PhD Oral Exam - Mitchell Dickau, Geography, Urban and Environmental Studies

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Limiting global warming to the temperature thresholds outlined in the Paris Agreement requires a clear understanding of the relationship between cumulative CO₂ emissions and temperature, the consequences of temporarily exceeding these thresholds, and the role of carbon dioxide removal in achieving temperature stabilization. This thesis advances understanding of the Earth system response to CO₂ emissions and removals through three complementary manuscripts. First, I provide a review of the Remaining Carbon Budget (RCB) framework where I examine how methodological choices and uncertainty affect its use as a decision-making tool, highlighting the importance of transparent communication of likelihoods and assumptions in setting net-zero targets. Second, I use an Earth system climate model to quantify the reversibility of climate responses following temperature overshoot, showing that slow-responding variables exhibit irreversible changes that scale linearly with the time-integrated magnitude of overshoot. Third, I investigate the climate value of temporary carbon storage, demonstrating that storage-years of carbon yield proportional short-term benefits across the same slow-responding variables, but that some of these benefits decline over multi-century timescales. Collectively, these studies reveal that the magnitude of positive emissions and the magnitude, timing, and durability of removals play an important role in determining climate outcomes. This research underscores the urgency of rapid and durable emissions reductions.