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Nature 484, 228-232 (2012) The accepted date should read 08 February 2012, rather than 2011. This error has been corrected in the PDF and HTML versions online. [...]

The vertical profile of aerosol is important for its radiative effects, but weakly constrained by observations on the global scale, and highly variable among different models. To investigate the controlling factors in one particular model, we investigate the effects of individual processes in HadGEM3-UKCA and compare the resulting diversity of aerosol vertical profiles with the inter-model diversity from the AeroCom Phase II control experiment. In this way we show that (in this model at least) the vertical profile is controlled by a relatively small number of processes, although these vary among aerosol components and particle sizes. We also show that sufficiently coarse variations in these processes can produce a similar diversity to that among different models in terms of the global-mean profile and, to a lesser extent, the zonal-mean vertical position. However, there are features of certain models' profiles that cannot be reproduced, suggesting the influence of further structural differences between models. In HadGEM3-UKCA, convective transport is found to be very important in controlling the vertical profile of all aerosol components by mass. In-cloud scavenging is very important for all except mineral dust. Growth by condensation is important for sulfate and carbonaceous aerosol (along with aqueous oxidation for the former and ageing by soluble material for the latter). The vertical extent of biomass-burning emissions into the free troposphere is also important for the profile of carbonaceous aerosol. Boundary-layer mixing plays a dominant role for sea salt and mineral dust, which are emitted only from the surface. Dry deposition and below-cloud scavenging are important for the profile of mineral dust only. In this model, the microphysical processes of nucleation, condensation and coagulation dominate the vertical profile of the smallest particles by number (e.g. total CN >3 nm), while the profiles of larger particles (e.g. CN>100 nm) are controlled by the same processes as the component mass profiles, plus the size distribution of primary emissions. We also show that the processes that affect the AODnormalised radiative forcing in the model are predominantly those that affect the vertical mass distribution, in particular convective transport, in-cloud scavenging, aqueous oxidation, ageing and the vertical extent of biomass-burning emissions. [ABSTRACT FROM AUTHOR]

Correction to: Climate Dynamics https://doi.org/10.1007/s00382-021-05941-3 The original html version of the article contained an error in the formatting of Fig. The original article can be found online at https://doi.org/10.1007/s00382-021-05941-3. The caption was swapped and below is the corrected caption. [Extracted from the article]

We evaluate a 12-member perturbed parameter ensemble of regional climate simulations over Europe at 12 km resolution, carried out as part of the UK Climate Projections (UKCP) project. This ensemble is formed by varying uncertain parameters within the model physics, allowing uncertainty in future projections due to climate modelling uncertainty to be explored in a systematic way. We focus on present day performance both compared to observations, and consistency with the driving global ensemble. Daily and seasonal temperature and precipitation are evaluated as two variables commonly used in impacts assessments. For precipitation we find that downscaling, even whilst within the convection-parameterised regime, generally improves daily precipitation, but not everywhere. In summer, the underestimation of dry day frequency is worse in the regional ensemble than in the driving simulations. For temperature we find that the regional ensemble inherits a large wintertime cold bias from the global model, however downscaling reduces this bias. The largest bias reduction is in daily winter cold temperature extremes. In summer the regional ensemble is cooler and wetter than the driving global models, and we examine cloud and radiation diagnostics to understand the causes of the differences. We also use a low-resolution regional simulation to determine whether the differences are a consequence of resolution, or due to other configuration differences, with the predominant configuration difference being the treatment of aerosols. We find that use of the EasyAerosol scheme in the regional model, which aims to approximate the aerosol effects in the driving model, causes reduced temperatures by around 0.5 K over Eastern Europe in Summer, and warming of a similar magnitude over France and Germany in Winter, relative to the impact of interactive aerosol in the global runs. Precipitation is also increased in these regions. Overall, we find that the regional model is consistent with the global model, but with a typically better representation of daily extremes and consequently we have higher confidence in its projections of their future change. [ABSTRACT FROM AUTHOR]

The radiative forcing of anthropogenic aerosol is important for understanding climate change. However, the uncertainty in anthropogenic aerosol forcing is large, despite decades of research on aerosols and their effect on climate. The reasons for the associated model spread in aerosol forcing are numerous, but their relative importance is largely unclear due to the model complexity. In this presentation, we see how a deliberate reduction of the model complexity aids improving our understanding of the effective radiative forcing of anthropogenic aerosol (ERF). We investigate the impact on ERF from uncertainties that remain when we pretend to know the spatio-temporal distribution of anthropogenic aerosols and their properties. For doing so, we present an ensemble of atmosphere-only experiments of a selection of contemporary climate models (Fiedler et al., 2018a, in review). All five models use the same prescribed anthropogenic aerosol optical properties and an associated effect on clouds from the new observationally informed simple-plumes parameterisation MACv2-SP for use in CMIP6 (Fiedler et al., 2017, Stevens et al., 2017). The highlights of the results are (1) a strong impact of year-to-year variability internal to the models on estimating the ERF, (2) a small change in the global mean ERF between the mid-1970s and the mid-2000s, despite the substantially different spatial distribution of anthropogenic aerosol, and (3) a persistently large model diversity in clouds on the macro- and micro-scale. In the future, MACv2-SP will be used for CMIP6 experiments, e.g., in the radiative forcing model inter-comparison project (RFMIP), and for other scientific endeavours, e.g., decadal climate predictions. These projects will make use of MACv2-SP's time series of anthropogenic aerosol for the past (Stevens et al., 2017) and for the projections into the future, based on CMIP6 emission scenarios (Fiedler et al., 2018b, in review).PublicationsFiedler, et al. (2018a) Anthropogenic aerosol forcing - insights from multi-estimates from aerosol-climate models with reduced complexity, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2018-639, in review.Fiedler, et al. (2018b) First forcing estimates from the future CMIP6 scenarios of anthropogenic aerosol optical properties and an associated Twomey effect, Geosci. Model Dev. Discuss., doi:10.5194/gmd-2018-244, in review.Fiedler, et al. (2017) On the sensitivity of the anthropogenic aerosol forcing to model-internal variability and parameterizing a Twomey effect, J. Adv. Mod. Earth Syst., 9, doi:10.1002/2017MS000932. Stevens, et al. (2017) Simple Plumes: A parameterization of anthropogenic aerosol optical properties and an associated Twomey effect for climate studies, Geosci. Model Dev. 10, 433-452, doi:10.5194/gmd-10-433-2017. [ABSTRACT FROM AUTHOR]

The absorption of radiation by carbonaceous aerosol can result in mean local heat perturbations of several Kelvin per day. Subsequent rapid-adjustments to the atmosphere lead to changes in radiation fluxes, referred to as semi-direct radiative effects.The SE Atlantic is a region that experiences periodic plumes of strongly absorbing smoke that are transported over extensive semi-permanent marine stratocumulus. Before mixing with the cloud layer the elevated plumes are often separated by smoke-free gaps for extended periods of time.We use the high resolution MetOffice large eddy model to investigate the diurnal cycle of the semi-direct radiative effect from elevated smoke layers above marine stratocumulus cloud. We use a range of experiments to investigate the sensitivity of the semi-direct effect to the gap to layer, thickness of the layer, and smoke aerosol optical depth (AOD). We find a pronounced diurnal cycle driven by changes in cloud-top entrainment that impact daytime coupling to the surface and boundary layer water content. Subsequent changes to the cloud liquid water path drive periods of both positive and negative semi-direct radiative effects, which are sensitive to the amount of time the plume persists over the cloud.Contrary to published literature, the presence of an overlying layer of absorbing aerosol does not systematically exert a negative semi-direct radiative effect. Instead we show that both the sign and magnitude of the daily mean semi-direct radiative effect of above-cloud smoke layers is sensitive to the smoke layer position, thickness, AOD, persistence, and properties of the boundary layer. Thus, rapid adjustments to atmospheric temperature perturbations caused by above-cloud absorbing aerosol both enhance or mitigate the direct radiative effect. [ABSTRACT FROM AUTHOR]

Estimation of the perturbation to the Earth's energy budget by contrail outbreaks is required for estimating the climate impact of aviation and verifying the climate benefits of proposed contrail avoidance strategies such as aircraft rerouting. Here we identified two successive large‐scale contrail outbreaks developing in clear‐sky conditions in geostationary and polar‐orbiting satellite infrared images of Western Europe lasting from 22–23 June 2020. Their hourly cloud radiative effect, obtained using geostationary satellite cloud retrievals and radiative transfer calculations, is negative or weakly positive during daytime and positive during nighttime. The cumulative energy forcing of the two outbreaks is 7 PJ and −8.5 PJ, with uncertainties of 3 PJ, stemming each from approximately 15–20 flights over periods of 19 and 7 hr, respectively. This study suggests that an automated quantification of contrail outbreak radiative effect is possible, at least for contrails forming in clear sky conditions. Plain Language Summary: Contrail cirrus is produced by aircraft and perturb the energy budget of the Earth. However, the actual size of the perturbation is uncertain. In this study, we calculate the energy budget perturbation of two successive contrail‐cirrus outbreaks over Western Europe from 22–23 June 2020. An infrared image composite allows the identification and tracking of contrails with a 15 min frequency, which is verified by comparison to satellite images with better horizontal resolution from several polar‐orbiting platforms. Cloud properties of the contrail‐cirrus clusters, estimated from geostationary satellite data, are used in radiative transfer calculations. We find that one contrail cirrus outbreak adds an average power of 2 TW over 20 hr, while the other removes 3.3 TW over 8 hr. This cumulative energy depends on the lifespan and cloud properties of the outbreaks. This case study suggests that geostationary satellite observations allow the estimation of the energy perturbation of a contrail outbreak, with encouraging implications for contrail‐cirrus monitoring and the verification of contrail avoidance strategies. Key Points: The cloud radiative effect (CRE) of two successive contrail‐cirrus outbreaks is estimated from geostationary satellite measurementsThese two outbreaks have different CRE sign and magnitude, which can be explained by their different cloud properties and time evolutionsThe study suggests that automated quantification of contrail‐cirrus CRE for monitoring or verification of contrail avoidance is feasible [ABSTRACT FROM AUTHOR]

This corrects the article DOI: 10.1038/nature22974 [ABSTRACT FROM AUTHOR]

The article discusses research done on the interactions between particles of black carbon and cloud processes in the atmosphere. It references a study by M. Sand and colleagues published in the 2015 issue of the "Journal of Climate." Topics discussed include the importance of rapid responses in the atmosphere to changes in black carbon and the role of the two-way black carbon-atmosphere interactions in exploring the full climate response.

The article explores the study on the formation of colours from particles of smoke from natural and human made fire, which contributes to the development of global warming. The study examines the biomass-burning aerosols in combustion chamber experiments and reveals the reflectivity of aerosol soot content. The article also discusses the absorption properties of organic aerosols from biomass burning.

Stratocumulus (Sc) clouds and stratocumulus‐to‐cumulus transitions (SCTs) are challenging to represent in global models and they contribute to a large spread in modeled subtropical cloud feedbacks. We evaluate the impact of increasing the horizontal model resolution (∼135, 60 and 25 km, respectively) and increasing the complexity of the aerosol–cloud interaction parameterization (interactive versus non‐interactive at medium resolution) on springtime subtropical marine Sc properties and SCTs in the atmosphere‐only version of HadGEM3‐GC3.1. No significant impact on the spatial location of the SCT could be found between the different model versions. Increasing horizontal resolution led to small but significant increases in liquid water content and a stronger (more negative) shortwave (SW) cloud radiative effect (CRE), in particular over the southern‐hemisphere Sc regions. However, for two out of the four studied regions, the stronger SW CRE also brought the model outside the range of satellite‐derived values of the SW CRE. Applying non‐interactive aerosols instead of interactive aerosols also led to significantly higher liquid water content and a stronger SW CRE over the southern‐hemisphere Sc regions, while over the northern‐hemisphere Sc regions, a competition between a substantial increase in the cloud droplet number concentration and small changes in the liquid water content resulted in a weaker SW CRE or non‐significant changes. In general, using interactive instead of non‐interactive aerosol–cloud interactions brought the model closer to satellite‐retrieved mean values of the SW CRE. Our results suggest that increasing the horizontal resolution or the complexity of the aerosol–cloud parameterization has a small but statistically significant effect on the SW CRE of marine Sc, in particular over regions with high liquid water content. For these regions, the effect of introducing non‐interactive versus interactive aerosol–cloud interactions is about as large as increasing the horizontal resolution from medium to high. [ABSTRACT FROM AUTHOR]

Accurate assessment of anthropogenic carbon dioxide (CO 2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize datasets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO 2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric CO 2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO 2 sink (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based data products. The terrestrial CO 2 sink (SLAND) is estimated with dynamic global vegetation models. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ± 1 σ. For the first time, an approach is shown to reconcile the difference in our ELUC estimate with the one from national greenhouse gas inventories, supporting the assessment of collective countries' climate progress. For the year 2020, EFOS declined by 5.4 % relative to 2019, with fossil emissions at 9.5 ± 0.5 GtC yr -1 (9.3 ± 0.5 GtC yr -1 when the cement carbonation sink is included), and ELUC was 0.9 ± 0.7 GtC yr -1 , for a total anthropogenic CO 2 emission of 10.2 ± 0.8 GtC yr -1 (37.4 ± 2.9 GtCO 2). Also, for 2020, GATM was 5.0 ± 0.2 GtC yr -1 (2.4 ± 0.1 ppm yr -1), SOCEAN was 3.0 ± 0.4 GtC yr -1 , and SLAND was 2.9 ± 1 GtC yr -1 , with a BIM of - 0.8 GtC yr -1. The global atmospheric CO 2 concentration averaged over 2020 reached 412.45 ± 0.1 ppm. Preliminary data for 2021 suggest a rebound in EFOS relative to 2020 of + 4.8 % (4.2 % to 5.4 %) globally. Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2020, but discrepancies of up to 1 GtC yr -1 persist for the representation of annual to semi-decadal variability in CO 2 fluxes. Comparison of estimates from multiple approaches and observations shows (1) a persistent large uncertainty in the estimate of land-use changes emissions, (2) a low agreement between the different methods on the magnitude of the land CO 2 flux in the northern extra-tropics, and (3) a discrepancy between the different methods on the strength of the ocean sink over the last decade. This living data update documents changes in the methods and datasets used in this new global carbon budget and the progress in understanding of the global carbon cycle compared with previous publications of this dataset (Friedlingstein et al., 2020, 2019; Le Quéré et al., 2018b, a, 2016, 2015b, a, 2014, 2013). The data presented in this work are available at 10.18160/gcp-2021 (Friedlingstein et al., 2021). [ABSTRACT FROM AUTHOR]

The impact of diffuse radiation on photosynthesis has been widely documented in field measurements. This impact may have evolved over time during the last century due to changes in cloudiness, increased anthropogenic aerosol loads over polluted regions, and to sporadic volcanic eruptions curtaining the stratosphere with sulfate aerosols. The effects of those changes in diffuse light on large-scale photosynthesis (GPP) are difficult to quantify, and land surface models have been designed to simulate them. Investigating how anthropogenic aerosols have impacted GPP through diffuse light in those models requires carefully designed factorial simulations and a reconstruction of background diffuse light levels during the preindustrial period. Currently, it remains poorly understood how diffuse radiation reconstruction methods can affect GPP estimation and what fraction of GPP changes can be attributed to aerosols. In this study, we investigate different methods to reconstruct spatiotemporal distribution of the fraction of diffuse radiation (Fdf) under preindustrial aerosol emission conditions using a land surface model with a two-stream canopy light transmission scheme that resolves diffuse light effects on photosynthesis in a multi-layered canopy, ORCHIDEE_DF. We show that using a climatologically averaged monthly Fdf, as has been done by earlier studies, can bias the global GPP by up to 13 PgC yr -1 because this reconstruction method dampens the variability of Fdf and produces Fdf that is inconsistent with shortwave incoming surface radiation. In order to correctly simulate preindustrial GPP modulated by diffuse light, we thus recommend that the Fdf forcing field should be calculated consistently with synoptic, monthly, and inter-annual aerosol and cloud variability for preindustrial years. In the absence of aerosol and cloud data, alternative reconstructions need to retain the full variability in Fdf. Our results highlight the importance of keeping consistent Fdf and radiation for land surface models in future experimental designs that seek to investigate the impacts of diffuse radiation on GPP and other carbon fluxes. [ABSTRACT FROM AUTHOR]

One major source of uncertainty in the cloud-mediated aerosol forcing arises from the magnitude of the cloud liquid water path (LWP) adjustment to aerosol-cloud interactions, which is poorly constrained by observations. Many of the recent satellite-based studies have observed a decreasing LWP as a function of cloud droplet number concentration (CDNC) as the dominating behavior. Estimating the LWP response to the CDNC changes is a complex task since various confounding factors need to be isolated. However, an important aspect has not been sufficiently considered: the propagation of natural spatial variability and errors in satellite retrievals of cloud optical depth and cloud effective radius to estimates of CDNC and LWP. Here we use satellite and simulated measurements to demonstrate that, because of this propagation, even a positive LWP adjustment is likely to be misinterpreted as negative. This biasing effect therefore leads to an underestimate of the aerosol-cloud-climate cooling and must be properly considered in future studies., (© 2022. The Author(s).)

Aerosols have a dimming and cooling effect and change hydrological regimes, thus affecting carbon fluxes, which are sensitive to climate. Aerosols also scatter sunlight, which increases the fraction of diffuse radiation, increasing photosynthesis. There remains no clear conclusion whether the impact of aerosols on land carbon fluxes is larger through diffuse radiation change than through changes in other climate variables. In this study, we quantified the overall physical impacts of anthropogenic aerosols on land C fluxes and explored the contribution from each factor using a set of factorial simulations driven by climate and aerosol data from the IPSL-CM6A-LR experiments during 1850-2014. A newly developed land surface model which distinguishes diffuse and direct radiation in canopy radiation transmission, ORCHIDEE_DF, was used. Specifically, a subgrid scheme was developed to distinguish the cloudy and clear sky conditions. We found that anthropogenic aerosol emissions since 1850 cumulatively enhanced the land C sink by 22.6 PgC. Seventy-eight percent of this C sink enhancement is contributed by aerosol-induced increase in the diffuse radiation fraction, much larger than the effect of the aerosol-induced dimming. The cooling of anthropogenic aerosols has different impacts in different latitudes but overall increases the global land C sink. The dominant role of diffuse radiation changes found in this study implies that future aerosol emissions may have a much stronger impacts on the C cycle through changing radiation quality than through changing climate alone. Earth system models need to consider the diffuse radiation fertilization effect to better evaluate the impacts of climate change mitigation scenarios., (© 2021. The Authors.)

Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation. Depending on the model, the top-of-the-atmosphere BB aerosol effect can range from cooling to warming. By relating aerosol absorption relative to extinction and carbonaceous aerosol composition from 12 observational datasets to nine state-of-the-art Earth system models/chemical transport models, we identify varying degrees of overestimation in BB aerosol absorptivity by these models. Modifications to BB aerosol refractive index, size, and mixing state improve the Community Atmosphere Model version 5 (CAM5) agreement with observations, leading to a global change in BB direct radiative effect of -0.07 W m -2 , and regional changes of -2 W m -2 (Africa) and -0.5 W m -2 (South America/Temperate). Our findings suggest that current modeled BB contributes less to warming than previously thought, largely due to treatments of aerosol mixing state.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

The cooling of the Earth's climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. An increase in the amount of water inside liquid-phase clouds induced by aerosols, through the suppression of rain formation, has been postulated to lead to substantial cooling, which would imply that the Earth's surface temperature is highly sensitive to anthropogenic forcing. Here we provide direct observational evidence that, instead of a strong increase, aerosols cause a relatively weak average decrease in the amount of water in liquid-phase clouds compared with unpolluted clouds. Measurements of polluted clouds downwind of various anthropogenic sources-such as oil refineries, smelters, coal-fired power plants, cities, wildfires and ships-reveal that aerosol-induced cloud-water increases, caused by suppressed rain formation, and decreases, caused by enhanced evaporation of cloud water, partially cancel each other out. We estimate that the observed decrease in cloud water offsets 23% of the global climate-cooling effect caused by aerosol-induced increases in the concentration of cloud droplets. These findings invalidate the hypothesis that increases in cloud water cause a substantial climate cooling effect and translate into reduced uncertainty in projections of future climate.

The ability of 11 models in simulating the aerosol vertical distribution from regional to global scales, as part of the second phase of the AeroCom model intercomparison initiative (AeroCom II), is assessed and compared to results of the first phase. The evaluation is performed using a global monthly gridded data set of aerosol extinction profiles built for this purpose from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Layer Product 3.01. Results over 12 subcontinental regions show that five models improved, whereas three degraded in reproducing the interregional variability in Z α 0-6 km , the mean extinction height diagnostic, as computed from the CALIOP aerosol profiles over the 0-6 km altitude range for each studied region and season. While the models' performance remains highly variable, the simulation of the timing of the Z α 0-6 km peak season has also improved for all but two models from AeroCom Phase I to Phase II. The biases in Z α 0-6 km are smaller in all regions except Central Atlantic, East Asia, and North and South Africa. Most of the models now underestimate Z α 0-6 km over land, notably in the dust and biomass burning regions in Asia and Africa. At global scale, the AeroCom II models better reproduce the Z α 0-6 km latitudinal variability over ocean than over land. Hypotheses for the performance and evolution of the individual models and for the intermodel diversity are discussed. We also provide an analysis of the CALIOP limitations and uncertainties contributing to the differences between the simulations and observations.