  We propose and implement a new method, called the Ice Sheet Coupled
Approximation Levels (ISCAL) method, for simulation of ice sheet flow in large
domains under long time-intervals. The method couples the exact, full Stokes
(FS) equations with the Shallow Ice Approximation (SIA). The part of the domain
where SIA is applied is determined automatically and dynamically based on
estimates of the modeling error. For a three dimensional model problem where
the number of degrees of freedom is comparable to a real world application,
ISCAL performs almost an order of magnitude faster with a low reduction in
accuracy compared to a monolithic FS. Furthermore, ISCAL is shown to be able to
detect rapid dynamic changes in the flow. Three different error estimations are
applied and compared. Finally, ISCAL is applied to the Greenland Ice Sheet,
proving ISCAL to be a potential valuable tool for the ice sheet modeling
community.
