Examining the sensitivity of ice sheet models to updates in rheology (n=4)

Abstract

Ice is a non-Newtonian fluid whose rheology is typically described using Glen's flow law, a power-law relationship between stress and strain rate with a stress exponent, n, generally taken to be 3. Recent observation-based work suggests that a more accurate choice for the Glen’s law exponent in high-strain regions like ice shelves may be n=4, implying that ice viscosity is more sensitive to changes in stress than is generally assumed. The implications of a higher stress exponent for ice sheet models and their projections of ice sheet response to climate forcing are unclear and likely to be complex. Rheological parameters, such as ice viscosity, are fundamental to ice sheet dynamics and influence the evolution of marine ice sheets.  Here, we present work that explores the rheological parameter space within the idealized MSIMIP+ marine ice sheet configuration using the BISICLES model. We explore the impacts of increasing  the stress exponent from 3 to 4, highlighting the considerable changes to the ice sheet system caused by increasing the stress exponent. Beyond dynamic changes in the ice behavior, changes become necessary to the other flow law parameters generally computed during initialization. For example, it may be that viscosity modifiers typically interpreted as “damage” may instead be indications of mismatches in rheology.  This study underscores the dynamic sensitivity of glacial ice to changes in the rheological parameters and calls attention to the key variables influencing ice sheet evolution.