Title: Hydroclimate variability and environmental change in Eurasia over the past millennium and its impacts
Description: Streamflow records in many regions of Eurasia including South Asia are short and fragmentary. This makes it challenging to contextualise natural climate variability relative to anthropogenic climate change and evaluate the severity of recent extreme events. In the first section of the dissertation (Chapter 1 and 2) we use tree rings to reconstruct centennial-scale streamflow of the Indus and Brahmaputra Rivers in South Asia for the past six centuries. These two rivers and their tributaries are fed by Himalayan glaciers and the South Asian Monsoon and are the basis of economic, social, and cultural life of over 700 million people in the region. These centennial reconstructions of past discharge provide valuable information about long-term hydroclimate variability, drought and flood hazard. They also help us to interpret recent climate extremes relative to those in the past and benchmark projections of climate change for the region. Large tropical (and extratropical) volcanic eruptions can release large quantities of reflective sulphate aerosols into the stratosphere where they may persist for up to 3 years.

In Chapter 3 we discuss how these aerosols can impact European and Mediterranean hydroclimate by causing dry conditions over northwestern Europe and the British Isles and wet conditions over the western Mediterranean. We examine this hydroclimate response using Superposed Epoch Analysis (SEA), a statistical method used to identify consistent responses to events by testing for the possibility of random occurrence. Finally, in Chapter 4 we describe in further detail (cf. Chapter 3) the modified double-bootstrap SEA that we developed in Chapter 3 to examine the uncertainty inherent in SEA within a probabilistic framework. We describe our modified SEA by applying it to two datasets, a reconstruction of northern hemisphere summer temperature for the past millennium, and a compilation of tree ring fire scar records for the western U.S. Using these two datasets we examine post-eruption northern hemisphere cooling following volcanism and the synchrony between drought conditions and fire events in the western U.S. respectively.