Large eruptions are well known. Mention Pinatubo, Mount St Helens, Krakatoa, Thera, and everyone knows something about it. The Tambora eruption of 1815 is the largest of this set: it caused the “year without summer”, left tell-tale marks in the ice sheets, and remains a terrifying example of the destructive power of volcanoes.
But the Tambora eruption has been a bit of a problem. It caused several cold years, but in the temperature records, it seems as if this cold period already started before the eruption. As if the climate knew what was coming. And that, of course, is impossible.
When people looked in detail at the ice records, from annual layers in the glaciers of Greenland and Antarctica, a second surprise came. The signal of Tambora was clearly visible: a huge spike of sulphate, lasting 2-3 years, among the largest such events of the past 1000 years (but perhaps not the largest: that is a different story). But the rise seemed to start before the Tambora eruption. When a higher resolution data set became available, it became clear there were two spikes, one about 5 years before the other. There was a major eruption before Tambora which no one had known about.
The figure above shows these sulphate spikes, clearly showing two different events, with the first one not that much smaller than Tambora. This fitted in with the climate record. The figure below shows the temperature record of central England, which is rather a too small region to show global temperatures but which does show the effect of Tambora very well. The blue bar shows the Tambora years: especially the summers were very cold. But you can also see that the cooling began already in 1810 and there had been several cold years before 1815. Tambora’s effect was so large in part because it came on top of previous cooling. Weather reports from the UK show that spring and summer 1812 was very cold and wet. The harvest was late and poor. The following winters were severe, very cold and very snowy: 1814 was in fact the very last time the London ‘frost fair’ could be held on the frozen Thames. 1816 was the year without summer, cold, cheerless, and wet (but Scotland was sunnier, suggesting low pressure was traveling further south than usual). 1817 was not as cold but still very wet. 1818 was a warm summer, and marked the end of this depressing spell.
The conclusion seems inescapable: there was a large eruption, VEI6 or perhaps even VEI7, about 5 years before Tambora. But where was it, and why had it not been noticed by anyone? The mystery has remained unsolved. In a way, it shows how poorly explored the world still was 200 years ago. On the other hand, the major volcanic regions were well known and occupied. For instance, if something major had happened in Indonesia, the Philippines, or central America it would have been noticed.
And this how the situation stayed for more than a decade. Only in 2014 did a team with Caroline Williams,Alvaro Guevara-Murua and Erica Hendy uncover a new clue to the mystery eruption, in the records of the Bogota Observatory. In the ‘Semanario del Nuevo Reyno de Granada’, a regular publication compiled by its director Francisco Caldas, a description was given of a peculiar haze, starting in late 1808:
As of 11 December of last year, the disk of the sun has appeared devoid of irradiance, its light lacking that strength which makes it impossible to observe it easily and without pain. Its natural fiery colour has changed to that of silver, so much so that many have mistaken it for the moon. This phenomenon is very noticeable at sunrise, and particularly when the sun sets. When [the sun] is at its zenith, it shines more brightly and cannot be looked at with the naked eye. Near the horizon, it has been seen to take on a light rosy hue, [or] a very pale green, or a blue-grey close to that of steel. […] The whole vault of the sky has been covered by a light cloud as widespread as it is transparent. […] [Also] missing have been the emphatic coronas which are so frequently seen around the sun and the moon when those clouds that meteorologists know by the name of veil are present. The stars of the first, second and even the third magnitude have appeared somewhat dimmed, and those of the fourth and fifth have completely disappeared, to the observer’s naked eye. This veil has been constant both by the day and by night.
This phenomenon has been observed in Pasto, in Popayan, in Neiva, in Santa Marta, in Tunja and no doubt throughout the entire Viceroyalty. To a physicist it would not be at all surprising were it to be seen in all countries located within the tropic
But in fact, almost no other records have been found. No haze has been reported from anywhere else in the tropics, in South America or in Central America, with only one exception: a report from Lima states:
At sundown in the middle of the month of December, there began to appear towards the S.W, between cerro de los Chorrillos and the sea, an evening twilight that lit up the atmosphere. From a N.S. direction on the horizon, it rose towards its zenith in the form of a cone, [and] shone with a clear light until eight [o’clock] at night, when it faded. This scene was repeated every night until the middle of February, when it vanished.
No exact date for the onset of this is given, which is a pity since it would have allowed to determine in which direction the haze spread. The description is consistent with a volcanic haze. (But a similar haze in 1950 in the UK was caused by peat fires in Canada, so other explanations may exist.)
Assuming that the eruption occured in the tropics, the jet stream would have traveled around the world in two weeks. This has been used to date the eruption to no more than two weeks earlier, i.e. Dec 4 1808, plus or minus 1-2 weeks, as suggested by researchers. This is indeed how fast the Krakatoa ejecta traveled. Of course, the eruption may not have hit the tropical jet stream in which case dispersal could have been slower. The eruption could in principle have happened anywhere in the tropics, with a higher probability for south of the equator. Excluding well known regions, there are some options in Africa and others in pacific ocean.
There are some more cautious reports. Glass fragments from this eruption have been found in Antarctica (comprising andesite) and in Alaska. But the Alaskan ones have been shown to be different, and caused by a different eruption. Possibly there was another, perhaps much smaller, eruption in the northern hemisphere, at high latitude. There are tentative reports of red sunsets from London, around April 1809, attributed to the 1808 eruption, but this could have been related to this second eruption. The sulphate at Greenland is clearly from the same volcano as that in Antarctica, but if there a contribution from a northern volcano, this could put the big eruption a bit further south than otherwise thought
( this blog has previously estimated the 1808/1809 eruption to be located at 5 degrees south). Not too much further though, as an eruption further from the equator than 20 degrees does not easily spread to the other hemisphere. The climate deterioration shows that the ejecta did manage to quickly spread north.
Another concern is that the only observations of this haze are from the region on the west side of the Andes. This suggests that the haze was worse here than anywhere elsewhere, and makes it more likely that the eruption occured closer to here. For a local volcano, a smaller eruption may have sufficed to create the haze. In this case, the volcano that caused the haze may be different from the one that caused the sulphate spike and the poor weather. There are plenty of volcanoes around the region where the haze was seen, including some that are not easy to observe. Reventador comes to mind: it is very active, but eruptions are easily missed. It is about equidistant from the furthest locations from which the haze was reported (Lima and Santa Marta). The fact that in Lima the haze came in from the south west may point at one of the many volcanoes in the south of Peru. Huaynaputina is here: it had a catastrophic eruption in 1600. Further south, Putana (a volcano with an unfortunate name) at the deserted northern end of Chile may have had a large eruption around 1810, and it should be considered a realistic possibility.
Even though smoke was seen, the smoking gun is still missing. That there was a large eruption in late 1808 or first half of 1809 seems certain. The Bogota and Lima haze may well be due to this, but it cannot be excluded that this was a separate event. It would be helpful to find out whether any of the major volcanoes in South America may have erupted around this time. Lacking this, a good look at isolated volcanoes in the Pacific may be needed.
And a final point: we have always assumed that large eruptions are rare. But this is not the only case where a big event was hiding behind another one. The huge eruption of 1459 came after one in 1453, as was only recently discovered. We know now of 4 possible VEI7’s over the past 600 years, although for two do we not know which volcano it was. Larger eruptions are not as rare as once thought.
Observations of a stratospheric aerosol veil from a tropical volcanic
eruption in December 1808: is this the Unknown ∼ 1809 eruption? A. Guevara-Murua, C. A. Williams, E. J. Hendy, A. C. Rust, and K. V. Cashman
Clim. Past, 10, 1707-1722, 2014