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Black carbon (BC) absorbs solar radiation, increasing the buoyancy and vertical ascent of absorbing aerosol in the atmosphere. This self‐lofting process has been observed for individual plumes in the troposphere and lower stratosphere but here we show it occurring at broader scales through enhanced large‐scale ascent over BC‐rich regions. This is demonstrated in a pair of simulation using the UKESM1 Earth‐System model where BC aerosols were modeled either with or without the ability to absorb radiation. With absorption included the annual global mean concentration of BC in the upper troposphere and lower stratosphere (8–22 km) rose by up to 50% and the column loading over some remote oceanic regions more than doubled. The increase in aerosol height was particularly notable over the southeast Atlantic where biomass burning aerosol from Africa was elevated up to 1 km higher when their absorption was included. Similar effects were seen over the Arctic where the absorbing haze was transported in at higher levels and surface concentrations were halved. The absorption by BC also increased ascent over southern Asia, which tended to thicken the Asian brown cloud during the dry season but in the wet season enhancing ascent promoted deep convection and had the tendency to deplete the aerosol through wash‐out. We conclude that representing aerosol absorption accurately is important in simulating the vertical distribution, transport and abundance of aerosol in the Earth‐system that will affect their interactions with climate. Plain Language Summary: Black carbon is an important component of atmospheric aerosols as it absorbs solar radiation thereby heating the atmosphere and potentially warming climate. The localized heating can also affect clouds and atmospheric motions making the regional and climate effects more complex and uncertain. The vertical distribution and geographic spread of the aerosol is also key to how it interacts with the climate system. In this paper we highlight the fact that the absorption taking place in BC aerosols can affect its vertical ascent in the atmosphere and therefore how it becomes distributed across the globe. This self‐lofting process was demonstrated in a climate model and was particularly important in aiding the elevation and downwind transport of absorbing smoke layers from regions such as central Africa, as well as increasing the amount of BC reaching the upper troposphere and lower stratosphere. Key Points: Absorption of solar radiation helps to raise the altitude of black carbon aerosol by increasing buoyancy and vertical ascentThis self‐lofting mechanism has been captured in a climate model where radiative heating affects the large‐scale ascent and circulationSelf‐lofting increased the altitude and long‐range transport of black carbon to remote regions and the upper troposphere [ABSTRACT FROM AUTHOR]

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 Summary: Human 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 cloud droplets and ice crystals. But because aerosols are much more varied in their chemical composition and much more heterogeneous in their spatial and temporal distributions than greenhouse gases, their perturbation to the energy budget, called radiative forcing, is much more uncertain. This review uses traceable and arguable lines of evidence, supported by aerosol studies published over the past 40 years, to quantify that uncertainty. It finds that there are two chances out of three that aerosols from human activities have increased scattering and absorption of solar radiation by 14% to 29% and cloud droplet number concentration by 5 to 17% in the period 2005–2015 compared to the year 1850. Those increases exert a radiative forcing that offsets between a fifth and a half of the radiative forcing by greenhouse gases. The degree to which human activities affect natural aerosol levels, and the response of clouds, and especially ice clouds, to aerosol perturbations remain particularly uncertain.Key Points: An assessment of multiple lines of evidence supported by a conceptual model provides ranges for aerosol radiative forcing of climate changeAerosol effective radiative forcing is assessed to be between ‐1.6 and ‐0.6 W m−2 at the 16–84% confidence levelAlthough key uncertainties remain, new ways of using observations provide stronger constraints for models [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols strongly absorb solar radiation, but their effective radiative forcing and impacts on regional climate remain highly uncertain owing to strong feedbacks of BC heating on clouds, convection, and precipitation. This study investigates the role of large‐scale circulation changes in governing such feedbacks. In the HadGEM3 climate model BC emissions were increased to 10 times present‐day values while keeping sea surface temperatures fixed, to assess the rapid adjustments to increased BC absorption. The BC perturbation led to an effective radiative forcing of 2.7 W/m2 and a 0.13‐mm/day reduction in global precipitation. There were also large shifts in the spatial distribution of tropical convection, increased low cloud over oceans, and a weakening and poleward shift of midlatitude storm tracks, especially in the Northern Hemisphere. In a parallel experiment, horizontal winds were nudged toward meteorological reanalyses to deliberately suppress circulation responses while allowing changes to the thermodynamic structure of the atmosphere. Surprisingly, BC had approximately the same impact on global‐mean radiation and global precipitation in the nudged experiment, even though regional changes in clouds and convection were not fully captured. The results show that large‐scale dynamical responses to BC are important for regional impacts but have a limited role in determining the effective radiative forcing and global‐mean climate response. The rapid adjustments of clouds, radiation, and global precipitation were primarily a response to increased radiative absorption and atmospheric stability. This implies that short nudged simulations may be sufficient to assess absorbing aerosol impacts on global‐mean radiation and precipitation. Plain Language Summary: Black carbon (BC) aerosols are emitted during the combustion of various types of fuel. These soot‐like particles absorb solar radiation and therefore exert a warming influence on climate. However, their impact is highly uncertain as localized heating of the atmosphere also alters clouds, convection, and atmospheric circulation. This leads to strong feedbacks and changes in regional precipitation that vary widely among global models. This study investigates how important changes in atmospheric circulation are in explaining such feedbacks by comparing experiments where BC emissions were increased but changes in atmospheric flow were either permitted or deliberately suppressed via nudging. We find that regional changes in precipitation and cloud over the tropics and midlatitude storm tracks are indeed strongly linked to changes in atmospheric circulation, especially vertical motion. However, some of the impacts on regional cloud and precipitation were connected to more basic thermodynamic adjustments in the atmosphere. Moreover, the impact of BC on global‐mean radiation and precipitation was approximately the same whether the atmospheric flow was constrained or free to respond and adjust. We conclude that accurately capturing the feedback of BC on atmospheric circulation is essential to predicting regional changes in precipitation but may be less important for global‐mean climate impacts. Key Points: Black carbon aerosol alters atmospheric circulation patterns due to strong absorption of solar radiationAdjustments in circulation are important in driving regional changes in precipitation, convection and cloudCirculation adjustments have little impact on global‐mean precipitation and effective radiative forcing [ABSTRACT FROM AUTHOR]

This article presents observations and model simulations of the low-level nocturnal structure of the atmosphere over West Africa. The measurements are taken from the dry-season Special Observing Period (SOP-0) of the African Monsoon Multidisciplinary Analysis (AMMA), at Niamey, Niger, on 23 and 24 January 2006. During this time, mesoscale structures in the atmospheric aerosol loadings were observed. The available observations indicate that these mesoscale features at Niamey are consistent with the passage of gravity currents or bores in the northerly Harmattan winds. Model simulations at resolutions down to 1 km indicate that the mesoscale structures are caused by nocturnal frontogenesis in the baroclinic zone to the south of the Sahara in the winter months. This frontogenesis is a continental-scale phenomenon, which has significant implications for the uplift and transport of dust and biomass-burning aerosols in the region. An accompanying frontogenetic feature appears further south in the model simulations, associated with the winter intertropical front. The frontogenesis is possibly linked with the development of structures showing characteristics of canonical mesoscale phenomena, including internal bores and gravity currents. Representation of these features in the UK Met Office Unified Model at different resolutions (from 12 km to 1 km) is discussed. [ABSTRACT FROM AUTHOR]