CEE Seminars and Conferences

Urban Humid Heat Stress, Exposure, and Adaptation Under a Changing Climate

Advisor: Professor Lei Zhao

Abstract

Cities are uniquely vulnerable to many climate-driven threats, extreme heat being among the deadliest of these. Cities are also uniquely positioned to adapt successfully to extreme heat. Research to-date has tended to focus on air temperature alone, yet there is increasing recognition of the relevance of humid heat for capturing the human experience. Despite this recognition, there are key gaps in the knowledge-to-action spectrum which hinder effective, widespread heat adaptation in cities worldwide. First, there is a lack of understanding about the drivers of urban humid heat, specifically of how changes in air temperature and humidity, driven by local urbanization and global climate change, shape urban heat stress. Second, there is a lack of tools that capture local physical realities to support peer identification for knowledge sharing on a global scale. This dissertation aims to support urban heat adaptation by assessing urban humid heat stress, exposure, and adaptation. I achieve this goal by leveraging recent advancements in urban datasets, models, and climate science to improve the mechanistic understanding of urban humid heat and improve peer identification for knowledge sharing on urban adaptation.

First, I quantify the combined impact of urbanization and climate change on urban humid heat. I project that urban humid heat will increase substantially across the globe by 3.1C by the end of the century under a high emission scenario. This increase is largely attributed to climate change-driven increases in specific humidity (1.8C), followed by air temperature (1.4C). Hotspots of heat stress are concentrated in coastal, equatorial areas. I also show a critical, climate-driven dilemma between cooling efficacy and water limitation of urban greenery-based heat adaptation. This work reveals the substantial influence of both climate change and humidity increases on humid heat stress, especially in hotspot regions, and raises important considerations for greenery-based urban cooling strategies.

Next, I investigate the role of urban humidity changes in urban heat stress due to urbanization and climate change. I present a physics-based theoretical framework that enables a unified attribution of humid heat to temperature and humidity across humidity metrics. My results show that global urban moisture changes are evenly split between drying and humidifying. Under urbanization, temperature contributes slightly more to changes in heat stress. This pattern flips under climate change and twice as many urban grid cells attribute the majority of heat stress increases to humidity. Together, these results elucidate global urban moisture changes and

highlight the importance of shifting towards more humidity-conscious cooling strategies under a changing climate.

Finally, I propose and apply a globally federated framework to identify peer cities with similar climate hazards and urban forms. I categorize 4187 cities worldwide into 15 urban climate-form typologies of urban humid heat stress. I find that typologies connect peers across geographies and size, suggesting that traditional connections based on geography, size, and existing networks may not be the most appropriate for peer identification. I also identify peers whose current climate risks are analogous to their projected climates and find significant international collaboration potential. This work highlights the importance of considering urban climate risk and urban form in tandem when identifying relevant peers for knowledge sharing and emphasizes both the potential for and importance of international collaboration.

This dissertation advances understanding on future urban humid heat stress, exposure, and adaptation. It leverages global urban climate simulations and datasets and demonstrates how recent advancements on global urban climate science can support urban heat adaptation along the knowledge-to-action spectrum. This work has broader implications for urban heat adaptation, emphasizing the need to shift towards more humidity-conscious cooling strategies and providing a framework for future applications of urban climate science for knowledge sharing.