Stratospheric aerosols, most of which originate from explosive volcanic sulfur emissions into the stratosphere, are a key natural driver of climate variability. They are thus a forcing provided by the Coupled Model Intercomparison Project (CMIP) Climate Forcings Task Team to climate modelling groups participating in phase 7 of CMIP. For the historical period, we provide two datasets covering 1750-2023: (i) a volcanic upper tropospheric-stratospheric sulfur emission dataset, documented in a companion paper; and (ii) a stratospheric sulfate aerosol optical property dataset, documented in this paper. For the satellite era (from 1979 onwards), stratospheric aerosol optical properties are derived from the Global Space-based Stratospheric Aerosol Climatology (GloSSAC) dataset. For the pre-satellite era (1750-1978), optical properties are derived from our volcanic SO2 emission dataset using a new version of the reduced-complexity volcanic aerosol model Easy Volcanic Aerosol (Height) (EVA_H v2). A background, non-volcanic stratospheric aerosol climatology is derived from the 1998-2001 period with a trend over 1850-1978 accounting for increasing anthropogenic aerosols. A monthly stratospheric aerosol climatology is derived from the 1850-2021 average for both pre-industrial and scenario (future) simulations, with a 9-year ramp over 2022-2030 for scenario simulations to ensure a smooth transition from the historical period. CMIP7 simulations should use version 2.2.1 of our pre-industrial and historical datasets, and version 2.2.2 of our scenario dataset. Our methodology to produce historical aerosol optical properties significantly differs from CMIP6 for the pre-satellite era, and the resulting forcings in turn largely differ. In particular, the CMIP6 dataset was mostly based on the sparse and uncertain pyrheliometer record, which resulted in strongly underrepresented emissions from small-to-moderate magnitude eruptions. The resulting bias is addressed in CMIP7, which is entirely emission-derived in the pre-satellite era and uses more recent ice-core-based volcanic sulfur emission inventories than CMIP6. Our approach results in an overall larger volcanic aerosol forcing for CMIP7, with the 1850-2014 mean mid-visible global mean stratospheric aerosol optical depth (SAOD) in CMIP7 (0.0138) being 29 % higher than in CMIP6 (0.0107). The pre-industrial mean of the same variable is 26 % higher in CMIP7 (0.0135, derived from the historical 1850-2021) than CMIP6 (0.0107, derived from the historical 1850-2014 mean). Using a reduced-complexity climate model, we simulate a global mean surface temperature that is 0.07 degrees C colder for 1850-1900 when using the CMIP7 dataset instead of CMIP6, whereas 2000-2014 is 0.03 degrees C warmer in CMIP7. Our dataset also exhibits lower forcing for 1960-1980, resulting in temperatures 0.06 degrees C warmer when averaged over 1960-1990, a period for which CMIP6 climate models exhibit a cold bias. Given the large uncertainties characterizing the dataset, in particular for the pre-satellite era, we advise against treating the CMIP7 or CMIP6 dataset as uniquely superior for any specific year and highlight the need for further evaluation. We conclude the study by discussing sources of uncertainty for the dataset, future research avenues to improve it, as well as requirements to operationalize the production of the dataset, i.e. extend it and update it on an annual basis instead of every 5-7 years following CMIP cycles.

Injection of sulfur and water vapour by the Hunga volcanic eruption significantly altered chemical composition and radiative budget of the stratosphere. Yet, whether the eruption could also affect surface climate, especially via indirect pathways, remains poorly understood. Here we investigate these effects using large ensembles of simulations with the CESM2(WACCM6) Earth system model, incorporating interactive chemistry and aerosols in both coupled ocean and atmosphere-only configurations.We find some statistically significant extratropical regional climate responses to the eruption driven by circulation changes; these are partially linked to the modulation of El Ni & ntilde;o-Southern Oscillation, and its associated teleconnections, and to perturbations of the stratospheric polar vortex in both hemispheres. The stratospheric anomalies affect surface climate through modulating the North Atlantic Oscillation in the Northern Hemisphere (up to three boreal winters following the eruption) and the Southern Annular Mode in the Southern Hemisphere in late 2023. The latter is partly related to a concurrent reduction in Antarctic ozone, as increased stratospheric aerosols and water vapor reach the polar vortex.Our study suggests that the eruption could have had a non-negligible influence on regional surface climate, and discusses the mechanisms via which such an influence could occur. However, the results also highlight that this forcing is relatively weak compared to interannual variability, and is subject to model uncertainties in the representation of key processes. More research is thus needed before definitive statements on the role of the eruption in contributing to surface climate and weather events in the following years are made.

Volcanic eruptions are one of the most important drivers of climate variability, but climate model simulations typically show stronger surface cooling than proxy-based reconstructions. Uncertainties associated with eruption source parameters, aerosol-climate modelling, and internal climate variability might explain those discrepancies, but their quantification using complex global climate models is computationally expensive. In this study, we combine a reduced-complexity volcanic aerosol model (EVA_H) and a climate model (FaIR) to simulate global-mean surface temperature from 6755 BCE to 1900 CE (8705 to 50 BP) accounting for volcanic forcing, solar irradiance, orbital, ice sheet, greenhouse gases, land-use forcing, and anthropogenic aerosols and ozone forcing for the historical period (1750-1900 CE). The negligible computational cost of the models enables us to use a Monte Carlo approach to propagate uncertainties associated with eruption source parameters, aerosol and climate modelling, and internal climate variability. Averaging over the last 9000 years, we obtain a global-mean volcanic forcing of -0.15 W m-2 and an associated surface cooling of 0.12 K. Averaged over the 14 largest eruptions (injecting more than 20 Tg of SO2) of 1250-1900 CE, the mean temperature response in tree-ring-based reconstructions is in good agreement with the our simulations, scaled to Northern Hemisphere summer temperature. For individual eruptions, discrepancies between the simulated and reconstructed surface temperature response are almost always within uncertainties. At multimillennial timescales, our simulations reproduce the Holocene global warming trend typically derived from simulations and data assimilation products but exhibit some discrepancies on centennial to millennial timescales. In particular, the Medieval Climate Anomaly to Little Ice Age transition is weaker in our simulations, and we also do not capture a relatively cool period between 3000 and 1000 BCE (5000 and 3000 BP), visible in climate reanalyses. We discuss how uncertainties in land-use forcing and model limitations might explain these differences. Our study demonstrates the value of reduced-complexity volcanic aerosol-climate models to simulate climate at annual to multimillennial timescales.

Stratospheric aerosol forcing causes only a small global-mean temperature change compared to CO2 forcing of equal magnitude. It has been shown that the dampened temperature response to aerosol forcing originates from enhanced surface temperature change in the tropical Indian and Western Pacific Ocean, relative to the global mean. Due to the pronounced temperature change in this “warm pool” region, strong negative feedback processes are activated. These stabilizing processes strengthen the global mean radiative feedback and abate Earth’s global mean temperature response. In comparison, CO2 forcing has a smaller effect on warm pool temperatures and therefore produces relatively weak feedback, i.e. a strong temperature change. However, it has remained unclear why stratospheric aerosol forcing affects warm pool temperatures more strongly than CO2 forcing. We address this problem using simulations of aerosol and CO2 forcing in MPI-ESM. At the top of the atmosphere (TOA), aerosol forcing is stronger in the warm pool than in the global mean, while CO2 forcing is relatively homogeneous, which could explain the different temperature patterns. However, we find that the forcing pattern at the TOA is not sufficient to explain the aerosols’ strong influence on warm pool temperatures. The effect can only be explained when taking into account the effective forcing pattern at the surface, which is substantially different from the effective forcing at the TOA. In the case of stratospheric aerosol forcing, the stratospheric heating causes an acceleration of the Brewer-Dobson circulation, which induces an enhanced energy transport from the tropics to the extratropics. Although the transport occurs in the stratosphere, it affects the troposphere and causes a strongly negative forcing at the surface of the tropics. In contrast, CO2 does not substantially affect the Brewer-Dobson circulation, and therefore the surface response is not amplified in the tropics. Our results stress the importance of circulation adjustments for the climate response. In the case of stratospheric aerosol forcing, the troposphere is impacted by changes to the wave-driven stratospheric circulation. The accelerated Brewer-Dobson circulation affects the forcing pattern at the surface, and in consequence the pattern of surface temperatures and the climate feedback. Furthermore, we argue that the commonly used method of measuring effective forcing at the TOA is not sufficient for understanding the evolution of surface temperature patterns.

The amount of time that volcanic aerosols spend in the stratosphere is one of the primary factors influencing the climate impact of volcanic eruptions. Stratospheric aerosol persistence has been described in different ways, with many works quoting an approximately 12-month “residence time” for aerosol from large tropical eruptions. Here, we aim to develop a framework for describing the evolution of global stratospheric aerosol after major volcanic eruptions and quantifying its persistence, based on global satellite-based aerosol observations, tracer transport simulations, and simple conceptual modelling. We show that the stratospheric residence time of air, which is estimated through passive tracer pulse experiments and is one factor influencing the lifetime of stratospheric aerosols, is strongly dependent on the injection latitude and height, with an especially strong sensitivity to injection height in the first 4 km above the tropical tropopause. Simulated stratospheric tracer evolution is best described by a simple model which includes a lag between the injection and initiation of removal from the stratosphere. Based on analysis of global stratospheric aerosol observations, we show that the stratospheric lifetime of stratospheric aerosol from the 1991 Pinatubo eruption is approximately 22 months. We estimate the potential impact of observational uncertainties on this lifetime, finding it unlikely the lifetime of Pinatubo aerosol is less than 18 months.

Large volcanic eruptions can have significant impacts on climate. Due to their unpredictable nature, when a major volcanic eruption occurs, decadal forecasts issued prior to the eruption will be inaccurate. Consequently, new decadal forecasts including updated estimates of the stratospheric sulfate aerosol evolution must be produced. To rapidly generate such volcanic forcing once the initial eruption characteristics are known, the Easy Volcanic Aerosol (EVA) forcing generator, and its updated version EVA_H, can be used. Comparing the volcanic forcings generated with these tools and the one from Coupled Model Intercomparison Project phase 6 for the recent eruptions of Mount Agung (1963), El Chich & oacute;n (1982) and Mount Pinatubo (1991), we identify some differences in the magnitude and latitudinal structure, particularly for the eruptions of Mount Agung and El Chich & oacute;n. Using these forcings, we conduct a set of retrospective prediction experiments for these eruptions with the Barcelona Supercomputing Center decadal forecast system, following a specifically designed protocol. The predictions driven by the three forcing datasets show similar post-eruption radiative responses, with particularly good agreement for the eruption of Mount Pinatubo. The global mean top-of-atmosphere flux and global mean surface temperature responses in the hindcast experiments are indistinguishable across the three forcing sets and three eruptions. However, we find differences in the zonal mean and regional responses due to the latitudinally-varying structure of the volcanic forcings, particularly for the eruptions of Mount Agung and El Chich & oacute;n. Significant differences among the datasets are found in the global mean lower stratospheric warming, where the responses are strongest. Comparing the predicted anomalies in these hindcasts with observations we show that overall there is better agreement when volcanic forcing is included, highlighting its importance to accurate predictions. Our study suggests that either EVA and EVA_H forcings can be used for predicting the post-volcanic radiative response, although the generated forcing datasets and simulations should be interpreted with care given the limitations of these reduced-complexity empirical models.

Abstract. Large volcanic eruptions can have significant climatic impacts. Due to their unpredictable nature, such eruptions can render operational decadal forecasts inaccurate. To benefit from the strong climate signals they exert, which enhance climate predictability, decadal forecasts must be rerun with updated estimates of the stratospheric sulfate aerosol evolution. Two tools to rapidly generate the volcanic forcings are the Easy Volcanic Aerosol (EVA, Toohey et al., 2016) and its updated version, EVA_H (Aubry et al., 2020). In order to validate the use of the volcanic forcings generated with these simple models in decadal forecasts, we compare the volcanic forcings generated with EVA and EVA_H with CMIP6 for the recent eruptions of Mount Agung (1963), El Chichón (1982) and Mount Pinatubo (1991) and investigate the consistency in their associated climate responses in decadal predictions produced with the BSC decadal forecast system. Our findings reveal differences in the magnitude and latitudinal structure of the forcings generated by EVA and EVA_H compared to the official CMIP6 forcings, particularly for the eruptions of Mount Agung and El Chichón. These differences in the volcanic forcings lead to some global and regional quantitative differences in the predicted radiative responses, as evidenced in variables like the top-of-atmosphere (TOA) net radiative fluxes, surface temperature, and lower stratospheric temperature. Despite these differences, comparing the predicted anomalies in those variables with observations, we show that either of the forcings considered allows to make skillful predictions after the major volcanic eruptions. Our study thus supports both EVA and EVA_H generated forcings as reasonable choices for predicting the post-volcanic radiative responses.

Climate-driven changes in high-elevation forest distribution and reductions in snow and ice cover have major implications for ecosystems and global water security. In the Greater Yellowstone Ecosystem of the Rocky Mountains (United States), recent melting of a high- elevation (3,091 m asl) ice patch exposed a mature stand of whitebark pine (Pious albicaulis) trees, located similar to 180 m in elevation above modern treeline, that date to the develop tree-ring- based temperature estimates for the upper-elevation climate conditions that resulted in ancient forest establishment and growth and the subsequent regional ice- patch growth and downslope shift of treeline. Results suggest that mid-Holocene forest establishment and growth occurred under warm-season (May-Oct) mean temperatures of 6.2 degrees C (+/- 0.2 degrees C), until a multicentury cooling anomaly suppressed temperatures below 5.8 degrees C, resulting in stand mortality by c. 5,440 y BP. Transient climate model simulations indicate that regional cooling was driven by changes in summer insolation and Northern Hemisphere volcanism. The initial cooling event was followed centuries later (c. 5,100 y BP) by sustained Icelandic volcanic eruptions that forced a centennial-scale 1.0 degrees C summer cooling anomaly and led to rapid ice-patch growth and preservation of the trees. With recent warming (c. 2000-2020 CE), warm-season temperatures now equal and will soon exceed those of the mid-Holocene period of high treeline. It is likely that perennial ice cover will again disappear from the region, and treeline may expand upslope so long as plant-available moisture and disturbance are not limiting.

Explosive volcanic eruptions injecting gases and aerosols into the stratosphere are a key natural driver of climate variability at annual to centennial timescales. They are thus one of the forcings considered by the Coupled Model Intercomparison Project (CMIP) Climate Forcings Task Team, in charge of identifying and implementing the next generation forcings for current and future generations of Earth System models. This presentation will provide an overview of ongoing work to produce volcanic forcing datasets for phase 7 of CMIP (CMIP7). The datasets we produce will cover the period from 1750 to 2022 at version 1 to meet to the need of modelling groups who might run extended historical simulations starting in 1750 instead of 1850. We are producing one volcanic stratospheric sulfur emission dataset catering for the needs of models which have a prognostic interactive stratospheric aerosol scheme, as well as a stratospheric sulfate aerosol optical property dataset required by models that cannot interactively simulate stratospheric sufate aerosols. For the satellite era (from 1979 onwards), sulfur emissions and sufate aerosol optical properties are based on NASA’s MSVOLSO2L4 and GloSSAC datasets, respectively. For the pre-satellite era (1750-1978), the emission dataset is based on ice-core datasets complemented by the geological record, whereas the aerosol optical property dataset is directly derived from emissions using the latest version of the Easy Volcanic Aerosol (EVA) model. This ensures methodological consistency between our emission and optical property datasets, further enhanced by the fact that EVA is calibrated using the same datasets we use for the satellite era. Our choice of methods aims to maximize consistency with methodologies used in individual model intercomparison projects (e.g. PMIP and VolMIP). A major focus of our task team is to produce well-documented datasets, which includes extensive meta-data and flags, detailed documentation, and provision of open-access scripts used to create the datasets, which should facilitate future development and operationalization by the community. We also discuss the most critical challenges for providing accurate volcanic forcing datasets, including the under-recording of small-to-moderate magnitude eruptions before the satellite era, and the Hunga Tonga-Hunga Ha'apai 2022 eruptions, which injected relatively small amounts of sulfur, but 150 Tg of water into the stratosphere.

The Easy Volcanic Aerosol (EVA) family of simple models offers an approach to the generation of stratospheric aerosol fields from estimates of volcanic emissions. EVA takes as input a time series of volcanic eruption data, including the mass of sulfur injected into the stratosphere and location of the eruptions, and outputs aerosol optical properties as a function of time, latitude, height and wavelength based on a simple box-model of stratospheric transport. These aerosol properties are tailored for use as volcanic aerosol forcing in climate models. They are also useful as general quantitative estimates of the impact of volcanic eruptions on climate. EVA version 1 was based on observations of the aerosol from the 1991 Mt. Pinatubo eruption, while EVA_H was parameterized to improve agreement with a range of smaller magnitude eruptions observed over the 1979-2015 period, taking account of the estimated injection height of the emitted sulfur. Here, we present progress in the development of EVA version 2, which improves the fidelity of its output based on various important updates. The model accounts for bi-modal particle size distributions, in line with in-situ observations of Pinatubo aerosol plume. It can also account for the uncertainty in aerosol forcing due to the uncertainty in measurements of the refractive index of sulfuric acid solution. Further updates include implementation of a new method for incorporating injection height and its impact on aerosol growth and evolution. Improvements in the fidelity of aerosol properties is balanced with the aim of simplicity, making EVA2 well-suited for idealized model experiments as well as reconstructions of past volcanic forcing. We compare the results of EVA2 with observational data sets and quantify the impact of updates on reconstructions of volcanic forcing over periods relevant to upcoming CMIP7 experiments.

The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite provides aerosol extinction measurements in multiple solar wavelength bands. In this study, we evaluate the quality and utility of MAESTRO version 3.13 stratospheric aerosol extinction retrievals, from February 2004-February 2021, through comparison with measurements from other satellite instruments. Despite significant scatter in the MAESTRO data, we find that gridded median MAESTRO aerosol extinctions and stratospheric aerosol optical depth (SAOD) values are generally in good agreement with those from other instruments during volcanically quiescent periods. After volcanic eruptions and wildfire injections, gridded median MAESTRO extinction and SAOD are well correlated with other measurement sets but generally biased low by 40 %-80 %. The & Aring;ngstr & ouml;m exponent (AE), which can provide information on aerosol particle size, is derived from the MAESTRO spectral extinction measurements in the lowermost stratosphere, showing perturbations after volcanic eruptions qualitatively similar to those from the Stratospheric Aerosol and Gas Experiment on the International Space Station (SAGE III/ISS) for the eruptions of Ambae (2018) and Ulawun (2019). Differences in AE anomalies after the 2019 extratropical Raikoke eruption may be due to the different spatiotemporal sampling of the two instruments. Furthermore, we introduce a method to adjust MAESTRO extinction data based on comparison with extinction measurements from SAGE III/ISS during the period from June 2017-February 2021, resulting in improved comparison during volcanically active periods. Our work suggests that empirical bias correction may enhance the utility of MAESTRO aerosol extinction data, which can make it a useful complement to existing satellite records, especially when multi-wavelength solar occultation data from other instruments are unavailable.

Stratospheric aerosols, most of which originate from explosive volcanic sulfur emissions into the stratosphere, are a key natural driver of climate variability. They are thus one of the forcings provided by the Coupled Model Intercomparison Project (CMIP) Climate Forcings Task Team for the CMIP7 Fast Track, a set of climate model experiments designed to deliver the Intergovernmental Panel on Climate Change (IPCC) 7th assessment cycle. In this work, we document the final version of the stratospheric aerosol forcing datasets delivered to modelling groups for CMIP7 Fast Track. Our datasets cover the 1750-2023 period to meet to the need of modelling groups who might run extended historical simulations starting in 1750 instead of 1850. We produced one volcanic stratospheric sulfur emission dataset catering for the needs of models which have a prognostic interactive stratospheric aerosol scheme, as well as a stratospheric sulfate aerosol optical property dataset required by models that cannot interactively simulate stratospheric sufate aerosols. For the satellite era (from 1979 onwards), sulfur emissions and sufate aerosol optical properties are based on the MSVOLSO2L4 and GloSSAC datasets, respectively. For the pre-satellite era (1750-1978), the emission dataset is based on ice-core datasets complemented by the geological record for small-moderate magnitude eruptions not captured in ice-core records. Although inferring emissions of these eruptions from the geological record is highly uncertain, our approach minimizes an important bias in the pre-satellite era forcing, both in terms of mean and variability. The pre-satellite aerosol optical property dataset is directly derived from emissions using an updated version of EVA_H, a reduced-complexity volcanic aerosol model. This ensures methodological consistency between our emission and optical property datasets, and maximizes consistency with methodologies used in the paleoclimate (PMIP) and volcanic forcing (VolMIP) model intercomparison projects in CMIP6. We will present extensive comparison between our CMIP7 Fast Track dataset and the CMIP6 dataset. Last, we will discuss the main challenges to improve stratospheric aerosol datasets in the future and to move to high frequency (yearly or less) extension and update instead of an ad-hoc production for each CMIP phase.

The High-Altitude Aerosols, Water Vapor, and Clouds (HAWC) mission is an observing system, with a planned launch around 2031, that is being developed by the Canadian Space Agency to provide collocated global measurements of aerosols, water vapor, and thin ice clouds in the upper troposphere and lower stratosphere with vertical coverage that extends into the troposphere in the polar regions. The mission is the Canadian contribution to NASA's Atmospheric Observing System (AOS), a satellite constellation will include multiple instruments to monitor aerosols, clouds, and precipitation as part of Earth System Observatory (ESO). The HAWC mission includes three innovative Canadian instruments: the Aerosol Limb Imager (ALI), the Thin Ice Clouds and Far Infrared Emissions (TICFIRE) instrument, and the Spatial Heterodyne Observations of Water (SHOW) instrument. ALI and SHOW will provide limb profiles of aerosol and water vapor with high spatial resolution (vertical and along track) and high sensitivity to fine aerosols and dry conditions. TICFIRE will provide nadir measurements of infrared radiation, thin ice cloud content, and cloud microphysical properties. These coordinated measurements will help build a more comprehensive picture of high-altitude aerosols, clouds, and water vapor in the atmosphere. In this paper, we present the HAWC concept and discuss the primary science objectives and requirements of the mission. The instrument payloads and data products are introduced, and synergies between HAWC and other AOS instruments are identified.

Understanding the response of large-scale atmospheric circulation to radiative forcing agents is important for climate prediction. The radiative forcing from volcanic stratospheric aerosol is one of the most important natural climate forcings, with impacts on surface temperature and atmospheric dynamics. In this study, we explore changes in the energetic properties of the Hadley and Ferrel systems under the influence of radiative forcing associated with large volcanic eruptions in multi-model simulations performed as part of the Model Intercomparison Project on the Climatic Response to Volcanic Forcing (VolMIP) within the Coupled Model Intercomparison Project Phase 6 (CMIP6). In the Earth’s atmosphere, the Hadley and Ferrel systems are examples of thermally direct (warm air rises and cold air sinks) and indirect (cold air rises and warm air sinks) circulations, respectively. Being the part of Lorenz cycle of energy transformation in the atmosphere, the direct circulation converts zonal-mean available potential energy into zonal-mean kinetic energy. The indirect circulation in the midlatitude, however, converts some of the zonal-mean kinetic energy back into zonal-mean available potential energy. Averaged over the 4 models that provided the required model output from the VolMIP Pinatubo simulations, the mean power associated with the Hadley system in preindustrial simulations is 235.6 TW. The mean decrease of the power in VolMIP simulations of the 1991 Pinatubo eruption is 7.58 TW (3.22%) for the first post-eruption northern-hemisphere (NH) winter and 6.59 TW (2.80%) for the second one. For the Ferrel system, the preindustrial mean DJF power is 326.10 TW, and post-volcanic anomalies are 16.3 TW (5.00%) and 18.3 TW (5.61%) in NH winters 1 and 2, showing a stronger anomaly in the second NH winter than the first one. In additional VolMIP experiments, we also explore the response of the Hadley and Ferrel cells to the relatively strong forcing associated with the 1815 Tambora eruption and find the Hadley system weakening by 15.3 TW (6.48%) and 11.5 TW (4.90%) for the first two NH winters. We explore how post-eruption changes in the meridional atmospheric circulation strength and the cells' location can be explained with simple theoretical models of atmospheric thermodynamics.

Volcanic eruptions impact the climate and environment. The volcanic forcing is determined by eruption source parameters, including the mass and composition of volcanic volatiles, eruption season, eruption latitude, and injection altitude. Moreover, initial atmospheric conditions of the climate system play an important role in shaping the volcanic forcing and response. However, our understanding of the combination of these factors, the distinctions between tropical and extratropical volcanic eruptions, and the co-injection of sulfur and halogens remains limited. Here, we perform ensemble simulations of volcanic eruptions at 15 and 64° N in January, injecting 17 Mt of SO2 together with HCl and HBr at 24 km altitude. Our findings reveal that initial atmospheric conditions control the transport of volcanic volatiles from the first month and modulate the subsequent latitudinal distribution of sulfate aerosols and halogens. This results in different volcanic forcing, surface temperature and ozone responses over the globe and Northern Hemisphere extratropics (NHET) among the model ensemble members with different initial atmospheric conditions. NH extratropical eruptions exhibit a larger NHET mean volcanic forcing, surface cooling and ozone depletion compared with tropical eruptions. However, tropical eruptions lead to more prolonged impacts compared with NH extratropical eruptions, both globally and in the NHET. The sensitivity of volcanic forcing to varying eruption source parameters and model dependency is discussed, emphasizing the need for future multi-model studies to consider the influence of initial conditions and eruption source parameters on volcanic forcing and subsequent impacts.

Perturbations in stratospheric aerosol due to explosive volcanic eruptions are a primary contributor to natural climate variability. Observations of stratospheric aerosol are available for the past decades, and information from ice cores has been used to derive estimates of stratospheric sulfur injections and aerosol optical depth over the Holocene (approximately 10 000 BP to present) and into the last glacial period, extending back to 60 000 BP. Tephra records of past volcanism, compared to ice cores, are less complete but extend much further into the past. To support model studies of the potential impacts of explosive volcanism on climate variability across timescales, we present here an ensemble reconstruction of volcanic stratospheric sulfur injection (VSSI) over the last 140 000 years that is based primarily on terrestrial and marine tephra records. VSSI values are computed as a simple function of eruption magnitude based on VSSI estimates from ice cores and satellite observations for identified eruptions. To correct for the incompleteness of the tephra record, we include stochastically generated synthetic eruptions assuming a constant background eruption frequency from the ice core Holocene record. While the reconstruction often differs from ice core estimates for specific eruptions due to uncertainties in the data used and reconstruction method, it shows good agreement with an ice-core-based VSSI reconstruction in terms of millennial-scale cumulative VSSI variations over the Holocene. The PalVol reconstruction provides a new basis to test the contributions of forced vs. unforced natural variability to the spectrum of climate and the mechanisms leading to abrupt transitions in the palaeoclimate record with low- to high-complexity climate models. The PalVol volcanic forcing reconstruction is available at https://doi.org/10.26050/WDCC/PalVolv1 (Toohey and Schindlbeck-Belo, 2023).