Extreme precipitation and flooding in a changing climate : new quantitative approaches and process understanding

Extreme climate events like severe precipitation, extreme heat, and drought, have large, negative impacts on society and ecosystems, but are challenging to model and predict due to their complexity. Thus, there remain many unresolved questions about the characteristics of extreme events and their impacts in a changing climate. This dissertation develops new methodological approaches to understand how and why the risks of climate extremes are changing, focusing specifically on two of the most widespread hazards: extreme precipitation and flooding. In Chapter 1, I quantify the impact of long-term changes in precipitation on the cost of flooding in the United States. This research finds that historical changes in precipitation have contributed to around 36% of U.S. flood damages between 1988-2017, providing the first empirical evidence that historical climate changes have already increased the cost of flooding in the U.S. I also analyze precipitation changes simulated by an ensemble of global climate models, finding that pattern of historical change is consistent with human-caused climate change, and that further global warmer will lead to additional increases in extreme precipitation. In Chapter 2, I analyze the changes in flood risk that result from warmer conditions that cause a shift from winter snow to winter rain. Using causal inference regression techniques to analyze data from over 400 watersheds in the western U.S., I show that this shift from snow to rain leads to non-linear increases in flood size due to larger rain-driven floods compared to snowmelt. However, this analysis also indicates some heterogeneity in this response. I find that (i) larger changes in flood risk occur for watersheds with higher average precipitation and (ii) the coldest watersheds may temporarily see decreases in flood risk with initial warming. In Chapter 3, I demonstrate a new application of explainable deep learning to identify the processes that have led to increases in extreme precipitation. Using the U.S. Midwest region as a case study, I find that large-scale atmospheric circulation conditions associated with extreme precipitation have become more frequent over the past 20 years, at a rate of around one additional day per year. These changes in the frequency of atmospheric circulation conditions have co-occurred with increases in atmospheric moisture flux to the Midwest region. As a result, these atmospheric circulation conditions are more likely to result in extreme precipitation now than in the past. Combined, this dissertation (i) provides new empirical evidence quantifying the impacts of climate change, which can inform policy decisions and climate adaptation, and (ii) provides new understanding into the processes that shape extreme precipitation and flooding risks in a warmer climate. Further, this dissertation demonstrates new quantitative approaches to analyze climate hazards and climate impacts that can be extended broadly to study other hazards and regions

Keywords: Earth System Science.
Advisor: Burke Marshall
Diffenbaugh Noah S.
Field Christopher B.