| The Hindu Kush-Himalayan (HKH) region epitomizes a geographic location at the forefront of global environmental change where cryospheric, hydrological, and ecological processes are under threat owing to a warming climate. This region, also known as the 'water towers of Asia', contains the largest amount of ice outside of the polar regions, provides sustenance to ∼200 million inhabitants, feeds large Asiatic river basins, hosts global biodiversity hotspots with numerous ecoregions and protected areas, and is warming at a rate that significantly exceeds the global rate. Given the susceptibility of the HKH region to climate change, improved understanding of biophysical processes and climatological trajectories is necessary for impacts assessment and for long-term sustainability of resources throughout the region. Through basin- to regional-scale analyses, this study examines snowmelt processes, snowmelt hydrology, regional climate projections and climate extremes, and vegetation dynamics across the HKH region by integrating remotely sensed datasets with available field observations, global climate model outputs, and hydrological modeling. Results from a first comprehensive freeze/thaw detection study using satellite-based radar scatterometer provides spatial and temporal patterns of snowmelt dynamics (melt onset, freeze-up and melt duration) and shows the eastern Himalayan region with a significantly longer melt season compared to the central and western Himalayan and Karakoram regions. Multi-model analyses based on climate simulations from the Coupled Model Intercomparison Project phase 3 archive indicate a continued trend for more extremes in the 21 st century, consistent with a warmer, wetter climate. Precipitation projections indicate a more intense monsoon in the eastern Himalayan region, yet a wetter cold season in the western Himalayan region. Significant changes in snowmelt processes and regional hydrology may occur owing to such projected changes in climate. However, through a robust approach of hydrological modeling coupled with a data assimilation technique for parameterization, I emphasize the need for evaluation of snowmelt runoff models, examination of model uncertainty, and determination of uncertainty in snowmelt contributions to runoff prior to impacts assessment. Results quantify snowmelt contributions in an eastern Himalayan basin to be 29.7 ± 2.9% on average for the 2002-2006 period, with the 4000-5500 m elevation zone being the most critical with regards to its snowmelt contributions. Finally, a comprehensive regional analysis of vegetation dynamics as observed through satellite-derived Normalized Difference Vegetation Index (NDVI) identifies large-scale vegetation patterns and trends for the 1982-2006 period. Although there is a significant overall greening trend in most areas such as grassland regions in the Tibetan Plateau, other regions such as the southeastern HKH are undergoing significant losses in vegetation. Taken collectively, the overall research not only lays out cryospheric, hydrological, ecological, and climatological baselines necessary for the HKH region but also emphasizes regional variability in these responses and the need for an integrated approach to understanding based on monitoring and systematic modeling approaches. |
