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Extreme melt and rainfall events can induce temporary acceleration of Greenland Ice Sheet motion, leading to increased advection of ice to lower elevations where melt rates are higher. In a warmer climate, these events are likely to become more frequent. In September 2022, seasonally unprecedented air temperatures caused multiple melt events over the Greenland Ice Sheet, generating the highest melt rates of the year. The scale and timing of the largest event overwhelmed the subglacial drainage system, enhancing basal sliding and increasing ice velocities by up to ∼240% relative to pre‐event velocities. However, ice motion returned rapidly to pre‐event levels, and the speed‐ups caused a regional increase in annual ice discharge of only ∼2% compared to when the effects of the speed‐ups were excluded. Therefore, although late melt‐season events are forecast to become more frequent and drive significant runoff, their impact on net mass loss via ice discharge is minimal. Plain Language Summary: Extreme melt and rainfall events can cause the flow of ice on the Greenland Ice Sheet to accelerate, potentially causing more ice to move to lower elevations, where temperatures are warmer and melt rates are higher. In September 2022, there were multiple unprecedented melt events. Their intensity caused some glaciers on the ice sheet to speed up by 240% relative to pre‐event speeds. Despite these accelerations, our analyses show that these events had only a minimal long‐term impact on how much ice was moved to lower elevations due to the short duration of the speed‐ups. As a result, while these melt‐induced speed‐ups are expected to become more common in a warmer climate, their effect on the amount of ice transported toward the ice margins is minimal. Key Points: September 2022 saw multiple melt events over the west Greenland Ice Sheet, with the largest daily runoff of any late melt‐season since 1950Large quantities of surface‐generated meltwater overwhelmed the subglacial drainage system causing brief increases in ice velocityLate‐season runoff‐induced speed‐ups have only minimal impact on the mass balance of the Greenland Ice Sheet via dynamical processes [ABSTRACT FROM AUTHOR]

Supraglacial rivers and lakes are important for the routing and storage of surface meltwater during the summer melt season across the Greenland Ice Sheet (GrIS) but remain poorly mapped and quantified across the northern part of the ice sheet, which is rapidly losing mass. Here we produce, for the first time, a high-resolution record of the supraglacial drainage network (including both rivers and lakes) and its seasonal behaviour at Humboldt Glacier, a wide-outlet glacier draining a large melt-prone hydrologic catchment (13 488 km 2), spanning the period 2016 to 2020 using 10 m spatial resolution Sentinel-2 imagery. Our results reveal a perennially extensive yet interannually variable supraglacial network extending from an elevation of 200 m a.s.l. to a maximum of ∼ 1440 m a.s.l. recorded in 2020, with limited development of the network observed in the low-melt years of 2017 and 2018. The supraglacial drainage network is shown to cover an area ranging between 966 km 2 (2018) and 1566 km 2 (2019) at its maximum seasonal extent, with spatial coverage of up to 2685 km 2 recorded during the early phases of the melt season when a slush zone is most prominent. Up-glacier expansion and the development of an efficient supraglacial drainage network as surface runoff increases and the snowline retreats is clearly visible. Preconditioning of the ice surface following a high-melt year is also observed, with an extreme and long-lasting 2019 melt season and over-winter persistence of liquid lakes, followed by low snow accumulation the following spring, culminating in earlier widespread exposure of the supraglacial drainage network in 2020 compared to other years. This preconditioning is predicted to become more common with persistent warmer years into the future. Overall, this study provides evidence of a persistent, yet dynamic, supraglacial drainage network at this prominent northern GrIS outlet glacier and advances our understanding of such hydrologic processes, particularly under ongoing climatic warming and enhanced runoff. [ABSTRACT FROM AUTHOR]

B Replying to b Rott et al. I Nature Communications i 10.1038/s41467-020-16658-y (2020) In Tuckett et al.[1], we report short-term speed-up events of Antarctic Peninsula outlet glaciers. Here, we defend the meltwater hypothesis, present evidence against the sea-ice hypothesis, and examine potential bias in our glacier velocity measurements. Although sea-ice evacuation is coincident with the March 2018 speed-up event observed in Tuckett et al.[1], changes in sea-ice characteristics are not synchronous across all studied glaciers. Finally, the sea-ice hypothesis cannot explain how glaciers return to pre-event velocities[1], as the sea ice does not rapidly reform to its pre-breakup structure (Fig. [Extracted from the article]

Atmospheric warming is increasing surface melting across the Antarctic Peninsula, with unknown impacts upon glacier dynamics at the ice-bed interface. Using high-resolution satellite-derived ice velocity data, optical satellite imagery and regional climate modelling, we show that drainage of surface meltwater to the bed of outlet glaciers on the Antarctic Peninsula occurs and triggers rapid ice flow accelerations (up to 100% greater than the annual mean). This provides a mechanism for this sector of the Antarctic Ice Sheet to respond rapidly to atmospheric warming. We infer that delivery of water to the bed transiently increases basal water pressure, enhancing basal motion, but efficient evacuation subsequently reduces water pressure causing ice deceleration. Currently, melt events are sporadic, so efficient subglacial drainage cannot be maintained, resulting in multiple short-lived (<6 day) ice flow perturbations. Future increases in meltwater could induce a shift to a glacier dynamic regime characterised by seasonal-scale hydrologically-driven ice flow variations. Surface meltwater is known to influence the dynamics of some glaciers and the Greenland ice sheet. Here, the authors have identified the first examples of the drainage of surface meltwater to the bed of outlet glaciers on the Antarctic Peninsula that trigger large and rapid accelerations of ice flow. [ABSTRACT FROM AUTHOR]

Ice thickness measurements near the margin of the Greenland Ice Sheet (GrIS) are relatively sparse, presenting issues for modeling ice-flow dynamics, ice-sheet change, and subglacial hydrology. We acquired near-margin ice thickness data at Leverett Glacier, west Greenland, using a highly portable, low power, ground-penetrating radar operating at 10-80 MHz. Ice-thickness measurements, to a maximum of 270 m, were incorporated into the BedMachine model of ice thickness, created using mass conservation methods. The new data significantly modified the modeled ice thickness, and hence bed elevation and routing of subglacial water, in both the Leverett and adjacent Russell Glacier. Although the revised modeled basal topography and subglacial hydrology are consistent with observations, our new data unrealistically reduced the overall size of the Leverett Glacier hydrological catchment. Additional ice-thickness measurements are therefore required to realistically constrain subglacial topography and subglacial hydrological routing in this area. Our work improves understanding of the basal topography and the subglacial hydrology of Leverett Glacier, with implications for glacier dynamics and assessments of water piracy between catchments in the marginal zone of the GrIS, and for the interpolation of ice-thickness grids using mass conservation methods. [ABSTRACT FROM AUTHOR]

Iceberg discharge is a major component of the mass balance of the Greenland Ice Sheet (GrIS). While bulk estimates of discharge variation over time exist, inferred remotely from measurements of grounding line ice velocities or surface mass balance calculations, few detailed measurements of discharge itself from individual marine-terminating glaciers existed until recent years. Recently, it has been shown, through a combination of ocean–iceberg modelling and non-linear system identification, that the century-long record of iceberg numbers crossing 48 o N in the West Atlantic is a good first-order proxy for discharge from at least south and west Greenland. Here, we explore the varying relative importance of ice sheet, oceanic and climatic forcing of iceberg discharge from these areas over the twentieth century, by carrying out sensitivity studies of a non-linear auto-regressive mathematical model of the 48 o N time series. We find that the relationships are mainly non-linear, with the contribution of the GrIS surface mass balance to iceberg discharge likely to be dominant in the first half of the century. This period is followed by several decades where oceanic temperature effects are most important in determining the model variation in iceberg discharge. In recent decades, all physical processes play a non-negligible part in explaining the iceberg discharge and the model suggests that the glacial response time to environmental changes may have decreased. [ABSTRACT FROM AUTHOR]