One of the largest climate forcing eruptions of the nineteenth
century was, until recently, believed to have taken place at the Babuyan Claro
volcano, in the Philippines, in 1831. However, a recent investigation found
no reliable evidence of such an eruption, suggesting that the 1831 eruption
must have taken place elsewhere. We here present our newly compiled dataset of reported
observations of a blue, purple and green sun in August 1831, which we use to
reconstruct the transport of a stratospheric aerosol plume from that
eruption. The source of the aerosol plume is identified as the eruption of
Ferdinandea, which took place about 50 km off the south-west coast of Sicily
(37.1
Volcanic eruptions that produce sulfate aerosols in the stratosphere are important climate forcing events (Robock, 2000). Ranked in order of the mass of stratospheric sulfate aerosol produced, the most important climate forcing volcanic eruptions of the nineteenth century are as follows: Tambora, in Indonesia, in 1815 (120 Tg); an unidentified eruption in 1809 (59 Tg); Cosegüina, in Nicaragua, in 1835 (40 Tg); Krakatoa, in Indonesia, in 1883 (27 Tg); and another unidentified eruption in 1831 (17 Tg) (Gao et al., 2008). The combination of the 1831 eruption and the 1835 Cosegüina eruption contributed to delaying the end of the Little Ice Age and, hence, the onset of modern anthropogenic warming, until 1850 (Brönnimann et al., 2019). Until recently, the 1831 eruption had commonly been assumed to be an eruption of Babuyan Claro, in the Philippines, which had notionally been assigned a volcanic explosivity index (VEI) of 4 (Zielinski, 1995; Global Volcanism Program, 2013; Arfeuille et al., 2014; Toohey and Sigl, 2017). However, Garrison et al. (2018) found no reliable evidence of such an eruption in 1831, suggesting that the climate forcing eruption must have taken place elsewhere.
Observations of a blue, purple and green sun occurred around the world in
August 1831 (Arago, 1832; Kiessling, 1888; Symons et al., 1888). The sun is
white when viewed from above Earth's atmosphere. At Earth's surface,
however, its observed colour varies due to scattering and absorption by
atmospheric gases and aerosols (Bohren and Huffman, 2004). A sufficiently
dense aerosol of solid particles or liquid droplets with a radius of about
0.5
Here, we present our newly compiled dataset of reported observations of a blue,
purple and green sun in August 1831, which we use to reconstruct the transport of a
stratospheric aerosol plume from the 1831 eruption. We are thus able to
constrain the location of the eruption to a mid-latitude site between
30 and 45
For simplicity, we use the term “blue
Thirty one primary sources reporting observations of a blue
Three representative reported observations of a blue
The reported blue
Location of blue
Number of blue
Additional observations of blue
The reported observations of a blue
Longitude of blue
At locations to the east of Sicily from Malta to India, including those
across the 30–45
Locations of observations of a blue
Given the geographical and temporal distribution of these blue
Parameters for use with Eq. (2) in Sect. 3.6.
Calculating the elapsed time between the earliest and latest progressively
further westward blue
Rate of westward transport of the leading (left) and trailing (right) edges of the aerosol plume, based on sources [A5], [A6], [A17] and [A18] (left) and sources [A6] and [A30] (right). Elapsed time is measured in hours from 18:00 LT (local time, UTC +1) in Sicily on 8 August 1831 (source [A5]).
The prevailing winds in the mid-latitude upper troposphere in the Northern
Hemisphere are westerly (Barry and Hall-McKim, 2014). As the aerosol
plume transport took place from east to west, it must have been
transported at an even higher altitude, above the tropopause. The source
event type is most plausibly either a volcanic eruption or a very large
forest or bush fire, either of which can inject aerosol into the
stratosphere (Robock, 2000; Khaykin et al., 2020); super-eruptions with a VEI
of 7 or more may even inject aerosol into the mesosphere (Costa et al.,
2018). Given that a super-eruption in the vicinity of Sicily in August 1831
could not have gone un-recorded, however, it is reasonable to assume that
the aerosol plume must have been transported in the stratosphere. An
easterly stratospheric wind direction at around 40
In addition to altering the colour of the sun, the scattering and absorption
of sunlight by atmospheric gases and aerosols dims its observed brightness
(Bohren and Huffman, 2004). The sun has a visual magnitude
Horvath et al. (1994) report that, due to the physiology of human colour
perception, the colour of a blue
Thus, for suitable aerosol optical depth values (
These three observational phases are illustrated, for example, in two of the
reports reproduced in Table 1 (sources [A10] and [A22]). It is noteworthy
that the ratio between the brightness of the typical zenithal sun and a
white object diffusely reflecting its light is about
Nine of the sources (Appendix A) report the local time at which a
blue
Estimated instantaneous aerosol optical depth ranges. Those marked
with a circle represent ranges derived from observations of a blue
Five of the sources (Appendix A) report the local time at which the sun was
observed with the naked eye after having been observably blue
Based on the observations reported in sources [A8]–[A30] between 8 and 17
August 1831, the mean instantaneous optical depth of the aerosol plume is
estimated to be
Had the aerosol plume produced between 8 and 13 August 1831 been homogenous,
however, it would have taken 6 d to pass over each site and reports of
6 consecutive days of identical blue
Adapting Stothers (1984b, 1996), the mass
This is a minimum value for the total mass of stratospheric aerosol produced
by the source. It does not include the portion of the aerosol plume outside
the 30–45
The 1831 eruption of Ferdinandea (also known as “Campi Flegrei Mar Sicilia”
and “Graham Island”) occurred about 50 km off the south-west coast of Sicily
(Gemmellaro, 1831; Washington, 1909; Dean, 1980; Global Volcanism Program,
2013). Starting from a submarine base approximately 150 m b.s.l. (below sea level),
it produced a volcanic island that first rose above sea level around 16 July
1831 and subsequently grew to about 60 m high and 2 km in circumference by
the time the eruption ceased around 16 August 1831 (Dean, 1980; Spatola et
al., 2018). Based on the close coincidence in place (“in the vicinity of
Sicily”) and date (“between 31 July 1831 and 13 August 1831”), we
identify the Ferdinandea eruption as the source of the stratospheric aerosol
that was responsible for the blue
The compositional dynamics of volcanic aerosol plumes can be complex (Mather
et al., 2004). However, volcanogenic aerosol in the stratosphere is typically
treated as being composed of sulfate droplets containing three-quarters sulfuric
acid (
The Ferdinandea eruption is described as a small phreatomagmatic
(“surtseyan”) eruption (Self et al., 1989). The remnant cone today lies
underwater and has a volume of about 0.06 km
We hypothesize (hypothesis “H1”) that the release of the additional sulfur
was the result of magma interacting with layers of sulfur-rich sedimentary
deposits recently identified at the site of the Ferdinandea eruption
(Spatola et al., 2018). These include Messinian evaporites (Spatola et al., 2018).
Evaporitic sequences are typically associated with gypsum
(
This hypothesis (H1) is supported by eye-witness observations. During the eruption in July and August 1831, a very strong and unpleasant smell described as a “stink” of sulfur (“una puzza di zolfo”) or of sulfur and bitumen (“una puzza di zolfo e di bitume”) was reported in towns along or near the south-west coast of Sicily (Gemmellaro, 1831; Russo Ferrugia, 1831). The distance at which it was reported peaked at 100 km on 11 August 1831 (Russo Ferrugia, 1831), i.e. during the generation of the reconstructed stratospheric aerosol plume between 8 and 13 August 1831. Silver objects became tarnished at the same time (Gemmellaro, 1831; Russo Ferrugia, 1831), suggesting that sufficiently high atmospheric concentrations of hydrogen sulfide (and water vapour) reacted with the silver to form a blackened (“tarnished”) surface layer of silver sulfide (Inaba, 1996).
A VEI of 3 is associated with “possible” stratospheric injection rather than
the “definite” stratospheric injection associated with a VEI of 4 (Newhall
and Self, 1982; Global Volcanism Program, 2013). We hypothesize (hypothesis
“H2”) that injection of the volcanic aerosol into the stratosphere by the
Ferdinandea eruption was supported by favourable meteorological conditions.
The 2018 eruption of Anak Krakatau, an island volcano on the rim of the
Krakatau volcano, in Indonesia, was similar in style and magnitude to that
of Ferdinandea (Table 3; Fig. 9a, b). It is likewise assigned a VEI of 3 (Global Volcanism Program, 2013). Crucially, sustained phreatomagmatic
activity during the eruption produced a column with an updraught in which
the vertical velocity was enhanced by convective instability (Prata et al., 2020). Thus, for 6 continuous days, a plume of positively buoyant aerosol
was able to reach an altitude which varied between 16 and 18 km a.s.l. (above sea level), at times above the local tropopause at
Comparison of the eruptions of Ferdinandea and Anak Krakatau. References: Dean (1980), Retalis and Cartalis (1997), Global Volcanism Project (2013), Spatola et al. (2018), Gouthier and Paris (2019), and Prata et al. (2020).
Anticyclones with subsiding air typically inhibit deep convection in high summer in the central Mediterranean region, even though large amounts of convective instability may be present in the mid-troposphere above an inversion (Taszarek et al., 2018). The mean height of the tropopause over the south-eastern Mediterranean in the summer is approximately 14 km (Retalis and Cartalis, 1997). We hypothesize (H2) that phreatomagmatic activity during the Ferdinandea eruption was sufficiently sustained, at times, to produce a column with updraughts enhanced by environmental convective instability above any inversion, such that positively buoyant aerosol was able to breach the tropopause, similar to the Anak Krakatau eruption.
This hypothesis (H2) is also supported by eye-witness observations.
Approaching the site of the eruption on 5 August 1831, i.e. just prior to the
generation of the reconstructed stratospheric aerosol plume between 8 and 13
August 1831, Smythe (1831) reported from a distance of 55 km that:
“… a stupendous column of white steam was observed, rising
majestically far above the western horizon, splendidly illumined (sic) by the
setting sun…”. At that time, on 6 August 1831, continuous
episodes of violent “cypress-tree-like” phreatomagmatic explosions were
separated by quiescent periods of 2–3 h (Smythe, 1831). By 11
August 1831, however, phreatomagmatic activity had significantly
intensified: Gemmellaro (1831) reported that the continuous episodes lasted
between 30 and 45 min (Fig. 9) and were separated by
quiescent periods of only 2–3 min. Stratospheric injection may
also have been achieved earlier in the eruption, even if it did not lead to
the formation of an aerosol with the parameters necessary to produce
observations of a blue
Profiles of temperature, humidity and winds in the atmospheric column in the central Mediterranean in late July and early August 1831 will be required to test the hypothesis (H2) that environmental convective instability increased the height of the eruption column, permitting aerosol injection above the tropopause. Although no observations comparable to modern radiosonde ascents exist for this period, a proxy is provided by the recent extension of global atmospheric reanalysis datasets to the early nineteenth century, for example, the Twentieth Century Reanalysis (20CR) versions 2 and 3 (Compo et al., 2011; Slivinski et al., 2019).
Zonal mean wind fields derived from twentieth century data suggest that
easterly wind velocity (40
A reanalysis-based reconstruction of atmospheric circulation will also
permit investigation of whether aerosol transport in different wind
directions from the eruption site at different altitudes was responsible for
the only blue
Assuming that the sulfate aerosol plume was transported in the stratosphere, it could have been expected to produce occasional observations of a fiery twilight glow from 8 August 1831 onward. Such twilight glows are often referred to as “volcanic sunsets” and are characteristic of volcanogenic sulfate aerosols in the stratosphere (Meinel and Meinel, 1991). The stratospheric altitude is sufficiently high for the sulfate aerosol to continue to scatter (“reflect”) sunlight, so long as it is not blocked by tropospheric clouds, even when the sun is well below the horizon (Symons et al., 1888; Meinel and Meinel, 1991). Thus, stratospheric aerosol to the east of an observer may produce the observation of a twilight glow before dawn, whereas stratospheric aerosol to the west may produce the observation of a twilight glow after sunset.
Consistent with this expectation, nine of the sources that report
observations of a blue
Aerosol transport path A–B–C–D–A around Earth in
1831. The locations of the blue
Twilight glow observations reported in Palermo, Sicily, Italy, between August and October 1831 (source [A5]). Elapsed time is calculated in days from the twilight glow observation on 8 August 1831 (source [A5]). The periodicity of the observations is consistent with the aerosol responsible completing one circuit of Earth around transport path A–B–C–D–A (Fig. 10) in approximately 18 d.
Six of the null observation sources also report observations of a twilight glow in August 1831 (Appendix B), appearing to reflect the initial transport of the stratospheric aerosol plume or the two smaller precursory bodies of stratospheric aerosol (e.g. sources [B12], [B13] and [B14]) and/or the return of the aerosol plume after a first (e.g. sources [B5] and [B14]) or second circuit (e.g. source [B16]) of Earth.
As the stratospheric sulfate aerosol plume continued to be transported, it
would have dispersed to form a more homogenous stratospheric sulfate
aerosol. Assuming that sulfate aerosol typically has a stratospheric
residence time with an
Consistent with this expectation, Sigl et al. (2013) report a peak in
sulfate deposition in a Greenland ice core between
Based on this close coincidence in the expected and actual sulfate deposition profile (and although more minor contributions from other sources cannot be ruled out), we identify the Ferdinandea eruption as the source of the climate forcing stratospheric sulfate aerosol in 1831.
One of the largest climate forcing volcanic eruptions of the nineteenth
century took place in 1831 (Zielinski, 1995; Gao et al., 2008; Arfeuille et al., 2014; Toohey and Sigl, 2017). Here, we have used a newly compiled
dataset of reported observations of a blue, purple and green sun in August
1831 to reconstruct the transport of a stratospheric aerosol plume from that
eruption. Thus, we are able to constrain the location of the eruption to a
mid-latitude site between 30 and 45
Using additional reports where a blue, purple or green sun was not recorded,
despite active observation, we are also able to constrain the longitude of
the eruption and, hence, are able to identify it as the eruption of
Ferdinandea, which took place about 50 km off the south-west coast of Sicily
(37.1
Our reconstruction of the transport of the stratospheric aerosol plume can be tested and improved upon with a search for further sources reporting the observation of a blue, purple or green sun in 1831. Our two hypotheses (H1, H2) as to the Ferdinandea eruption can be tested through further study of its magmatic system and geological context as well as with a reanalysis-based reconstruction of atmospheric circulation in July and August 1831. If we are correct, one of the largest climate forcing volcanic eruptions of the nineteenth century would effectively have been hiding in plain sight. This would arguably “lower the bar” for the types of eruptions capable of having a substantial climate forcing impact. This example serves as a useful reminder that VEI values were not intended to be reliably correlated with eruption sulfur yields unless supplemented with compositional analyses (Newhall and Self, 1982).
Analysis of reported observations of unusual atmospheric optical phenomena both in 1831 and in 1883 may support further investigation in a number of additional directions.
For example, the first observation of a blue
Further, the durations of reported twilight glow observations in 1883 were used to constrain the altitude of the aerosol responsible (Symons et al., 1888; Meinel and Meinel, 1991). In the context of testing hypothesis H2, an analysis of the duration of the reported twilight glow observations mentioned in Sect. 4.2 as well as an analysis of a supplementary collected body of contemporary observations of twilight glows (and other unusual twilight phenomena) should provide independent evidence as to the altitude reached by the aerosol responsible in 1831.
Thus, the examples of 1831 and 1883 again underline that eye-witness accounts of historical geophysical events should not be neglected as a source of valuable scientific data (Guidoboni, 2010; Pyle and Barclay, 2020). The further before the nineteenth century an event took place, the more difficult it is likely to be to be able to collect primary sources reporting any associated observations of a blue, purple or green sun, or of other unusual atmospheric optical phenomena such as twilight glows. Nevertheless, where an adequate collection of primary sources can be collated, it can likewise be expected to be helpful in terms of, for example, reconstructing the circulatory state of the stratosphere (Hamilton and Sakazaki, 2018) or constraining the latitude, longitude and date of the source event which produced the aerosol responsible and the altitude at which it was present (Symons et al., 1888).
Observations of a blue, green or purple sun in August 1831.
“est.” denotes estimated.
Continued.
“est.” denotes estimated.
Continued.
Continued.
Continued.
“est.” denotes estimated.
Null Observations.
“est.” denotes estimated.
Continued.
All of the data necessary to repeat our analyses are found in the tables in this paper.
CG carried out the analysis presented in this study and drafted the paper. CK and SE provided guidance and expertise on the volcanological and geological contexts and critically reviewed the paper. DS provided guidance and expertise on the meteorological context (including proposing the pertinence of the convective instability mechanism) and critically reviewed the paper.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “Interdisciplinary studies of volcanic impacts on climate and society”. It is not associated with a conference.
The authors would like to thank James Lequeux (Observatoire de Paris) for undertaking a search of the papers of François Arago at the Observatoire de Paris; Donatella Randazzo (Osservatorio Astronomico di Palermo) for assistance with a search of the archives of the Osservatorio Astronomico di Palermo; Marc Prohom Duran (Servei Meteorològic de Catalunya) for providing an extract from Quintana i Mari (1938) reproducing the observations of Francesc Bolòs (source [A6]); Julia Rodriguez Sanchez (University College London) for assistance with translation of materials from Catalan; Sabine Rodda for assistance with the translation of materials from German; Kai Deng (University College London) and Zheyu Tian (University College London) for assistance with translation of materials from Mandarin; Colin Graham (Gide Loyrette Nouel) for assistance with translation and historiographical interpretation of materials from Mandarin; Alessandro Aiuppa (Università degli Studi di Palermo), Attilio Sulli (Università degli Studi di Palermo), Sergio Calabrese (Università degli Studi di Palermo) and Walter D'Alessandro (Università degli Studi di Palermo) for valuable discussions regarding the geological context of the 1831 Ferdinandea eruption; and Michael Sigl (Universität Bern) for valuable discussions regarding sulfate deposition peaks in Greenland ice cores in 1831. The authors would also like to thank Fred Prata and two anonymous reviewers for the improvements to the paper which resulted from their comments. Christopher Garrison is also grateful for an E.A. Milne Travelling Fellowship grant from the Royal Astronomical Society (Burlington House, Piccadilly, London, W1J 0BQ, UK) to support a research visit to Sicily in 2018.
This paper was edited by Francis Ludlow and reviewed by Fred Prata and two anonymous referees.