Rates of Sea‐Level Rise Are Highly Sensitive to Ice Viscosity Parameters in Model Benchmarks

Glacier flow plays a major role in current and future rates of globally averaged sea‐level rise. Theviscosity of glacial ice, controlling the rate of flow, decreases as stress increases and is highly sensitive to the value of the stress exponent, n, in the constitutive equation for viscous flow. Glaciologists and climate modelers almost exclusively assume n = 3 when modeling ice flow and projecting sea‐level rise through forward modeling. However, recent work suggests that n ≈ 4 better fits observations, prompting the question: How sensitive are projections of sea‐level rise to the value of n? We use an established community ice flow model and standard benchmark experiments designed as an idealized representation of Pine Island Glacier, WestAntarctica. While initializing an n = 3 model to match observations of an n = 4 ice sheet is possible, we find that incorrectly assuming n = 3 when in fact n = 4 dramatically underestimates rates of sea‐level rise. The scale of this error grows nonlinearly with the magnitude of the climate forcing, acting to increase projection uncertainties. Additionally, we find that models often account for this stress‐dependent rheology mismatch during model initialization in a way that masks this rheological effect in the short term while leaving model outputs vulnerable to larger biases in longer‐term projections. Initializations to observations of Pine Island Glacier display similar rheology‐mismatch fingerprints to our idealized example.