A Surface Energy Budget Perspective on Aerosol-Climate Interactions: Role of the Evaporative Pathway

Abstract:

Aerosols can enhance the productivity of terrestrial ecosystems by enhancing absorption of solar radiation by the shaded portion of the plant canopy – the diffuse radiation fertilization effect. Although this fertilization effect can, in principle, alter surface evaporation due to the coupling between water loss and carbon uptake in plants, with the potential to change the surface temperature, aerosol-climate interactions have been traditionally viewed in light of the radiative effects within the atmosphere. Here, we use global models and a theoretical framework to illustrate a surface energy budget perspective on these interactions. Results show that aerosols increase the land evaporative fraction, that is, the fraction of radiation energy consumed by evaporation, by over 4% globally and by as much as ~40% regionally. The main mechanism for this is the reduction in Bowen ratio, or a shift in energy allocation from sensible to latent heat flux, in response to a decrease in solar radiation, or solar dimming, and is augmented (around 20% in tropical climate) by the diffuse radiation fertilization effect. In areas with moderately dense vegetation (leaf area index > 2), it is this non-radiative pathway, rather than the reduction of solar radiation incident on the surface, that dominates the local surface cooling response to aerosols. Diffuse radiation fertilization alone has a stronger impact on gross primary productivity (+2.18 Pg C y^-1 or +1.8%) than on land evaporation (+0.18 W m^-2 or +0.48%) and surface temperature (-0.01 K). Our results suggest that it is important for land surface models to distinguish both the quantity and quality of radiative forcing for properly simulating the terrestrial carbon, water, and energy budgets, as well as surface temperature.