Why we didn’t die – The Short Story of Toba Tuff

Click the link to get a very high resolution image of the calderas.

We humans are great at imagining really bad things. It is part of our particular survival strategy. Thinking of all possible disaster scenarios and then planning for them. The thinking of doomsday scenarios is what we are particularly good at. The planning not so much. So, if you take two disaster scenarios, why not combine them and come up with a truly nibiruistic scenario? [Editors note; Nibiruistic = The art of seeing the end of the world in everything, especially through a great conspiracy]

Recipe:

Ingredient one: a nuclear winter (those of us who lived through the 80s might remember how tangible this particular disaster scenario was back then) Ingredient two: Pinatubo and the associated global cooling.

Mix dry ingredients well and add water. Hey presto, what have you got? A volcanic winter! The basic line of reasoning is this: If a comparatively small VEI 6 eruption can do that to the climate, what’s a “supervolcano” going to do?

Well, by far the most compelling argument for the impact of a massive eruption on the human population is the Toba catastrophe theory first suggested by Ann Gibbons in 1993 and subject to vigorous debate ever since.

The Young Toba Tough  (YTT)

Put simply, the evidence for the Toba catastrophe theory is that the DNA diversity in the population of homo sapiens is much lower than that of other great apes with some scholars suggesting at the time, that the entire population of homo sapiens was reduced to roughly 3000 individuals at about the time of the Toba event.

Now, such a radical reduction in the size of the population (yes, the human population was much larger prior to the date of the YTT) may have been caused by any number of things, such as disease, the sudden appearance of a new predator, Playstation IV, or some other factor. But given the lack of any hard evidence of some other factor, there does appear to be some correlation between the YTT and the human bottleneck. So for the sake of argument, let’s just say it was the YTT that just about did us all in back then.

If you are interested in the paleontological aspects of the Toba debate, there is a whole lot of material out there. This site might be a good place to start.

http://soundingthedepths.blogspot.de/2011/03/chapter­ten­bottleneck.html

Now, regardless of the final conclusions of the Toba debate, there is something that even lay people like me can take from it. Simply look at any map of the distribution of the early human population and correlate it to the fall­out of Toba:

Source: Evaluating the mitochondrial timescale of human evolution. Phillip Endicott, Simon Y.W. Ho, Mait Metspalu and Chris Stringer.

Source, Clive Oppenheimer.

Note how the ash fall from the YTT extends westwards from the vent. Ash thicknesses have been reported of 9m in Malaysia, 1 to 3 meters on the Indian subcontinent (yes, that’s right, a whopping three meters of ash that far from the vent), thinning out as the cloud extends to the Arabian peninsula and the fertile crescent and the eastern coast of  Africa. That is an awful lot of ash.

I don’t think it takes much imagination to realize that life within this area of heavy ash fall became pretty well impossible until growth recovered. Much more interesting, however, is how neatly this ash fall represents a more or less direct hit on the habitat of homo sapiens at the time. Those populations outside of the exclusion zone (had there been one) did fine. For example, upwind of the vent, there were survivors of Toba, just a couple of hundred kilometers from the vent. (homo erectus on Java and homo floresiensis on Flores).

Going anywhere Mr. Homo Sapiens?

Likewise, the habitat of gorillas and chimpanzees in the jungles of Africa west of the Rift Valley survived the event well enough for there to be no comparable bottleneck in their DNA. From this I think we can infer that IF Toba is responsible for the human bottleneck (and, despite the big if,  it looks like it was), then the primary cause was ash fall rendering their habit temporarily inhabitable and NOT (this is the main point I finally wanted to get at) a global volcanic winter as this would have affected other species in a comparable fashion.

We can see the same pattern in many other places around the globe. Let’s take New Britain. There are a number of large calderas located along the northern coast of this island that have all erupted in the last 50,000 years.  While nothing on the scale of the YTT, some of these eruptions have been pretty big monsters. Take, for example, the Witori caldera,  Long Island,  Dakataua, Rabaul, Tavui, etc.

Why is this significant? Well, this region of the world just happens to include some of the longest lived civilizations on the planet. One would think that if large eruptions were to have the devastating impact that we think they might have, then those regions with a high concentration of large caldera eruptions would be places of low population density. New Britain proves the opposite to be true.

New Guinea is home to 1000 of the world’s 6000 languages and has maintained this huge diversity of relative stable human civilizations for the last 50,000 years and it is downwind of and more important, relatively close to this belt of extremely powerful caldera volcanoes that have erupted within that same time frame.

In a paper on Wiktori and Dakataua calderas, Machida, Blong et al. show that the areas buried by the tephra from these volcanoes contain human artefacts, indicating that the area was repeatedly occupied after each major eruption. The same is true, by the way for Southern India after Toba. So much for local bottlenecks.

And if we move our anthropocentric view away from our own species for a change, New Zealand’s Taupo Volcanic Zone provides a similar example. Despite recording 34 ignimbrite eruptions over the last 300 000 years, the flora and fauna of New Zealand is doing very well, thank you ­ even without wings ­ well it was until humans arrived but that is another story. Japan has a similar history and we all know how big the eruptions are there.

For these reasons, I am fairly confident that it is ashfall (and pyroclastic falls), which render regions temporarily uninhabitable, and not a volcanic winter that accounts for the most devastating impact on human life. In other words, if you keep far enough away from a volcanic cloud, you have a pretty good chance of survival, even when that cloud comes from a “super” volcano.

But… but… but… what about the volcanic winter? We’re all going to starve!!! Look at Tambora!!!  100,000 dead in Russia!

For sure, the famous year without a summer is attributed to the large eruption of Tambora in 1815 with many dying of famine as a result. To quote from Wikipedia:

“In the spring and summer of 1816, a persistent “dry fog” was observed in the northeastern US. The fog reddened and dimmed the sunlight, such that sunspots were visible to the naked eye. Neither wind nor rainfall dispersed the “fog”. It has been characterized as a stratospheric sulfate aerosol veil.”

But here I sometimes wonder if we have fallen in for the old “correlation is not causation” trap. Look at the temperature anomalies for 1816 from Wikipedia:

Source: Wikipedia creative commons license

Note how the anomaly hit Western Europe but not Russia? People in America and western Europe thought it was cold and they went hunting for something to blame. If they lived somewhere else on the globe, like the sunny beaches of Irkutsk, they might have come to a totally different conclusion than that the cold snap was due to Tambora. And who knows if the cold was not due to any number of other factors? There was, after all, a historic low in solar activity at the time. Moreover, there had been a number of large (VEI4 or larger) eruptions in the couple of years prior to Tambora that may have loaded the atmosphere.

Despite the correlation between a winter famine and Tambora there is not actually a hell of a lot of evidence for any correlation between major eruptions and volcanic winters. Try this from a paper released a couple of months ago:

“The bipolar linking gives no support for a long­term global cooling caused by the Toba eruption as Antarctica experiences a major warming shortly after the event.” (Svensson et al. Direct linking of Greenland and Antarctic ice cores at the Toba eruption (74 ka BP))

I don’t think I need to remind people just how big Toba was. Another example is Taupo. Did it leave an ice record? Forget it.

Yeah, but what about SO2 and other nasty gases?

We, and by we I primarily mean GeoLurking [Frequent commenter and article-writer at Volcanocafé, editors comment], tried to identify any correlation firstly between SO2 in the ice cores and known large eruptions, and secondly, between large known eruptions and falls in global temperature indicative of a volcanic winter.

Well, based on what GeoLurking uncovered, I am not convinced that volcanic eruptions have such a massive impact on our climate that we are likely to die from mass starvation. Firstly, not all large eruptions leave a trace in the ice record and that is due to a number of factors: the season, the weather patterns, the amount of SO2 released by the eruption, how high the plume, the latitude of the eruption, and the evolution of the SO2 in the atmosphere (see GeoLurking’s ruminations on this topic in the archive).

And this is just to explain the correlation between the ice cores and the eruption. It totally ignores what cooling effect these aerosols might have had on the climate. To correlate this to past global temperature changes is fraught with uncertainty. I tried and quickly gave up, not least because it is pretty well impossible to find reliable information on what the global temperature actually was in any particular 5 year time frame let alone sift out the volcanic signal from the all the noise of other factors. If someone else with more tenacity than me (not difficult, I readily admit) wants to try, I’d love to see the results, but until someone convinces me otherwise I do not believe that volcanic winters on their own have such a massive impact as our doomsdayers might wish.

And Laki?

As for Laki, there is no doubt that the high fluorine content of the tephra poisoned crops and affected agriculture in Europe. But, again, the impact here is limited to a wider region. Not the whole globe. Possibly, it is not the size of an eruption that is critical, but how poisonous it is. While huge, large caldera forming eruptions of remelted crustal material might in fact have less impact on our civilization than nasty little ones.

Final words from a flightless bird

The little Kiwi bird.

Now, of course I don’t want to downplay the impact that a couple of hundred, or even thousand cubic kilometers of ash dumped on a major population would have on our global  civilization. This would be a calamity the like of which we have never seen. There would be all sorts of economic implications and the stress could indeed lead to the breakdown of civilization as we know it.

It is not something I would want to witness. That said, I think we should shift our focus away from Yellowstone­type doomsday scenarios to planning for disaster relief because the impact of a massive volcanic eruption, while devastating, is going to be regional first and foremost. Possibly there will be some kind of global impact like a volcanic winter, but I don’t think we need to worry ourselves that we are all going to die. I just can’t see enough evidence for that in the historic record. I mean, we have already lived through a number of VEI 7 and 8 eruptions and we’re still here. And anyway, a regional disaster is going to be bad enough to deal with.

BRUCE STOUT

References:

Eruptions that Shook the World Clive Oppenheimer The Toba Super­eruption: http://toba.arch.ox.ac.uk/project.htm this site also has an entire page of scientific references)

Sounding the Depths a blog by Victor Grauer http://soundingthedepths.blogspot.de/2011/03/chapter­ten­bottleneck.html

Evaluating the mitochondrial timescale of human evolution Phillip Endicott, Simon Y.W. Ho,

Direct linking of Greenland and Antarctic ice cores at the Toba eruption (74 ka BP)) http://www.clim­past.net/9/749/2013/cp­9­749­2013.pdf Mait Metspalu and Chris Stringer (paywalled)