On the role of natural laboratories and natural experiments in elucidating cloud-aerosol-climate interactions: A story of ships, smoke, and shutdowns

Abstract

Uncertainties in how airborne particles (aerosol) influence cloud reflectivity hinder our understanding of anthropogenic climate change. The study of aerosol-cloud interactions (ACI) is complicated by the difficulty of disentangling aerosol effects from other phenomena like meteorological variability. Natural laboratories (large-scale perturbations of known origin) and natural experiments (more abrupt and localized perturbations), in which a phenomenon of interest may be clearly separated from other sources of variability, offer promising means to better understand causality in ACI.Ship tracks — curvilinear cloud features following individual ships — are the quintessential example of natural experiments in ACI. Prior studies have not detected large-scale cloud changes from shipping, despite climate model predictions indicating sizable effects. In Chapter 2, we attribute increased cloud reflectivity within a major shipping corridor in the southeast Atlantic to enhanced cloud droplet numbers by estimating what cloud properties would have been without shipping. Increased cloud brightness from microphysical changes is partially offset by decreases in the total amount of cloud water. Extrapolating our results globally, we calculate an effective radiative forcing due to ACI of approximately -1 W/m2. Smoke from southern Africa blankets the southeast Atlantic Ocean from June-October, producing a natural laboratory with competing aerosol radiative effects. In Chapter 3, we investigate smoke effects on the transition between overcast stratocumulus and scattered cumulus clouds in regional climate and large eddy simulation models and compare the results with observations from recent field campaigns. Interactions between smoke and the large-scale circulation alter above-cloud temperature and moisture in ways that strongly affect cloud evolution. The COVID-19 outbreak and subsequent shutdown in China during February 2020 resulted in a sharp economic contraction. In Chapter 4, we show that nitrogen dioxide pollution declined by an unprecedented amount from its expected unperturbed value, but regional-scale column aerosol loadings and cloud microphysical properties were not detectably affected. The disparate impact is tied to differential economic impacts of the shutdown, in which transportation underwent much more drastic declines than industry and power generation. Anomalously warm and humid meteorological conditions and complex chemical interactions further decreased nitrogen dioxide but enhanced secondary aerosol formation.