In our series about the ‘Real’ Global Temperature Trend we’ve learned not to exaggerate the climate cooling potential of volcanoes. That is because many volcanoes are of the wrong type, lie on the wrong latitude – or, the vast majority, are simply too small to cause a significant dip in global temperatures.

Webcam image of Popocatépetl explosive eruption of April 3 2016, the strongest in three years. This volcano has the potential to emit large quantities of sulphur

Clear examples of globally cooling volcanoes are Krakatau in 1883 and Mount Pinatubo in 1991. Also El Chichón, another Mexican volcano, has had some measurable effect on global temperatures. It erupted in 1982 – but its potential cooling effect was masked by a subsequent strong El Niño.

Volcanic cooling mechanism – Popocatépetl is a candidate all right

The main factor for volcanoes to cool – and beware, this is only a temporary cooling, around a year or so – is the amount of sulphur they emit, mostly as SO2 gas, possibly (and very smelly) also as H2S. The sulphur in these gases oxidises swiftly (within weeks after the eruption) and then forms sulphate, a very reflective and therefore cooling aerosol.

Also important is how the sulphur is emitted. An aerosol in the troposphere easily rains down back to Earth’s surface. For aerosols to have prolonged cooling effect – and spread out across the globe – emissions would have to puncture the troposphere/stratosphere boundary, and do this preferably somewhere close to the equator. This means an explosive terrestrial volcano (composite or dome volcano) in Mexico is a better cooling candidate than a less-explosive shield volcano or fissure volcano in Hawaii (Mauna Loa for instance) or Iceland – that simply don’t reach high enough. It also means an explosive volcano in for instance Indonesia or the Philippines is a better (global) cooling candidate than a similar volcano in Alaska.

2016 eruption of Mexico’s Popocatépetl volcano

Popocatépl, popularly referred to as ‘El Popo’, is a very active Mexican volcano and at 5,426 meters of altitude has the second-highest mountain peak of the country. It is situated close (just 70 kilometres) from Mexico City.

The current eruption started late in March 2016 and had an explosive event on April 3 that created a 2-kilometre-high column of smoke and ash and shooting fragments to an altitude of 3.5 kilometres. 2013 and 2015 saw similar activity of the volcano, that awoke from a 50-year dormant period around 1994. The explosion on April 3 2016 was the strongest in three years time, the Mexican National Disaster Prevention Center (Cenapred) states.

Current activity still too low – but a major eruption (or a pre-eruptive gas phase) of Popocatépetl would have global consequences

If we compare recent eruptions of Popocatépetl to the 20^th century most potent volcanic eruption, that of Mount Pinatubo in 1991, we find there’s still a large difference in scale. Daily SO2 emissions during active episodes of Popocatépetl is recent years were in the range of several gigagrammes/day (equalling several thousands of a megatonne), we find in a 2008 publication in Atmospheric Chemistry and Physics. Such a level of daily activity would have to continue for over three years to match the SO2 emissions of Mexico’s El Chichón eruption of 1982.

Daily average SO2 emissions do indeed count up to the megatonnes scale, we learn from another study, that was published in Geology in 2008. This research group calculated total SO2 emissions from Popocatépetl between 1995 and 1997 amounted to 9 megatonnes. Not dramatic, but already quite substantial.

Process of SO2 and CO2 formation and emissions of Popocatépetl volcano, according to below-mentioned 2008 Geology publication. Popocatépetl produces relatively high amounts of both short-term cooling SO2 and long-term warming CO2.

And of course activity could increase further – and then Popocatépetl’s emissions could rise swiftly, and reach globally measurable climatic effects.

The volcano was mentioned by name in a 1997 Nature study performed by Victor Kress of the Department of Geological Sciences of the University of Washington. Kress tried to explain why Pinatubo had emitted such extreme amounts of sulphur just 6 years before. It was due to ‘magma mixing’ he concluded – both the sulphur and the eruption itself were the product of redox reactions following underground mixing of basaltic and dacitic magma’s – exactly the process that takes place under Popocatépetl volcano.

Kress’ work was supported in an American Geophysical Union presentation at the fall meeting of 2007, by a research group of the Mexican National Polytechnical Institute – that say they have found high concentrations (2,000 ppm) of sulphur in the magma underneath Popocatépetl – evidence of active degassing processes starting at a depth of 25 kilometres.

…these same researchers also found evidence for relatively high concentrations of CO2 under Popocatépetl…

In the end, volcanoes cannot ‘save the climate’ – as they both mask and contribute to further rise in CO2

As is the case with fossil fuels’ ‘cooling’ global dimming pollution, if we were to enter a period of increased global volcanic activity, this would, in the long run, make global climate warming worse, not better. That is because volcanoes are also major emitters of CO2. And as you will be well aware by now – cooling aerosols are very short lived, whereas additional CO2 can not be quickly sequestered in the biosphere (because the biosphere is more or less in balance) and therefore acts – on human time scales – cumulative (waiting for the very slow process of geological sequestration – due to rock weathering).

This is why it is really rather difficult to reconstruct volcanic cooling events on a centuries time scale – and why it becomes much harder still to find proof of those temporary volcanic cooling events if we increase that time scale by a factor thousand to get to the Pleistocene – the age of the ice ages.

And if we again increase the time scale by a thousand to get to the 100 million year scale – we can only link dramatic warming events to periods of increased volcanic activity. And for warming it really doesn’t matter what type of volcano – or where exactly they’re situated, the Siberian Traps taught us some 250 million years ago.

© Rolf Schuttenhelm | www.bitsofscience.org