Published: Feb 4, 2016 by The PISM Authors
Today’s publication of Aschwanden et al. (2016) in Nature Communications is certainly a milestone in PISM development. However, it is also a milestone in ice sheet modeling generally. Here’s why.
The paper is based on PISM simulations with grid resolution down to 600 m over the entire Greenland ice sheet. To start, each of an initial ensemble of 14 lower-resolution (1500 m) experiments has a single ice-sheet-wide value for all parameters. The best of these, in an ice-sheet-wide measure, is re-run at the 600 m resolution and various coarser resolutions. The quality of this flow model for 29 outlet glaciers is assessed; each outlet glacier sees the same physics. The main result is that the majority of the outlet glaciers show strong correlation between modeled and present-day-observed velocity, when it is compared along cross-flow and near-ocean profiles.
Before this paper one might suppose, based on the most prominent literature on the subject, that a detailed, measurably-accurate, outlet-glacier-resolving model of the present-day velocity of an entire ice sheet was dependent both on removing shallow assumptions from the stress balance and on tuning a very large number of basal parameters. Both of these “required” properties would be very bad news for the prospect of using ice sheet simulations to do science! On the one hand, Stokes models are computationally-expensive, while on the other hand only present-day, and not past or future, data are available to set all these basal parameters through inversion.
Such a pessimistic view turns out to be substantially false. Aschwanden et al. (2016) show that four things do matter: (i) an accurate map of bedrock topography, (ii) a stress regime in which viscous membrane stresses are part of the balance with basal sliding resistance, (iii) an energy-conservation-driven basal stress model derived (conceptually) from a model of a wet, pressurized, deformable basal layer, and (iv) high model resolution over all areas of the ice sheet where sliding is possible and/or steep/rough basal topography exists.
NASA IceBridge missions, and the mass-conserving-bed technology of Morlighem et al (2014), are shown by this paper to represent major progress on item (i). Items (ii) and (iii) are properties of the PISM continuum model, and item (iv) of its implementation as parallel-scalable software. Certainly all of these “things that matter” are improvable. More-complete stress balances and the use of inversion of present-day velocities will both be essential to improvements. The main idea remains, however: if the modeled flowing ice has the right bottom geometry, and if the dynamical model has certain key features, then the resulting dynamics are already inside the ballpark!
This research has been featured in Alaska’s largest newspaper, the Alaska Dispatch News.