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Atmospheric aerosols play an important role in Earth’s radiative balance, directly interacting with solar radiation or influencing cloud formation and properties. In order to assess their radiative impact, it is necessary to accurately characterise their optical properties, together with their spatial and vertical distribution. The information on aerosol vertical profile is often scarce, in particular in mountainous, complex terrains. This study presents the first attempt to evaluate the shortwave aerosol direct radiative effect in the Aosta Valley, a mountainous region in the Northwestern Italian Alps. Ground-based, remote sensing instruments (a sky radiometer and an Automated Lidar Ceilometer) are used to derive two descriptions of the aerosol properties and vertical distribution: a first, more accurate description, which includes the whole spectral information about the aerosol extinction coefficient, phase function and single scattering albedo; a second, more approximate one, which only relies on spectrally constant values of aerosol single scattering albedo and asymmetry factor. This information is used as input for radiative transfer simulations, which allow to estimate, in cloudless conditions, the shortwave aerosol direct radiative effect and the vertical profile of the instantaneous heating rates in the lower layers of the atmosphere. The simulations obtained with the two descriptions do not differ significantly: they highlight a strong surface dimming (between − 25 and − 50 W m− 2) due to the presence of aerosol, with a considerable radiative absorption inside the atmospheric column (around + 30 W m− 2), and an overall small cooling effect for the Earth-atmospheric system. The absorption of solar radiation within the atmospheric column due to aerosol leads to instantaneous heating rates up to 1.5 K day− 1 in the tropospheric layers below 6 km a.s.l. These results show that, in some conditions, the shortwave aerosol direct radiative effect can be considerable even in this Alpine environment, usually considered as relatively pristine (yearly average PM10 concentration about 20 μg m− 3).

This study compares aerosol direct radiative effects on numerical weather forecasts made by the NCEP Global Forecast System (GFS) with two different aerosol datasets, the Optical Properties of Aerosols and Clouds (OPAC) and MERRA-2 aerosol climatologies. The underestimation of aerosol optical depth (AOD) by OPAC over northwest Africa, central to East Africa, the Arabian Peninsula, Southeast Asia, and the Indo-Gangetic Plain, and overestimation in the storm-track regions in both hemispheres are reduced by MERRA-2. Surface downward shortwave (SW) and longwave (LW) fluxes and the top-of-the-atmosphere SW and outgoing LW fluxes from model forecasts are compared with CERES satellite observations. Forecasts made with OPAC aerosols have large radiative flux biases, especially in northwest Africa and the storm-track regions. These biases are also reduced in the forecasts made with MERRA-2 aerosols. The improvements from MERRA-2 are most noticeable in the surface downward SW fluxes. GFS medium-range weather forecasts made with the MERRA-2 aerosols demonstrated slightly improved forecast accuracy of sea level pressure and precipitation over the Indian and East Asian summer monsoon region. A stronger Africa easterly jet is produced, associated with a low pressure over the east Atlantic Ocean and west of northwest Africa. Impacts on large-scale skill scores such as 500-hPa geopotential height anomaly correlation are generally positive in the Northern Hemisphere and the Pacific and North American regions in both the winter and summer seasons.

The direct perturbation of anthropogenic aerosols on Earth’s energy balance [i.e., direct aerosol radiative forcing (DARF)] remains uncertain in climate models. These uncertainties critically depend on aerosol optical properties, primarily aerosol optical depth (AOD), single scattering albedo (SSA), and the asymmetry factor g. In this study, we investigate the intermodel spread of DARF across 14 global models within phase 6 of the Coupled Model Intercomparison Project (CMIP6), using unified radiative transfer calculation and aerosol optical parameter assumptions. The global mean DARF for clear sky in 2014 with respect to 1850 is estimated as −0.77 ± 0.52 W m−2 assuming an externally mixed state and −0.68 ± 0.53 W m−2 for an internally mixed state. We further conduct a quantitative analysis and find that globally, for the external mixing assumption, AOD is the dominant factor, whose intermodel spread results in 36% of the total DARF uncertainty. For the internal mixing assumption, SSA becomes the major factor, which also leads to 36% DARF uncertainty. The g parameter and aerosol vertical distribution combined contribute to ∼30% of the DARF uncertainty. Regionally, DARF uncertainty is typically more sensitive to SSA where the absorbing aerosol fraction is high, such as South Asia and central Africa. Substantial differences between model-averaged and observed aerosol optical parameters are still noticed, with external mixing in general yielding closer agreement with observations. Our results highlight the importance of aerosol scattering and absorption properties in DARF estimation.

A lower bound on the uncertainty in observational estimates of the aerosol direct radiative effect (DRE; the direct interaction with solar radiation by all aerosols) and the aerosol direct radiative forcing [DRF; the radiative effect of just anthropogenic aerosols (RFari)] is quantified by making the optimistic assumption that global aerosol observations can be made with the accuracy found in the Aerosol Robotic Network (AERONET) sun photometer retrievals. The global-mean all-sky aerosol DRE uncertainty was found to be 1.1 W m−2 (one standard deviation). The global-mean all-sky aerosol DRF (RFari) uncertainty was determined to be 0.31 W m−2. The total uncertainty in both quantities is dominated by contributions from the aerosol single scattering albedo uncertainty. These uncertainty estimates were compared to a literature survey of mostly satellite-based aerosol DRE/DRF values. Comparisons to previous studies reveal that most have significantly underestimated the aerosol DRE uncertainty. Past estimates of the aerosol DRF uncertainty are smaller (on average) than our optimistic observational estimates, including the aerosol DRF uncertainty given in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). This disconnect between our observation-based uncertainty and that found in past aerosol DRF studies that rely, at least in part, on modeling is discussed. Also quantified is a potential reduction in the current observational uncertainty possible with a future generation of satellite observations that would leverage aerosol typing and more refined vertical information.

The direct perturbation of anthropogenic aerosols on Earth’s energy balance [i.e., direct aerosol radiative forcing (DARF)] remains uncertain in climate models. In this study, we investigate the uncertainty of DARF associated with aerosol vertical distribution, using simulation results from 14 global models within phase 6 of the Coupled Model Intercomparison Project (CMIP6). The column mass loading for each aerosol species is first normalized to the multimodel average for each model, which is called the mass-normalization process. The unified radiative transfer model and aerosol optical parameter are used, so that the differences in the calculated DARF are solely attributed to the difference in aerosol vertical profiles. The global mean DARF values in 2014 with respect to 1850 before and after mass normalization are −0.77 ± 0.52 and −0.81 ± 0.12 W m−2 respectively, assuming external mixing, which indicates that the intermodel difference in aerosol vertical distribution accounts for ∼20% of the total DARF uncertainty. We further conduct two separate experiments by normalizing aerosol optical depth (AOD) and aerosol single scattering albedo (SSA) profiles, respectively, and find that the vertical distribution of SSA results in larger DARF uncertainty (0.17 W m−2) than that of AOD (0.10 W m−2). Finally, compared with CALIPSO observation, CMIP6 models tend to produce higher aerosol layers. The bias in modeled aerosol profile with respect to CALIPSO leads to stronger DARF, especially for land regions.

Aerosols emitted from wildfires could significantly affect global climate through perturbing global radiation balance. In this study, the Community Earth System Model with prescribed daily fire aerosol emissions is used to investigate fire aerosols’ impacts on global climate with emphasis on the role of climate feedbacks. The total global fire aerosol radiative effect (RE) is estimated to be −0.78 ± 0.29 W m−2, which is mostly from shortwave RE due to aerosol–cloud interactions (REaci; −0.70 ± 0.20 W m−2). The associated global annual-mean surface air temperature (SAT) change ΔT is −0.64 ± 0.16 K with the largest reduction in the Arctic regions where the shortwave REaci is strong. Associated with the cooling, the Arctic sea ice is increased, which acts to reamplify the Arctic cooling through a positive ice-albedo feedback. The fast response (irrelevant to ΔT) tends to decrease surface latent heat flux into atmosphere in the tropics to balance strong atmospheric fire black carbon absorption, which reduces the precipitation in the tropical land regions (southern Africa and South America). The climate feedback processes (associated with ΔT) lead to a significant surface latent heat flux reduction over global ocean areas, which could explain most (~80%) of the global precipitation reduction. The precipitation significantly decreases in deep tropical regions (5°N) but increases in the Southern Hemisphere tropical ocean, which is associated with the southward shift of the intertropical convergence zone and the weakening of Southern Hemisphere Hadley cell. Such changes could partly compensate the interhemispheric temperature asymmetry induced by boreal forest fire aerosol indirect effects, through intensifying the cross-equator atmospheric heat transport.

Clouds and aerosols provide the greatest uncertainty in estimating and interpreting Earth’s energy budget. This study not only focuses on surface brightening/dimming, but also explores Earth’s energy balance. The validation results of the CERES-SYN, ISCCP-FH, and GEWEX-SRB datasets with Baseline Surface Radiation Network (BSRN), Surface Radiation Budget Network (SURFRAD), and CMA observations show that CERES data have the highest accuracy and the longest temporal coverage. The role of clouds and aerosols in Earth’s energy budget was explained using CERES and MERRA-2 products. The results show that Earth’s energy increases at a rate of 0.63 W m −2 decade −1 in 2000–21. The global surface brightens at a rate of 0.57 W m −2 decade −1 , with surface energy decreasing at a rate of 0.19 W m −2 decade −1 . Brightening was found over Australia, central Asia, and southern Africa, mainly associated with cloud reduction, with aerosol emissions reductions contributing to the East Asian surface brightening. The surface brightening in South America and Southeast Asia is also due to the reduction of clouds. The increase of aerosols in South Asia is the main factor for its surface dimming, while we infer that the climatic effect from the increase of black carbon (BC) aerosols in South Asia is the inducing factor for the dimming in southern China. The surface darkening in West Asia is the result of the combined effect of clouds and aerosols, while in northern Africa it may be related to the increase of clouds caused by the decrease of dust aerosols. Surface energy increases only in Southeast Asia, South America, and Europe.

Black carbon aerosol (BC) has a significant influence on regional climate changes because of its warming effect. Such changes will feed back to BC loadings. Here, the interactions between the BC warming effect and the East Asian monsoon (EAM) in both winter (EAWM) and summer (EASM) are investigated using a regional climate model, RegCM4, that essentially captures the EAM features and the BC variations in China. The seasonal mean BC optical depth is 0.021 over East Asia during winter, which is 10.5% higher than that during summer. Nevertheless, the BC direct radiative forcing is 32% stronger during summer (+1.85 W m−2). The BC direct effect would induce lower air to warm by 0.11–0.12 K, which causes a meridional circulation anomaly associated with a cyclone at 20°–30°N and southerly anomalies at 850 hPa over East Asia. Consequently, the EAM circulation is weakened during winter but enhanced during summer. Precipitation is likely increased, especially in southern China during summer (by 3.73%). Relative to BC changes that result from EAM interannual variations, BC changes from its warming effect are as important but are weaker. BC surface concentrations are decreased by 1%–3% during both winter and summer, whereas the columnar BC is increased in south China during winter. During the strongest monsoon years, the BC loadings are higher at lower latitudes than those during the weakest years, resulting in more southerly meridional circulation anomalies and BC feedbacks during both winter and summer. However, the interactions between the BC warming effect and EAWM/EASM are more intense during the weakest monsoon years.

Since China implemented the Air Pollution Prevention and Control Action Plan in 2013, the aerosol emissions in East Asia have been greatly reduced, while emissions in South Asia have continued to increase. This has led to a dipole pattern of aerosol emissions between South Asia and East Asia. Here, the East Asian summer monsoon (EASM) responses to the dipole changes in aerosol emissions during 2013–17 are investigated using the atmosphere model of Community Earth System Model version 2 (CESM2). We show that decreases in East Asian emissions alone lead to a positive aerosol effective radiative forcing (ERF) of 1.59 (±0.97) W m−2 over central-eastern China (25°–40°N, 105°–122.5°E), along with a 0.09 (±0.07)°C warming in summer during 2013–17. The warming intensified the land–sea thermal contrast and increased the rainfall by 0.32 (±0.16) mm day−1. When considering both the emission reductions in East Asia and increases in South Asia, the ERF is increased to 3.39 (±0.89) W m−2, along with an enhanced warming of 0.20 (±0.08)°C over central-eastern China, while the rainfall insignificant decreased by 0.07 (±0.16) mm day−1. It is due to the westward shift of the strengthened western Pacific subtropical high, linked to the increase in black carbon in South Asia. Based on multiple EASM indices, the reductions in aerosol emissions from East Asia alone increased the EASM strength by almost 5%. Considering the effect of the westward shift of WPSH, the dipole changes in emissions together increased the EASM by 5%–15% during 2013–17, revealing an important role of South Asian aerosols in changing the East Asian climate.

Unlike greenhouse gases (GHGs), anthropogenic aerosol (AA) concentrations have increased and then decreased over the past century or so, with the timing of the peak concentration varying in different regions. To date, it has been challenging to separate the climate impact of AAs from that due to GHGs and background internal variability. We use a pattern recognition method, taking advantage of spatiotemporal covariance information, to isolate the forced patterns for the surface ocean and associated atmospheric variables from the all-but-one forcing Community Earth System Model ensembles. We find that the aerosol-forced responses are dominated by two leading modes, with one associated with the historical increase and future decrease of global mean aerosol concentrations (dominated by the Northern Hemisphere sources) and the other due to the transition of the primary sources of AA from the west to the east and also from Northern Hemisphere extratropical regions to tropical regions. In particular, the aerosol transition effect, to some extent compensating the global mean effect, exhibits a zonal asymmetry in the surface temperature and salinity responses. We also show that this transition effect dominates the total AA effect during recent decades, e.g., 1967–2007.