What is the Parallel Ice Sheet Model for? If you are working in climate science or the physics of glaciers then you have a sense of what an “ice sheet model” does. But if you are not a scientist, or need the very basics of ice sheets, these questions and answers might help.
Ice sheets, which are just big, continent-size glaciers, are moving ice fluids, like the ocean is a fluid and the atmosphere is a fluid. Glaciers flow downhill, driven by gravity. PISM simulates the movement of the ice fluid, and its temperature, and so on, just as a weather forecasting model simulates the atmosphere. Scientists are interested in how changing climate might affect the ice flow. Ice moves slower than the ocean or the atmosphere, so they use PISM to simulate decades, centuries, or millenia of flow, while a weather model works over just days.
See this excellent 5 minute movie introducing what a glacier flow model does. (It is not the PISM model but a new model by Guillaume Jouvet.)
Ice sheets contain a large amount of (frozen) water which is currently above sea level. Ice sheets are not sea ice—sea ice is already floating and changing will not change sea level directly. Ice sheets in Greenland and Antarctica are more than two miles (more than four kilometers) thick and sitting on land. If ice sheets flow faster or slower, or the way they flow changes, then this affects the rate at which they can raise sea level. The potential amount is big—tens of meters—but the question is how fast these changes can happen. In the past, as the earth went in and out of the ice ages, there were huge changes over very short periods. If the present-day ice sheets flow faster into the ocean in the next century then sea level will rise and there may be other relatively-fast affects on the climate. The question is whether sea level will rise a fraction of a meter or up to one or two meters in the next century or two. And there could be tipping points we don't see now. An ice sheet flow model like PISM is part of understanding such possibilities.
Of course, a big part of understanding the ice sheets is to observe them better, and do field work, and so on … not just work on a computer simulation. Computers can only integrate the observations we have into a more complete picture; they do not create new facts.
We have written software which is free and open source. Our international partners, like the Potsdam Institute (PIK) and other groups in Denmark, Germany, New Zealand, and the Netherlands, use our software, and they even add to it. The collaboration is not very formal, but we have traveled both ways across the pole quite a bit! They have chosen PISM over the several other available ice sheet models because it works well for them.
The purpose of a model like this is to put your imperfect understanding of ice sheets into a form where you can see the consequences of your current understanding, and then try to understand better, and then change the model accordingly. That's what we do, and that's what they do.
At UAF and PIK we have contributed estimates of the amount of sea level change coming from changes to the Greenland and Antarctic ice sheets using PISM. These estimates will be combined into the next report of the Intergovernmental Panel on Climate Change, which will appear next year after an extensive evaluation of the current science, including these model-based estimates. But at UAF we are mostly building the model and writing code, and trying to understand the physics of glaciers and ice sheets.
In preparation for a radio interview about a news item, reporter Ariel van Cleave of KDLG, Dillingham, Alaska asked Ed Bueler the above questions, and the above were his DRAFT replies. What he actually was recorded as saying was different, at least in detail. Hopefully he did not make a big fool of himself …
PISM has been used for modeling every present-day ice sheet (Greenlandic and Antarctic), many paleo-ice sheets, and many glaciers.