Committed retreat: controls on glacier disequilibrium in a warming climate

Abstract

The widespread retreat of mountain glaciers is a striking emblem of recent climate change. Yet mass-balance observations indicate that many glaciers are out of equilibrium with current climate, meaning that observed retreats do not show the full response to warming. This is a fundamental consequence of glacier dynamics: mountain glaciers typically have multidecadal response timescales, and so their response lags centennial-scale climate trends. A substantial difference between transient and equilibrium glacier length persists throughout the warming period; we refer to this length difference as 'disequilibrium'. Forcing idealized glacier geometries with gradual warming shows that the glacier response timescale fundamentally governs the evolution of disequilibrium. Comparing a hierarchy of different glacier models suggests that accurate estimates of ice thickness and climatology, which control the timescale, are more important than higher order ice dynamics for capturing disequilibrium. Current glacier disequilibrium has previously been estimated for a selection of individual glaciers; our idealized modeling shows that sustained disequilibrium is a fundamental response of glacier dynamics, and is robust across a range of glacier geometries. This implies that many mountain glaciers are committed to additional, kilometer-scale retreats, even without further warming. Disequilibrium must also be addressed when calibrating glacier models used for climate reconstructions and projections of retreat in response to future warming.

Impact of glacier loss and vegetation succession on annual basin runoff

“…First, we assume that the catchment becomes increasingly vegetated following deglaciation and that the type of vegetation within a basin depends only on the time since deglaciation. The assumption is based on the time since deglaciation being highly correlated with vegetation types, biomass, and cover (Crocker and Major, 1955;Burga et al, 2010;Chapin et al, 1994;Klaar et al, 2015;Whelan and Bach, 2017;Fickert et al, 2017;Wietrzyk et al, 2018), and does not account for the effect that altitude has on vegetation levels (Cowie et al, 2014;Whelan and Bach, 2017). However, in some cases succession rates during glacier recession are comparable at different altitudes because changes in air temperature with altitude can be offset by climate warming (Fickert et al, 2017).…”

On the Imbalance and Response Time of Glaciers in the European Alps

“…The use of mass balance as a metric for the glacier‐climate imbalance is furthermore complicated by the fact that by losing mass, the glacier does not only reduce its area but also thins, which further decreases the local mass balance (Huss et al, ). An alternative and more complete approach for quantifying the evolution of the glacier‐climate imbalance consists of quantifying the dynamic response of glaciers (e.g., Christian et al, ). Recent efforts in this direction have been undertaken by Marzeion et al (), who utilized a simple glacier evolution model based on volume scaling to analyze the present‐day committed glacier loss at the global scale.…”

On the attribution of industrial-era glacier mass loss to anthropogenic climate change

“…1 # ′ is the anomalous mass-balance forcing with respect to the equilibrium geometry, or equivalently, a reference surface (e.g., Huss et al, 2012). That is, 1 # ′ is not affected by the glacier's geometric adjustment; Equation (1) reflects three overlapping dynamical stages of glacier adjustment (Roe and Baker, 2014;Hooke, 2019), and accurately reproduces the transient length response of numerical flowline models, for both the shallow-ice approximation and full-Stokes ice dynamics (Christian et al, 2018).…”