Past-present-future Monsoon variability revealed by stable water isotopes: combining observations and climate models to assess its impacts on Bhu Stabila vattenisotoper för undersökningen av Monsunens variabilitet i för-, nu- och framtid: kombination av observationer och klimatmodellering The present project focuses on understanding the processes that govern the variability of the South Asian Monsoon over various time-scales from days to decades. We have chosen to focus primarily on the monsoon impacts in Bhutan, with a planned extension of the study area to neighbouring Nepal: this region is mainly affected by the Indian Monsoon subsystem, but it is likely to be influenced by the South-East Monsoon subsystem as experienced e.g. in Thailand. We emphasise in particular the use of stable water isotopes (SWI) to better understand and constrain processes in the atmospheric water cycle. The suggested approach combines ground- based (precipitation sampling, shallow ice-cores) and remote-sensed (tropospheric vapour) ob- servations with global and regional climate models with embedded SWI diagnostics. The periods of interest cover present-day, recent (1981-2010) and future (2021-2050) Mon- soon variability. Beyond the inherent scientific challenges in improving our understanding of the Asian Monsoon, the present project is highly relevant for supporting sustainable develop- ment policies in the partner countries. Analysing the regional SWI signature will also contribute to a refined interpretation of isotopic climate archives in Southern and Eastern Asia. Anthropogenic climate change, whose historical responsibility mainly lies with industrial nations, represents a critical challenge for developing countries. It is commonly accepted that the mean global temperature will increase by approximately 1 to 5 °C by 2100. This considerable temperature increase does not fully express the consequences of global climate change for, in this case, the populations of Bhutan and Nepal. Bhutan is a small country on the southern foothills of the Himalayas, bordered by India to the south and the Chinese Autonomous Region of Tibet to the north. The water resource issue is critical for Bhutan: 67% of its population lives in rural areas, with a limited use of irrigation techniques. Furthermore, Bhutan has recently developed its hydro-electrical capacities; electricity production exceeding the domestic demand is exported to India, which constitutes a major source of foreign currencies (42% of all exports). Hence, the people and economy of Bhutan are very vulnerable to any modification of precipitation, both in terms of total amount and seasonality. Bhutanese authorities, which for several decades have been particularly concerned with environmental conservation and sustainable development, need therefore a detailed assessment of the impacts of the global climate change on Bhutan. Evaluating the consequences of global climate change for Bhutan is not a trivial task. Projections for future climate change, as delivered by the Intergovernmental Panel on Climate Change (IPCC), are generally obtained from global climate models (GCM), whose coarse spatial resolution fail to represent the local climate characteristics over Bhutan, marked by a steep topography. Furthermore, most GCM were developed, evaluated and optimized in developed countries for temperate climates, and are therefore likely to present significant biases in their representations of the South-East Asian Monsoon. Therefore, we plan to use a high-resolution (up to 15 km) regional climate model over Bhutan, in order to evaluate its performance over present-day and recent (1981-2010) periods, before performing a dynamical downscaling of global IPCC projections for the near future (2021-2050). Yet the evaluation of the model performance over Bhutan is complicated by the fact that long observational series, as required for statistical significance assessment, are not available in Bhutan. Therefore, we opted for including stable water isotopes (SWI) as an additional indicator of the atmospheric water cycle. The SWI signal integrates processes along the air-mass trajectory, such as the source region, cumulative rain-out, evapo-transpiration over land etc. SWI will be studied both in observations (precipitation monitoring network over Bhutan, shallow ice-cores, satellite retrieval) and modeling (SWI module in the regional climate model REMOiso). Combining observations and model results, we will concentrate on distinguishing between natural climate variability and underlying climate change over Bhutan during 1981-2010. Thanks to our improved understanding of Monsoon dynamics in Bhutan, and the metrics characterizing the statistical properties of Monsoon variability, we intend to deliver not only a validated climate projection for Bhutan, but also an assessment of the reliability (i.e. statistical level of confidence) of the simulated climate projection.