2025 Theses Doctoral

Extreme Heat in a Warming World: Causes, Changes, and Air Quality Connections

Bartusek, Samuel

Extreme heat is a significant climate hazard, posing a major threat to human lives, social systems, and ecosystems. Global warming is certain to intensify extreme heat and its impacts, yet key uncertainties remain that motivate work to better understand extreme heat in the current and future climate. Two emerging goals in the field of extreme heat research are (1) determining how severe extreme heat may become under global warming and how its physical mechanisms may change, and (2) investigating how extreme heat interacts with other environmental hazards. This dissertation advances these research frontiers through the following four chapters.

Chapter 1 investigates the physical drivers of the unprecedented 2021 Pacific Northwest (PNW) heatwave and its connections to climate change. The event's primary cause was extreme atmospheric dynamical forcing provided by the interaction of a hemispheric-scale wave in the polar jet stream and a smaller wavetrain emanating from the subtropical Pacific. Dry soils likely amplified the heating in parts of the region, and a model experiment provides evidence that land–atmosphere feedbacks are capable of amplifying heatwaves in the PNW. Global warming dramatically increased the potential for this event's occurrence over the past decades (from a virtually-impossible to a multi-hundred-year event) and will continue to do so, while ongoing soil drying is probably making land–atmosphere amplification of heatwaves more likely in the PNW.

Chapter 2 examines, across global land area, how the severity of heatwaves has changed relative to warm-season-average warming over recent decades, and what mechanisms have been responsible. Multiple regions worldwide have experienced strong amplified (or suppressed) warming of the hottest days of the year, but the trends in most of these regions lie outside the spread of climate model simulations over the same time period. We apply two independent methods, based on a model experiment and statistical analysis of observations, to disentangle dynamic versus thermodynamic drivers of relative hottest-day warming: both agree that it is driven by atmospheric dynamics in the extratropics but by surface energy balance factors within the subtropics, and that while the highest-magnitude trends are driven by dynamics, the highest-significance trends are driven by surface energy balance factors.

Chapter 3 explores how extreme humid heat affects near-surface ozone and particulate matter pollution across global land area, using chemical and meteorological reanalysis datasets. There are several regions worldwide where pollution tends to be worsened during extreme humid heat relative to extreme non-humid heat, revealing a compounding tendency. Many of these regions experience some of the globally most-severe humid heat, and many are densely-populated. A more urban background chemistry regime is a strong predictor of higher co-occurrence of humid heat and pollution, and in such regions, stagnation and suppressed boundary layer heights during extreme humid heat likely help precursor species and pollutants accumulate.

Chapter 4, placing Chapter 3's findings in a broader atmospheric chemistry context, assesses the role of troposphere–stratosphere dynamics in transporting stratospheric ozone downward throughout the troposphere. Combining an advanced chemical reanalysis and a high-resolution meteorological reanalysis reveals that more of the behavior of ozone in the free troposphere can be attributed to stratospheric intrusions (as opposed to other processes of stratosphere-to-troposphere transport) than implied if the tropopause is under-resolved and vertical folds in it are obscured. However, the influence of tropopause dynamics on near-surface ozone is much weaker than at higher altitudes, as surface processes like pollution remain much more important.