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. The model shown is not PISM but a new glacier-scale model by Guillaume Jouvet. PISM does this kind of thing too, but for each part of a big ice sheet like Greenland or Antarctica.)

Ice sheets in Greenland and Antarctica are more than two miles (more than four kilometers) thick and sitting on land. They contain a large amount of (frozen) water which is currently above sea level. Note that ice sheets are not sea ice—sea ice is already floating and changing its thickness will not change sea level directly.

If ice sheets flow faster or slower, or their flow direction 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. In the future there could be tipping points we don't see now. An ice sheet flow model like PISM is part of understanding such possibilities.

If present-day Antarctic ice sheet flows faster into the ocean during the next century then sea level will rise. There may be other relatively-fast effects on the climate. One question is whether sea level will rise a only a fraction of a meter, or up to one or two meters, in the next century.

A major part of predicting the ice sheets is to observe them better now through field work. Computer simulations depend on observations; they can can only integrate the data we have into a more complete picture, and 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.

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.

• The PISM User's Manual shows some realistic and idealized cases.
• The publications page shows published applications of PISM from hundreds of different scientists all over the world.