DYNAMICS OF DEBRIS-COVERED GLACIERS IN THE HKH

The high mountains of Asia are often referred to as the Third Pole as they contain the greatest concentration of ice on the planet outside of the real poles. Meltwater from glaciers in this area contributes to current global sea level rise and regional water resources and ongoing changes in the glaciers is associated with increasing natural hazards such as floods and rockfalls. We still dont know much about the state of the glaciers in these mountain ranges, or how they are changing under current climate conditions. Part of the reason for this is because many of the glaciers in these ranges are covered in rock, sand and gravel, from the surrounding landscape. This cover of debris changes the melt rate of the underlying glacier and, through a series of feedback loops, this in turn changes when and how far the glacier tongue advances or retreats in response to changes in climate conditions. This means that knowledge and models developed for clean-ice glaciers cannot be applied to these so-called debris-covered glaciers. In order to calculate how debris-covered glaciers will change in the future, or have changed in the past, in response to different climate conditions we need to understand how debris from the surrounding landscape ends up on the glacier surface, how long it takes to form the debris cover, and how this debris affects the melting and glacier flow. Existing computer models that allow scientists to simulate glacier systems do not account for debris within the glacier, and so do not recreate the behavior of debris-covered glaciers. The effect of the debris cover on the glacier changes through time as the debris-covered area expands or contracts, and representing this time-varying aspect of the debris cover in calculations of the glacier melt and ice flow is particularly tricky. In this research project an existing glacier system model will be further developed to include calculations of how rock debris is transported through the glacier, how it builds up at the surface to form a debris cover, and how this ultimately affects the glacier flow and behavior. Running the model for different climate conditions and glacier settings will make it possible to understand how debris-covered glaciers can be expected to respond over 100s to 1000s of years, and how their behavior differs to that of clean ice glaciers. The results of computer simulations with this model can then be used to understand what changes lie in store for the debris-covered glaciers of the high Asian mountains, and how these changes might impact global sea level and local communities.

Some glaciers in mountain regions are covered with a layer of rock rubble that alters the melt rate of the underlying glacier and how the glacier responds to a warming climate. In the future, due to climate change, more and more glaciers are expected to develop a rock cover like this. This project aimed to better understand how this rubble affects glacier behavior and to develop computer models that represent the impact of surface rubble on the glacier, so we can better predict the glacier meltwater runoff and glacier change in the decades to come. We developed the first computer model capable of reproducing the way rubble is transported through a mountain glacier and how and where it melts out at the surface, so that its impact on ice melt can be properly accounted for. We also developed a second model that can reproduce the way a glacier surface changes over time in response to a changing climate. We made a large number of new field measurements to determine parameters such as the thickness of the glacier beneath the rock debris cover, how variations in the thickness of the surface rock layer affect the total glacier meltwater production, and how the rock debris layer alters the energy exchange between the ground surface and the atmosphere. These data and computer models provide essential information or tools for accurately calculating the way that glaciers are expected to change, and impact their local environment and global sea level in the near future.