Search

Predicting the amplitude and distribution of surface undulations on ice sheets and glaciers is useful because of their influence on surface mass and energy balance, atmospheric boundary layer processes, and supraglacial meltwater routing. We develop an approximate method of calculating the surface elevation response due to spatial perturbations in basal topography and slipperiness, on two‐dimensional flow sections whose thickness, surface slope, and basal slip ratio vary along flow. Our main result is an integral expressing nonuniform transfer of basal variability to the surface. It uses published Fourier transfer functions derived through perturbing plane‐slab Stokes flow but circumvents the need to subwindow the spatial domain to estimate the response. We test the method on ice flow synthesized by a finite‐element model of Stokes flow with constant viscosity and known basal topography and slipperiness perturbations; in this case, it predicts the observed size and shape of the surface undulations well, capturing more than 90% of their variance. Application of the method to the central flowline of Columbia Glacier, Alaska, and a flowline on the Greenland Ice Sheet ending on Nordenskiöld Glacier, using knowledge of the approximate bed topography and ignoring the unknown slipperiness forcing, yields less faithful prediction of their surface undulations (40–50% of their variance) but demonstrates the method's potential to reproduce their qualitative features. We discuss the factors limiting the method's performance on real flowlines. We present an extended mathematical theory of how ice flow transfers basal variations upward to form undulating surface topography, for nonuniform flow in two dimensionsWe formulate an integral transform method for calculating such transfer along flowlinesValidation of the method on synthetic flowlines informs its application to flowlines on valley glaciers and ice sheets

An analysis suggests that ice geometry and flow speeds control how meltwater affects the slipperiness of the bed beneath the Greenland ice sheet. Changes in these conditions could therefore influence future ice-mass loss.

Ice stream bed topography contains key evidence for the ways ice streams interact with, and are potentially controlled by, their beds. Here we present the first application of two‐dimensional Fourier analysis to 22 marine and terrestrial topographies from 5 regions in Antarctica and Canada, with and without mega‐scale glacial lineations (MSGLs). We find that the topography of MSGL‐rich ice stream sedimentary beds is characterized by multiple, periodic wavelengths between 300 and 1200 m and amplitudes from decimeters to a few meters. This periodic topography is consistent with the idea that instability is a key element to the formation of MSGL bedforms. Dominant wavelengths vary among locations and, on one paleo ice stream bed, increase along the direction of ice flow by 1.7 ± 0.52% km−1. We suggest that these changes are likely to reflect pattern evolution via downstream wavelength coarsening, even under potentially steady ice stream geometry and flow conditions. The amplitude of MSGLs is smaller than that of other fluvial and glacial topographies but within the same order of magnitude. However, MSGLs are a striking component of ice stream beds because the topographic amplitude of features not aligned with ice flow is reduced by an order of magnitude relative to those oriented with the flow direction. This study represents the first attempt to automatically derive the spectral signatures of MSGLs. It highlights the plausibility of identifying these landform assemblages using automated techniques and provides a benchmark for numerical models of ice stream flow and subglacial landscape evolution. MSGL topography consists of superimposed periodic wavelengths as expected in the instability theory of subglacial bedform formationMost of the dominant wavelengths present within one extensive MSGL field increase downstream, suggesting that MSGLs evolve via pattern coarseningFor MSGLs to generate periodic topography, sediment must be able to accumulate or erode (freely move) without fixed anchor points

The formation and rapid drainage of supraglacial lakes (SGL) influences the mass balance and dynamics of the Greenland Ice Sheet (GrIS). Although SGLs are expected to spread inland during the 21st century due to atmospheric warming, less is known about their future spatial distribution and volume. We use GrIS surface elevation model and regional climate model outputs to show that at the end of the 21st century (2070–2099) approximately 9.8 ± 3.9 km3(+113% compared to 1980‐2009) and 12.6 ± 5 km3(+174%) of meltwater could be stored in SGLs under moderate and high representative concentration pathways (RCP 4.5 and 8.5), respectively. The largest increase is expected in the northeastern sector of the GrIS (191% in RCP 4.5 and 320% in RCP 8.5), whereas in west Greenland, where the most SGLs are currently observed, the future increase will be relatively moderate (55% in RCP 4.5 and 68% in RCP 8.5). We present a comprehensive new data set of potential supraglacial lake locations on the Greenland Ice SheetSupraglacial lakes are predicted to become more prevalent on the ice sheet during the 21st century with an increase in volume of 113–174%According to our results, by the end of the 21st century, the majority of supraglacial lakes will be found in northeastern Greenland

The dynamic response of large mountain glaciers to climatic forcing operates over timescales of several centuries and therefore understanding how these glaciers change requires observations of their behavior through the Holocene. We used Be‐10 exposure‐age dating and geomorphological mapping to constrain the evolution of glaciers in the Khumbu Valley in the Everest region of Nepal. Khumbu and Lobuche Glaciers are surrounded by high‐relief lateral and terminal moraines from which seven glacial stages were identified and dated to 7.4 ± 0.2, 5.0 ± 0.3, 3.9 ± 0.1, 2.8 ± 0.2, 1.3 ± 0.1, 0.9 ± 0.02, and 0.6 ± 0.16 ka. These stages correlate to each of the seven latest Holocene regional glacial stages identified across the monsoon‐influenced Himalaya, demonstrating that a coherent record of high elevation terrestrial palaeoclimate change can be extracted from dynamic mountain landscapes. The time‐constrained moraine complex represents a catchment‐wide denudation rate of 0.8–1.4 mm a−1over the last 8 kyr. The geometry of the ablation area of Khumbu Glacier changed around 4 ka from a broad, shallow ice tongue to become narrower and thicker as restricted by the topographic barrier of the terminal moraine complex. Satellite observations indicate that glaciers in the monsoon‐influenced Himalaya are changing rapidly in response to climate change. However, understanding why glaciers are changing requires observing glacier behavior over longer timescales using the glacial geological record. We mapped the geometry and measured the ages of ice‐marginal moraines built by two adjacent glaciers in the Everest region of Nepal. The moraines represent a complete record of glacier expansion during cold periods in the monsoon‐influenced Himalayan region over the last 8,000 years. Moraines formed by the large Khumbu Glacier and the smaller, steeper Lobuche Glacier also reveal differences in how these glaciers have changed since the last Ice Age. The results are useful to understand how monsoon‐influenced Himalayan glaciers respond to climate change, and to improve projections of their future behavior. Ice‐marginal moraines in the upper Khumbu Valley represent a complete record of regional glacier change between 7.4 ka and the present dayMoraine building modified the response of Khumbu Glacier to climate change after 8 kaHolocene moraine volume indicates a catchment‐wide denudation rate of 0.8–1.4 mm per year Ice‐marginal moraines in the upper Khumbu Valley represent a complete record of regional glacier change between 7.4 ka and the present day Moraine building modified the response of Khumbu Glacier to climate change after 8 ka Holocene moraine volume indicates a catchment‐wide denudation rate of 0.8–1.4 mm per year

ABSTRACTIce 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.