Asymmetries between the climate responses to CO2 and stratospheric aerosol forcing

Abstract

Understanding Earth’s temperature response to radiative perturbations is pivotal in climate science, as emphasized in the World Climate Research Program’s Grand Challenge on Clouds, Circulation and Climate Sensitivity. Surprisingly, a radiative perturbation from stratospheric aerosol produces a smaller temperature response than a radiative perturbation of the same magnitude from CO2. Resolving this apparent paradox is the core motivation behind this thesis, with the ambition to understand the atmospheric processes that cause this climate response asymmetry. The climate response asymmetry can be quantified by the feedback parameter, which is more negative (i. e., more stabilizing) for stratospheric aerosol than CO2 forcing. The pattern effect framework provides a modern interpretation of variations of the feedback parameter in time. I apply this framework to explain differences of the feedback parameter among forcing agents, in this particular case between CO2 and stratospheric aerosol forcing. I employ an earth system modelling approach, testing hypotheses with simulations with the general circulation model MPI-ESM 1.2, and demonstrating the identified key mechanisms in the CMIP6 multi model ensemble. I identify two essential ingredients to explain the climate response asymmetry between CO2 and stratospheric aerosol forcing: temperature change in the warm pool (tropical Indian and Western Pacific Ocean, 30°S - 30°N, 50°E - 160°W), and the Brewer-Dobson circulation. I show that the warm pool is the dominant region causing feedback differences between CO2 and stratospheric aerosol forcing. In an ensemble of 120 decadal simulations forced with CO2 and stratospheric aerosol forcing, a simple measure for warm pool temperature change explains 50 % of the variance of the feedback parameter. I prove that the warm pool cools particularly strongly from stratospheric aerosol forcing, and that the stratospheric circulation is a key ingredient to explain this. Stratospheric aerosol heats the stratosphere, leading to an acceleration of the Brewer-Dobson circulation and hence to an increased poleward energy transport. This provides additional cooling to the tropical surface that is not reflected in the forcing pattern at the top of the atmosphere, highlighting the role of surface forcing for understanding the evolution of temperature patterns. My results engender conclusions about climate feedback beyond the response to stratospheric aerosol forcing. To understand temperature pattern formation, the surface forcing must be considered, which is in contrast to the established method of measuring forcing at the top of the atmosphere. Although climate feedback mainly refers to processes at the surface and in the troposphere, I show that the stratospheric circulation influences surface forcing patterns and thus temperature patterns and feedback.