Characterising the evolution of Himalayan debris covered glaciers.

The majority of the 20,000 glaciers found in the Himalaya are in a state of negative mass balance, and have been for decades. Broad spatial trends in ice mass loss have been identified by large scale geodetic mass balance studies, but regional averaging of mass loss data has masked catchment or glacier scale variability. This thesis has the broad aim of examining the catchment scale variability of ice mass loss, in order to identify factors that might promote, or inhibit, more substantial ice mass loss from the region in the future. Ice mass loss rates from Everest region glaciers were calculated using the geodetic approach, over the period 2000-2015, and compared depending on glacier terminus type. Lake-terminating glaciers were found to have lost 32% more ice mass than land-terminating glaciers, and maximum surface lowering rates of lake-terminating glaciers peaked at more than twice the rate of land-terminating counterparts. Glacier hypsometry was found to be contrasting at the catchment scale, and predicted accumulation area ratio (AARs) change in response to different RCP warming scenarios emphasises the importance of considering glacier area-altitude distribution in future ice loss estimates. A more detailed assessment of the evolving geometry, dynamics and ice loss rates of nine lake-terminating glaciers suggested two phases of glacier-lake interaction may exist. A phase of dynamic lake-terminating glacier retreat was evident where terminus proximal surface lowering rates were high (up to 3 m a-1), ice front retreat rates were steady or accelerating, and surface velocities increased (by up to 10 m a-1, between 1999 and 2015). Alternatively, a phase of retreat typified by surface lowering rates akin to land-terminating glaciers (~1 m a-1), where ice front retreat rates were steady or diminishing, and where surface velocity reduction occurred. The dynamic phase of ice loss observed on lake-terminating glaciers in the Everest region is not of the same magnitude as larger waterter-minating glaciers found in other glacierised regions, probably because of the topographic confinement of host glaciers and the dominance of resistive stresses, but the now populous nature of glacial lakes in the region means the potential for amplified future ice loss exists. The impact of long-term ice loss on the topographic characteristics of debris covered glacier surfaces was also examined. Ice cliff and supraglacial pond expansion was identified as the main driver of topographic change on slow flowing, land-terminating glaciers. A more pitted surface topography of greater relief developed on most glaciers, which has implications for the energy balance at the glacier surface, and for supraglacial hydrology. Overall, the results of this thesis emphasise the need to incorporate a range of glacier dynamics scenarios and melt processes into simulations of future ice loss in the Himalaya