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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]

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]