Anthropogenic CO2 Far Exceeds Volcanic

By Bryan Walker 07/08/2011

I was a little startled a few weeks back to see in a Waikato Times column written by former National Party MP Michael Cox the extraordinary claim that the 1991 Mt Pinatubo eruption ’shot out more greenhouse gases into the atmosphere than the entire human race had emitted in its entire years on Earth’.  I don’t know where he derived this from — no doubt it’s floating around somewhere in the denial world, though even there it seems possible that he misunderstood what he was reading. Anyway it served to support his view that talk of human-caused warming is a Left-inspired crusade to compensate for the collapse of communism! I was able to say in a letter to the paper how ridiculous the Pinatubo statement was, and pretty much everything else he said as well.

Ian Plimer’s book Heaven and Earth claims that “Volcanoes add far more carbon dioxide to the oceans and atmosphere than humans”. Perhaps that was Cox’s source. Plimer obviously still carries weight with those looking for alternatives to climate science, as a recent Herald piece from cartoonist Peter Bromhead revealed. Bromhead’s denialism survived unscathed a flight to New York sitting next to a climate scientist. It was difficult ’for a bewildered old cartoonist to try to verbally outrun somebody professionally clued up on his subject’. But he did detect that ’my companion’s viewpoint appeared disturbingly contaminated with doctrine that leaned heavily on ideology rather than absolute fact.’ And Bromhead had that ’admirably comprehensive book’ Heaven and Earth as counterweight. ’This volume, with thousands of scientific references on every aspect of climate change – through the history of the planet – is a must-read for those bewildered by climate contradictions.’

All of which is a rather circuitous introduction to my purpose in this post to draw attention to a very worthwhile article on volcanic CO2 emissions. Terry Gerlach, a retired volcanic gas geochemist who worked with the US Geological Survey, has a guest commentary in Real Climate introducing his article Volcanic Versus Anthropogenic Carbon Dioxide which was published in the American Geophysical Union’s publication Eos, and is now available online. In the article he addresses the widespread misperception that volcanic CO2 emissions greatly exceed anthropogenic CO2 emissions. He presents an overview of the subject, using only published peer-reviewed data with a minimum of technical jargon and aimed at a broad readership. I’ve read it and I can confirm its accessibility for the general reader. A few points from it:

Anthropogenic CO2 emissions were estimated to be responsible for a projected 35 gigatons of CO2 in 2010. This clearly dwarfs all estimates of the annual present-day global volcanic CO2 emission rate of around 0.26 gigaton, which Gerlach points out is comparable to the global CO2 emissions from the flaring of waste gases, or to the CO2 emissions of nations such as Pakistan (0.18 gigaton), Kazakhstan (0.25 gigaton), Poland (0.31 gigaton), and South Africa (0.44 gigaton). Anthropogenic emissions were projected to be 135 times as great as volcanic emissions in 2010.

Occasional volcanic paroxysms such as Mount Pinatuboin 1991  or Mount St. Helen’s in 1980 may for a few hours equal or even exceed the human output during those same few hours, but ’volcanic paroxysms are ephemeral, while anthropogenic CO2 is emitted relentlessly from ubiquitous sources.’ Gerlach considers what would be needed by way of volcanic activity to exceed the CO2 levels due to human activity, and concludes ’… the belief that volcanic CO2 exceeds anthropogenic CO2 implies either unbelievable volumes of magma production or unbelievable concentrations of magmatic CO2’.  It would take 700 Mt Pinatubo-equivalent volcanic paroxysms annually to match the yearly level of anthropogenic emissions.

Even supereruptions don’t compare.  He describes them as extremely rare, with recurrence intervals of 100,000—200,000 years; none have occurred historically, the most recent examples being Indonesia’s Toba volcano, which erupted 74,000 years ago, and the United States’ Yellowstone caldera, which erupted 2 million years ago. ’[Calculations] strongly suggest that present-day annual anthropogenic CO2 emissions may exceed the CO2 output of one or more supereruptions every year.’

The article performs a very useful function in gathering and referencing the work that scientists have done on the issue and spelling out the comparisons. In a quiet and restrained way it underlines just how massive a disruption the human release of fossil fuel CO2 is to the natural processes by which Earth’s vital carbon cycle is maintained. It’s very difficult to believe that the consequences can’t be severe. The author in conclusion stresses the need for educators, climate change policy makers, the media, and the general public to understand that anthropogenic CO2 dwarfs volcanic emissions. ’Discussions about climate policy can only benefit from this recognition,’ is his final sentence. He’d have been justified in using stronger language.

I should also mention a shorter and simpler article by Gerlach published by Earth Magazine last year in which he specifically addressed Plimer’s assertions on the size of volcanic emissions and took them apart.