From Source to Sink: Measuring and Modeling Processes Affecting Methane Emissions and Loss

Abstract

Methane is a key target for climate change mitigation efforts. With a radiative forcing 85 times stronger than CO2 over a 20-year period and an atmospheric lifespan of only a decade, mitigating methane emissions will slow climate change in the near-term. However, quantifying methane emissions from specific sectors accurately poses a significant challenge. This is because top-down estimations of methane emissions demand precise observations and constraints on a range of physical and chemical processes. In this thesis, I seek to enhance the accuracy of methane emissions calculations by resolving these processes in detail and advocating for an expansion of the methane monitoring network.

The primary mechanism for atmospheric methane destruction is its oxidation by the Hydroxyl radical (OH). Chemical feedbacks due to temporal variations in OH availability can substantially influence the methane lifetime and, consequently, emissions trends over recent decades. In Chapter 2, I quantify the impact of this predominant chemical loss mechanism on methane emissions calculations.

Methane loss to the stratosphere represents the second most significant methane destruction mechanism, although the processes involved remain highly uncertain. Accurately quantifying methane loss via stratospheric-tropospheric exchange is crucial for improving the accuracy of methane emissions calculations. In Chapter 3, I utilize chemical tracers to determine how stratospheric-tropospheric exchange influences global methane emissions trends.

Current understanding of greenhouse gas fluxes from a top-down perspective typically relies on atmospheric inversions, which depend on spatial and temporal gradients in observed greenhouse gas concentrations. However, maintaining highly accurate ground-based measurements poses logistical and financial challenges, while satellites currently do not provide the requisite accuracy and spatial resolution for long-term monitoring. In Chapter 4, I explore the potential of frequency combs in measuring environmental impacts on greenhouse gas sensing and as tools to expand the observation network.

In summary, this thesis contributes to a more profound understanding of the two primary methane sinks and how their variations affect methane emissions trends over recent decades. It also lays the groundwork for the next-generation greenhouse gas observation network using laser frequency combs by quantifying environmental impacts on greenhouse gas spectroscopy directly in the field. Future advances should focus on a more accurate understanding of methane sink processes, improved spectroscopy, and expanded measurement networks. This will require advances in both modeling and measurements.

Ultimately, rapid and efficient mitigation of methane emissions remains the most feasible approach to curb anthropogenic climate change. To do this however, accurate assessments of methane trends and emissions necessitate bringing methane measurements and modeling of methane destruction processes closer to the real world.