Superscale volcanic eruptions can disappoint as climate coolers, Pleistocene record shows

Yes, during ice ages it can be a bit chilly. That’s why stuff that happened in the Pleistocene is easily linked to climate cooling. Like asteroids falling from the sky. Or volcanoes erupting.

supervolcano sulphate bipolar seesaw
Matching sulphate peaks from Greenland and Antarctica ice cores indicate the world must have suffered from acid rain (and acid snow at higher latitudes) after the Toba supervolcano eruption, a new study led by the Niels Bohr Institute of the University of Copenhagen states. But did these enormous amounts of sulphur create the ‘predicted’ 10 degrees cooling? There’s something mysterious about the Pleistocene bipolar temperature seesaw…

But some stuff happens anyway – without offering the prime correlation to surrounding events.

So let’s not too easily throw away Mr Milankovitch’s contributions to the Pleistocene climate debate. Good thing about his astronomical cycles is they act as continuous climate forcers on timescales of (at least) multiple millennia – indeed, the timescales of glaciations and interglacials.

And that’s when these fancy aerosol disaster theories start to somewhat fade away in comparison. A volcanic ash cloud from Iceland can dominate the news for months. An asteroid can really, really hurt your head. But over the paleoclimatic course of events – once the mist has cleared, give it a couple of years for the dust to settle (literally) – a planetary climate system can recover pretty well, it seems.

ice age temperature graph volcano
Temperature reconstruction of last 140,000 years based on isotope proxies for Antarctica and Greenland. Shown to the right is the mild Eemian interglacial, shown at the very left the mild Holocene interglacial. In between is ‘the last glacial period’ (ice age), which we northern Europeans prefer to call the Weichsel glaciation (or Weichselian), but that goes by many different names (Wisconsin/Pinedale/Fraser glaciation depending where you live in North America, Devensian in the UK, Midlandian in Ireland, Würm in the European Alpes, etc.). If you look carefully you can even distinguish the Last Glacial Maximum, somewhere between 30,000 and 18,000 years BP. Now try to zoom in as much as the graph’s resolution allows you – and go hunt for that Toba supervolcano eruption! Image: Wikimedia Commons (not derived from the new Copenhagen study’s findings).

Big volcano, bigger volcano, supervolcano

Not all volcanoes are equally equipped to change the climate. In order for emitted aerosols to reach the stratosphere and spread out across the Earth, volcanoes firstly have to be of the explosive kind (so not the oceanic shield volcanoes of for instance Hawaii and Iceland) – and secondly, preferably, be located close to the tropics.

After that it is all down to size. By coincidence a couple of days ago in our story about mountain meadows we mentioned the famous Mount St. Helens volcano. If you live in the US, you will know it had a big eruption in 1980, which practically blew off half the mountain.

So that sounds like a big eruption doesn’t it? Well indeed, not wise to be standing anywhere near [what is it that these poor overly loyal volcanologists have to ‘monitor’ there anyway? – ‘Yeah, I think it is coming, I think it is coming…’] when it erupted.

Mount St. Helens blew away some 1.2 km3 of ejecta, in the form of lava, rock and ash.

From the 20th century back to the middle ages

One other volcanic eruption in recent history was of considerably larger size though. Indeed, Mount Pinatubo on the Philippines, in 1991 emitting 10km3 of Earth to its atmosphere (of which most of course fell straight back). To get to yet another larger order we have to go back to the explosive eruption of Krakatau in modern Indonesia in 1883, that by estimates ejected some 25 km3.

But all of these volcanoes are still ‘regular’ volcanoes – like the medieval volcanoes that some scientists link to the onset of the Little Ice Age.

[But most studies connect the LIA to a couple subsequent Grand Solar Minima in the natural solar cycle. One cooling mechanism does not have to exclude the other, but without strong evidence from the temperature record (most show a pretty flat (gently declining from ~1000 AD) line from the Middle Ages) there is of course no necessity to include extra hypotheses either.]

The Pleistocene supervolcanoes – big explosions in a frozen world

There is however another category of volcanic eruptions possible on Earth. Just a couple of weeks back we’ve paid attention to the largest eruption of the northern hemisphere (excluding equatorial tropics) in the past 100,000 years – the ‘European supervolcano‘ close to Naples, Italy – which erupted some 39,000 years ago and ejected between 350-500 (depending on source) km3 of lava and debris.

Now that’s quite something, compared to Mount St. Helens, or even lesser eruptions of modern times. But as you may recall from the paleoclimatic temperature reconstruction graph in that same article Earth was (indeed) very cold during the eruption, but it did not get any cooler afterwards – at least not visibly on a timescale of centuries to millennia**.

Hmm… that’s odd, considering you hear about the volcano-global-cooling hypotheses so often, you’d almost take their value* for granted.

[*) And that of extrapolations in SRM geoengineering analyses – 'we need one Mount Pinatubo every 18 months'].

[**) On such timescales any sulphur aerosol-induced cooling would have to be translated to cooling climate feedbacks – ice as most likely candidate – to remain visible. But when we talk centuries and millennia we must not forget that when the aerosols have all settled down volcanic CO2 is still around. That's why in the paleoclimate record volcanology is easier associated with climate warming than with cooling and massive and long-lasting volcanic activity is suggested as the trigger (through CO2 and warming) of both the end-Triassic Mass Extinction and the end-Permian Mass Extinction. Not the subject for today though.]

The Lake Toba supervolcano

Well then, let’s just travel a couple of tens of thousands of years further back in Pleistocene time – to get to the real deal, perhaps the biggest volcanic eruption on our planet in 2 million (possibly even 25 million) years, the Lake Toba eruption, of 74,000 years ago.

The Toba volcano is located on the Indonesian island of Sumatra and during that famous eruption, scientists think, it expelled 2.500-2.800 km3 (some sources say it could have been even more) of lava. You can build two Mount Everests out of the rubble – and still have some left over. So Toba, that’s a proper supervolcano.

And its absence from the temperature record

Now what does a climatologist do when he is in a melancholic mood? Indeed, he sits down by the fire, pours himself (they’re mostly men) a glass of wine – and opens a journal called Climate of the Past.

And its latest edition happens to contain a publication about the Toba eruption by a group of scientists from the University of Copenhagen, the University of Bern, the Japanese National Institute of Polar Research and other institutions.

Toba it is thought by some archeologists, has caused massive population loss among early modern humans. But question remains, how bad was the volcanic winter?

Ice shows acid rain, models ‘predict’ extreme cooling, poles even out

The University of Copenhagen-led group is the first to be able to define the Toba eruption in ice cores from both Greenland and Antarctica. As there was no ash, the researchers used sulphate deposites (see corresponding peaks in graph on top of this article), evidence of a planet-wide period of acid rain.

That does not sound like a healthy environment to be in. But what about the temperature? Climate models have suggested an eruption of such a magnitude (through solar-reflecting sulphur aerosol emissions) could create a climate cooling of as much as 10 degrees Celsius, lasting for decades. Through sea ice albedo feedbacks such a period of cooling can then be considerably prolonged.

But that’s not what the researchers have found. There was a slight temperature response they say – a cooling on the northern hemisphere, but simultaneous warming on the southern hemisphere – feeding the Pleistocene bipolar seesaw hypothesis.

And that’s all the wiser we got for now.

© Rolf Schuttenhelm | www.bitsofscience.org

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