From time to time we hear the claim that volcanoes inject more CO2 into the atmosphere than human activity. Its typical form is exemplified by a comment at RealClimate which was (quite appropriately) consigned to the “Borehole.”



When the volcano, Mt Pinatubo, erupted in the Philippines in 1991, it spewed out more greenhouse gases into the atmosphere than the entire human race had emitted in its entire YEARS on earth.



This claim is almost as ubiquitous as it is ridiculous, and seems to be championed by Australian geologist Ian Plimer, author of the execrable book “Heaven and Earth: Global Warming, the Missing Science.” Science seems to be missing from all of Plimer’s musings on global warming.



There’s an article in EOS by Terrance Gerlach, and a press release about it, which attempts to lay some of these myths to rest. Gerlach notes that this is a common misunderstanding, not just among the general public but among geoscientists who don’t work in this field:



The most frequent question that I have gotten (and still get), in my 30 some years as a volcanic gas geochemist from the general public and from geoscientists working in fields outside of volcanology, is ‘Do volcanoes emit more carbon dioxide than human activities?’



Gerlach doesn’t just echo the question, he answers it:



Which emits more carbon dioxide (CO2): Earth’s volcanoes or human activities? Research findings indicate unequivocally that the answer to this frequently asked question is human activities. However, most people, including some Earth scientists working in fields outside volcanology, are surprised by this answer. The climate change debate has revived and reinforced the belief, widespread among climate skeptics, that volcanoes emit more CO2 than human activities [Gerlach, 2010; Plimer, 2009]. In fact, present-day volcanoes emit relatively modest amounts of CO2, about as much annually as states like Florida, Michigan, and Ohio.



Gerlach surveys the literature and reports the scientific findings:



Global estimates of the annual present-day CO2 output of the Earth’s degassing subaerial and submarine volcanoes range from 0.13 to 0.44 billion metric tons (gigatons) per year [Gerlach, 1991; Allard, 1992; Varekamp et al., 1992; Sano and Williams, 1996; Marty and Tolstikhin, 1998]; the preferred global estimates of the authors of these studies range from 0.15 to 0.26 gigaton per year. Other aggregated volcanic CO2 emission rate estimates — published in 18 studies since 1979 as subaerial, arc, and mid-oceanic ridge estimates — are consistent with the global estimates.



Considering that human activity released some 30 Gt CO2 into the atmosphere last year, human emissions are likely 100 (or more) times as large as volcanic emissions. Those who make claims about the Mt. Pinatubo explosion emitting more CO2 than all of human activity for all time, should be made aware that the estimated CO2 emissions from Mt. Pinatubo are 0.05 Gt CO2, about the amount released by human activity in half a day, not our entire history. In fact, in less than 3 days we outstrip the volcanic emissions for an entire year:



On average, humanity’s ceaseless emissions release an amount of CO2 comparable to the 0.01 gigaton of the 1980 Mount St. Helens paroxysm every 2.5 hours and the 0.05 gigaton of the 1991 Mount Pinatubo paroxysm every 12.5 hours. Every 2.7 days, they emit an amount comparable to the 0.26 gigaton preferred estimate for annual global volcanic CO2 emissions.



Annual CO2 emissions from human activity are greater even than what results from supereruptions, volcanic events which spew forth more than 450 cubic kilometers of magma:



Supereruptions are 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. Interestingly, these calculations strongly suggest that present-day annual anthropogenic CO2 emissions may exceed the CO2 output of one or more supereruptions every year.



Supereruptions are a significant contributor to adding CO2 to the atmosphere on geologic time scales. Yet they pale by comparison to human emissions. Yes, you read that right — while supereruptions only happen every 100,000 to 200,000 years or so, we’re presently adding CO2 to the atmosphere at a rate of one or more supereruptions every year.

Those who continue to claim that volcanic activity puts more CO2 into the atmosphere than human activity (including Ian Plimer) have been corrected — many times — by those who actually do the research. Yet the claim, like a zombie, refuses to die. Those who cling to it do so, not just out of ignorance, but out of willful ignorance.