A single volcano does not emit more than all mankind

 

Mt Cleveland, Alaska (Wikipedia Commons)

From Snopes

The myth that a single volcanic eruption puts more CO₂ into the atmosphere than all of mankind to date, let alone 10,000 times more, is one of the most pervasive as well as one of the most demonstrably false climatological claims out there. It stems, ultimately, from a geologist named Ian Rutherford Plimer, infamous for writing a widely discredited book titled Heaven and Earth, which attempted to argue that humans have had an insignificant effect on global climate. [Plimer is a well-known Australian climate denialist]

In a 2009 editorial written for Australia’s ABC news, he echoed a sentiment he had argued with similar inelegance in his book by providing the following statement, widely spread nearly word-for-word in climate skeptic circles, without any supporting citation: “Over the past 250 years, humans have added just one part of CO₂ in 10,000 to the atmosphere. One volcanic cough can do this in a day.”

This brief statement — a mere 28 words — yields a remarkably dense buffet of spurious claims and outright falsehoods. It also is rife with ambiguity. What numbers is he actually comparing? What is a volcanic “cough”? From a fact-checking standpoint, there are no interpretations of Plimer’s second sentence that can produce a factual assertion.

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A more scientifically valid approach, perhaps, would be to compare annual volcanic emissions fluxes to annual anthropogenic fluxes, as the carbon cycle is an ever-shifting network of sources and sinks of CO₂ that need to be accounted for. A 2013 review attempted to estimate the annual contribution of CO₂ emitted from all volcanoes (active and passive) and other tectonic sources on Earth per year, coming up with a figure of 540 megatons per year (note that these measurements, unlike the ones above, represent the total mass of CO₂ not solely the carbon component):

[CO2 from the plumes of actively erupting volcanoes]:

Using the available data from plume measurements from 33 degassing volcanoes we determine a total CO2 flux of 59.7 Mt/yr. Extrapolating this to ~150 active volcanoes produces a total of 271 Mt/yr CO2.

[CO2 passively vented by active volcanoes]:

Extrapolation of the measured 6.4 Mt/yr of CO2 emitted from the flanks of 30 historically active volcanoes to all 550 historically active volcanoes produces a global emission rate of 117 Mt/yr.

[CO2 from other volcanic sources]:

Perez et al. (2011) calculated the global emission from volcanic lakes to be 94 Mt/yr CO2. The sum of these fluxes produces an updated estimate of the global subaerial volcanic CO2 flux of 474 Mt/yr. Emissions from tectonic, hydrothermal and inactive volcanic areas contribute a further 66 Mt/yr to this total […], producing a total subaerial volcanic emission of 540 Mt/yr.

While the authors of this study note that this is an exceedingly rough estimate, they also point out that it is orders of magnitude lower than estimates of the annual flux of CO₂ added to the atmosphere through human activity, currently estimated to be around 35,000 Mt/year:

The global subaerial CO2 flux we report is higher than previous estimates, but remains insignificant relative to anthropogenic emissions, which are two orders of magnitude greater at 35,000 Mt/yr. 

Once again, the actual numbers bear no resemblance to Plimer’s claims. It would have to be a pretty heavy volcanic “cough” from a “single volcano” to, by itself, increase Earth’s annual volcanic CO₂ flux by a factor of 65.

Absurdity notwithstanding, numerous online claims reference specific volcanic eruptions purported to have added more than the total emission of anthropogenic carbon ever released (a value, estimated above, to be more than 282 Gt of carbon). The most commonly cited are the 15 June 1991 eruption of Mt Pinatubo and the 18 May 1980 eruption of Mount St Helens. According to the United States Geological Survey (USGS), Mount St. Helens released 0.01 Gt to the atmosphere and Mount Pinatubo released 0.05 Gt. Put another way:

There is no question that very large volcanic eruptions can inject significant amounts of carbon dioxide into the atmosphere. The 1980 eruption of Mount St. Helens vented approximately 10 million tons of CO2 into the atmosphere in only 9 hours.

However, it currently takes humanity only 2.5 hours to put out the same amount. While large explosive eruptions like this are rare and only occur globally every 10 years or so, humanity’s emissions are ceaseless and increasing every year.