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Explosive volcanic eruptions affect climate, but how climate change affects the stratospheric volcanic sulfate aerosol lifecycle and radiative forcing remains unexplored. We combine an eruptive column model with an aerosol-climate model to show that the stratospheric aerosol optical depth perturbation from frequent moderate-magnitude tropical eruptions (e.g. Nabro 2011) will be reduced by 75% in a high-end warming scenario compared to today, a consequence of future tropopause height rise and unchanged eruptive column height. In contrast, global-mean radiative forcing, stratospheric warming and surface cooling from infrequent large-magnitude tropical eruptions (e.g. Mt. Pinatubo 1991) will be exacerbated by 30%, 52 and 15% in the future, respectively. These changes are driven by an aerosol size decrease, mainly caused by the acceleration of the Brewer-Dobson circulation, and an increase in eruptive column height. Quantifying changes in both eruptive column dynamics and aerosol lifecycle is therefore key to assessing the climate response to future eruptions. How climate change influences the lifecycle of stratospheric volcanic aerosols and the associated radiative forcing is unknown. Here, the authors present model experiments suggesting that climate change amplifies the forcing of large-magnitude tropical eruptions but reduces the forcing of moderate-magnitude tropical eruptions.

Standard climate projections represent future volcanic eruptions by a constant forcing inferred from 1850 to 2014 volcanic forcing. Using the latest ice‐core and satellite records to design stochastic eruption scenarios, we show that there is a 95% probability that explosive eruptions could emit more sulfur dioxide (SO2) into the stratosphere over 2015–2100 than current standard climate projections (i.e., ScenarioMIP). Our simulations using the UK Earth System Model with interactive stratospheric aerosols show that for a median future eruption scenario, the 2015–2100 average global‐mean stratospheric aerosol optical depth (SAOD) is double that used in ScenarioMIP, with small‐magnitude eruptions (<3 Tg of SO2) contributing 50% to SAOD perturbations. We show that volcanic effects on large‐scale climate indicators, including global surface temperature, sea level and sea ice extent, are underestimated in ScenarioMIP because current climate projections do not fully account for the recurrent frequency of volcanic eruptions of different magnitudes. Plain Language Summary Climate projections are the simulations of Earth's climate in the future using complex climate models. Standard climate projections, as in Intergovernmental Panel on Climate Change Sixth Assessment Report, assume that explosive volcanic activity over 2015–2100 are of the same level as the 1850–2014 period. Using the latest ice‐core and satellite records, we find that explosive eruptions could emit more sulfur dioxide into the upper atmosphere for the period of 2015–2100 than standard climate projections. Our climate model simulations show that the impacts of volcanic eruptions on climate, including global surface temperature, sea level and sea ice extent, are underestimated because current climate projections do not fully account for the recurrent frequency of volcanic eruptions. We also find that small‐magnitude eruptions occur frequently and can contribute a significant effect on future climate. Key Points There is a 95% chance that the time‐averaged 2015–2100 volcanic SO2 flux from explosive eruptions exceeds the time‐averaged 1850–2014 flux Standard climate projections very likely underestimate the 2015–2100 stratospheric aerosol optical depth and volcanic climate effects Small‐magnitude eruptions (<3 Tg SO2) contribute 30%–50% of the volcanic climate effects in a median future eruption scenario

Rapid and simple estimation of the mass eruption rate (MER) from column height is essential for real‐time volcanic hazard management and reconstruction of past explosive eruptions. Using 134 eruptive events from the new Independent Volcanic Eruption Source Parameter Archive (IVESPA, v1.0), we explore empirical MER‐height relationships for four measures of column height: spreading level, sulfur dioxide height, and top height from direct observations and as reconstructed from deposits. These relationships show significant differences and highlight limitations of empirical models currently used in operational and research applications. The roles of atmospheric stratification, wind, and humidity remain challenging to detect across the wide range of eruptive conditions spanned in IVESPA, ultimately resulting in empirical relationships outperforming analytical models that account for atmospheric conditions. This finding highlights challenges in constraining the MER‐height relation using heterogeneous observations and empirical models, which reinforces the need for improved eruption source parameter data sets and physics‐based models. Plain Language Summary Explosive volcanic eruptions expel gas and tephra in the form of a volcanic column (or plume) that rises into the atmosphere. Two important metrics characterizing these eruptions are the maximum rise height and the eruptive intensity, that is, the rate at which material is emitted from the eruptive vent. Understanding the relationship between these parameters is critical for reconstructing past volcanic events and managing hazards during volcanic crises. In this study, we use a new database of well‐characterized eruptions to constrain simple relationships between column height and eruptive intensity. We distinguish four different measurements of column height: the maximum height reached by tephra from observations and from analysis of deposits, the height at which ash spreads in the atmosphere, and the height reached by volcanic sulfur gases. We show that each height category has a distinct relationship with the eruption intensity, enabling volcanologists and risk managers to use the relationship most appropriate to the measurements available to them. Despite the improved level of detail, our data set cannot resolve any systematic influence of atmospheric conditions such as wind and humidity on eruption column height, highlighting difficulties in measuring volcanic eruption characteristics and understanding their dynamics. Key Points We provide empirical scaling relationships between mass eruption rate (MER) and column height using a new database with 134 volcanic events We constrain bespoke relationships and their uncertainties for four height metrics to support ash dispersion forecasters and researchers We detect no clear atmospheric influence on scaling relationships, highlighting required improvements of scaling models and the database

The evolution of volcanic sulfur and the resulting radiative forcing following explosive volcanic eruptions is well understood. Petrological evidence suggests that significant amounts of halogens may be co-emitted alongside sulfur in some explosive volcanic eruptions, and satellite evidence indicates that detectable amounts of these halogens may reach the stratosphere. In this study, we utilise an aerosol–chemistry–climate model to simulate stratospheric volcanic eruption emission scenarios of two sizes, both with and without co-emission of volcanic halogens, in order to understand how co-emitted halogens may alter the life cycle of volcanic sulfur, stratospheric chemistry, and the resulting radiative forcing. We simulate a large (10 Tg of SO2) and very large (56 Tg of SO2) sulfur-only eruption scenario and a corresponding large (10 Tg SO2, 1.5 Tg HCl, 0.0086 Tg HBr) and very large (56 Tg SO2, 15 Tg HCl, 0.086 Tg HBr) co-emission eruption scenario. The eruption scenarios simulated in this work are hypothetical, but they are comparable to Volcanic Explosivity Index (VEI) 6 (e.g. 1991 Mt Pinatubo) and VEI 7 (e.g. 1257 Mt Samalas) eruptions, representing 1-in-50–100-year and 1-in-500–1000-year events, respectively, with plausible amounts of co-emitted halogens based on satellite observations and volcanic plume modelling. We show that co-emission of volcanic halogens and sulfur into the stratosphere increases the volcanic effective radiative forcing (ERF) by 24 % and 30 % in large and very large co-emission scenarios compared to sulfur-only emission. This is caused by an increase in both the forcing from volcanic aerosol–radiation interactions (ERFari) and composition of the stratosphere (ERFclear,clean). Volcanic halogens catalyse the destruction of stratospheric ozone, which results in significant stratospheric cooling, offsetting the aerosol heating simulated in sulfur-only scenarios and resulting in net stratospheric cooling. The ozone-induced stratospheric cooling prevents aerosol self-lofting and keeps the volcanic aerosol lower in the stratosphere with a shorter lifetime. This results in reduced growth by condensation and coagulation and a smaller peak global-mean effective radius compared to sulfur-only simulations. The smaller effective radius found in both co-emission scenarios is closer to the peak scattering efficiency radius of sulfate aerosol, and thus co-emission of halogens results in larger peak global-mean ERFari (6 % and 8 %). Co-emission of volcanic halogens results in significant stratospheric ozone, methane, and water vapour reductions, resulting in significant increases in peak global-mean ERFclear,clean (> 100 %), predominantly due to ozone loss. The dramatic global-mean ozone depletion simulated in large (22 %) and very large (57 %) co-emission scenarios would result in very high levels of UV exposure on the Earth's surface, with important implications for society and the biosphere. This work shows for the first time that co-emission of plausible amounts of volcanic halogens can amplify the volcanic ERF in simulations of explosive eruptions. It highlights the need to include volcanic halogen emissions when simulating the climate impacts of past or future eruptions, as well as the necessity to maintain space-borne observations of stratospheric compounds to better constrain the stratospheric injection estimates of volcanic eruptions.

​​​​​​​The 21 June 2019 Raikoke eruption (48° N, 153° E) generated one of the largest amounts of sulfur emission to the stratosphere since the 1991 Mt. Pinatubo eruption. Satellite measurements indicate a consensus best estimate of 1.5 Tg for the sulfur dioxide (SO2) injected at an altitude of around 14–15 km. The peak Northern Hemisphere (NH) mean 525 nm stratospheric aerosol optical depth (SAOD) increased to 0.025, a factor of 3 higher than background levels. The Volcano Response (VolRes) initiative provided a platform for the community to share information about this eruption which significantly enhanced coordination efforts in the days after the eruption. A multi-platform satellite observation subgroup formed to prepare an initial report to present eruption parameters including SO2 emissions and their vertical distribution for the modeling community. It allowed us to make the first estimate of what would be the peak in SAOD 1 week after the eruption using a simple volcanic aerosol model. In this retrospective analysis, we show that revised volcanic SO2 injection profiles yield a higher peak injection of the SO2 mass. This highlights difficulties in accurately representing the vertical distribution for moderate SO2 explosive eruptions in the lowermost stratosphere due to limited vertical sensitivity of the current satellite sensors (±2 km accuracy) and low horizontal resolution of lidar observations. We also show that the SO2 lifetime initially assumed in the simple aerosol model was overestimated by 66 %, pointing to challenges for simple models to capture how the life cycle of volcanic gases and aerosols depends on the SO2 injection magnitude, latitude, and height. Using a revised injection profile, modeling results indicate a peak NH monthly mean SAOD at 525 nm of 0.024, in excellent agreement with observations, associated with a global monthly mean radiative forcing of −0.17 W m−2 resulting in an annual global mean surface temperature anomaly of −0.028 K. Given the relatively small magnitude of the forcing, it is unlikely that the surface response can be dissociated from surface temperature variability.

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 Chichón (1982) and Mount Pinatubo (1991), we identify some differences in the magnitude and latitudinal structure, particularly for the eruptions of Mount Agung and El Chichón. 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 Chichón. 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.

Most climate projections represent volcanic eruptions as a prescribed constant forcing based on a historical average, which prevents a full quantification of uncertainties in climate projections. Here we show that the contribution of volcanic forcing uncertainty to the overall uncertainty in global mean surface air temperature projections reaches up to 49% in 2029, and is comparable or greater than that from internal variability throughout the 21st century. Furthermore, compared to a constant volcanic forcing, employing a stochastic volcanic forcing reduces the probability of exceeding 1.5 °C warming above pre-industrial level by at least 5% for high climate mitigation scenario, and enhances the probability of negative decadal temperature trends by up to 8%. Intermediate to high climate mitigation scenarios are particularly sensitive to the choice of future volcanic forcing implementation. We recommend the use of either a stochastic approach or prescribed constant forcing levels that sample volcanic uncertainty in future climate simulations. While volcanic forcing uncertainty is typically ignored in climate projections, it can represent up to 49% of the total climate uncertainty and systematically exceed internal variability uncertainty, according to simulations with stochastic forcing.

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.

The 852/3 CE eruption of Mount Churchill, Alaska, was one of the largest first-millennium volcanic events, with a magnitude of 6.7 (VEI 6) and a tephra volume of 39.4–61.9 km3 (95 % confidence). The spatial extent of the ash fallout from this event is considerable and the cryptotephra (White River Ash east; WRAe) extends as far as Finland and Poland. Proximal ecosystem and societal disturbances have been linked with this eruption; however, wider eruption impacts on climate and society are unknown. Greenland ice core records show that the eruption occurred in winter 852/3 ± 1 CE and that the eruption is associated with a relatively moderate sulfate aerosol loading but large abundances of volcanic ash and chlorine. Here we assess the potential broader impact of this eruption using palaeoenvironmental reconstructions, historical records and climate model simulations. We also use the fortuitous timing of the 852/3 CE Churchill eruption and its extensively widespread tephra deposition of the White River Ash (east) (WRAe) to examine the climatic expression of the warm Medieval Climate Anomaly period (MCA; ca. 950–1250 CE) from precisely linked peatlands in the North Atlantic region. The reconstructed climate forcing potential of the 852/3 CE Churchill eruption is moderate compared with the eruption magnitude, but tree-ring-inferred temperatures report a significant atmospheric cooling of 0.8 ∘C in summer 853 CE. Modelled climate scenarios also show a cooling in 853 CE, although the average magnitude of cooling is smaller (0.3 ∘C). The simulated spatial patterns of cooling are generally similar to those generated using the tree-ring-inferred temperature reconstructions. Tree-ring-inferred cooling begins prior to the date of the eruption suggesting that natural internal climate variability may have increased the climate system's susceptibility to further cooling. The magnitude of the reconstructed cooling could also suggest that the climate forcing potential of this eruption may be underestimated, thereby highlighting the need for greater insight into, and consideration of, the role of halogens and volcanic ash when estimating eruption climate forcing potential. Precise comparisons of palaeoenvironmental records from peatlands across North America and Europe, facilitated by the presence of the WRAe isochron, reveal no consistent MCA signal. These findings contribute to the growing body of evidence that characterises the MCA hydroclimate as time-transgressive and heterogeneous rather than a well-defined climatic period. The presence of the WRAe isochron also demonstrates that no long-term (multidecadal) climatic or societal impacts from the 852/3 CE Churchill eruption were identified beyond areas proximal to the eruption. Historical evidence in Europe for subsistence crises demonstrate a degree of temporal correspondence on interannual timescales, but similar events were reported outside of the eruption period and were common in the 9th century. The 852/3 CE Churchill eruption exemplifies the difficulties of identifying and confirming volcanic impacts for a single eruption, even when the eruption has a small age uncertainty.