Contrasting features of scattering and absorbing aerosol direct radiative forcings and climate responses

Abstract

Human activities have greatly increased the amount of aerosols in the atmosphere since the Industrial Revolution. Anthropogenic aerosols are comprised of optically scattering and absorbing particles, with the principal concentrations located in the Northern Hemisphere. This thesis investigates the contrasting features between scattering and absorbing aerosol radiative forcings and the accompanying climate responses by employing the GFDL CM2.1 global climate model. Anthropogenic sulfate and black carbon aerosols are used as examples of a strong scatterer and strong absorber, respectively.

Model aerosol distributions are evaluated by comparing optical properties with ground-based, aircraft, and satellite measurements. Geographical forcing distributions of absorbing and scattering aerosols show approximately similar magnitudes with opposite signs, and an interhemispheric forcing asymmetry, in complete contrast to long-lived greenhouse gases. Uncertainties in the forcings are addressed by examining the quantitative roles of cloud coverage, surface albedo, relative humidity, and mixing state in governing the magnitudes; clouds and high-albedo surfaces weaken sulfate forcing and strengthen black carbon forcing. Sulfate forcing is strengthened by hygroscopic growth. Black carbon forcing is strengthened by internal mixing.

The contrast in climate responses to scattering and absorbing aerosol features (including the influence on clouds) is analyzed by employing different aerosol forcing configurations in preindustrial to present-day climate simulations. This thesis goes beyond the analysis of the typical temperature response by investigating important hydrological and dynamical climate responses, such as precipitation, atmospheric circulation, and heat transport. Aerosol responses are also contrasted to the long-lived greenhouse gases' climate response.

Aerosol climate responses are governed by the sign of the forcing and the interhemispheric forcing asymmetry. Precipitation, atmospheric circulation, and heat transport responses are anti-correlated for scattering and absorbing aerosols. Compared to the interhemispherically symmetric long-lived greenhouse gas forcing effects, aerosols induce asymmetric patterns of changes in precipitation north and south of the equator accompanied by cross-equatorial heat fluxes in the atmosphere and ocean. Because responses to scattering and absorbing aerosols are opposite in sign and occur simultaneously, there is a significant dilution of their individuality in the net aerosol effect on climate. The findings are relevant for future climate changes and aerosol emission policy decisions.