This time last week I was at Los Alamos National Laboratory for a meeting that discussed building a Community Ice Sheet Model, inspired by the success of the Community Climate System Model. (Eventually CISM will be part of CCSM).
Ice sheets are getting a lot of attention because that's where one of the biggest potential impacts of climate disruption, sea level rise, comes from. The mechanics of ice sheets have some big known unknowns. The biggest is what is happening at the bottom of the ice sheet--at the ice-land interface. Is it sticky or slippery? What can change it? We only have a few observations and some guesses at theory. Another group of "known unknowns" are the processes that control ice sheet calving. Calving is when chunks of the ice sheet edge break off and form icebergs, a very photogenic process whose exact physics are poorly understood. There's lots of theories, but only trial and error in simulations and more observations will help weed out the good from the bad. Both of these processes affect how fast ice sheets can shed mass to the ocean. Its these mechanical processes, rather then simple melting alone, that has the potential to lead to rapid sea level rise. As you might guess from the immaturity of the field, none of this was considered in the last IPCC report. With efforts like CISM, we will hopefully be able to say a little more in the next report, due out in 2013.