Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
07 July 2020

Aquaplanets as a Framework for Examination of Aerosol Effects

Science

Aerosols are airborne particles that arise from natural or human-caused surface emissions. Aerosols impact the flow of energy through the atmosphere by scattering and absorbing light and acting as seeds for cloud droplets. The aerosol impact on the energy entering the climate system is a major uncertainty in climate simulations. This study proposes that idealized aquaplanet configurations of climate models could be useful for studying aerosol effects, both for model development and hypothesis testing. A series of experiments demonstrates that altering the geographic pattern of aerosol emissions leads to different radiative forcing while fixing total emissions. Most of this forcing arises through aerosol-cloud interactions.

 

Impact

The results from the simple experiments changing the patterns of aerosol emissions plainly show that where aerosols are emitted matters to the climate system. This is salient because it suggests that changes in aerosol sources could lead to changes in their impact, for example, if emissions shift from midlatitude to tropical regions. The idealized configuration makes this analysis straightforward and could be applied across models to identify why models have different aerosol effects, thus providing a path for reducing uncertainty in aerosol effects and improving climate projections. The aquaplanet framework also provides an economical way to test aerosol-cloud interactions in developmental versions of climate models and a testbed for hypothesis testing.

Summary

This study proposes that aquaplanets are a suitable and useful framework for studying aerosol effects in climate models. A few previous aerosol studies have used aquaplanets, but this is the first one to propose a configuration that does not alter the atmospheric physics or dynamics, including the representation of aerosol effects. This bridges a conceptual gap between simplified configurations and the most comprehensive configurations of climate models. The results show that the CESM2 aquaplanet configuration captures aspects of the realistic configuration’s aerosol-cloud interactions. The configuration provides an efficient testbed because statistics are robust for relatively short simulations. As an example application, effective radiative forcing is shown to be a function of the geographical pattern of aerosol emissions, and that this dependence arises because different cloud regimes respond to aerosol changes differently. CESM2 appears to have a strong sensitivity to aerosol perturbations in the tropics and subtropics, with analyses indicating this is largely driven by increases in cloud liquid water content with increased aerosols.

Contact
Brian Medeiros
National Center for Atmospheric Research (NCAR)
Publications
Medeiros, B.  2020.  "Aquaplanets as a Framework for Examination of Aerosol Effects."  Journal of Advances in Modeling Earth Systems 12(7).  https://doi.org/10.1029/2019ms001874.