Search

The magnitude of tropical cooling during the Last Glacial Maximum (LGM; ∼19–26.5 ka) remains controversial, with sea‐surface temperatures cooling by several degrees less than most temperatures reconstructed at high elevations. To explain this discrepancy, past studies proposed a steeper (increased) lapse rate—the temperature decrease with elevation—during the LGM relative to today. For instance, LGM temperatures in East Africa reconstructed from branched GDGTs from multiple elevations support an ∼0.9°C/km increase in the lapse rate during the LGM relative to present day. Lapse rates are a critical part of the Earth's climate sensitivity and atmospheric energy transfer, and it is vital to know whether and by how much the tropical lapse rate steepened during the LGM. Here, we simulate LGM glacier extents in the Rwenzori Mountains of Uganda with and without a change in lapse rate using a range of temperature and precipitation estimates. We find that the lapse rate must have been steeper than present for glaciers to reach their LGM positions using available sea‐level temperature and precipitation estimates for East Africa. Plain Language Summary: Glaciers are sensitive to changes in temperature and precipitation, and during the last ice age (∼20,000 years ago), glaciers in the tropics grew in cold (5°C–9°C) and dry conditions while sea surface temperatures changed relatively little (∼1°C–3°C). Perhaps the temperature estimates are telling different stories, or perhaps they are both correct and the rate of cooling with elevation (lapse rate) was steeper (increased) during the drier conditions of the ice age. Here, we use a glacier model and a range of temperature, precipitation, and lapse rate estimates to grow glaciers to moraines (glacial deposits) that mark their ice age extent at 20,000 years ago. Results indicate that glaciers can reach the moraines using the modest sea surface temperature change and a steeper lapse rate, which is supported by available biogeochemical analysis in this area or a large change in temperature and no lapse rate change, which is not supported by sea surface temperature estimates. Climate alterations in the tropics have global implications, and understanding how the lapse rate can change with time and location is vital for informing climate change models. Key Points: Tropical glaciers in East Africa required significant cooling (5.4°C–7.3°C) to reach their Last Glacial Maximum (LGM) extentsA large magnitude of cooling in Afro‐alpine regions conflicts with Indian Ocean sea surface temperatures (1°C–3°C)A steeper (increased) lapse rate during the LGM could explain this altitudinal discrepancy [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]

Understanding the climate-glacier dynamics of debris-covered glaciers is notoriously difficult given a multitude of controlling factors and feedback mechanisms involving climate forcing, debris-load properties, supraglacial water bodies, and multi-scale topographic effects. Recent studies have provided insights into controlling factors, and have reported the presence of anomalies that contradict the general consensus of the protective influence of debris loads on ablation dynamics. Nevertheless, numerous processes that regulate glacier dynamics at various spatial and temporal scales have not been adequately accounted for in statistical and numerical modeling studies. Furthermore, important feedbacks involving ablation, topography, irradiance, gravitational debris flux, and supraglacial ponding are often neglected or oversimplified in existing models, which poses a challenge to our understanding of conflicting field observations such as the accelerated mass loss of many Himalayan glaciers, and glacier-subsystem responses (ice-flow, debris flux, surface morphology, and supraglacial water bodies) to climate forcing. This paper provides insights into the complexity of debris-covered glacier systems by addressing concepts and issues associated with forcing factors and glacial processes, and highlights the importance of understanding system couplings and feedbacks. Specifically, we review recent studies on debris-covered glaciers and utilize simulation results based on the Baltoro Glacier in the central Karakoram to discuss important concepts and issues. Our results demonstrate that climate forcing, the properties and transport of debris, topography and supraglacial water bodies are the key controlling factors in a debris-covered glacier system, and that their coupled effects and positive feedbacks may increase the ice loss of a debris-covered glacier. We also recommend new research directions for future studies.

Assessment of the response of the Himalayan river flows to climate change is complex due to multiple contributors: rainfall, snowmelt, and glacier‐melt. The number of studies is limited in this direction due to lack of data availability as well as non‐availability of models considering all the above‐mentioned components. As for example, the state‐of‐the‐art variable infiltration capacity (VIC) model does not account for the glacier melt. Here we integrate a glacier‐melt model with VIC and validate the model output with observed streamflow in five river basins in the Himalayas, at daily scale. Our model simulates the streamflow with Nash‐Sutcliffe estimates greater than 0.65 in all basins. The sensitivity analysis shows that the contribution from snowmelt decreases substantially in all the five basins with highest decrease of 36% in Dudh Kosi (DK), in a warm and dry scenario. The glacier‐melt increases (15%–70%) in a warmer environment with its present volume, but decreases (3%–38%) substantially, when the volumes are reduced to half. However, such a decrease is found to be compensated by increased precipitation in a wetter scenario with a net increase of 3%–13%. Climate model simulations show a decrease in the spring onset times for Sutlej basin while increase for DK basin for both RCP4.5 and RCP8.5 scenarios. Sutlej and Arun basins show decreases of more than 6 days in the center of volume of streamflow, which suggests that there will be increased flows in the early part of year and reduced flows later in the year. Plain Language Summary: Water is one of the resources essential for our survival. Climate change impacts water resources globally, and the impacts are adverse in regions which rely on water from snowpacks and glaciers. The Himalayas is the most magnificent mountain range and has vast reserves of ice and snow. The climate change has affected the Himalayas and reduced storages of ice and snow; lesser snowfall and more melt are evident across the mountain range. These changes affect about a billion people that reside in the basins of the Indus, Ganges, and Brahmaputra rivers. The water in these rivers is contributed by rainfall, snowmelt and glacier‐melt. Here, we studied changes in streamflow and its components in the Central Himalayas with changing precipitation and temperature scenarios. We found that in warming climate, snowmelt will reduce substantially, and glacier melt will increase initially, but with significantly reduced volumes in the latter end of the century, it will start to decrease. However, in a wetter scenario, increased precipitation may compensate for such a decrease. The changes in streamflow in early spring and autumn vary from basin to basin in the Himalayas. Key Points: Integration of temperature index glacier model with state of art variable infiltration capacity model to consider melts from clean and debris covered glaciersThe rainfall dominates the Central Himalayan water resources, and the streamflow is highly sensitive to precipitation changesSutlej basin shows decrease in the sprint onset times while Dudh Kosi shows increases for RCP 4.5 and 8.5 scenarios [ABSTRACT FROM AUTHOR]

Observed past climate data used as input in glacier models are expected to differ among datasets, particularly those for precipitation at high elevations. Differences among observed past climate datasets have not yet been described as a cause of uncertainty in projections of future changes in glacier mass, although uncertainty caused by varying future climate projections among general circulation models (GCMs) has often been discussed. Differences among observed past climate datasets are expected to propagate as uncertainty in future changes in glacier mass due to bias correction of GCMs and calibration of glacier models. We project ensemble future changes in the mass of glaciers in Asia through the year 2100 using a glacier model. A set of 18 combinations of inputs, including two observed past air temperature datasets, three observed past precipitation datasets, and future air temperature and precipitation projections from three GCMs were used. The uncertainty in projected changes in glacier mass was partitioned into three distinct sources: GCM uncertainty, observed past air temperature uncertainty, and observed past-precipitation uncertainty. Our findings indicate that, in addition to the differences in climate projections among GCMs, differences among observed past climate datasets propagate fractional uncertainties of about 15% into projected changes in glacier mass. The fractional uncertainty associated with observed past precipitation was 33–50% that of the observed air temperature. Differences in observed past air temperatures and precipitation did not propagate equally into the ultimate uncertainty of glacier mass projection when ablation was dominant. [ABSTRACT FROM AUTHOR]

Recent model studies suggest that interannual climatic variability could be confounding the interpretation of glacier fluctuations as climate signals. Paleoclimate interpretations of moraine positions and associated cosmogenic exposure ages may have large uncertainties if the glacier in question was sensitive to interannual variability. Here we address the potential for interannual temperature and precipitation variability to cause large shifts in glacier length during the Holocene. Using a coupled ice-flow and mass-balance model, we simulate the response of Cameron Glacier, a small mountain glacier in New Zealand's Southern Alps, to two types of climate forcing: equilibrium climate and variable climate. Our equilibrium results suggest a net warming trend from the Early Holocene ( 10.69 ± 0.41 ka; 2.7 °C cooler than present) to the Late Holocene (CE 1864; 1.3 °C cooler than present). Interannual climatic variability cannot account for the Holocene glacier fluctuations in this valley. Future studies should consider local environmental characteristics, such as a glacier's climatic setting and topography, to determine the magnitude of glacier length changes caused by interannual variability. [ABSTRACT FROM AUTHOR]

Abstract: The paper focuses on reconstructing a plateau icefield surface from field geomorphological data and a physical-based glacier model. The results allow the analysis of the patterns of glacial erosion and the estimation of the palaeo-Equilibrium Line Altitudes (ELA). The study area is the Serra da Estrela, a plateau in Central Portugal rising to ∼2000 m ASL. The glaciated area during the Last Maximum of the Serra da Estrela Glaciation (LMGSE) was ∼66 km2. The reconstruction of the topography of the icefield and valley glaciers in the LMGSE is based on the Schilling and Hollin model. It iterates along the valley longitudinal profile and is based on the gradient, on valley-shape indices and on yield basal shear stresses. These variables influence ice thickness and therefore, also the slope of the ice surface allowing for its reconstruction. The key variable is basal yield shear stress and its value was included in the model manually starting from the points of maximum extent of the valley glaciers and following a range known to occur in contemporary conditions. The input values are validated by matching the resulting ice surface to geomorphological features. Where these are absent, a constant value of 100 kPa was used. The icefield was reconstructed from a radiating set of long-sections, along which ice thicknesses were calculated. A DEM of the ice surface was constructed, allowing the estimation of the hypsometric curves of the distinct glacier catchments and the calculation of the palaeo-ELAs. The results show a regional ELA at 1650 m ASL with spatial variations across the icefield reflecting mainly the effect of eastward snow drift. The LMGSE glaciers were very sensitive to minor climatic changes, especially due to the large area of the plateau icefield, and to the positioning of the ELAs, close to, or in the, flat part of the hypsometric curve. The model of the ice surface is of significant value for the analysis of the patterns of glacial erosion at the landscape level. In the Serra da Estrela most of the glacial erosion occurred near the plateau margins and in valley heads, where glacier surface slope was steeper allowing for a faster ice flow and where ice flow concentrated. Strong glacier erosion in the Zêzere valley is linked to the tectonic setting, but also to the confluence of glaciers and to the overfeeding of snow from the plateau. [Copyright &y& Elsevier]

Synthetic glacier length records are generated for the Holocene epoch using a process-based glacier model coupled to the intermediate-complexity climate model ECBilt. The glacier model consists of a massbalance component and an ice-flow component. The climate model is forced by the insolation change due to variations in the Earth's orbital parameters. We consider three glaciers, ranging from maritime to continental. At Nigardsbreen (southern Norway), the simulated long-term trend in the annual mass-balance is primarily determined by summer temperature, with a smaller contribution from winter precipitation. In the early Holocene, summers were warmer and winters wetter than today in the ECBilt simulation. Both signals seem consistent with proxy data. The simulated glacier length shows a phase of rapid expansion during the mid-Holocene, followed by more gradual growth. At Rhonegletscher (the Swiss Alps), ECBilt simulates warmer and wetter summers in the early Holocene. The temperature signal seems realistic, but proxy data and earlier modelling results are not conclusive with respect to the precipitation signal. The implied glacier length shows a maximum extent at 3–5 kyr BP, which seems unlikely. This suggests that the simulated precipitation response is not realistic. The simulated early-Holocene climate at Abramov glacier (Kirghizia) is characterized by high summer precipitation, associated with a northward extension of the Asian monsoon. The precipitation signal reaches its maximum around 6 kyr BP, which is consistent with the timing of the maximum in lake-level data. The simulated glacier length shows a pronounced postglacial maximum at the time of maximum monsoon intensity. There is considerable centennial-timescale variability in the simulated glacier length records. These length variations are generated by internal climatic variability. They are typically asynchronous among the three different glaciers. ECBilt has reasonable skill in simulating the relative importance of temperature and precipitation as well as the seasonality of the forcing, although the overall level of variability is underestimated. Length variations are shown to behave as a lagged moving-average process, with a glacier-specific memory. [ABSTRACT FROM AUTHOR]