TITLE: The Gordian Knot of aerosol-cloud interactions: insight from global climate models
Since industrialization humans have emitted substantial quantities of aerosol and green house gases (GHGs). These pollutants have opposing effects on Earth’s energy balance. Emissions of GHGs act to warm the Earth by trapping heat (a positive radiative forcing). Emissions of aerosol cause many different radiative effects, but on average lead to an increased reflection of light back to space (a negative radiative forcing). The observed increases in surface temperature during the industrial era may tell us very different things about the Earth system’s sensitivity to GHGs, depending on how much aerosol brightening has masked GHG-induced warming during this period. Thus, accurate knowledge of the strength of aerosol radiative forcing is key to predicting future warming. In turn, the most uncertain radiative forcing from aerosol is how it interacts with cloud as a cloud condensation nuclei.
In this presentation I will discuss constraints on aerosol-cloud interactions (aci) based on observations of clouds and aerosol interpreted through modelling experiments. Constraints on aci present a difficult problem because of the short time and length scale of aci and because the causal link between aerosol and cloud is bidirectional. In the first part of my talk I will examine the information contained in injections of aerosol by volcanoes on remote islands, changes in aerosol emissions due to air quality legislation in the US and China, and the pristine Southern Ocean to constrain anthropogenically-driven increases in droplet number concentration within clouds, and by extension cloud brightness. In the second part of the talk I examine how increased cloud droplet number concentration due to increased aerosol affects cloud microphysical processes and, by extension, cloud macrophysical quantities, such as coverage and thickness. Recent studies are divided as to whether enhanced aerosol thins or thickens cloud. I will present novel analysis techniques using global models to account for causal ambiguity and interpret observations to show that extratropical cloudiness is enhanced by enhanced aerosol and exerts a negative radiative forcing. The combined evidence offered by this analysis shows that a weak aerosol forcing, and by extension low climate sensitivity, is not consistent with our understanding of cloud and aerosol behavior from observations.