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Tropical low‐cloud feedback is the largest source of uncertainty in climate sensitivity, yet multi‐century records of surface shortwave radiation are scarce. We calibrate Porites coral δ13C against satellite photosynthetically available radiation (PAR) and reconstruct monthly PAR for the northern South China Sea during the Medieval Climate Anomaly (1129–1264 CE) and the Little Ice Age (1631–1771 CE). After correcting for the Suess effect and propagating errors via Monte Carlo resampling techniques, annual PAR during the Little‐Ice‐Age is ∼22% lower and seasonality slightly weaker. The dimming aligns with regional proxies for cooler, wetter conditions and is best explained by brighter low clouds, likely boosted by volcanic aerosol–cloud interactions. CMIP6/PMIP4 past1000 simulations, however, yield <0.2% change over the same interval, indicating that current models understate volcanic microphysics and tropical low‐cloud sensitivity. The coral PAR record thus provides a quantitative pre‐industrial target for evaluating tropical cloud processes and reducing uncertainty in equilibrium climate sensitivity.

Aerosol from the Hunga Tonga‐Hunga Ha'apai (HT‐HH) volcanic eruption (20.6°S) in January 2022 were not incorporated into the austral polar vortex until the following year, March 2023. Within the polar vortex in situ profiles of aerosol size spectra were completed in the austral autumns of 2019 and 2023, from McMurdo Station, Antarctica (78˚S), 30 months prior to and 15 months after the HT‐HH eruption. The measurements indicate that the HT‐HH impact on aerosol size was primarily confined to particles with diameters >0.5 μm leading to differences in aerosol mass, surface area, and extinction from factors of 2–4 at the volcanic layer's peak below 20 km, increasing to ratios of 5–10 above 20 km. Effective radius, with radiative and microphysical implications, increased from ∼0.2 to ∼0.3 μm. An Earth system model with a modal aerosol package compares favorably with the in situ measurements of the HT‐HH aerosol impact. Plain Language Summary In austal autumn of 2019 and 2023 balloon‐borne measurements of aerosol size distributions were completed above McMurdo Station, Antarctica (78˚S, 167˚E). Size spectra were measured for particles between 0.01 and 16 μm from the surface to over 30 km. Between these two sets of measurements the Hunga Tonga‐Hunga Ha'apai (HT‐HH) volcano (20.6°S) started an eruptive phase culminating with a massive underwater explosion on 15 January 2022. Due to the timing of the eruption, the HT‐HH volcanic aerosol were not incorporated into the austral polar vortex until 2023, which formed about a month before the 2023 aerosol measurements were completed. The 2019 measurements were in a volcanically quiescent period and capture the background stratospheric aerosol. The two sets of measurements are used to characterize the impact of HT‐HH on stratospheric aerosol within the austral polar vortex, particularly on quantities important for assessing the volcanic impact on climate and heterogeneous chemistry: aerosol mass, surface area, and light extinction. Such quantities were observed to increase by factors of 2–4 below, and 5–10 above, 20 km after the eruption. An Earth System Model with an aerosol package compared favorably wih the measurements. Polar ozone loss in 2023 was, however, not impacted significantly. Key Points Austral polar vortex aerosol concentrations from Hunga Tonga increased, for particles >0.5 μm, by factors of 2–5 below, >10 above, 18 km Concomittant increases in SO4 mass, surface area, and near IR extinction were by factors of 2–4 below, and 5–10 above, 18 km Effective radius, with radiative and microphysical implications, increased from ∼0.2 to ∼0.3 μm at the peak of the layer

In the framework of regular European Aerosol Research Lidar Network (EARLINET) observations, aerosol layers have been monitored with a multiwavelength aerosol Raman lidar in the upper troposphere and lower stratosphere over Leipzig (51.4°N, 12.4°E), Germany, since the summer of 2008. The origins of these layers are eruptions of different volcanoes on the Aleutian Islands, Kamchatka, Alaska, and on the Kuril Islands. FLEXPART transport simulations show that the volcanic aerosol is advected from Alaska to central Europe within about 7 days. The aerosol layers typically occurred in the upper troposphere above 5 km height and in the lower stratosphere below 25 km height. The optical depths of the volcanic aerosol layers are mostly between 0.004 and 0.025 at 532 nm. The wavelength dependence of the backscatter coefficients and extinction coefficients indicate Ångström exponents from 1.0–2.0. Lidar ratios in the stratosphere are found in the range from 30–60 sr (355 nm) and 30–45 sr (532 nm). The estimation of the effective radius, surface‐area, and mass concentrations of a volcanic aerosol layer, observed well within the stratosphere at end of August 2009, reveals values of 0.1–0.2 μm, 5–10 μm2 cm−3, and 0.3–0.5 μg m−3, respectively.

Volcanic eruptions have long been studied for their wide range of climatic effects. Although global-scale climatic impacts following the formation of stratospheric sulfate aerosol are well understood, many aspects of the evolution of the early volcanic aerosol cloud and regional impacts are uncertain. In the last twenty years, several advances have been made, mainly due to improved satellite measurements and observations enabling the effects of small-magnitude eruptions to be quantified, new proxy reconstructions used to investigate the impact of past eruptions, and state-of-the-art aerosol-climate modelling that has led to new insights on how volcanic eruptions affect the climate. Looking to the future, knowledge gaps include the role of co-emissions in volcanic plumes, the impact of eruptions on tropical hydroclimate and Northern Hemisphere winter climate, and the role of eruptions in long-term climate change. Future model development, dedicated model intercomparison projects, interdisciplinary collaborations, and the application of advanced statistical techniques will facilitate more complex and detailed studies. Ensuring that the next large-magnitude explosive eruption is well observed will be critical in providing invaluable observations that will bridge remaining gaps in our understanding.

Aerosol-cloud interactions have a potentially large impact on climate, but are poorly quantified and thus contribute a significant and long-standing uncertainty in climate projections. The impacts derived from climate models are poorly constrained by observations, because retrieving robust large-scale signals of aerosol-cloud interactions are frequently hampered by the considerable noise associated with meteorological co-variability. The Iceland-Holuhraun effusive eruption in 2014 resulted in a massive aerosol plume in an otherwise near-pristine environment and thus provided an ideal natural experiment to quantify cloud responses to aerosol perturbations. Here we disentangle significant signals from the noise of meteorological co-variability using a satellite-based machine-learning approach. Our analysis shows that aerosols from the eruption increased cloud cover by approximately 10%, and this appears to be the leading cause of climate forcing, rather than cloud brightening as previously thought. We find that volcanic aerosols do brighten clouds by reducing droplet size, but this has a significantly smaller radiative impact than changes in cloud fraction. These results add substantial observational constraints on the cooling impact of aerosols. Such constraints are critical for improving climate models, which still inadequately represent the complex macro-physical and micro-physical impacts of aerosol-cloud interactions.

The Calbuco volcano in southern Chile (41.3° S, 72.6° W) underwent three separate eruptions on 22–23 April 2015. Following the eruptions, distinct layers of enhanced lidar backscatter at 532 nm were observed in the lower stratosphere above Buckland Park, South Australia (34.6° S, 138.5° E), and Kingston, Tasmania (43.0° S, 147.3° E), during a small set of observations in April–May 2015. Using atmospheric trajectory modelling and measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) space-borne lidar and the Ozone Mapping Profiler Suite (OMPS) instrument on the Suomi National Polar-orbiting Partnership (NPP) satellite, we show that these layers were associated with the Calbuco eruptions. Buckland Park measurements on 30 April and 3 May detected discrete aerosol layers at and slightly above the tropopause, where the relative humidity was well below saturation. Stratospheric aerosol layers likely associated with the eruptions were observed at Kingston on 17 and 22 May in narrow discrete layers accompanied by weaker and more vertically extended backscatter. The measurements on 22 May provided a mean value of the particle linear depolarisation ratio within the main observed volcanic aerosol layer of 18.0 ± 3.0%, which was consistent with contemporaneous CALIOP measurements. The depolarisation measurements indicated that this layer consisted of a filament dominated by ash backscatter residing above a main region having likely more sulfate backscatter. Layer-average optical depths were estimated from the measurements. The mean lidar ratio for the volcanic aerosols on 22 May of 86 ± 37 sr is consistent with but generally higher than the mean for ground-based measurements for other volcanic events. The inferred optical depth for the main volcanic layer on 17 May was consistent with a value obtained from OMPS measurements, but a large difference on 22 May likely reflected the spatial inhomogeneity of the volcanic plume. Short-lived enhancements of backscatter near the tropopause of 17 May likely represented the formation cirrus that was aided by the presence of associated volcanic aerosols. We also provide evidence that gravity waves potentially influenced the layers, particularly in regard to the vertical motion observed in the strong layer on 22 May. Overall, these observations provide additional information on the dispersal and characteristics of the Calbuco aerosol plumes at higher southern latitudes than previously reported for ground-based lidar measurements.

The impact of volcanic eruptions on the climate has been studied over the last decades and the role played by sulfate aerosols appears to be major. S-bearing volcanic gases are oxidized in the atmosphere into sulfate aerosols that disturb the radiative balance on earth at regional to global scales. This paper discusses the use of the oxygen and sulfur multi-isotope systematics on volcanic sulfates to understand their formation and fate in more or less diluted volcanic plumes. The study of volcanic aerosols collected from air sampling and ash deposits at different distances from the volcanic systems (from volcanic vents to the Earth poles) is discussed. It appears possible to distinguish between the different S-bearing oxidation pathways to generate volcanic sulfate aerosols whether the oxidation occurs in magmatic, tropospheric, or stratospheric conditions. This multi-isotopic approach represents an additional constraint on atmospheric and climatic models and it shows how sulfates from volcanic deposits could represent a large and under-exploited archive that, over time, have recorded atmospheric conditions on human to geological timescales.

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) version 4.10 (V4) level 2 aerosol data products, released in November 2016, include substantial improvements to the aerosol subtyping and lidar ratio selection algorithms. These improvements are described along with resulting changes in aerosol optical depth (AOD). The most fundamental change in the V4 level 2 aerosol products is a new algorithm to identify aerosol subtypes in the stratosphere. Four aerosol subtypes are introduced for stratospheric aerosols: polar stratospheric aerosol (PSA), volcanic ash, sulfate/other, and smoke. The tropospheric aerosol subtyping algorithm was also improved by adding the following enhancements: (1) all aerosol subtypes are now allowed over polar regions, whereas the version 3 (V3) algorithm allowed only clean continental and polluted continental aerosols; (2) a new “dusty marine” aerosol subtype is introduced, representing mixtures of dust and marine aerosols near the ocean surface; and (3) the “polluted continental” and “smoke” subtypes have been renamed “polluted continental/smoke” and “elevated smoke”, respectively. V4 also revises the lidar ratios for clean marine, dust, clean continental, and elevated smoke subtypes. As a consequence of the V4 updates, the mean 532 nm AOD retrieved by CALIOP has increased by 0.044 (0.036) or 52 % (40 %) for nighttime (daytime). Lidar ratio revisions are the most influential factor for AOD changes from V3 to V4, especially for cloud-free skies. Preliminary validation studies show that the AOD discrepancies between CALIOP and AERONET–MODIS (ocean) are reduced in V4 compared to V3.

The 2022 eruption of the Hunga Tonga‐Hunga Ha'apai volcano caused substantial impacts on the atmosphere, including a massive injection of water vapor, and the largest increase in stratospheric aerosol for 30 years. The Ozone Mapping and Profiler Suite (OMPS) Limb Profiler instrument has been critical in monitoring the amount and spread of the volcanic aerosol in the stratosphere. We show that the rapid imagery from the OMPS instrument enables a tomographic retrieval of the aerosol extinction that reduces a critical bias of up to a factor of two, and improves vertical structure and agreement with coincident lidar and occultation observations. Due to the vertically thin and heterogeneous nature of the volcanic aerosol, this impacts integrated values of aerosol across latitude, altitude, and time for several months. We also investigate the systematic impact of uncertainty in assumed particle size that result in an underestimation of the aerosol extinction at the peak of the volcanic aerosol layer. Plain Language Summary The Hunga Tonga‐Hunga Ha'apai volcano erupted in 2022. The eruption plume went higher into the atmosphere than ever observed before in the modern age. It also carried large amounts of water vapor and other gases and particles, called aerosols, into the stratosphere. The NASA satellite instrument, called the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler, has given us valuable measurements of these aerosols, which are helpful in understanding the impact the volcanic eruption might have on climate. We use an advanced technique to analyze the OMPS measurements that provides a clearer view of the plume. This analysis gives somewhat different results about the thickness of the volcanic plume than the standard method. Key Points Tomographic retrievals reduce a critical bias in Ozone Mapping and Profiler Suite Limb Profiler volcanic aerosol extinction, improving agreement with Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation and Stratospheric Aerosol and Gas Experiment III/International Space Station Biases of up to a factor of two extend beyond the early plume, with zonal, temporal, and altitude integrated values affected for months Uncertainty in particle size distribution also has an impact that should be considered when analyzing aerosol loading

A previous model intercomparison of the Tambora aerosol cloud has highlighted substantial differences among simulated volcanic aerosol properties in the pre-industrial stratosphere and has led to questions about the applicability of global aerosol models for large-magnitude explosive eruptions prior to the observational period. Here, we compare the evolution of the stratospheric aerosol cloud following the well-observed June 1991 Mt. Pinatubo eruption simulated with six interactive stratospheric aerosol microphysics models to a range of observational data sets.Our primary focus is on the uncertainties regarding initial SO2 emission following the Pinatubo eruption, as prescribed in the Historical Eruptions SO2 Emission Assessment experiments (HErSEA), in the framework of the Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP). Six global models with interactive aerosol microphysics took part in this study: ECHAM6-SALSA, EMAC, ECHAM5-HAM, SOCOL-AERv2, ULAQ-CCM, and UM-UKCA. Model simulations are performed by varying the SO2 injection amount (ranging between 5 and 10 Tg S) and the altitude of injection (between 18–25 km).The comparisons show that all models consistently demonstrate faster reduction from the peak in sulfate mass burden in the tropical stratosphere. Most models also show a stronger transport towards the extratropics in the Northern Hemisphere, at the expense of the observed tropical confinement, suggesting a much weaker subtropical barrier in all the models, which results in a shorter e-folding time compared to the observations. Furthermore, simulations in which more than 5 Tg S in the form of SO2 is injected show an initial overestimation of the sulfate burden in the tropics and, in some models, in the Northern Hemisphere and a large surface area density a few months after the eruption compared to the values measured in the tropics and the in situ measurements over Laramie. This draws attention to the importance of including processes such as the ash injection for the removal of the initial SO2 and aerosol lofting through local heating.