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The Arctic is warming at almost four times the global rate. An estimated sixty percent of greenhouse‐gas‐induced Arctic warming has been offset by anthropogenic aerosols, but the contribution of aerosols to radiative forcing (RF) represents the largest uncertainty in estimating total RF, largely due to unknown preindustrial aerosol abundance. Here, sulfur isotope measurements in a Greenland ice core show that passive volcanic degassing contributes up to 66 ± 10% of preindustrial ice core sulfate in years without major eruptions. A state‐of‐the‐art model indicates passive volcanic sulfur emissions influencing the Arctic are underestimated by up to a factor of three, possibly because many volcanic inventories do not include hydrogen sulfide emissions. Higher preindustrial volcanic sulfur emissions reduce modeled anthropogenic Arctic aerosol cooling by up to a factor of two (+0.11 to +0.29 W m−2), suggesting that underestimating passive volcanic sulfur emissions has significant implications for anthropogenic‐induced Arctic climate change. Plain Language Summary: Sulfate aerosols are particles in the atmosphere that have a net cooling effect on the climate. One of the most uncertain aspects of climate modeling is the abundance of sulfate aerosols during the preindustrial era. Without knowing the amount of sulfate aerosols during the preindustrial, it is difficult to estimate how much anthropogenic sulfate aerosols have offset warming from anthropogenic greenhouse gases. In this study, we examine preindustrial sulfate aerosols in a Greenland ice core. We find that sulfate aerosols from passive (i.e., non‐eruptive) volcanic degassing contribute almost two thirds of preindustrial Arctic sulfate aerosols in years without major volcanic eruptions. We compare this result to a state‐of‐the‐art global model and find that most climate models use a volcanic emissions inventory that underestimates preindustrial passive volcanic sulfur emissions. That volcanic inventory only includes one type of sulfur emission (sulfur dioxide), but studies have shown that volcanoes emit hydrogen sulfide, which can also form sulfate aerosols. We show that higher emissions of volcanic sulfur during the preindustrial era decrease the estimated cooling effect of anthropogenic aerosols during the industrial era. Thus, the underestimate of preindustrial volcanic emissions in current climate models has significant implications for anthropogenic climate change in the Arctic. Key Points: Sulfur isotopes in a Greenland ice core show that passive volcanic degassing contributes 66% of preindustrial Arctic sulfateThe volcanic inventory used by most climate models underestimates passive degassing, possibly due to missing hydrogen sulfide emissionsElevated preindustrial passive volcanic degassing reduces the estimated cooling effect of anthropogenic sulfate in the Arctic [ABSTRACT FROM AUTHOR]

An important aspect of volcanic hazard assessment is determination of the level and character of background activity at a volcano so that deviations from background (called unrest) can be identified. Here, we compile the instrumentally recorded eruptive and noneruptive activity for 161 US volcanoes between 1978 and 2020. We combine monitoring data from four techniques: seismicity, ground deformation, degassing, and thermal emissions. To previous work, we add the first comprehensive survey of US volcanoes using medium‐spatial resolution satellite thermal observations, newly available field surveys of degassing, and new compilations of seismic and deformation data. We report previously undocumented thermal activity at 30 volcanoes using data from the spaceborne ASTER sensor during 2000–2020. To facilitate comparison of activity levels for all US volcanoes, we assign a numerical classification of the Activity Intensity Level for each monitoring technique, with the highest ranking corresponding to an eruption. There are 96 US volcanoes (59%) with at least one type of detected activity, but this represents a lower bound: For example, there are 12 volcanoes where degassing has been observed but has not yet been quantified. We identify dozens of volcanoes where volcanic activity is only measured by satellite (45% of all thermal observations), and other volcanoes where only ground‐based sensors have detected activity (e.g., all seismic and 62% of measured degassing observations). Our compilation provides a baseline against which future measurements can be compared, demonstrates the need for both ground‐based and remote observations, and serves as a guide for prioritizing future monitoring efforts. Plain Language Summary: We have compiled the instrumentally recorded eruptive and noneruptive activity in terms of earthquakes, ground deformation, degassing, and thermal emissions for 161 US volcanoes between 1978 and 2020. There are 96 US volcanoes (59%) with at least one type of detected activity. But we think that more than 96 volcanoes had activity during this time period because of the limits in the data available. We report previously undocumented thermal activity at 30 volcanoes using data from the spaceborne ASTER sensor measured in the Thermal Infrared during 2000–2020. We identify dozens of volcanoes where volcanic activity is only measured by satellite (45% of all thermal observations), and other volcanoes where only ground‐based sensors have detected activity (e.g., all seismic and 62% of measured degassing observations). Our compilation provides a baseline against which future measurements can be compared, demonstrates the need for both ground‐based and remote observations, and serves as a guide for prioritizing future monitoring efforts. Key Points: In the United States, 96 of 161 volcanoes have at least one type of detected activity (seismicity, deformation, and gas or thermal emissions)Forty‐five percent of volcanoes with thermal emissions are only seen by medium‐spatial resolution satellites (<100 m/pixel)Each volcano has an Activity Intensity Level; a higher score from multiple data types indicates a greater likelihood of magmatic activity [ABSTRACT FROM AUTHOR]

Volcanoes are hazardous to local and global populations, but only a fraction are continuously monitored by ground‐based sensors. For example, in Latin America, more than 60% of Holocene volcanoes are unmonitored, meaning long‐term multiparameter data sets of volcanic activity are rare and sparse. We use satellite observations of degassing, thermal anomalies, and surface deformation spanning 17 years at 47 of the most active volcanoes in Latin America and compare these data sets to ground‐based observations archived by the Global Volcanism Program. This first comparison of multisatellite time series on a regional scale provides information regarding volcanic behavior during, noneruptive, pre‐eruptive, syneruptive, and posteruptive periods. For example, at Copahue volcano, deviations from background activity in all three types of satellite measurements were manifested months to years in advance of renewed eruptive activity in 2012. By quantifying the amount of degassing, thermal output, and deformation measured at each of these volcanoes, we test the classification of these volcanoes as open or closed volcanic systems. We find that ~28% of the volcanoes do not fall into either classification, and the rest show elements of both, demonstrating a dynamic range of behavior that can change over time. Finally, we recommend how volcano monitoring could be improved through better coordination of available satellite‐based capabilities and new instruments. Key Points: Seventeen years of degassing, thermal, deformation, and Global Volcanism Program ground‐based data are analyzed for the 47 most active volcanoes in Latin AmericaThis study demonstrates the advantages of using a multiparameter approach for monitoring volcanoes while demonstrating the need for greater coordinationThe data are used to test the open and closed volcanic classification scheme, demonstrating the need for additional classifications [ABSTRACT FROM AUTHOR]

Volcanic SO2 column amount and injection height retrieved from the Ozone Monitoring Instrument (OMI) with the Extended Iterative Spectral Fitting (EISF) technique are used to initialize a global chemistry transport model (GEOSChem) to simulate the atmospheric transport and lifecycle of volcanic SO2 and sulfate aerosol from the 2008 Kasatochi eruption, and to subsequently estimate the direct shortwave, top-of-the-atmosphere radiative forcing of the volcanic sulfate aerosol. Analysis shows that the integrated use of OMI SO2 plume height in GEOS-Chem yields: (a) good agreement of the temporal evolution of 3-D volcanic sulfate distributions between model simulations and satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP), and (b) an e-folding time for volcanic SO2 that is consistent with OMI measurements, re- flecting SO2 oxidation in the upper troposphere and stratosphere is reliably represented in the model. However, a consistent (~25 %) low bias is found in the GEOS-Chem simulated SO2 burden, and is likely due to a high (~20 %) bias of cloud liquid water amount (as compared to the MODIS cloud product) and the resultant stronger SO2 oxidation in the GEOS meteorological data during the first week after eruption when part of SO2 underwent aqueous-phase oxidation in clouds. Radiative transfer calculations show that the forcing by Kasatochi volcanic sulfate aerosol becomes negligible 6 months after the eruption, but its global average over the first month is -1.3 Wm-2, with the majority of the forcing-influenced region located north of 20° N, and with daily peak values up to -2 Wm-2 on days 16-17. Sensitivity experiments show that every 2 km decrease of SO2 injection height in the GEOS-Chem simulations will result in a ~25% decrease in volcanic sulfate forcing; similar sensitivity but opposite sign also holds for a 0.03 µm increase of geometric radius of the volcanic aerosol particles. Both sensitivities highlight the need to characterize the SO2 plume height and aerosol particle size from space. While more research efforts are warranted, this study is among the first to assimilate both satellite-based SO2 plume height and amount into a chemical transport model for an improved simulation of volcanic SO2 and sulfate transport. [ABSTRACT FROM AUTHOR]

Volcanic SO2 column amount and injection height retrieved from the Ozone Monitoring Instrument (OMI) with the Extended Iterative Spectral Fitting (EISF) technique are used to initialize a global chemistry transport model (GEOS-Chem) to simulate the atmospheric transport and lifecycle of volcanic SO2 and sulfate aerosol from the 2008 Kasatochi eruption, and to subsequently estimate the direct shortwave, top-of-the- atmosphere radiative forcing of the volcanic sulfate aerosol. Analysis shows that the integrated use of OMI SO2 plume height in GEOS-Chem yields: (a) good agreement of the temporal evolution of 3-D volcanic sulfate distributions between model simulations and satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP), and (b) a e-folding time for volcanic SO2 that is consistent with OMI measurements, reflecting SO2 oxidation in the upper troposphere and stratosphere is reliably represented in the model However, a consistent (~25%) low bias is found in the GEOS-Chem simulated SO2 burden, and is likely due to a high (~20%) bias of cloud liquid water amount (as compared to the MODIS cloud product) and the resultant stronger SO2 oxidation in the GEOS meteorological data during the first week after eruption when part of SO2 underwent aqueous-phase oxidation in clouds. Radiative transfer calculations show that the forcing by Kasatochi volcanic sulfate aerosol becomes negligible 6months after the eruption, but its global average over the first month is -1.3Wm-2 with the majority of the forcing-influenced region located north of 20° N, and with daily peak values up to -2Wm-2 on days 16-17. Sensitivity experiments show that every 2 km decrease of SO2 injection height in the GEOS-Chem simulations will result in a ~25% decrease in volcanic sulfate forcing; similar sensitivity but opposite sign also holds for a 0.03 µm increase of geometric radius of the volcanic aerosol particles. Both sensitivities highlight the need to characterize the SO2 plume height and aerosol particle size from space. While more research efforts are warranted, this study is among the first to assimilate both satellite-based SO2 plume height and amount into a chemical transport model for an improved simulation of volcanic SO2 and sulfate transport. [ABSTRACT FROM AUTHOR]

The article focuses on an opportunistic validation of Ozone Monitor Instrument (OMI) sulfur dioxide retrievals from Sarychev Peak's stratospheric volcanic cloud in Kurile Islands, Russia. The validation was made through opportunistic deployment of a ground-based ultraviolet (UV) spectrometer as the volcanic cloud moving over central Alaska. Such effort demonstrates the need for a network of rapidly deployable instruments for validating space-based volcanic sulfur dioxide measurements.

The sulfur dioxide (SO2) output from Nyamuragira volcano has been monitored by the Total Ozone Mapping Spectrometer (TOMS) since 1979, and is evaluated here to quantify the emissions from this highly productive volcano. The majority of Nyamuragira's emissions were emplaced in the lower to middle troposphere, with SO2 removal rates of 30-90% per day (k = 4.13×10-6 to 2.66×10-5 s-1). We have tested a new method of back-calculating persistent, effusive emission fluxes from once-daily observations, which accounts for this rapid daily removal of SO2 that cannot be measured using satellite 'snapshots'. Twelve of the 14 eruptions during this period each produced ≥0.8 teragrams (Tg) of SO2. Nyamuragira erupted nearly 25 Tg of SO2 during these eruptions, and probably emitted significantly more than we could measure by TOMS. Nyamuragira may be the largest volcanic source of sulfur to the atmosphere for the past few decades. [ABSTRACT FROM AUTHOR]

Volcanic eruptions emit gases, ash particles and hydrometeors into the atmosphere, occasionally reaching heights of 20 km or more, to reside in the stratospheric over-world where they affect the radiative balance of the atmosphere and the Earth's climate. Here we use satellite measurements and a Lagrangian particle dispersion model to determine the mass loadings, vertical penetration, horizontal extent, dispersion and transport of volcanic gases and particles in the stratosphere from the volcanic cloud emitted during the 20 May 2006 eruption of Soufrière Hills volcano, Montserrat, West Indies. Infrared, ultraviolet and microwave radiation measurements from two polar orbiters are used to quantify the gases and particles, and track the movement of the cloud for 23 days, over a distance of ~18 000 km. Approximately, 0.1±0.01 Tg(S) was injected into the stratosphere in the form of SO2: the largest single sulphur input to the stratosphere in 2006. Microwave Limb Sounder measurements indicate an enhanced mass of HCl of ~0.003-0.01 Tg. Geosynchronous satellite data reveal the rapid nature of the stratospheric injection and indicate that the eruption cloud contained ~2 Tg of ice, with very little ash reaching the stratosphere. These new satellite measurements of volcanic gases and particles can be used to test the sensitivity of climate to volcanic forcing and assess the impact of stratospheric sulphates on climate cooling. [ABSTRACT FROM AUTHOR]

Sulfate aerosol produced after injection of sulfur dioxide (SO2) into the stratosphere by volcanic eruptions can trigger climate change. We present new satellite data from the Ozone Monitoring Instrument (OMI) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) missions that reveal the composition, structure and longevity of a stratospheric SO2 cloud and derived sulfate layer following a modest eruption (0.2 Tg total SO2) of Soufriere Hills volcano, Montserrat on 20 May 2006. The SO2 cloud alone was tracked for over 3 weeks and a distance of over 20 000 km; unprecedented for an eruption of this size. Derived sulfate aerosol at an altitude of ∼20 km had circled the globe by 22 June and remained visible in CALIPSO data until at least 6 July. These synergistic NASA A-Train observations permit a new appreciation of the potential effects of frequent, small-to-moderate volcanic eruptions on stratospheric composition and climate. [ABSTRACT FROM AUTHOR]

The Internet now harbours vast amounts of cheap and potentially useful remote sensing data. Advanced Very High Resolution Radiometer (AVHRR) data are being increasingly used for volcano surveillance, and the provision of AVHRR Global Area Coverage (GAC) imagery at no cost over the Internet offers the possibility of cheap volcano monitoring on a global scale. Herein we use an extensive, 690-scene AVHRR GAC dataset to observe volcanic activity in the Indonesian island arc between January 1996 and November 1997. Indonesia contains over 70 active volcanoes, with styles of activity during the observation period including active lava domes, lava flows, pyroclastic flows and hot crater lakes, many in close proximity to major centres of population. The detection potential of these and other phenomena in GAC data is assessed. Thermal anomalies were identified at ~18 volcanoes during the observation period, including lava flows at Anak Krakatau, persistent open-vent activity at Semeru and a previously unreported eruption at Sangeang Api volcano. Using these results, a classification scheme for night-time Indonesian GAC data is presented. Routine use of freely available high temporal resolution data such as AVHRR GAC could help elucidate cyclic activity at active volcanoes, which would contribute significantly to hazard mitigation in affected areas. Browse images of higher resolution data (e.g. SPOT) from the daily updated archives of the Centre for Remote Imaging, Sensing and Processing (CRISP) in Singapore also show potential as an aid to volcano monitoring in the region. [ABSTRACT FROM AUTHOR]

Volcanic sulfur dioxide (SO2) emissions have been measured by ultraviolet sensors on polar‐orbiting satellites for several decades but with limited temporal resolution. This precludes studies of key processes believed to occur in young (~1–3 hr old) volcanic clouds. In 2015, the launch of the Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory (DSCOVR) provided an opportunity for novel observations of volcanic eruption clouds from the first Earth‐Sun Lagrange point (L1). The L1 vantage point provides continuous observations of the sunlit Earth, offering up to eight or nine observations of volcanic SO2 clouds in the DSCOVR/EPIC field of view at ~1‐hr intervals. Here we demonstrate DSCOVR/EPIC's sensitivity to volcanic SO2 using several volcanic eruptions from the tropics to midlatitudes. The hourly cadence of DSCOVR/EPIC observations permits more timely measurements of volcanic SO2 emissions, improved trajectory modeling, and novel analyses of the temporal evolution of volcanic clouds. Plain Language Summary: Satellite measurements of sulfur dioxide (SO2) and ash emissions by volcanic eruptions are crucial for assessment of volcanic impacts on climate and mitigation of hazards to aviation. Until recently, the vast majority of such observations were made using satellites in low‐Earth (or polar) orbit at altitudes of ~700–800 km, which only provide one measurement per day at most latitudes. This precludes studies of dynamic processes in volcanic clouds, which could radically alter their composition and potential impact. Here we report the first measurements of volcanic SO2 emissions from an entirely new perspective: the Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory, located at the first Earth‐Sun Lagrange point (L1), 1.6 million kilometers from Earth. From L1, EPIC views the sunlit Earth continuously as it rotates and can measure volcanic SO2 hourly from sunrise to sunset, as we demonstrate using several recent volcanic eruptions as examples. EPIC measurements allow us to detect volcanic eruptions sooner, and track their emissions for longer, than was previously possible with a single sensor. Our paper thus demonstrates a new Earth observation paradigm that could revolutionize studies of volcanic cloud chemistry and impacts and potentially reduce the societal impacts of volcanic eruptions. Key Points: Volcanic eruption clouds can be detected and tracked with hourly temporal cadence from L1 orbitThe hourly cadence of EPIC volcanic SO2 observations can be used to attribute gas emissions to specific events during multiphase eruptionsObservations of transient variations in SO2 loading will provide more constraints on processes such as H2S oxidation in volcanic clouds [ABSTRACT FROM AUTHOR]

The global flux of sulfur dioxide (SO2) emitted by passive volcanic degassing is a key parameter that constrains the fluxes of other volcanic gases (including carbon dioxide, CO2) and toxic trace metals (e.g., mercury). It is also a required input for atmospheric chemistry and climate models, since it impacts the tropospheric burden of sulfate aerosol, a major climate-forcing species. Despite its significance, an inventory of passive volcanic degassing is very difficult to produce, due largely to the patchy spatial and temporal coverage of ground-based SO2 measurements. We report here the first volcanic SO2 emissions inventory derived from global, coincident satellite measurements, made by the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite in 2005-2015. The OMI measurements permit estimation of SO2 emissions from over 90 volcanoes, including new constraints on fluxes from Indonesia, Papua New Guinea, the Aleutian Islands, the Kuril Islands and Kamchatka. On average over the past decade, the volcanic SO2 sources consistently detected from space have discharged a total of ~63 kt/day SO2 during passive degassing, or ~23 ± 2 Tg/yr. We find that ~30% of the sources show significant decadal trends in SO2 emissions, with positive trends observed at multiple volcanoes in some regions including Vanuatu, southern Japan, Peru and Chile. [ABSTRACT FROM AUTHOR]

The article shows that aerial measurements of volcanic gases using unoccupied aerial systems (UAS) transform our ability to measure and monitor plumes remotely and to constrain global volatile fluxes from volcanoes. It emphasizes the need to account for time averaging of temporal variability in volcanic gas emissions in global flux estimates.