title: Extreme hydrological events during the last 3500 years and their changes in the future
Creators: name: Kim, Woon Mi
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 530 Physics
subject: 550 Earth sciences &amp; geology

description: The Earth's climate has constantly varied over time due to the influence of internal processes and external factors. Together with the mean climate, spatial and temporal characteristics of extreme hydrological events, such as droughts and extreme precipitation, have also changed over time. For instance, droughts with multi-decadal duration, which have never been detected in the instrumental era, occurred during the Medieval climate anomaly (approximately 950-1200 CE) in North America and northern Europe. Devastating floods associated with heavy rainfall occurred more frequently during the Little Ice Age (about 1250-1850 CE) compared to other periods in the Common Era (the last 2k years) over Europe. These past extreme events significantly impacted the environment and ancient societies, sometimes influencing societal disruptions.

Despite their impacts on the environment and society, until now, less attention has been paid to the variations of past extreme hydrological events and their drivers compared to the variations in mean precipitation. Among many factors that hinder the investigations of past extreme hydrological events, the main problem arises from the limited availability of observations and past reconstructions for this kind of sporadic events.

Nowadays, complex climate models have become essential tools to examine the underlying dynamics of the Earth's climate. As these models can simulate the response of the climate to internal and external perturbations, they offer a possibility to address responsible drivers of climate variations and also extreme events on the global scale. In addition, simulations with climate models cover long time periods that can go far beyond the modern instrumental era. Hence, information from simulations can complement observations and reconstructions to illustrate better the characteristics of past droughts and extreme precipitation.

This thesis uses a state-of-the-art earth system model, the Community Earth System Model (CESM), as the main investigation tool to understand the variability and dynamics of past extreme hydrological events, namely droughts and extreme precipitation, during the past three millennia. It also aims to address the effects of an external factor, i.e., volcanic eruptions, on the climate and the impacts of past change in the climate on ancient European society. The thesis mainly consists of three studies.

The first study focuses on the dynamics of persistent Mediterranean droughts during 850-2099 CE. The Mediterranean region is one of the drought hot spots which is projected to experience an intensified drying by the end of the 21st century compared to the historical period. Hence, a more comprehensive understanding of the underlying mechanisms of past droughts over the region is necessary to better assess the changes of drought drivers in the historical and future period. In the study, temporal characteristics of Mediterranean droughts are assessed, and drivers of persistent long droughts are identified. In addition, the sensitivity of Mediterranean droughts to various drought metrics is tested.

The second study deals with daily extreme precipitation during the last three millennia previous to the Industrial Era (1501 BCE-1849 CE). The study is based on the newly conducted 3500-year long CESM simulations, which include a new proxy record of reconstructed volcanic eruptions. Internal and externally generated processes that influence the long-term variability of extreme precipitation are identified across the globe using a statistical method based on the extreme value theory. Among the externally generated processes, the impacts of volcanic eruptions on extreme precipitation are analyzed in more detail.

The third research topic concentrates on examining the impacts of the 43 BCE Okmok eruption in Alaska on the climate and early Mediterranean civilization. Abrupt large-scale changes of the Mediterranean climate after this large extratropical volcanic eruption are detected in various climate-related records, and the magnitudes of these changes are quantified with CESM. This change in climate is as a possible driver of the societal changes that occurred during the ancient Roman period.

Lastly, an outlook and a general conclusion of the thesis are presented, also proposing some potential follow-up investigations.