The effects of ozone depletion, increased greenhouse gas emissions, and increased aerosols on precipitation in High-Mountain Asia

High-Mountains Asia (HMA) forms the largest mountain mass on Earth and provides the water needs for more than one billion people living in downstream regions. They play a crucial role by interacting with regional atmospheric circulations, such as the summer monsoon and winter mid-latitude westerlies. However, despite the importance of precipitation in this region our understanding of how human-related activities such as ozone depletion, increasing greenhouse gas (GHG) concentrations and aerosol loading influences precipitation are still uncertain. In particular, although studies have tried to explain the sensitivity of precipitation in HMA due to e.g. aerosols and GHG, no study has additionally investigated the effect of all anthropogenic forcings (i.e. a combination of increasing GHG, ozone depletion, and aerosols). This study undertakes an analysis of five global atmosphere-only climate simulations (which use forcing representative of pre-industrial conditions, present-day ozone depletion, present-day GHG concentrations, present-day aerosol concentrations, and ‘all’ present-day anthropogenic forcing) to explore the sensitivity of the distribution of precipitation over HMA, and particularly the Hindu-Kush Karakoram (HKK) and Himalaya regions. The analysis indicates a complex set of interactions involving increasing GHG and anthropogenic aerosols affects precipitation over the Himalayas in summer, with a reduction in precipitation caused by anthropogenic aerosols tending to offset increases caused by increasing GHG. It is further shown that increasing GHG has a strong positive impact in summer in the Himalaya regions below around 2000 m, while aerosols decrease precipitation at all altitudes in the Himalayas in summer. Examination of the all forcing experiment suggests that the associated precipitation response is highly non-linear and regionally varying, and much more complex than a simple addition of individual effects. By comparison, the various anthropogenic forcings had little influence on winter precipitation. It is shown that the increase in summer precipitation over the Himalayas in response to increasing GHG is due to a strengthening of the summer monsoon, while the decrease in precipitation in response to aerosols is due to a weakened circulation. In the all forcing experiment the effects on the monsoon due to increasing GHG and aerosols largely offset each other. This perhaps suggests that the aerosols also play an important role by acting as condensation nuclei and therefore altering cloud distributions.