf, one of Antarctica's many floating shelves. This particular one, along with nearby Thwaites, have been called the "weak underbelly" of Antarctica, in part because they hold back a disproportionately large amount of land-based ice.
Around the entire perimeter of Antarctica, floating ice shelves are common, and they buttress the ice sheets that are pushing outwards towards the ocean. This buttressing effectively slows the flow of the land-based ice sheets.
In contrast to floating ice shelves, ice that rests on Antarctica's bedrock is often referred to as land-based ice.
The land-based ice is thousands of meters thick in many areas, and under its own weight it deforms in a plastic flow - like a strange thick syrup. Temperature, friction, and physical barriers (e.g. mountains) are some of the critical elements that regulate flow speeds of the ice. At the edges of the Antarctic continent, the floating ice shelves are the final restraining element before the ice reaches the ocean. When an ice shelf thins, the buttressing effect is reduced, and the rate of ice flow from land-based ice into the ocean accelerates significantly.
Alley said that the floating shelves themselves are subject to friction and physical barriers in some cases - for example, where they are confined to fjords or if their keel runs aground on underwater rises. And because warmer ocean water has gotten into the cavities underneath the ice shelves, "that has thinned the ice shelves and made it in many ways easier to calve - so you see a little more break-off on the ends of some of them. That in turn has reduced the friction - that has let the ice go faster - that has caused thinning of the non-floating ice and sea level rise."
While the size of the new Pine Island iceberg is certainly large, sea level rise from the thinning of land-based ice is the most important if less-immediately-visible concern.
Richard Alley was also the host of "Earth: The Operator's Manual," recommended for anyone interested in global climate change solutions.