Ruth Digby

Topic

Changes in aerosols and clouds following a rapid reduction in emissions: Case studies from the COVID-19 pandemic

School of Earth and Ocean Sciences

Date & location

• Wednesday, July 24, 2024
• 10:00 A.M.
• Clearihue Building, room B019

Examining Committee

Supervisory Committee

• Dr. Adam Monahan, School of Earth and Ocean Sciences, University of Victoria (Supervisor)
• Dr. Nathan Gillett, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Knut von Salzen, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Jason Cole, Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada (Outside Member)

External Examiner

• Dr. Andrew Gettelman, Senior Earth Scientist, Pacific Northwest National Laboratory

Chair of Oral Examination

• Dr. Valentin Schaefer, School of Environmental Studies, UVic

Abstract

This thesis explores the uncertainties in several key aerosol- and cloud-related fields. Although the three projects described here investigate different topics, they are united by a common theme: how well do we understand the impacts of potential changes in emissions?

The first two science chapters revolve around the COVID-19 pandemic and associated changes in anthropogenic activity. Each of these chapters asks two questions. First, did the emissions change in question lead to a statistically significant anomaly in satellite observations of relevant fields? And second, how well do current models reproduce the observed changes?

The first of the COVID-19 chapters explores the impact of reduced aviation on cirrus cloud. Aviation-induced cirrus is the largest source of uncertainty in, and by some estimates may be the largest component of, aviation's net radiative forcing. Despite a sustained global reduction in air traffic, with April 2020 flight numbers less than 40% of their 2019 levels (FlightRadar24, 2020c), aviation-weighted cirrus coverage was not significantly reduced in Spring 2020. A regression model based on the contrail cirrus simulations of Bock and Burkhardt (2016a) did not exhibit a statistically significant dependence of cirrus anomalies on aviation intensity, and suggested that current models may overestimate the impacts of aviation on cirrus. We apply these results to calculate the first observationally constrained estimate of the radiative forcing of aviation-induced cirrus: (-3, 22) mW/m², much lower than but consistent with the current best estimate of 57 (17, 98) mW/m² (Lee 2021).

The second of the COVID-19 chapters explores the impact of reduced aerosol emissions on aerosol optical depth (AOD). Of the regions considered, only India experienced statistically significant reductions in total or dust-subtracted AOD. Earth system model simulations from the CovidMIP experiment over-predict the magnitude of the Spring 2020 total and dust-subtracted AOD anomalies, but this bias can be attributed in part to biases in the model inputs. Through a series of sensitivity tests we demonstrate that, given sufficiently accurate inputs, Earth system models can reproduce the observed total and dust-subtracted AOD anomalies to within observational uncertainty. These results provide confidence in the ability of Earth system models to predict the large-scale aerosol responses to other emission scenarios as well.

The final science chapter of this thesis assesses the sensitivity of black carbon's (BC's) simulated climate impacts to the choice of its refractive index, which determines the extent to which BC absorbs or scatters radiation. We compare ensembles simulated with three values of the refractive index that are commonly used in the climate modeling community. Increasing from low to high absorption can increase simulated global-mean absorbing aerosol optical depth (AAOD) by 42% and effective radiative forcing from BC-radiation interactions (BC ERFari) 47%. The AAOD increase is comparable to that from recent updates to aerosol emission inventories, and in BC source regions, a third as large as the difference in AAOD retrieved from different satellites. The BC ERFari increase is comparable to the scale of the uncertainty in recent literature assessments. Although model sensitivity to the choice of BC refractive index is modulated by other parameterization choices, our results highlight the importance of considering refractive index diversity in model intercomparison projects.