Estimating the Indirect Climate Forcing Effects of Dust via Ice-containing Clouds for the Pliocene

Abstract:

Atmospheric aerosols exert a major control on the physical and radiative properties of clouds, which are a crucial component of radiative heating and cooling in the atmosphere. Aerosol-cloud radiative effects are a major uncertainty in climate models and the representation of aerosol processes varies greatly between models due to limitations in observational records, gaps in theoretical knowledge and difficulties in parameterising sub-grid scale processes in numerical models.

Large variations in natural and anthropogenic aerosols over the industrial era have made constraining aerosol-cloud climate forcing difficult. Paleoclimates represent pristine atmospheres where only natural aerosols are present and can be used to understand and constrain background, natural aerosol climate forcing.

Mineral dust is an ubiquitous natural aerosol which has varied considerably during the past. Whilst many studies have considered the direct effects of dust on the climate, the indirect effect of dust on the climate is often overlooked. Mineral dust is the most important ice nuclei in the atmosphere and the availability of dust in the atmosphere therefore is a strong control on the physical and radiative properties of ice-containing clouds i.e. cirrus and mixed phase clouds.

Previous work has highlighted the potential importance of aerosol-cloud forcing in other periods of amplified high-latitude warmth: i.e. the Eocene and Cretaceous. However, these approaches have only speculated possible mechanisms for these changes. Our model setup uses the CESM including CAM5 and DEAD (Dust Entrainment & Deposition Model) along with recent cloud micro-physics parameterisations in order to simulate aerosol dependent ice-nucleation in the atmosphere in response to low atmospheric dust load. Proxy data show evidence of very low dust deposition rates during the Pliocene, suggesting much lower atmospheric dust loading than the present. We quantify the changes in cloud physical properties and cloud radiative forcing, and examine how these contribute to global warmth and high-latitude amplification in the Pliocene.