Introduction

Accurate inventories of the current spatial and temporal distribution of volcanic gas emissions to the atmosphere are required for numerous applications, ranging from baseline volcano monitoring to assessment of the impacts of volcanic degassing on the broader Earth system1. Sulfur species, principally sulfur dioxide (SO2), are of most interest due to the ease of SO2 measurement via ground- and satellite-based remote sensing2,3 and their key role in the processes responsible for volcanic impacts on the environment, health, atmospheric chemistry and climate4,5,6,7. Recent advances in satellite remote sensing techniques have greatly improved constraints on the eruptive flux of SO2 (and several other volatile species) from volcanoes3,8,9,10, but the non-eruptive or passive volcanic degassing flux of SO2 (hereafter, PVF) remains poorly constrained. In addition to its relevance for impact assessment, an accurate global volcanic SO2 emissions inventory permits estimation of the volcanic output of other climate-relevant gas species and toxic trace metals (e.g., CO2 and mercury11,12), and the identification of potential targets for ground-based gas sampling to measure the complete chemical and isotopic composition of volcanic gases. The most widely used existing volcanic SO2 emissions inventory13 is now several decades old, but its enduring popularity reflects the high demand for global volcanic SO2 flux data.

Producing a database that faithfully reflects the contemporaneous PVF is a challenge due to the generally poor temporal and spatial coverage of ground-based volcanic gas measurements, which are often conducted on a campaign-style basis and/or during periods of heightened unrest14,15,16 and are hence unlikely to accurately represent long-term average degassing rates. Although the geographic extent and frequency of ground-based volcanic SO2 measurements is increasing17, they remain sparse in many highly active volcanic regions such as Indonesia, Papua New Guinea, Vanuatu, the Aleutian Islands, the Kuril Islands and Kamchatka, and will be a formidable challenge in some very remote regions (e.g., the South Sandwich Islands, southern Atlantic Ocean). As a solution to this problem, we report here a new satellite-based volcanic SO2 emissions inventory, based on more than a decade of measurements by the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite18, which is global in scope and provides estimates of the PVF from all of the strongest contemporary volcanic SO2 sources. This new database, the first volcanic SO2 emissions inventory to be derived from global, coincident measurements (rather than by collation of ground-based data widely distributed in space and time), benefits from several advantages of polar-orbiting satellite measurements, including global coverage and the use of a single, well-characterized sensor to detect and quantify all SO2 sources over the course of a long-term (multi-decadal) satellite mission. The use of a single instrument permits relatively straightforward reprocessing of archived data as SO2 retrieval algorithms improve, offering increasing sensitivity to volcanic SO219. Furthermore, unlike many spectroscopic instruments used for ground-based SO2 measurements, satellite instruments such as OMI are also subject to intensive calibration and validation20.

Although satellites have been used to measure eruptive SO2 emissions for several decades3,8,21,22, their use for quantification of passive volcanic degassing is relatively recent and concurrent with the advent of sufficiently sensitive space-borne instruments, such as OMI23,24,25. Previous application of OMI SO2 data to detection of non-eruptive volcanic degassing has focused on the stronger SO2 sources, detectable from space on a near-daily basis3,16,25. As recently demonstrated26,27,28,29,30, with specialized data processing techniques it is possible to enhance the sensitivity of ultraviolet (UV) satellite SO2 measurements to enable detection of persistent anthropogenic SO2 sources emitting on the order of 30 kilotons/year (kt/yr; equivalent to ~80 tons/day [t/d]), with the detection limit expected to be even lower for SO2 sources located at high elevation (including many volcanoes). Here, we present a new global volcanic SO2 emissions inventory derived from application of these techniques to more than a decade of OMI observations (2005–2015), which represents a timely replacement for existing databases13,31. We also compare the satellite-based SO2 fluxes to a recent compilation of independent ground-based measurements31, and other sources, and examine the global distribution of volcanic SO2 fluxes to reveal regional- and arc-scale trends in volcanic degassing.

Data and Methods

Volcanic SO2 emissions were estimated using a new operational OMI planetary boundary layer (PBL) SO2 column dataset produced using a principal component analysis (PCA) algorithm32. A detailed description of the techniques used to identify SO2 sources (both anthropogenic and volcanic) and calculate emissions is given in ref. 29, and is briefly summarized here. The OMI PCA SO2 data used in the analysis were restricted to ‘clear sky’ conditions by including only those OMI pixels with a cloud radiance fraction below 20%; solar zenith angles were also restricted to <70° to reduce noise at high latitudes. In addition, all pixels affected by the OMI row anomaly data gap since 2007 (see: http://www.knmi.nl/omi/research/product/rowanomaly-background.php) were excluded. After pixel screening, an OMI pixel averaging or oversampling procedure26,27 is used to resolve potential locations of SO2 emissions and produce global maps similar to those shown in Fig. 1. To further enhance the SO2 signal and identify sources, a wind rotation technique is applied to align all the OMI SO2 observations for each source along the same wind vector30, and then SO2 emissions are estimated by fitting an exponentially modified Gaussian function to the OMI data33. The variable altitude of passive volcanic SO2 plumes is accounted for by applying an air mass factor (AMF) correction to the OMI PBL SO2 columns based on volcano altitude. To calculate accurate estimates of the SO2 PVF, the effects of volcanic eruptions generating transient, large SO2 column amounts are removed by applying a threshold SO2 column amount of 5–15 Dobson Units (DU) to the OMI SO2 data. This threshold was selected based on typical SO2 column amounts measured by OMI in passive and eruptive volcanic plumes. However, we note that at some volcanoes it may be impossible to completely separate passive (i.e., involving no coincident eruption of magma) from eruptive SO2 emissions, or even to establish which mode of degassing dominates at any given time. This is particularly problematic at volcanoes undergoing lava dome extrusion (e.g., Merapi, Indonesia; Soufriere Hills, Montserrat) or persistent Vulcanian or Strombolian activity (e.g., Stromboli, Italy; Fuego, Guatemala; Sakura-jima, Japan; Yasur, Vanuatu). Hence, while we believe that passive SO2 degassing is the dominant process responsible for the emissions reported here, a contribution from eruptive degassing is inevitable at some volcanoes, as is the case for previous SO2 emissions inventories13,31. Total uncertainties (including contributions from AMF, SO2 mass, SO2 lifetime, and wind speed uncertainty) on annual SO2 flux estimates are 55% and >67% for sources emitting more than 100 kt/yr and under 50 kt/yr, respectively29. Some of the largest individual sources of error are systematic and hence will introduce a bias in absolute SO2 flux values but will not affect relative inter-annual flux variability (Fig. 2).

Figure 1
figure 1

Mean SO2 columns (in Dobson Units [DU]; 1 DU = 2.69 × 1016 molecules cm−2) for 2005–2007 over (a) the Aleutian Islands (USA) and (b) Indonesia. The volcanic SO2 sources (including paired sources) are labeled. The Aleutian map also shows locations of explosive eruptions since 2005 (red triangles), with symbol size proportional to total SO2 emission3,10. The Indonesian map also shows anthropogenic SO2 sources in Singapore and central Sulawesi, but does not show volcanic SO2 emissions from Sinabung, Rinjani and Sangeang Api, which first appeared after 2007. Maps were generated using Interactive Data Language (IDL) version 8.5.1 (http://www.harrisgeospatial.com/).

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Figure 2: OMI-derived annual mean SO2 fluxes in 2005–2015 for the ten strongest volcanic SO2 sources (including paired sources) in the inventory.
figure 2

Plots are titled with the volcanic source name and rank, and the trend (slope) and linear correlation coefficient (r) of an error-weighted linear regression fit of the annual mean SO2 fluxes. Each plot shows the annual mean SO2 fluxes (solid black line), mean SO2 flux in 2015 (labeled red dot), linear regression trend line (dashed orange line), decadal mean SO2 flux (horizontal red line), ±1 standard deviation of the decadal mean SO2 flux (gray band), and an independent estimate of SO2 flux (horizontal dashed blue line) from a recent compilation13 or another source. Here, SO2 flux data for Etna and Popocatepetl are from refs 58 and 59, respectively. If no independent measurements are available, the plot is labeled with ‘N/A’. See Supplementary Figures (Figs S9–S16) for similar plots for all other sources.

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For the inventory presented here, volcanic SO2 sources were identified based on 3-year averages of OMI data for 2005–2007, 2008–2010 and 2011–2014, then annual emissions were calculated for each source for the entire 11-year period studied (2005–2015). Note that the aforementioned 30 kt/yr (~80 t/d) detection limit was determined based on OMI observations of power plant SO2 emissions in the eastern USA28, which are typically confined to the PBL. The higher altitude of volcanic SO2 plumes translates into a higher AMF (greater sensitivity), which reduces the detection limit to values as low as ~6 kt/yr (~16 t/d). The detection limit will be lowest for low-latitude volcanoes, which benefit from more satellite observations under optimal conditions (e.g., low solar zenith angles). To assess the presence of significant decadal trends in the SO2 emissions, we applied a weighted linear regression fit to the annual SO2 emissions for each source, using the 1σ emission uncertainties (Supplementary Table S1) to weight the data, to derive a trend and linear correlation coefficient (r). Although it is possible to use satellite data to estimate SO2 fluxes on much shorter timescales for strong sources9,25, the focus here is on long-term average emissions and trends rather than short-term variations. Future updates to the volcanic SO2 emissions inventory will benefit from the recent release of new OMI PCA SO2 products tailored to the variable injection height of volcanic plumes19, which should further reduce the uncertainties.

Results and Discussion

A total of 91 persistently degassing volcanic SO2 sources have been detected in OMI measurements between 2005 and 2015 (Table 1; Supplementary Fig. S1). However, some of the detected SO2 signals originate from paired sources (see below), so the actual number of volcanoes contributing to the detected SO2 emissions is probably at least 100. For comparison, the Andres and Kasgnoc (1998) inventory13 includes 49 continuously emitting sources. Since 3-year averages of OMI SO2 data were used to identify the sources, the main criterion for detection is persistent emissions on that timescale. Hence it is possible that volcanoes exhibiting shorter-duration episodes of passive degassing may elude detection, but may subsequently be identified in more detailed analysis of shorter time periods. Table 1 lists the volcanic SO2 sources, ranked according to their mean SO2 flux for the entire 11-year period analyzed. Maps of the volcanic SO2 sources are shown in Fig. 1 and Supplementary Figs S1–S8. Figure 2 and Supplementary Figures (Supplementary Figs S9–S16) show the variation in annual mean SO2 fluxes at each source, with correlative ground-based SO2 measurements if available, and the trends and linear correlation coefficients (r) derived from a weighted linear regression of the annual emissions. The complete dataset, including annual emissions for each volcano, is provided in a Supplementary Table (Supplementary Table S1).

Table 1 Mean SO2 fluxes (2005–2015) for sources of passive volcanic SO2 degassing detected by OMI.
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One of the disadvantages of UV satellite measurements is low spatial resolution, and as a result SO2 emissions from clustered degassing volcanoes (within ~50 km) cannot be distinguished. Hence, some SO2 emissions in the inventory are attributed to paired sources (e.g., Fig. 2), such as Nyiragongo-Nyamuragira (DR Congo), Bromo-Semeru (East Java, Indonesia) and Batu Tara – Lewotolo (Lesser Sunda Islands, Indonesia). Kamchatka (Russia) is another region where assignment of SO2 emissions to specific volcanoes can be problematic (e.g., Mutnovsky – Gorely). Emissions reported for Chikurachki in the northern Kuril Islands may include a contribution from Ebeko (Table 1), where SO2 emissions of ~100 t/d have been reported34. Resolving these merged SO2 sources will require further field-based measurements in some regions, or the use of satellite data with higher spatial resolution35.

Notwithstanding some drawbacks, the strength of a satellite-derived emissions inventory is the global coverage. Most of the dominant sources (e.g., Ambrym, Kilauea, Bagana, Etna) are well established from prior measurements14,16,36,37. However, the OMI measurements (Table 1; Fig. 2) reveal, in some cases for the first time, significant, persistent SO2 degassing at remote volcanoes in the South Sandwich Islands (Michael and Montagu), the Kuriles (Ketoi, Kudriavy), the Aleutians (Gareloi, Korovin), Indonesia (e.g., Dukono, Batu Tara - Lewotolo, Sirung, Ebulobo), and the southwest Pacific (e.g., Tofua, Tinakula). Gas emissions from Erebus (Antarctica) are also detected from space for the first time (Table 1; Supplementary Fig. S16). The OMI database thus provides what is the first truly global picture of contemporary volcanic SO2 degassing, including sources where acquisition of frequent ground-based data will remain highly challenging.

Weighted linear regression reveals a range of temporal trends in the SO2 fluxes (Fig. 2; Supplementary Figs S9–S16). We acknowledge that a simple linear trend may not be applicable to many of the volcanic SO2 sources (indicated by a low correlation coefficient, −0.5 ≤ r ≤ 0.5; Fig. 2; Supplementary Figs S9–S16), but a detailed exploration of the trends in SO2 emissions at each volcano is beyond the scope of this study. Nevertheless, the SO2 data for some sources clearly indicate a long-term decline in SO2 discharge (e.g., Miyakejima, Manam, Soufriere Hills; Fig. 2, Supplementary Fig. S9). A weak or insignificant trend in SO2 emissions likely reflects relatively stable emissions (e.g., Bagana, Etna; Fig. 2), or more pulsatory degassing (e.g., Tavurvur, Anatahan, Huila; Fig. 2, Supplementary Figs S9 and S10); the latter could reflect cycles of magma intrusion followed by protracted gas release. Three of the top four sources feature active basaltic lava lakes (Ambrym, Kilauea and Nyiragongo-Nyamuragira), and in these cases the peak SO2 discharge can be clearly linked to the establishment of new and/or larger lava lakes (e.g., at Kilauea in 200838 and Nyamuragira in 201235). The significance of the observed trends in SO2 emissions is discussed further below.

In summing the SO2 emissions from all detected sources, we find that the total annual SO2 PVF is remarkably stable at 23.0 ± 2.3 Tg/yr (the highest annual total in the past decade was ~26 Tg in 2010). Andres and Kasgnoc (1998)13 estimated a total non-eruptive volcanic SO2 flux of ~12 Tg/yr for the 1970–1997 period (including a power-law extrapolation to estimate the contribution from unmeasured volcanoes); our higher estimate reflects the inclusion of more strong sources emitting >1000 t/d SO2 (Table 1). A comparison with eruptive SO2 fluxes3,10 confirms the common assumption that the SO2 PVF is typically around an order of magnitude larger (Fig. 3), except during years with major SO2-rich eruptions such as at Bárðarbunga-Holuhraun (Iceland) in 201439. The average total SO2 PVF from all detectable sources is ~63 kt/day (2005–2015 mean; Table 1), which is broadly commensurate with a global SO2 PVF of ~50.6 kt/day estimated by ref. 31 using a sparser dataset. Fluxes of SO2 during large eruptions (e.g., Holuhraun39) can greatly exceed the total PVF on short timescales.

Figure 3: Total, global eruptive and passive volcanic SO2 emissions (in Tg; 1 Tg = 1012 g) in 2004–2016.
figure 3

Eruptive emissions are derived from [10] and recent updates; passive emissions are from the inventory described here. Passive volcanic SO2 emissions in 2016 are assumed to continue at the mean annual rate observed in 2005–2015.

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The new volcanic SO2 emissions inventory includes numerous previously unquantified sources. Based on SO2 data reported in the literature (and we acknowledge that a substantial amount of SO2 emissions data collected by volcano observatories may not be published), we find that 36 of the 91 sources (i.e., ~40%) have no previously reported SO2 flux. The most prominent of these is Dukono (Halmahera, Indonesia), ranked 8th in our inventory (Table 1; Figs 1 and 2), but many of the stronger sources have relatively few SO2 flux determinations. Based on recent compilations31, 38 volcanoes (i.e., ~68% of the 56 volcanoes with prior measurements) have reported SO2 fluxes within the 1σ fitting uncertainty of the OMI-derived fluxes. For ~41% of the sources with prior measurements, the OMI-derived SO2 flux exceeds the independent estimate by at least 20%, and for ~36% the reverse is true (Fig. 2; Supplementary Figs S9–S16), whilst for the remainder (e.g., Ulawun, San Cristobal, Satsuma-Iwojima, Masaya, Fuego; Supplementary Figs S9–S16) the satellite- and ground-based SO2 emission rates show excellent agreement (to within 20%). Nonetheless, it is notable that the OMI-derived SO2 fluxes for most of the strongest sources are higher than previous estimates (Table 1; Fig. 2). For several sources (e.g., Ambrym, Bagana, Aoba, Manam) we believe that this is real and a result of infrequent prior measurements at these very active volcanoes coupled with significant variability in SO2 emissions. Furthermore, at Kilauea, where a significant discrepancy is observed (Fig. 2), it has recently been shown37 that ground-based techniques can underestimate SO2 emissions by a factor of 2 or more in dense plumes. However, with the exception of the high-flux volcanoes, we observe no significant high or low bias in the OMI-derived SO2 fluxes, but more detailed validation of the derived SO2 emissions is certainly required.

In addition to Dukono, the new database sheds considerable light on the SO2 flux from other Indonesian volcanoes (Fig. 1), which is noteworthy given the generally poor constraints on volcanic emissions in the archipelago40,41. Dukono (Halmahera) is the strongest volcanic SO2 source with no prior constraints on its SO2 flux (Figs 1 and 2). The SO2 signal in the Sunda Strait near Krakatau volcano (Fig. 1) was previously assigned to the Suralaya power plant in Cilegon, West Java29, but we now assume this to be dominated by volcanic emissions from Krakatau (Table 1). SO2 emissions of 190 ± 40 t/d were reported at Krakatau in 201442, well above the satellite detection limit, so if this is a sustained SO2 flux then it seems likely that most of the detected SO2 is volcanic. The OMI-derived SO2 flux for Krakatau is 303 ± 252 t/d (Table 1), i.e., within the range of ground-based measurements42. Degassing from Papandayan (West Java40) may also be detected in the OMI data (Fig. 1), although it is difficult to isolate from the larger SO2 signal associated with Slamet and hence is not treated as a separate source here. As noted earlier, several Indonesian volcanoes in East Java and the Lesser Sunda Islands are difficult to resolve using the OMI measurements, thus the reported emissions for Bromo and Semeru, Raung and Ijen, and Batu Tara and Lewotolo represent aggregated fluxes (Table 1). Ground-based SO2 measurements in Indonesia are also increasing in frequency and coverage40,41,42,43,44. The OMI-derived average SO2 flux from Bromo-Semeru (775 ± 298 t/d; Table 1) is higher than combined ground-based estimates for these volcanoes (~200 t/d41,44), but the ground-based campaigns only cover a few days of degassing. It is also possible that the satellite measurements are more effective than ground-based techniques at constraining SO2 flux at volcanoes that exhibit transitions from purely passive degassing to degassing via Vulcanian explosions (e.g., Semeru), due to the difficulty of measuring SO2 in proximal ash-laden plumes44.

Another notable feature apparent in the map of Indonesian SO2 sources is that some regions show lower emissions or an absence of subaerial SO2 degassing, despite the presence of numerous Holocene volcanoes; e.g., southern Sumatra and the western Lesser Sunda Islands (Fig. 1). It is perhaps no coincidence that the latter region is the location of several volcanoes responsible for large SO2-rich explosive eruptions (linked to significant climate impacts5) including Agung (1963)45, Samalas (1257)46 and Tambora (1815)47. The identification of such degassing gaps, where stored gas may be accumulating in magma reservoirs rather than being released to the atmosphere, could assist hazard mitigation and identification of potential sites of future explosive eruptions. The mutually exclusive relationship between strong subaerial SO2 degassing and large explosive eruptions during the past decade is also apparent in the Aleutian Islands (Fig. 1).

Further corroboration of the OMI-derived SO2 emissions is possible based on data collected at Japanese volcanoes. A recent assessment48 showed that 94% of the total volcanic SO2 flux in Japan originates from 6 volcanoes: Tokachi, Asama, Aso, Sakurajima, Satsuma-Iwojima, and Suwanosejima; plus Mijake-jima after 2000. A total of 17 degassing volcanoes are documented in Japan48. OMI is able to detect all seven of the strongest sources (Table 1), yielding a time-averaged total SO2 flux for Japan of 1.73 Tg/yr in 2005–2015, which is commensurate with a total SO2 flux of 2.2 Tg/yr (including the intense degassing from Miyake-jima after 2000, which continues to subside) or 1.4 Tg/yr pre-2000 based on ground-based data48. Thus the OMI measurements represent an accurate estimate of total volcanic SO2 emissions from Japan during the ongoing waning phase of Miyake-jima’s degassing activity.

Examination of the frequency-flux relationship of volcanic SO2 fluxes in Japan reveals that they do not fit a power law distribution48, as had been previously suggested for the global flux distribution49. A frequency-flux plot for the OMI-derived SO2 emissions confirms that the global volcanic SO2 sources also do not follow a power law distribution (Fig. 4). We also find a clear ‘roll-off’ of the distribution at an SO2 flux of ~500–600 t/d, remarkably similar to that found in the ground-based Japanese SO2 flux data48. This important result shows that the distribution of volcanic SO2 emissions on the scale of individual arcs can indeed mimic the global distribution, provided that large flux datasets are available from a range of source strengths (i.e., including very strong emitters such as Miyake-jima). It also indicates that the global volcanic SO2 flux is dominated by the ~30 largest sources (Table 1; Fig. 4), and quantifying the flux from these volcanoes would provide a good estimate of the global SO2 flux (in our database the 30 strongest sources emit ~80% of the total flux).

Figure 4: Cumulative frequency – SO2 flux plot for all volcanic SO2 sources detected by OMI.
figure 4

Symbol color indicates whether the CO2/SO2 ratio of the volcanic gases has been measured as of October 2016. Information on availability of CO2/SO2 ratios is from E. Hauri (DCO-DECADE, pers. comm.).

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Arc-scale trends in volcanic degassing

Another significant application of the global satellite SO2 measurements is the potential for detection of arc-scale trends in gas flux. Global, consistent SO2 measurements such as the OMI-derived database presented here pave the way to new insights into arc-scale volcanic processes, including correlations between volcanic SO2 emissions and other geophysical parameters such as arc length and subduction rate, since they provide a synoptic perspective on degassing that is not easily obtained from other techniques. The application of pattern recognition techniques to global SO2 emissions data, such as the example in Fig. 5 (also see Supplementary Fig. S17), will permit an epidemiological approach whereby analogous degassing patterns may be identified at similar volcanic systems on regional or global scales. Interpretation of SO2 data at individual volcanic systems can be ambiguous50, but analysis of arc-scale SO2 measurements potentially allows the identification of correlated trends at multiple volcanoes that can be more confidently ascribed to similar volcanic processes.

Figure 5
figure 5

(a) Decadal trends in SO2 emissions measured at 32 volcanic SO2 sources showing a significant linear correlation coefficient from a weighted linear regression fit (r ≤ −0.5 or r ≤ 0.5). Plots are ranked in order of calculated SO2 flux trend (i.e., the slope of the linear fit) from negative to positive values. Hence, cold and warm colors indicate sources showing a significant reduction or increase in SO2 emissions over the 11-year period of measurements, respectively. Each individual plot shows the annual mean SO2 fluxes for 2005–2015 (white-gray line), the decadal mean SO2 flux (red line) and the annual mean SO2 flux in 2015 (labeled red dot) for each source; axis labels are omitted for clarity. The vertical scale on each plot extends from zero to the maximum measured SO2 flux. For more detailed time-series plots, see Fig. 2 and Supplementary Figures (Figs S9–S16); (b) Location map of the 32 volcanic SO2 sources, colored based on SO2 flux trend in 2005–2015 (also see Supplementary Figure S1). Map generated using Interactive Data Language (IDL) version 8.5.1 (http://www.harrisgeospatial.com/).

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The recent status of SO2 emissions at the detected volcanic sources can be straightforwardly assessed by comparing the most recently measured annual mean SO2 flux (for 2015) with the decadal mean flux (Supplementary Figs S1 and S17). This simple metric shows some notable arc-scale consistency in several regions; for example, all the detected volcanic SO2 sources in Peru and Chile (Isluga, Villarrica, Lastarria, Ubinas, Copahue, and Sabancaya) have measured emissions in 2015 that are above the long-term average (Supplementary Figs S1 and S17). In southern Peru, both Ubinas and Sabancaya show particularly anomalous SO2 emissions in 2015 (Table 1; Supplementary Figs S1 and S17), suggesting that these volcanoes are currently in a period of elevated activity. In contrast, the volcanoes of Papua New Guinea (Tavurvur, Langila, Bagana, Manam, and Ulawun) all show recent SO2 emissions close to or below the decadal mean (Supplementary Figs S1 and S17).

A more rigorous evaluation of trends in SO2 emissions must be restricted to those sources with annual SO2 emissions showing a significant positive or negative linear correlation coefficient (i.e., r ≤ −0.5 or r ≥ 0.5; Fig. 5). Using this criterion, 32 volcanoes show significant decadal trends in SO2 emissions (Fig. 5), and although we highlight some potential arc-scale correlations here, further detailed analyses and other measurements are required to evaluate these findings. Trend analysis reveals that most volcanoes in the Vanuatu arc (Ambrym, Aoba and Yasur) show increased degassing in 2005–2015 (Fig. 5), and the only other detectable volcanic SO2 source in Vanuatu (Gaua) also shows a positive trend but with a weaker correlation coefficient (r = 0.38; Supplementary Fig. S11). Both Ebulobo and Paluweh (Flores, Indonesia) show significant positive trends (Fig. 5) and are located in the same region of the Sunda arc (Fig. 1). In the Ryukyu Islands and Kyushu regions of Japan, SO2 emissions from Satsuma-Iwojima, Sakura-jima, and Aso all show significant positive trends in 2005–2015 (Fig. 5), and the only other detected volcanic SO2 source in this region (Suwanose-jima) also shows a positive trend with a lower correlation coefficient (r = 0.39; Supplementary Fig. S9). In addition, there is independent evidence for increased volcanic activity in the Ryukyu Islands and Kyushu region, including a significant eruption at Aso in October 2016, and elevated unrest at Sakura-jima51. A recent study51 presents geophysical evidence for magma accumulation at Sakura-jima in the 1996–2007 period, with potential for a repeat of its 1914 Plinian eruption in ~25–30 years. The OMI SO2 observations show a substantial increase in SO2 degassing from Sakura-jima, particularly in 2011–13 (Fig. 5), indicating that the volcano was releasing more gas in this period largely via an increased frequency of vulcanian eruptions51. However, since 2013 the SO2 emissions from Sakura-jima have declined below the decadal mean (Fig. 5), and so the future evolution of its activity is unclear. Nevertheless, the observed degassing over the past decade may have important implications for future activity at Sakura-jima. For example, the sustained release of SO2 could be ‘defusing’ the potential climate impact of a future Plinian eruption, and/or could render a combined explosive-effusive eruption (such as the 1914 event) more likely due to limited gas supply. Gas overpressure and compressibility are rarely factored into models of volcano deformation52 and the SO2 emissions could also indicate a contribution to the deformation signal due to volatile overpressure in the magma reservoir.

In summary, while the correlated trends in SO2 emissions observed in some arcs could be purely coincidental, possible links to underlying regional- or arc-scale geophysical processes (e.g., a coincident pulse in shallow magma supply) merit further investigation but cannot be confirmed on the basis of SO2 emissions alone. Regardless of the underlying cause, our trend analysis (Fig. 5) provides new insight into the locations of increased volcanic SO2 degassing over the past decade, which would be good targets for increased monitoring (if not already in place), and into volcanoes undergoing long-term decline.

Pre-eruptive volcanic degassing

Global satellite-based SO2 surveillance also offers the potential for detection of pre-eruptive degassing at reawakening volcanoes. As noted above, increased SO2 emissions at Aso (Japan) beginning in 2011 (Fig. 5; Supplementary Fig. S10) preceded eruptions in 2014–201653. SO2 emissions were detected at Sarychev Peak (Kuril Islands) in 2005–2008 and showed a modest increase prior to its large eruption in June 20093 (Supplementary Fig. S13). At Alu-Dalafilla (Ethiopia), weak but detectable SO2 emissions were present in 2005–2007 (Supplementary Fig. S16) prior to an unexpected eruption in November 20083. A shallow (~1 km deep) magma chamber has been identified at Alu-Dalafilla54, refilling after the 2008 eruption, which is a likely source of the pre-eruptive SO2 emissions. Ground deformation data and the longevity of the magmatic system are consistent with the existence of a relatively thick sill54; the persistent low SO2 flux detected from 2005–2014 (Supplementary Fig. S16) also supports this, although it is possible that some of the SO2 detected by OMI may originate from nearby Erta ‘Ale volcano. Continued analysis of global space-based SO2 measurements will thus be valuable for volcanic hazard assessment, particularly at unmonitored volcanoes. Although the low temporal resolution of annual mean SO2 emissions precludes timely identification of pre-eruptive unrest (unless it spans several years), one possible approach would be to calculate SO2 emissions for all volcanic sources based on a 12-month moving average of satellite SO2 measurements (or shorter for stronger sources). This would conserve the sensitivity of the technique to the weak SO2 degassing expected in the initial stages of pre-eruptive unrest, whilst permitting more timely identification of increased emissions.

Missing sources and global volcanic CO2 emissions

Inevitably, an undetermined number of weaker SO2 sources, populating the tail of global SO2 flux distribution (Fig. 4), are missing from the inventory. Continued ground-based SO2 measurements at low-flux volcanoes43,55,56 are required to constrain these sources. Such measurements are also needed to improve the relatively poor constraints on the component of global volcanic CO2 emissions discharged in volcanic plumes11, which requires in-situ determination of the CO2/SO2 ratio in the emissions. As shown by Fig. 4, through coordinated efforts such as the Deep Carbon Observatory (DCO; https://deepcarbon.net/)57 significant progress has been made towards improving the spatial coverage of CO2/SO2 measurements, and around 50% of the detected SO2 sources in Table 1 have characterized CO2/SO2 ratios, including many of the strongest sources (Fig. 4), although the frequency of some measurements remains low. Based on our assessment, particular efforts should be made to pursue further CO2/SO2 measurements in regions such as Indonesia, Papua New Guinea and Kamchatka, in order to improve constraints on the global volcanic CO2 flux.

Conclusions

We believe that the volcanic SO2 emissions inventory described here represents the most accurate assessment of contemporary global volcanic SO2 degassing, and we encourage its use by the volcanological and atmospheric science communities as a substitute for existing databases13,31. Techniques such as this represent a major step forward in monitoring global volcanic degassing and ensure that few, if any, significant sources of volcanic SO2 will remain undetected in the future, provided that satellite instruments with comparable sensitivity to OMI continue to be deployed (e.g., the Tropospheric Monitoring Instrument [TROPOMI], scheduled for launch on board the Copernicus Sentinel 5-Precursor satellite in 2017; http://www.tropomi.eu). Efforts to further characterize and validate the derived SO2 emissions are strongly encouraged, particularly at those sources with no prior recorded measurements.

We have highlighted several potential applications of the new inventory, including the identification of regional- and arc-scale trends in SO2 emissions, and improvement of constraints on global volcanic CO2 emissions via measurement of CO2/SO2 ratios (and their temporal variation) at sources where this information is currently lacking. Ongoing updates to the inventory will potentially provide opportunities to identify pre-eruptive degassing at reawakening volcanoes, and correlate SO2 flux data with other geophysical data (e.g., ground deformation measured by InSAR) on a larger scale to elucidate volcanic processes. As a final point, the inventory demonstrates the remarkable persistence of passive volcanic degassing, and as anthropogenic SO2 emissions continue to steadily decline, the volcanic contribution to atmospheric sulfur loading will inexorably increase.

Additional Information

How to cite this article: Carn, S. A. et al. A decade of global volcanic SO2 emissions measured from space. Sci. Rep. 7, 44095; doi: 10.1038/srep44095 (2017).

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