Bounding aerosol radiative forcing of climate change

Abstract

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6 W m −2 , or -2.0 to -0.4 W m −2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds. Plain Language SummaryHuman activities emit into the atmosphere small liquid and solid particles called aerosols. Those aerosols change the energy budget of the Earth and trigger climate changes, by scattering and absorbing solar and terrestrial radiation and playing important roles in the formation of

Climate Impacts of COVID‐19 Induced Emission Changes

“…Decreases in drop numbers and increases in drop size further affect cloud microphysics potentially increasing precipitation and altering cloud lifetime (Albrecht, 1989). Any reduction of anthropogenic aerosols would reverse their effect on cooling the planet from anthropogenic emissions over the industrial era (Bellouin et al, 2020). Yang et al (2020) note simulated increases in surface temperature as a result of aerosol reductions.…”

“…BC and SO 4 aerosols are important for climate (Bellouin et al, 2020), both for direct scattering (SO 4 ) and absorption (BC) of radiation, as well as for their indirect effect on clouds. Aerosols act as Cloud Condensation Nuclei (CCN) and are locations for nucleating liquid drops and ice crystals.…”

Ensemble daily simulations for elucidating cloud–aerosol interactions under a large spread of realistic environmental conditions

“…The fact that the aerosol effect on clouds and precipitation is dependent on the cloud regime and meteorological conditions, makes the quantification of its global effect challenging and uncertain (Mülmenstädt and Feingold, 2018;Bellouin et al, 2019). One way to overcome this challenge is by examining the aerosol effect for an ensemble of realistic co-varying initial conditions (as opposed to perturbing each environmental condition separately).…”

The Impact of Resolving Subkilometer Processes on Aerosol‐Cloud Interactions of Low‐Level Clouds in Global Model Simulations

“…This study reveals that by resolving the scales of boundary layer eddies, we arrive at a conceptually different picture of the aerosol-cloud interaction than one might get from looking at a model that resolves up to the km-scale motions. Even as we move towards storm-or cloud-resolving global simulations (e.g., Sato et al, 2018;Stevens et al, 2019), we are still some years off from resolving the boundary layer eddies in global models (Schneider et al, 2017;Bellouin et al, 2020). There are subgrid turbulence parameterizations that can bridge those sub-kilometer unresolved scales (Larson et al, 2012;Bogenschutz & Krueger, 2013;Xu & Cheng, 2016;K.…”

“…Both an increase in N d and an increase in cloud LWP can contribute to a brightening of the cloud and a negative ERF aci . To estimate their relative importance in explaining the model differences between SPCAM and UPCAM, we predict the change in SW radiation ∆R sw as the sum of the contribution from relative N d changes ∆N d /N d and relative liquid water path (L) changes ∆L/L building on the relationship from (Ackerman et al, 2000) (see also Bellouin et al, 2020),…”