Observed mechanism for sustained glacier retreat and acceleration in response to ocean warming around Greenland

Abstract

Abstract. The dynamic loss of ice via outlet glaciers around the Greenland Ice Sheet is a major contributor to sea level rise. However, the retreat history and ensuing dynamic mass loss of neighboring glaciers are disparate, complicating projections of sea level rise. Here, we examine the stress balance evolution for three neighboring glaciers prior to; at the onset of; during; and, where possible, after retreat. We find no dynamic or thickness changes preceding retreat, implicating a retreat trigger at the ice–ocean boundary. Terminus retreat initiates large-scale changes in the stress state at the terminus. This includes a drop in along-flow resistance to driving stress followed by an increase in lateral drag and associated glacier acceleration. We find that the pre-retreat spatial pattern in stresses along-fjord may control retreat duration and thus the long-term dynamic response of a glacier to terminus retreat. Specifically, glaciers with large regions of low basal drag extending far inland from the terminus permit a chain of stress changes that results in sustained acceleration, increased mass loss, and continued retreat. Glaciers with similarly low basal stress conditions occur around Greenland. Our results suggest that for such glaciers, dynamic mass loss can be sustained into the future despite a pause in ocean forcing.

Glacier geometry limits the propagation of thinning in Patagonian Icefields

“…In PI, it has been suggested that 98% of the surface velocities are due to basal slip (Collao-Barrios et al, 2018). Depth-integrated resistive stresses are estimated with a stress exponent of n = 3 and a viscosity rate factor of B = 300 kPa yr 1/3 , similar to that used in previous investigations (Carnahan et al, 2022;Van Der Veen et al, 2011). The driving stress in the x-direction takes the form:…”

“…In our research, we use the force balance method according to van der Veen & Whillans (1989), which provides us with the stress balance in glaciers for a given instant. The force balance method has been used extensively in Greenland (Carnahan et al, 2022) to understand glacier behavior, either individually (Van Der Veen et al, 2011) or on multiple glaciers for given periods (Bartholomaus et al, 2016;Stearns and van der Veen, 2018;Van Der Veen et al, 2011;Enderlin et al, 2018; https://doi.org/10.5194/egusphere-2024-1053 Preprint. Discussion started: 14 May 2024 c Author(s) 2024.…”

“…The force balance method assumes that the driving stress (𝜏 2 ) is supported by basal drag (𝜏 3 ), depth-integrated longitudinal coupling (𝐹 4567 ), and lateral drag (𝐹 489 ). In this work, like Carnahan et al (2022), we used a sign convention where positive driving stress values act in the direction of flow; in turn, positive values of the other stresses represent opposition to flow:…”

“…Here, we assumed that ice flow does not vary with depth; that is, we ignored vertical shear, so the depthaveraged strain rate is equal to the surface strain rate (shallow-shelf approximation with basal drag) (Carnahan et al, 2022). In PI, it has been suggested that 98% of the surface velocities are due to basal slip (Collao-Barrios et al, 2018).…”

“…Kinematic wave theory establishes that glacial thinning can be modeled as a wave propagating upstream due to a perturbation in the terminus. In this sense, Carnahan et al (2022), based on the study of Greenland glaciers, suggest that regions with low Péclet numbers substantially impact the components of the force balance, highlighting that glaciers with large areas of low basal resistance extending inland from the terminus allow a chain of stress changes that results in sustained acceleration, more significant mass loss, and continued retreat. Despite significant advances, gaps remain in our knowledge about how glacial geometry limits changes in Patagonian Icefields and, if possible, through these conditions to define regions of vulnerability to thinning.…”