Learning about Ice:
First, thanks to Jeff for covering for me. I’ve been overwhelmed with some of those official duties provided by the U of Michigan. This included a trip to Florida to talk about climate change to alumni. Of course the question of the cold winter in the U.S. came up. It’s already old news that even with the cold in the East of the U.S., the warmth in the West was enough to make the January 2009 just about average
. Plus the fires in Australia brought attention to the record heat in Australia
Part of my job in the last month has led to me learning a lot more about ice sheets. Some may recall that when the 2007 Fourth Assessment Report
was issued by the IPCC
, that there was some controversy about whether or not the melting of the polar ice sheets had been underestimated.
It is my opinion
that the melting of the ice sheets was underestimated in the IPCC report. This is based on two reasons. The first reason is that the dynamics of ice sheets and the melting of ice sheets have only been crudely estimated in climate models. Only in the past few years have we been making serious progress on this problem. As we incorporate the melting of ice to form water and the flow of water through the ice sheets we discover processes that speed up melting. The second reason is – that in a situation of such uncertainty arguments of extreme melting are not well posed. These both lead to what, in my opinion, is an underestimate of ice sheet melting, and likely therefore, sea level rise.
Remember, ice sheet melting is compensated for by increased ice being deposited at high elevations in both Greenland and Antarctic. (I usually ask, why is this the case?) Sea level rise is related to whether or not there is net loss to ice sheets.
What I have learned in the last week is that we have made some progress in understanding the physics of ice sheet melting. Our old ways of thinking about ice melting because the air was warm, perhaps like the ice cubes that I drop on the floor, leads to it being centuries before the ice sheets melt. More recently we have seen the pictures of lakes forming on the ice sheets and disappearing, flowing into the ice sheets. This leads to a lubrication of the base of the ice sheet and increases the flow. This process definitely leads to fast melting, but the thermodynamics of this process suggest that it is not likely to lead to ice sheet collapse.
Some may recall early on in my blog a collection of blogs on melting ice. (Warm Snow
, Fast Ice
, The End of Ice?
). In these blogs the importance of ice shelves and, perhaps, sea ice in buttressing the ice sheets flowing into the sea was discussed. Both the buttressing and insulating effects of ice shelves and sea ice is emerging as important. This allows the glaciers flowing from the ice sheets to “feel” the sea water. The heat associated with the sea water leads to accelerated melting. There is a lot of heat in the sea. This interaction between the sea and the ice is emerging as the most important melt mechanism.
What does this mean? A couple of things come to mind. One is the possibility of episodic mass loss. As the glacier retreats from the sea, it losses contact with the sea, the warmth. Therefore it gets some time to recover, until the leading edge gets to the sea again. This, I assert, still leads to accelerated mass loss. (Assertion based on years of studying irreversible transport in the atmosphere.) The second thing that comes to mind is that we have a physical reason to expect cyclical behavior in ice melt associated with atmospheric-oceanic cycles, i.e. the North Atlantic Oscillation
The basic conclusions that the surface global temperature will rise, sea level will rise and the weather will change are robust. Also in the news this week is that the presidents of the Maldives and Kiribati
are aggressively planning the evacuation of their nations.