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During recent decades, both greenhouse gases (GHGs) and anthropogenic aerosols (AAs) drove major changes in the Earth's energy imbalance. However, their respective fingerprints in changes to ocean heat content (OHC) have been difficult to isolate and detect when global or hemispheric averages are used. Based on a pattern recognition analysis, we show that AAs drive an interhemispheric asymmetry within the 20°‐35° latitude band in historical OHC change due to the southward shift of the atmospheric and ocean circulation system. This forced pattern is distinct from the GHG‐induced pattern, which dominates the asymmetry in higher latitudes. Moreover, it is found that this significant aerosol‐forced OHC trend pattern can only be captured in analyzed periods of 20 years or longer and including 1975–1990. Using these distinct spatiotemporal characteristics, we show that the fingerprint of aerosol climate forcing in ocean observations can be distinguished from both the stronger GHG‐induced signals and internal variability. Plain Language Summary: The increase in global ocean heat content (OHC) is a good indicator of the combined effect of greenhouse gas (GHG) warming of Earth's climate system and anthropogenic aerosol (AA) cooling. In this study, we focus on the detection of the OHC change driven by AAs by separating it from the GHG‐driven changes. Our investigation demonstrates that in climate models, AAs do leave an imprint in the interior ocean which are detectable via an hemispheric asymmetry within the 20°–35° latitude band which develops in the 1975–1990 period. Observations share many features with results from climate models, revealing that AAs have driven distinct changes in the ocean at regional scales. Key Points: A pattern recognition method is applied to extract the aerosol‐induced pattern of interhemispheric ocean heat content (OHC) changeAnthropogenic aerosols dominate the historical interhemispheric OHC asymmetry within the 20°–35° latitude band over the 1975–1990 periodObserved OHC changes have an asymmetry that is consistent with the aerosol‐induced OHC fingerprint [ABSTRACT FROM AUTHOR]

Abstract During recent decades, both greenhouse gases (GHGs) and anthropogenic aerosols (AAs) drove major changes in the Earth's energy imbalance. However, their respective fingerprints in changes to ocean heat content (OHC) have been difficult to isolate and detect when global or hemispheric averages are used. Based on a pattern recognition analysis, we show that AAs drive an interhemispheric asymmetry within the 20°‐35° latitude band in historical OHC change due to the southward shift of the atmospheric and ocean circulation system. This forced pattern is distinct from the GHG‐induced pattern, which dominates the asymmetry in higher latitudes. Moreover, it is found that this significant aerosol‐forced OHC trend pattern can only be captured in analyzed periods of 20 years or longer and including 1975–1990. Using these distinct spatiotemporal characteristics, we show that the fingerprint of aerosol climate forcing in ocean observations can be distinguished from both the stronger GHG‐induced signals and internal variability.

Satellite-derived products and reanalyses show consistent increases in downward surface solar radiation (SSR) and decreases in cloud cover over North America and Europe from the 1980s to 2010s. These trends show a strong seasonality, with the largest changes in boreal summer. A set of timeslice experiments with an atmospheric general circulation model (AGCM) forced with prescribed changes in sea surface temperature/sea ice extent (SST/SIE), greenhouse gas (GHG) concentrations, and anthropogenic aerosol (AA) emissions, together and separately, is performed to assess the relative roles of different forcings in these observed trends. The model reproduces the main observed features over Europe and North America, including the seasonality of trends, suggesting a dominant role of forced changes in the recent trends in SSR and cloud cover. Responses to individual forcings indicate that recent decadal trends in SSR over Europe are predominantly driven by AA emission reductions, with an additional influence from SST/SIE and GHG changes. In contrast, changes in AA, SST/SIE, and GHG contribute more equally to simulated decadal trends in SSR and cloud cover over North America, although SST/SIE play the most important role. In our simulations, responses of SSR to AA emission reductions are primarily governed by aerosol-radiation interactions. Responses to SST/SIE and GHG changes are predominantly due to cloud cover changes, which are driven by atmospheric circulation and humidity changes. This process level understanding of how different forcing factors influence decadal trends in SSR and cloud cover is valuable for understanding past changes and future projections in global and regional surface energy budgets, surface warming, and global and regional hydrological cycles. [ABSTRACT FROM AUTHOR]

The global mean surface temperature cooled slightly in the mid-twentieth century despite a continuous increase in greenhouse gas concentrations. The cooling was strongest in the Northern Hemisphere mid-latitudes, while the Southern Hemisphere mid-latitudes experienced moderate warming. This apparent contradiction is often attributed to internal multi-decadal variability originating from Pacific and Atlantic ocean-atmosphere interactions. Given the rapid increase of industrial activities in North America and Europe during that period, it is also plausible that anthropogenic aerosol (AA) emissions (as an external forcing) contributed to the stronger Northern Hemisphere cooling. This paper aims to quantify the contributions of AA and decadal variability to the 1948–1978 cooling. We analyzed the latitudinal temperature trend asymmetry in 60° S–60° N throughout the troposphere, using multiple reanalysis datasets and the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble that bears significant similarity with the observed patterns. We show that both AA increase and the North Atlantic Variability Index (NAVI) transition into its negative phase are the major contributors to the latitudinal asymmetry of cooling. At the surface level, based on the horizontal pattern correlation method, AA and NAVI have similar contribution fractions (20 vs. 16%), but the contribution fraction of AA is much larger at 500 hPa (55 vs. 8%). Attributions based on vertical pattern correlation and latitudinal gradient show consistent results. Natural forcings (NAT) also contribute to the cooling asymmetry during mid-20C, but with a much smaller impact compared to AA and NAVI. Therefore, we argue that previous studies that mostly focused on surface variables may have underestimated the role of AA in the mid-twentieth-century climate change. The study suggests that the three-dimensional thermal structure and atmospheric circulation change should be closely examined in future climate attribution analysis. [ABSTRACT FROM AUTHOR]

The intricacy of the chemical components of atmospheric aerosols causes distinct or even converse climate-forcing effects. Investigating the interannual trends and relative contributions (RCs) of component-segregated aerosol optical depth (AOD) regionally and over land enhances comprehension of the climatic and environmental implications of natural and anthropogenic aerosols. Here, we utilized multiyear (2007–2019) AOD data from the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis to examine the trends in aerosol component loadings and identify the dominant components driving the total AOD (TAOD). Independent validations using ground-based Aerosol Robotic Network measurements revealed marked spatial consistency worldwide (Pearson correlation coefficient: 0.87, root mean square error: 0.08, mean fractional error: 34.04%, and index of agreement: 0.94), whereas the fractional gross error of 23.31% indicated widespread overestimation of CAMS AOD, particularly in the United States and Western Europe (WEU). Over the study period, the CAMS reanalysis featured notable reduction in sulfate (SO 4) in East China (EC), the Eastern United States (EUS), WEU, and Australia, together with considerable reduction in dust in EC, which played a substantial role in the trend of decline in TAOD globally. Conversely, positive trends in component-segregated AOD in South Asia (SA) (SO 4 , black carbon (BC), and organic matter (OM)) and Siberia (BC and OM) counteracted those reductions. Furthermore, we isolated the RCs of the components to regional trends in TAOD, which revealed that in EC (SA), anthropogenic aerosols, particularly SO 4 and OM, overwhelmingly contributed to the prevailing decrease (increase) in TAOD, with RCs of 31% and 45% (43.6% and 54.4%), respectively. We found that SO 4 was the predominant component controlling the decline in TAOD in EUS and WEU, although the rise in natural aerosols (sea salt) mitigated the impact of this reduction. These findings provide valuable insights into the roles of anthropogenic and natural aerosols in the interannual variation in AOD. • CAMS AOD has robust agreement with AERONET measurements. • Isolated aerosol components drive distinct dominance across dependent regions. • SO 4 and OM variations mainly account for the trend in TAOD in EC and SA. • Upward trends of SS counteract the TAOD reductions in EUS and WEU. [ABSTRACT FROM AUTHOR]

The South Asian high (SAH) experienced a decadal weakening in the late 1970s under global warming. Based on an evaluation of the historical runs from CMIP6 models, we quantitatively assessed the contributions of different external forcing using "good" models that reasonably simulated the decadal decline of the SAH. All‐forcing runs yielded the weakened SAH after the late 1970s, albeit the decadal decline was underestimated by most models. Compared to the insignificant contributions of greenhouse gas and natural forcing, anthropogenic aerosol played a dominant role in the decadal decline of the SAH. The increased aerosol likely drove a cooling surface over the Tibetan Plateau (TP) and East China via its effect on radiation. Consequently, the weakened heat source over the TP and associated thermodynamic effects over East China would have driven a cooling of eddy temperature and cyclonic anomalies in the upper troposphere, respectively, thereby causing the decline of the SAH. Plain Language Summary: Under global warming, the South Asian high (SAH) measured by eddy geopotential height experienced a decadal decline in the late 1970s. However, the mechanism responsible for the decadal decline of the SAH has not been investigated. In this study, the contributions of external forcing to the decadal decline of the SAH are accessed using "good" CMIP6 models that can reasonably simulate the decadal changes of SAH. Results indicate that anthropogenic aerosol plays an important role in weakening the SAH. The increased aerosol emission over the Tibetan Plateau (TP) and East China cools the ground through the radiation effect. The weakened heat effect of TP and East China can contribute to the weakening of SAH. By contrast, the contributions of greenhouse gas and natural forcing are almost completely insignificant due to their homogenized and only very slight effects on the upper‐tropospheric eddy temperature over the SAH region, respectively. Key Points: ALL forcing can yield a weakening of the South Asian high (SAH), and anthropogenic aerosol provided the dominant contributionAnthropogenic aerosol drove the decline of the SAH by weakening the effect of the heat source over the Tibetan PlateauThe contribution of greenhouse gas and natural external forcing to the decadal decline of the SAH is insignificant [ABSTRACT FROM AUTHOR]

Abstract The South Asian high (SAH) experienced a decadal weakening in the late 1970s under global warming. Based on an evaluation of the historical runs from CMIP6 models, we quantitatively assessed the contributions of different external forcing using “good” models that reasonably simulated the decadal decline of the SAH. All‐forcing runs yielded the weakened SAH after the late 1970s, albeit the decadal decline was underestimated by most models. Compared to the insignificant contributions of greenhouse gas and natural forcing, anthropogenic aerosol played a dominant role in the decadal decline of the SAH. The increased aerosol likely drove a cooling surface over the Tibetan Plateau (TP) and East China via its effect on radiation. Consequently, the weakened heat source over the TP and associated thermodynamic effects over East China would have driven a cooling of eddy temperature and cyclonic anomalies in the upper troposphere, respectively, thereby causing the decline of the SAH.

The direct radiative effect (DRE) of anthropogenic aerosols on meteorological elements and pollutants over Tianjin, China, was investigated using a fully coupled meteorology-chemistry model [Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem)] for the entire year of 2021. The results showed that the ground solar radiation decreased from 208.84 W m−2 to 194.52 W m−2 due to DRE in Tianjin, and the percentage of the reduction was 6.86%. The reduction of ground solar radiation resulted in a 0.90% decrease in temperature, 1.13% decrease in boundary layer height, and 6.01% decrease in visibility, respectively, and a 0.80% increase in relative humidity. Changes in meteorological parameters changed the diffuse conditions of pollutants, leading to an increase of PM2.5. Higher concentration of pollutants leads to stronger DRE, and the percentages of changes caused by DRE on polluted days are higher than those on clean days. The DRE influence on pollutants can be summarized as a weather → aerosol → weather → air quality circulation mechanism. The DRE had more severe influence on meteorological elements and pollutants on foggy days. With the emission reduction of pollutants, the influence of the DRE has declined.

With rapid warming since the mid‐20th century, China has experienced remarkable changes in the extreme temperatures. We use the updated observational data and the newest generation of climate models from Coupled Model Intercomparison Project Phase 6 (CMIP6) to investigate the relative contribution from different external forcing to the temperature extremes. We find that both intensity and frequency indices of extreme temperature experience continuous warming during 1951–2018. More intense and more frequent warm extremes and less intense and less frequent cold extremes are observed in most regions. An exception is a warming slowdown in the intensity of the coldest extremes since the late 1990s in northeast China. These observed changes are generally well reproduced by CMIP6 climate models, especially for the warm days and nights. Detection analyses based on an optimal fingerprinting method show that anthropogenic forcing (ANT) is the main driver for these changes, with cold extremes less detectability than warm extremes. Three‐signal detections show that both greenhouse gas (GHG) and anthropogenic aerosols (AA) influences can be detected and separated in most warm extreme indices but not in the cold extremes, while the natural forcing influence is negligible for most indices. GHG forcing plays a dominant role, accounting for about 1.6 (1.1–2) times of observed warming in changes of most indices, while the AA offset about 35% (10–60%) of GHG induced warming for warm extremes. Anthropogenic factors including land use and ozone may have a very small positive contribution to the extreme temperatures. [ABSTRACT FROM AUTHOR]

Anthropogenic impacts on terrestrial evapotranspiration (ET) changes during 1980–2020 were evaluated based on newly released observed‐based GLEAM ET and Coupled Model Intercomparison Project Phase 6 (CMIP6) through optional fingerprint method. Global assessments show that anthropogenic forcings dominate the increasing ET trend, other than natural forcing (NAT). On the global scale, anthropogenic forcings explain ∼84.2% of the observed ET trend, while the signal of natural forcing cannot be detected. Among anthropogenic forcings, greenhouse gases (GHG) are the primary driving factor for ET changes, and GHG‐only has already explained ∼78.8% of the observed ET trend. This study for the first time demonstrates that the GHG signal can be detected in the observed‐based ET and can be separable from NAT and aerosol (AER) signals. At the regional scale, CMIP6 simulations work well in North Hemisphere, but are deficient in South Hemisphere. GHG still dominates ET changes in the North Hemisphere, except for Europe, where the influence of AER forcing stands out, and attributable changes to AER is 2.4 times of GHG. Our first quantitative detection and attribution of ET contribute to advance the understanding of anthropogenic activities on hydrological cycle changes. Key Points: This study conducts a first quantitative assessment of terrestrial ET changes response to greenhouse gases and anthropogenic aerosolThe greenhouse gases forcing can be significantly detected and explains ∼80.6% of the global ET changesAnthropogenic aerosol forcing is detectable and contributes 66.9% of ET change over Europe [ABSTRACT FROM AUTHOR]

Anthropogenic aerosol emissions are expected to change rapidly over the coming decades, driving strong, spatially complex trends in temperature, hydroclimate, and extreme events both near and far from emission sources. Under-resourced, highly populated regions often bear the brunt of aerosols’ climate and air quality effects, amplifying risk through heightened exposure and vulnerability. However, many policy-facing evaluations of near-term climate risk, including those in the latest Intergovernmental Panel on Climate Change assessment report, underrepresent aerosols’ complex and regionally diverse climate effects, reducing them to a globally averaged offset to greenhouse gas warming. We argue that this constitutes a major missing element in society’s ability to prepare for future climate change. We outline a pathway towards progress and call for greater interaction between the aerosol research, impact modeling, scenario development, and risk assessment communities.

To disentangle the mechanisms how different types of aerosols influence the position of the Intertropical convergence zone (ITCZ), this study explores the impacts of black carbon (BC) and sulfate (SO4) aerosols on the ITCZ latitudinal position through altering different autoconversion schemes and cumulus parameterizations within a single climate model. To investigate their impacts with a detectable magnitude, we homogeneously perturb the emissions of BC and SO4 in the year 2000 by 10 and 5 times, respectively. It is found that for 5SO4 cases, altering the autoconversion scheme not only affects the total (atmosphere plus ocean) anomalous cross‐equatorial heat transport, but also modulates the atmospheric portion in it, and thus influences the magnitude of the corresponding ITCZ shift. The atmospheric portion in the 5SO4‐induced total anomalous cross‐equatorial heat transport increases with the cloud adjustment to 5SO4, as longer cloud lifetime can suppress more surface evaporation in the northern hemisphere and increase southern‐northern hemispheric energy contrast in the atmosphere. For 10BC cases, it is found that with the increase of cloud adjustment, the corresponding oceanic anomalous cross‐equatorial heat transport tends to shift from southward to northward, as larger cloud adjustment means more increase (decrease) of low (high) cloud fraction and less net downward radiation both at the top of the atmosphere and the surface in the northern hemisphere. However, the atmospheric anomalous cross‐equatorial heat transport and the consequent ITCZ shift caused by 10BC are affected little. Compared with the altering autoconversion scheme, altering cumulus parameterization has relatively small influences on the 10BC‐ and 5SO4‐induced anomalous cross‐equatorial heat transports and the consequent ITCZ shifts. Plain Language Summary: Many studies have found that anthropogenic aerosols (e.g., sulfate, black carbon) can cause the Intertropical convergence zone (ITCZ) to shift in the latitudinal direction, which is roughly attributable to the asymmetric distribution of anthropogenic aerosols about the equator and the consequent disturbance of inter‐hemispheric energy balance. Here we demonstrate that the efficiency of anthropogenic aerosols in causing ITCZ to shift in the latitudinal direction is closely related with the atmospheric role in compensating the corresponding inter‐hemispheric energy disturbance, which is further related with aerosol‐induced cloud adjustments. Key Points: Aerosols' efficiency in causing Intertropical convergence zone (ITCZ) shift is directly related with atmospheric portion in anomalous cross‐equatorial heat transportThe atmospheric and oceanic fractions in anomalous cross‐equatorial heat transport for aerosols are influenced by cloud adjustmentThe mechanisms of black carbon and sulfate aerosols in influencing the latitudinal position of the ITCZ are different [ABSTRACT FROM AUTHOR]

The relationship between electric field variations and microparticle concentrations in the atmospheric surface layer has been studied using the example of the Moscow region. Synchronous field observations of the electric field strength obtained at two stations (the Moscow Center of Geophysical Monitoring and the Mikhnevo Geophysical Observatory of the Institute of Geosphere Dynamics, Russian Academy of Sciences) have been used. Under megacity conditions, the amplitudes of electric field signals have been found to increase relative to their values outside the region. The ratios of average electric field strengths at the observation stations to the values obtained outside the region have been estimated using the model that is developed. Variations in the electric field have been compared with changes in the concentrations of CO, NOx, SO2, and fine particulate matter (PM2.5 and PM10); the electric field strength and the concentration of microparticles under megacity conditions have been found to be correlated. [ABSTRACT FROM AUTHOR]

The Arctic is warming two to three times faster than the global average, and the role of aerosols is not well constrained. Aerosol number concentrations can be very low in remote environments, rendering local cloud radiative properties highly sensitive to available aerosol. The composition and sources of the climate-relevant aerosols, affecting Arctic cloud formation and altering their microphysics, remain largely elusive due to a lack of harmonized concurrent multi-component, multi-site, and multi-season observations. Here, we present a dataset on the overall chemical composition and seasonal variability of the Arctic total particulate matter (with a size cut at 10 μ m, PM _10 , or without any size cut) at eight observatories representing all Arctic sectors. Our holistic observational approach includes the Russian Arctic, a significant emission source area with less dedicated aerosol monitoring, and extends beyond the more traditionally studied summer period and black carbon/sulfate or fine-mode pollutants. The major airborne Arctic PM components in terms of dry mass are sea salt, secondary (non-sea-salt, nss) sulfate, and organic aerosol (OA), with minor contributions from elemental carbon (EC) and ammonium. We observe substantial spatiotemporal variability in component ratios, such as EC/OA, ammonium/nss-sulfate and OA/nss-sulfate, and fractional contributions to PM. When combined with component-specific back-trajectory analysis to identify marine or terrestrial origins, as well as the companion study by Moschos et al 2022 Nat. Geosci. focusing on OA, the composition analysis provides policy-guiding observational insights into sector-based differences in natural and anthropogenic Arctic aerosol sources. In this regard, we first reveal major source regions of inner-Arctic sea salt, biogenic sulfate, and natural organics, and highlight an underappreciated wintertime source of primary carbonaceous aerosols (EC and OA) in West Siberia, potentially associated with the oil and gas sector. The presented dataset can assist in reducing uncertainties in modelling pan-Arctic aerosol-climate interactions, as the major contributors to yearly aerosol mass can be constrained. These models can then be used to predict the future evolution of individual inner-Arctic atmospheric PM components in light of current and emerging pollution mitigation measures and improved region-specific emission inventories.

Persistent heavy haze episodes have repeatedly shrouded North China in recent years. Besides anthropogenic emissions, natural dust also contributes to the aerosols in this region. Through continuous observation by a dual-wavelength Raman lidar, the primary aerosol types and their contributions to air pollution in North China were determined. The following three aerosol types can be classified: natural dust, anthropogenic aerosols, and the mixture of anthropogenic aerosols and dust (polluted dust). The classification results are basically consistent with the classification results from the cloud–aerosol lidar and infrared pathfinder satellite observations (CALIPSO) satellite measurements. The relative bias of the lidar ratio between the Raman lidar and CALIPSO is less than 25% over 90% of the cases, indicating that the CALIPSO lidar ratio selection algorithm is reasonable. The classification results show that approximately 45% of aerosols below 1.8 km are contributed by polluted dust during our one year observations. The contribution of dust increased with height, from 6% at 500 m to 28% at 1,800 m, while the contribution of anthropogenic aerosols decreased from 49% to 25%. In addition, polluted dust is the major aerosol subtype below 1.0 km in spring (over 60%) and autumn (over 70%). Anthropogenic aerosols contribute more than 75% of air pollution in summer. In winter, anthropogenic aerosols prevailed (over 80%) in the lower layer, while polluted dust (around 60%) dominated the upper layer. Our results identified the primarily aerosol types to assess the contributions of anthropogenic and natural sources to air pollution in North China, and highlight that natural dust plays a crucial role in lower-layer air pollution in spring and autumn, while controlling anthropogenic aerosols will significantly improve air quality in winter.

Long-term Aqua and Terra MODIS (MODerate resolution Imaging Spectroradiometer) Collections 5.1 and 6.1 (c051 and c061, respectively) aerosol data have been combined with AERONET (AERosol RObotic NETwork) ground-based sun photometer observations to examine trends in aerosol optical thickness (AOT, at 550 nm) over Northern Europe for the months April to September. For the 1927 and 1559 daily coincident measurements that were obtained for c051 and c061, respectively, MODIS AOT varied by 86 and 90%, respectively, within the predicted uncertainty of one standard deviation of the retrieval over land (ΔAOT = ±0.05 ± 0.15·AOT). For the coastal AERONET site Gustav Dalen Tower (GDT), Sweden, larger deviations were found for MODIS c051 and c061 (79% and 75%, respectively, within predicted uncertainty). The Baltic Sea provides substantially better statistical representation of AOT than the surrounding land areas and therefore favours the investigations of trends in AOT over the region. Negative trends of 1.5% and 1.2% per year in AOT, based on daily averaging, were found for the southwestern Baltic Sea from MODIS c051 and c061, respectively. This is in line with a decrease of 1.2% per year in AOT at the AERONET station Hamburg. For the western Gotland Basin area, Sweden, negative trends of 1.5%, 1.1% and 1.6% per year in AOT have been found for MODIS c051, MODIS c061 and AERONET GDT, respectively. The strongest trend of –1.8% per year in AOT was found for AERONET Belsk, Poland, which can be compared to –1.5% per day obtained from MODIS c051 over central Poland. The trends in MODIS and AERONET AOT are nearly all statistically significant at the 95% confidence level. The strongest aerosol sources are suggested to be located southwest, south and southeast of the investigation area, although the highest prevalence of pollution events is associated with air mass transport from southwest.

The Asian Summer Monsoon (ASM) affects ecosystems, biodiversity, and food security of billions of people. In recent decades, ASM strength (as represented by precipitation) has been decreasing, but instrumental measurements span only a short period of time. The initiation and the dynamics of the recent trend are unclear. Here for the first time, we use an ensemble of 10 tree ring‐width chronologies from the west‐central margin of ASM to reconstruct detail of ASM variability back to 1566 CE. The reconstruction captures weak/strong ASM events and also reflects major locust plagues. Notably, we found an unprecedented 80‐year trend of decreasing ASM strength within the context of the 448‐year reconstruction, which is contrary to what is expected from greenhouse warming. Our coupled climate model shows that increasing anthropogenic sulfate aerosol emissions over the Northern Hemisphere could be the dominant factor contributing to the ASM decrease. Plain Language Summary: Monsoonal rainfall has a certain influence on agriculture and industry in the regions of Asian Summer Monsoon (ASM). An understanding of the spatial‐temporal variability of the ASM and the associated dynamics is vital for terrestrial ecosystems, water resources, forests, and landscapes. We have developed a 448‐year ASM reconstruction back to 1566 CE using 10 tree ring chronologies from the margin region of ASM. We find that historical severe droughts and locust plague disasters during weak ASM events. The recent decreasing ASM trend persisting for over 80 years is unprecedented over the past 448 years. Coupled climate models show that increasing anthropogenic aerosol emissions are the dominant underlying factor. Our aim is that the time series will find a wide range of utility for understanding past climate variability and for predicting future climate change. Key Points: A 448‐year Asian Summer Monsoon reconstruction that extends back to 1566 CE was developed using an ensemble of 10 tree ring chronologies from northwest ChinaThe recent 80‐year decreasing trend of the Asian Summer Monsoon was unprecedented over the past 448 yearsCoupled climate models showed that the unprecedented decreasing trend was likely due to increasing anthropogenic aerosols [ABSTRACT FROM AUTHOR]

Observational analysis indicates significant changes in some temperature extremes over China across the mid-1990s. The decadal changes in hot extremes are characterized as a rise in annual hottest day and night temperature (TXx and TNx) and an increase in frequencies of summer days (SU) and tropical night (TR). The decadal changes in cold extremes are distinguished by a rise in annual coldest day and night temperature (TXn and TNn) and a decrease in frequencies of ice days (ID) and frost days (FD). These decadal changes manifest not only over China as a whole, but also over individual climate sub-regions. An atmosphere-ocean-mixed-layer coupled model forced by changes in greenhouse gases (GHG) concentrations and anthropogenic aerosol (AA) emissions realistically reproduces the general spatial patterns and magnitudes of observed changes in both hot and cold extremes across the mid-1990s, suggesting a pronounced role of anthropogenic changes in these observed decadal changes. Separately, changes in GHG forcing lead to rise in TXx, TNx, TXn and TNn, increase in frequencies of SU and TR and decrease in frequencies of ID and FD over China through increased Greenhouse Effect with positive clear sky longwave radiation and play a dominant role in simulated changes of both hot and cold extremes over China. The AA forcing changes tend to cool Southern China and warm Northern China during summer via aerosol-radiation interaction and AA-induced atmosphere-cloud feedback and therefore lead to some weak decrease in hot extremes over Southeastern China and increase over Northern China. Meanwhile, AA changes lead to warming over China during winter through cloud feedbacks related to aerosol induced cooling over tropical Indian Ocean and western tropical Pacific, and also induce changes in cold extremes the same sign as those induced by GHG, but with weak magnitude. [ABSTRACT FROM AUTHOR]

Long-term Aqua and Terra MODIS (MODerate resolution Imaging Spectroradiometer) Collections 5.1 and 6.1 (c051 and c061, respectively) aerosol data have been combined with AERONET (AERosol RObotic NETwork) ground-based sun photometer observations to examine trends in aerosol optical thickness (AOT, at 550 nm) over Northern Europe for the months April to September. For the 1927 and 1559 daily coincident measurements that were obtained for c051 and c061, respectively, MODIS AOT varied by 86 and 90%, respectively, within the predicted uncertainty of one standard deviation of the retrieval over land (ΔAOT = ±0.05 ± 0.15⋅AOT). For the coastal AERONET site Gustav Dalen Tower (GDT), Sweden, larger deviations were found for MODIS c051 and c061 (79% and 75%, respectively, within predicted uncertainty). The Baltic Sea provides substantially better statistical representation of AOT than the surrounding land areas and therefore favours the investigations of trends in AOT over the region. Negative trends of 1.5% and 1.2% per year in AOT, based on daily averaging, were found for the southwestern Baltic Sea from MODIS c051 and c061, respectively. This is in line with a decrease of 1.2% per year in AOT at the AERONET station Hamburg. For the western Gotland Basin area, Sweden, negative trends of 1.5%, 1.1% and 1.6% per year in AOT have been found for MODIS c051, MODIS c061 and AERONET GDT, respectively. The strongest trend of -1.8% per year in AOT was found for AERONET Belsk, Poland, which can be compared to -1.5% per day obtained from MODIS c051 over central Poland. The trends in MODIS and AERONET AOT are nearly all statistically significant at the 95% confidence level. The strongest aerosol sources are suggested to be located southwest, south and southeast of the investigation area, although the highest prevalence of pollution events is associated with air mass transport from southwest. [ABSTRACT FROM AUTHOR]

Atmospheric black carbon (BC) is the most important aerosol contributor to global warming. However, there is a lack of understanding about the climate impact of BC aerosols because of systematic discrepancies between model and observation estimates of light absorption enhancements (Eabs) in atmospheric processes after emissions, and such discrepancies are transferred directly into large uncertainties of aerosol radiative forcing assessments. In this study, we quantify Eabs of atmospheric BC aerosols with diverse particle morphology distributions using a multi-dimensional aerosol model. We show that current widely used Mie method may overestimate BC Eabs by ∼50% because variations in particle morphology are not considered. Although absorption calculation can be improved by including complex particle morphology and heterogeneity in composition, we find that neglect of the diverse particle morphology distributions in modeling may lead to 15% ∼ 30% relative deviations on Eabs estimations of BC aerosol ensembles. The results thus imply that particle morphology distribution should be included in models to accurately represent the radiative effects of BC aerosols.

The effect of high anthropogenic aerosols on the aerosol optical and radiative properties was aggravated from west to east in China. The annual mean (from 2004 to 2007) aerosol optical depth (AOD), Ångström exponents (α), absorptive aerosol optical depth (AAOD), and single-scattering albedo (SSA) were from 0.16 to 0.73, from − 0.01 to 1.15, from 0.03 to 0.04, and from 0.78 to 0.94, respectively. In addition, the annual mean of aerosol direct radiative forcing at the top of the atmosphere (TOA) were from − 7.2 to 18.5 W/m 2 . High anthropogenic aerosol exhibited the effect of heating the atmosphere (ATM) (48–52 W/m 2 ) and cooling the surface (SFC) (− 48 to − 56 W/m 2 ) in eastern China. In Lanzhou where there is more serious pollution of heavy industry, aerosol-induced surface cooling reached − 61 W/m 2 . The radiative forcing was higher in winter and spring than that in summer and autumn. Aerosol heating effect on the atmosphere was obvious; there was significantly linear correlation between AOD and TOA, SSA and TOA. The slopes of AOD vs TOA were from − 20.8 to − 42.6. The slopes of SSA vs TOA were from − 58 to − 302. The aerosol was cooling the Earth-atmosphere system with AOD or SSA increasing. In the meantime, AAOD (x) exhibited significantly linear correlation with the heating effect in the atmosphere (ATM: y) (y = 1053.7x + 10.5, R 2 = 0.85). And the effect of AOD (x) on the SFC (y) cooling was also obvious (y = − 47.1x − 24.5, R 2 = 0.56). Therefore, the thermodynamic effects of anthropogenic aerosols on the atmosphere circulation and structure should be taken into consideration in East Asia. [ABSTRACT FROM AUTHOR]

We carried out simulations with predefined and simulated aerosol distributions in order to investigate the improvement in the forecasting capabilities of an operational weather forecast model by the use of an improved aerosol representation. This study focuses on convective cumulus clouds developing after the passage of a cold front on 25 April 2008 over Germany. The northerly flow after the cold front leads to increased sea salt aerosol concentrations compared to prefrontal conditions. High aerosol number concentrations are simulated in the interactive scenario representing typically polluted conditions. Nevertheless, due to the presence of sea salt particles, effective radii of cloud droplets reach values typical of pristine clouds (between 7 µm and 13 µm) at the same time. Compared to the predefined continental and maritime aerosol scenarios, the simulated aerosol distribution leads to a significant change in cloud properties such as cloud droplet radii and number concentrations. Averaged over the domain covered by the convective cumuli clouds, we found a systematic decrease in precipitation with increasing aerosol number concentrations. Differences in cloud cover, short wave radiation and cloud top heights are buffered by systematic differences in precipitation and the related diabatic effects. Comparisons with measured precipitation show good agreement for the interactive aerosol scenario as well as for the extreme maritime aerosol scenario.

In recent years, haze pollution and high concentrations of particulate matter frequently occur in many mega cities of China, which has seriously impacted the regional air quality, and further caused harm to human health. Although satellite observation provides a convenient way to evaluate air quality in space and time, satellite measurements do not separate between natural and anthropogenic aerosols. To discriminate anthropogenic aerosol contribution from satellite observations, we proposed the concept of Local Aerosol Optical Depth (LAOD) to describe the localized aerosol loading. A comparative analysis was performed between seasonal/monthly Mean AOD (MAOD), LAOD and ground measured PM2.5/PM10. The comparison results show that LAOD has better linear relationship with PM2.5/PM10 than MAOD in central and eastern China with persistent localized aerosol emissions. Based on the MODIS Deep Blue AOD dataset from 2001 to 2015, we analyzed the spatio-temporal distribution of LAOD over China. Spatially, high LAODs are mainly distributed in Sichuan basin, North China Plain, and central China; temporally, LAOD over China presents an upward trend (+0.003 year −1 ) during 2001–2007 and a weak downward (−0.002 year −1 ) trend from 2008 to 2015. LAOD was also found to be highly correlated with haze frequency over most areas of central and eastern China, especially in North China Plain with a correlation coefficient of 0.87 (P < 0.01). It demonstrates the significant impact of local anthropogenic aerosol emission on regional haze pollution in China. [ABSTRACT FROM AUTHOR]