Ice surface steepening drives multi-decadal acceleration of the Greenland Ice Sheet interior

Annual ice flow along the land-terminating margins of the Greenland Ice Sheet has been negatively correlated with surface melt over recent decades, a trend commonly attributed to the seasonal evolution of efficient subglacial drainage in response to larger melt volumes. However, there remains scant observational evidence of ice flow behavior at higher elevations in the accumulation zone, despite increasing surface melt and runoff. Here, we employ in-situ GPS measurements to analyse multi-decadal (1996-2023) variations in ice motion and surface slope along the land-terminating K-Transect, West Greenland, between ~ 1400 and ~ 1900 m.a.s.l.

We show that below the equilibrium line altitude (ELA), annual ice motion is negatively correlated with surface melt, consistent with the self-regulation of ice flow previously reported across the ablation zone. In contrast, above the ELA, we observe a small but persistent ice flow acceleration, punctuated by slowdowns in the large melt years of 2012 and 2023. We find that this ice flow acceleration has largely been driven by surface steepening, with the resultant increase in driving stress calculated to account for 70.0 ± 26.7% of the acceleration observed at ~ 1700 m.a.s.l. between 2009 and 2021. However, together with continued muted seasonality in ice flow at ~ 1900 m.a.s.l. already identified during 2009-2012, we also find clear evidence of direct surface meltwater access to the ice sheet bed at ~ 1700 m.a.s.l for the first time in our observation record. It is therefore possible that the drainage of surface meltwater has driven some of the observed acceleration, through increasing the basal sliding component of ice flow.

With the rate of ice surface lowering at the ice sheet margin predicted to continue to exceed that in the ice sheet interior, we expect that ice surface steepening will likely persist, thereby driving sustained ice flow acceleration across the higher elevations in the coming years. The direct impact of this ice flow acceleration on mass loss by drawdown will likely be modest; whilst increased ice will be advected to lower elevations where air temperatures are higher, downstream self-regulation of ice flow is expected to constrain the resultant increase in ice flux. However, our findings show that surface-to-bed connections can form above the ELA, which has implications for the volume and timing of runoff from the high elevation regions which undergo summer melt increasingly often.