UPDATE: comment from glacierchange (http://glacierchange.
The above [below now] just does not make sense. We have glaciers in all sorts of climate settings, including relatively temperate settings with glaciers calving into the ocean and in tropical settings with very intense solar radiation. In none of these settings is the structure of the ice weakened beyond the immediate by melting in a substantial way that would lead to its losing its solid strength and becoming somewhat of a slurry as suggested. I have spent plenty of days on glaciers when the temperature was in the 70s and meltwater is running everywhere, and the ice we were on was plenty strong. Drop an ice cube in a glass and the ice does not disintegrate into shards. The glacier on Eyjafjallajökull did not just turn into a slurry despite this massive heat-induced meltwater running over and through the Gigjokull Glacier
Start of original post:
Below I am copying a comment by Aaron Lewis on the dynamics of heat transfer and how Greenland's ice sheet is moving ever closer to collapsing around the edges, with resulting enormous increase in outflow.
See first the graphs and article at this post (lifted from CEJournal post by Tom Yulsman):
http://climatechangepsychology.blogspot.com/2010/10/greenlands-jakobshavn-could-be-poised.html
- Ice is a fluid with structural and mechanical strength that is dependent on its heat content. The more heat, the weaker the ice. When ice warms to near its melting point, then the potential energy of the ice can provide the energy to drive a progressive structural failure with the ice sheet collapsing into a slurry.We now have 3 plumes of warm, moist air (seasonally) blowing across Greenland. Every gram of water vapor that condenses on the ice melts 7.5 gram of ice resulting in 8.5 gram of water absorbing sunlight. When that water falls through a moulin, its heat and potential energy is transferred to the interior of the ice sheet.Recently, we have not seen progressive structural failure of ice sheets because ice is a good insulator. Thus, glaciers melt from the outside in, with the interior retaining more mechanical strength as they flow from a colder region to a warmer (lower) region. In a glacier, only the lower tip, the calving face has a heat content of 0 °C plus some heat of fusion. However, in the current case of Greenland, we are warming very large volumes of ice all at once. And, this ice is currently supported by the (weak) ice around it.At some point, a moulin will disrupt this mutual support, and instead of the moulin healing after the surface water drains, the ice will start calving into the moulin, and the released potential energy will shock the surrounding ice, resulting in additional calving. Then, the ice around the moulin will undergo progressive structural collapse, rapidly forming a slurry of ice and water.Ice is not a good dam material. Ice will not dam water with a head of more than ~6 meters. Where we have large glacial lake outburst floods, part of the “ice dam” is actually terminal moraine – rock, not ice. And, such dams composed of terminal moraine and ice do not last very long and are less than 30 meters high, rather than the many hundreds proposed for the great ice dams. Thus, the great melt water pulses were not ice dams breaking, but ice sheets undergoing progressive collapse. The wave benches from Lake Missoula were from shallow lakes sitting on of top hundreds of feet of ice. That canyon of ice broke up suddenly, and repeatedly, to form the Lake Missoula Floods. Similar, but much larger, flow channels surround Greenland. Yes, over the next few years we can expect parts of the GIS to move much faster.The rock guys all say, “We have not seen progressive collapse of ice before,” to which I would reply, “Where are your notes from the Missoula Floods?"The moulins on Greenland prove that ice dams cannot support large heads of liquid water.Feynman did the basic calcs in 1964.Link: http://www.cejournal.net/?p=4225#comment-8097
