Understanding contrasts in high mountain hydrology in Asia

There is great scientific debate about the cryospheric response to climate change in the high mountains of Asia, caused by lack of data and the limited amount of studies. There is, however, indirect evidence of a complex pattern in glacier responses as a result of climate change. There are strong indications of a gradient in the cryospheric response to a changing climate from West to East in the Hindu-Kush-Karokoram-Himalaya region (HKKH). Glaciers in the Karakoram (West) show no volume loss or a slight mass gain and glaciers to the East exhibit negative mass balances. This so called Karokoram anomaly, which contrasts the global trend of glacier recession worldwide, is however still controversial, and seems confined to the highest and largest glaciers. Recent stud- ies have provided additional evidence but are limited to the most recent period from 2000. One of the explanations put forward for the distinct patterns in glacier response are differences in climate between the mainly monsoon dominated central and east areas of the HKKH region and the importance of midlatitude Westerlies in the Karokoram region. Other factors might be the differences in the accumulation and ablation regime and differences in glacier size and elevation range. One important feature in both regions is the presence of debris-covered glaciers. Debris is well known for its insulating effect and related reduction of melt rates starting from debris thicknesses of few centimetres. Several recent studies based on remote sensing, however, have obtained rates of thinning over debris-covered glaciers that are comparable to those of debris-free glaciers in the same region. Such estimates are spatially averaged and integrated in time over the period between two Digital Elevation Models (DEMs) and are therefore difficult to compare to point scale observations or results from numerical modelling. However, they point to an apparent inconsistency that should be further investigated which is partly associated with our lack of knowledge about processes controlling the functioning of debris-covered glaciers and their response to climate. Despite being such a prominent feature in the region, in fact, few studies have looked at debris-covered glaciers behind few point scale works. Another important knowledge gap in the region are a dramatic lack of representative, high elevation data about the meteorology and melt at high elevation. Our understanding of the main mechanisms controlling variability of air temperature and precipitation in high elevation, glacierised catchments is very limited and hinders a sound use of glacio-hydrological models. Finally, very few studies in the region are truly comparative in nature, beside those based on remote sensing, and no study has combined before observations in the fields, remote sensing and advanced glacio-hydrological modelling. In our proposed study, we intend to improve the understanding of the glacio-hydrological functioning of two contrasting catch- ments in the grater Himalaya and the Karokoram that are characterised by a considerable amount of debris-covered glaciers but strongly differ in climate, morphology and glacier characteristics, with the aim to improve the quality of future simulations of the catchments response to climate. The two catchments are the Shimshal catchment in Karokoram and the Langtang catchment in Nepal. We intend to achieve this goal by addressing three key knowledge gaps in process based modelling of glacierised catch- ments in the region: i) the impact of debris covered on the energy balance of a glacier at the glacier wide scale; ii) the spatial variations in temperature and therefore melt patterns over heterogeneous glaciers; and iii) the intra-catchment variation in precipi- tation and air temperature. We will use remote sensing to reconstruct glacier changes and long term geodetic mass balances, espe- cially for the earlier decades (from the 1970s on), for which basically no estimates exist, and we will interpret them in the light of the knowledge about processes gathered through field work and process based modelling. We will then integrate such knowledge into a distributed, physically-based glacio- hydrological model with improved mathematical representation of such processes, and we will finally use the model developed in this way to assess the impact of climate change on the cryosphere and hydrology of the two contrasting catchments. Uncertainties in the simulations associated both with the model parameters and uncertainty in climate projections will also be quantified. The project will be executed by a strong international consortium led by scientists from ETH Zurich and closely collaborating with the University of Zurich, Graz University, Utrecht University as well as the International Centre for Integrated Mountain Development (ICIMOD) in Nepal and Pakistan Meteorological Department (PMD) in Pakistan. The project key innovative com- ponent is in the integration of monitoring, remote sensing and glacio-hydrological modeling to address recognized knowledge gaps in our understanding of the HKKH glacierised catchments.

Glaciers under climate change are showing a complex pattern in the high mountains of Asia: while there is no volume loss or a slight mass gain in the West Himalayan region, glaciers in the East have shown negative mass balances over the last decades. This so called Karakorum anomaly is supposed to be based on different responses to climate change in large-scale phenomena (i.e. the Indian Summer Monsoon and mid-latitude Western Disturbances). In the framework of the project Understanding contrasts in high mountain hydrology in Asia (UNCOMUN) climate scientists from the Wegener Center for Climate and Global Change at the University of Graz (Austria) together with colleagues from ETH Zurich (Switzerland) and the University of Utrecht (Netherlands) have found that these climate change effects in the Indian Summer Monsoon and Western Disturbances remain in the majority of serious climate projections for the Himalayan region and may become even more pronounced until the end of the 21st century. As a consequence, the Karakorum anomaly is expected to remain at least for the near future. On the long-term perspective, however, human behaviour becomes the dominating factor. While climate change leads to increases in precipitation over the Himalayan slopes, rising temperatures impede glacier mass gains as precipitation no longer deposits in the form of snow, in particular during the Indian Summer Monsoon season. However, under the IPCC climate-protection scenario (RCP4.5) a further increase of snow melting is stabilised at a high level due to the stabilisation of global warming and the Karakorum anomaly becomes emphasised because of spatial differences in snow accumulation; under an IPCC business-as-usual scenario (RCP8.5) global warming continues undamped, leading to an accelerated melting and a strong reduction of snow accumulation, especially in the eastern Himalaya. This once more underscores the endangering of the farer existence of Himalayan glaciers by human behaviour.