Aquaplanets as a Framework for Examination of Aerosol Effects

Although fundamental to the planetary radiative balance, aerosol impacts are highly uncertain in climate simulations because of the uneven distribution of aerosol sources and the complex interactions with radiation and clouds that are difficult to represent in climate models. This study proposes that aquaplanet configurations represent an idealized framework to investigate aerosol effects. As a simple demonstration, a series of aquaplanet simulations with the Community Atmosphere Model version 6 shows that the spatial distribution of aerosol emissions changes the aerosol effective radiative forcing even with unchanged total emissions. Some statistical properties of the simulations are presented to show that relatively short model integrations yield robust results. Much of the aerosol effect is shown to arise from aerosol-cloud interactions, especially through rapid adjustments associated with the aerosol lifetime effect that alter the cloud optical thickness. Plain Language Summary 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 an idealized modeling framework could be useful for studying aerosol effects, both for model development and hypothesis testing. The framework idealizes a climate model; it removes the ocean, land, and sea ice, replacing them with an ocean surface with fixed temperature. Experiments demonstrate that different patterns of aerosol emissions lead to different climate forcing even when the total emissions are unchanged. Most of this forcing arises through aerosol-cloud interactions. This result plainly shows 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.