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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−1 over 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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Subglacial hydrologic systems regulate ice sheet flow, causing acceleration or deceleration, depending on hydraulic efficiency and the rate at which surface meltwater is delivered to the bed. Because these systems are rarely observed, ice sheet basal drainage represents a poorly integrated and uncertain component of models used to predict sea level changes. Here, we report radarderived basal melt rates and unexpectedly warm subglacial conditions beneath a large Greenlandic outlet glacier. The basal melt rates averaged 14 mm · d−1 over 4 months, peaking at 57 mm · d−1 when basal water temperature reached +0.88 ◦C in a nearby borehole. We attribute both observations to the conversion of potential energy of surface water to heat in the basal drainage system, which peaked during a period of rainfall and intense surface melting. Our findings reveal limitations in the theory of channel formation, and we show that viscous dissipation far surpasses other basal heat sources, even in a distributed, highpressure system. [ABSTRACT FROM AUTHOR]

Sustained mass loss from Himalayan glaciers is causing supraglacial debris to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier longevity. However, debris‐covered glaciers are losing mass at similar rates to clean‐ice glaciers in High Mountain Asia. This rapid mass loss is attributed to the combined effects of; (a) low or reversed mass balance gradients across debris‐covered glacier tongues, (b) differential ablation processes that locally enhance ablation within the debris‐covered section of the glacier, for example, at ice cliffs and supraglacial ponds, and (c) a decrease in ice flux from the accumulation area in response to climatic warming. Adding meter‐scale spatial variations in supraglacial debris thickness to an ice‐flow model of Khumbu Glacier, Nepal, increased mass loss by 47% relative to simulations assuming a continuous debris layer over a 31‐year period (1984–2015 CE) but overestimated the reduction in ice flux. Therefore, we investigated if simulating the effects of dynamic detachment of the upper active glacier from the debris‐covered tongue would give a better representation of glacier behavior, as suggested by observations of change in glacier dynamics and structure indicating that this process occurred during the last 100 years. Observed glacier change was reproduced more reliably in simulations of the active, rather than entire, glacier extent, indicating that Khumbu Glacier has passed a dynamic tipping point by dynamically detaching from the heavily debris‐covered tongue that contains 20% of the former ice volume. Plain Language Summary: Glaciers in the Himalaya are shrinking rapidly in response to ongoing climate change. Many of these glaciers are covered with thick layers of rock debris that insulate the ice surface from atmospheric warming. Recent observations suggest that, contrary to expectations, debris‐covered glaciers are losing mass at similar rates to clean‐ice glaciers. We explore the processes driving the rapid loss of ice from a debris‐covered glacier using a glacier model with a novel representation of sub‐debris melt. Our model shows that the rapid ice loss from Khumbu Glacier, Nepal, since 1984 cannot be explained solely by accounting for variations in debris thickness and the presence of ice cliffs and ponds across the glacier surface. Instead, the glacier has passed a dynamic tipping point in the last 100 years where the upper active glacier no longer provides ice to the stagnant debris‐covered tongue. We expect that Khumbu Glacier will pass another tipping point in the next 100 years and detach completely from the debris‐covered tongue at the base of the icefall as the active glacier shrinks in response to climate change. Key Points: Debris‐covered glaciers are losing mass at a similar rate to clean‐ice glaciers in High Mountain AsiaParameterizing differential ablation in an ice‐flow model increases net mass loss from Khumbu Glacier by 29%–47%Dynamic detachment of the upper active glacier from the stagnant debris‐covered tongue after 1900 CE could have accelerated recent mass loss [ABSTRACT FROM AUTHOR]

Abstract Although a substantial ice cover has been identified within the mid-latitudes of Mars, there is uncertainty regarding the formation, current and former volume, and dynamic evolution of these ice masses. Here, we present the first comprehensive ice volume estimate of martian glacier-like forms (GLFs) from systematic population scale mapping and volumetric analysis. The outlines of 1243 GLFs were manually delineated from 6 m per pixel Context Camera (CTX) images and the volume of each determined using a volume–area scaling approach. Our results show that GLFs cover a surface area of 11344 ± 393 km2 and have a total volume of 1744 ± 441 km3. Using two end-member scenarios for ice concentration by volume of 30% (pore ice) and 90% (debris-covered glacier ice), we calculate the volume of ice contained within GLFs to be between 523 ± 132 km3 (480 ± 121 Gt) and 1570 ± 397 km3 (1439 ± 364 Gt), equivalent to a mean global water layer 3 to 10 mm thick. We investigate the local topographic setting of each GLF by reference to the Mars Orbiter Laser Altimeter (MOLA) digital elevation model. Our analysis reveals that globally GLFs are on average larger in latitudes >36° and on slopes between 2 and 8°. In the northern hemisphere GLFs between 500 and 2500 m in elevation and in the southern hemisphere GLFs with a northern aspect are also larger on average. The observed spatial patterns of GLF landform and volume distribution suggests that regional to local meteorological and topographical conditions play an important role in GLF ice accumulation and/or preservation. Assuming a net accumulation rate of 10 mm a−1 typical of climatic excursions with high obliquity, we estimate a period of at least 13 ka is required to yield the average calculated GLF ice thickness of ∼130 m. Such a period is well within the timeframe of a high obliquity cycle (20–40 ka), suggesting that the current GLF volume could have formed during a single climate excursion. Highlights • We present a complete inventory of martian glacier-like form (GLFs) outlines. • Martian GLFs cover 11344 ± 393 km2 and contain between 523 ± 132 km3 and 1570 ± 397 km3 of ice. • This ice volume corresponds to a global water layer 3 to 10 mm thick. • Local microclimates, as well as solar forcing, control GLF distribution and size. [ABSTRACT FROM AUTHOR]

The mid-latitudes of Mars host numerous ice-related landforms that bear many similarities to terrestrial ice masses. This collection of landforms, termed viscous flow features (VFFs), is composed primarily of H2O ice and shows evidence of viscous deformation. Recent work has hypothesised that VFFs are the diminishing remains of once larger ice masses, formed during one or more previous ice ages, and the landscape therefore records evidence of polyphase glaciation. However, debate persists concerning the former extent and volume of ice, and style of former glaciations. The accompanying map (1:100,000 scale) presents a geomorphic and structural assessment of a glacial landscape in eastern Hellas Planitia, Mars. Here, we present a description of the features identified, comprising four geomorphic units (plains, lobate debris apron, degraded glacial material, and glacier-like form) and 16 structures (craters, moraine-like ridges, flow unit boundaries, arcuate transvers structures, longitudinal surface structures, ring-mold craters, terraces, medial moraine-like ridges, raised textured areas, flow-parallel and flow-transverse lineations, crevasses and crevasse traces, and ridge clusters). [ABSTRACT FROM PUBLISHER]

Near-surface water ice, either pure or mixed with regolith, forms a widespread suite of distinctive landforms in Mars’ mid-latitudes. These landforms are in many cases sufficiently similar to cryospheric features on Earth to allow analogue-based comparisons to be made. However, our understanding of glacial processes in Mars’ subpolar latitudes remains far from complete, and crucial fundamental issues remain unresolved. These include basic glaciological information such as the internal physical and thermal structure of martian ice masses, whether (and how) those ice masses move, and the nature of any mass-balance regime they are subject to. Addressing these issues is complicated by the fact that any current knowledge, being based overwhelmingly on the visual interpretation of remotely sensed images, is insufficient to determine the extent to which these landforms are currently active or relict. Addressing these issues would contribute not only to our understanding of Mars’ current landscape but also to our knowledge of Mars’ long-term climatic variability and to our awareness of where and in what form H2O exists on Mars – information that would be of value to future space missions. The aim of this review is to bring the state of knowledge regarding mid-latitude glaciation on Mars to the attention of the wider research community. As background, we provide an overview of the geological and planetary framework within which Mars’ mid-latitude ice was deposited. We then define and describe the various ice-related landforms that have been identified within Mars’ mid-latitudes, and we review the processes that have been proposed to explain the origin and physical characteristics of those landforms. Finally, we present what we consider to be key avenues for further research. [ABSTRACT FROM AUTHOR]