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We all know that volcanoes emit a tremendous amount of CO₂ when they erupt. I've often heard people argue that the amount of CO₂ an erupting volcano emits dwarfs the amount of CO₂ that humans emit in an entire year. Is this true?

Update: I don't mean human biological emissions (alone). I mean all natural and human-produced CO₂ emissions.

Sklivvz
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Javid Jamae
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    For some reason, when I read your question, I envisioned humans _exhaling_ CO2, and completely disregarded the whole fossil fuel thing. – voithos May 25 '11 at 19:05
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    Whatever the answer, the point is that these volcanoes contribute to a base level that was always there. A certain amount of volcanoes is even necessary to sustain our atmosphere. The problem is that humans add a whole bunch of CO2 to this. It doesn't have to be more than the volcanoes contribute to become a problem! – Lagerbaer May 25 '11 at 19:10
  • comparison of planes vs volcano (single event): http://www.informationisbeautiful.net/2010/planes-or-volcano/ –  May 25 '11 at 23:27
  • volcanoes also emit other harmful gasses besides CO2 you know – ratchet freak Aug 21 '11 at 12:30

2 Answers2

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No, that’s not true. In fact,

Humans emit 100 times more CO2 than volcanoes. [source]

so it’s the exact opposite.

For example, in 2008 humans emitted about 36 billion metric tons of CO2. In that same year, the highest (!) estimates for all volcanoes combined (submarine volcanoes included) were just 270 million metric tons (Gerlach, 2010).

The claim to the contrary, for instance voiced here by Ian Plimer on ABC:

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.

is an artful lie. This is exposed wonderfully in a comment to that article:

Our emissions since [before the Industrial Revolution] have raised the level [from 280 ppm] to around 390 ppm, an increase in the CO2 concentration of around 40%!! The increase of 110 ppm is 1.1 per 10,000 - roughly Ian's magic number. […]

1 in 10,000 of CO2 [of 390 ppm] would add 0.039 ppm which a [volcanic] 'cough' could easily do.

So Ian Plimer arrives at his assertion by comparing two different numbers: the number of overall increase in CO2 in the atmosphere (110 ppm), and a relative percentage of the atmospheric concentration (0.039 ppm), and alleges that these numbers are the same. That’s like saying that 10$ and 10% of 10$ (= 1$) are identical.

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Konrad Rudolph
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In addition to Konrad's excellent answer, another way we can know that volcanic CO2 does not exceed anthropogenic CO2 is to look for signs of volcanic eruptions in CO2 data. Here are the Mauna Loa data collected by the [Scripps Institution]:1

http://www.esrl.noaa.gov/gmd/ccgg/trends/

N.B. the pre-industrial CO2 level had been around 280ppm for several thousand years (see comments below).

There were large volcanic eruptions in 1963 (Agung), 1980 (mt St Helens), 1982 (El Chichon) and 1991 (Pinatubo), none of which left so much as a blip on the measurements taken at Mauna Loa.

Or, looking back further in time, we can look at super-volcanoes in the proxy record, such as the Yellowstone supervolcano 640,000 years ago, which again left no trace in the proxy record:

enter image description here

Taken from a very good article by Tamino, archived here, which gives a further three examples.

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    I have to say that I recognize the good reasons for researchers using a suppressed zero in the upper figure, but this topic is so politicized that it bothers me to see it in a more public context. That curve represents a (quite significant) 25% change, not the factor of several ballooning which an unwary or unsophisticated reader could take away from it. Hmmm ... both figures, actually. – dmckee --- ex-moderator kitten Mar 03 '14 at 19:17
  • @dmckee since nobody is claiming that CO2 levels are "balooning", I can't see why an unwary or unsophisticated reader would take that away from the figures. A graph that started the CO2 axis at zero would be misleading as an atmospheric CO2 concentration anywhere near zero would be physically impossible on a planet with abundant carbon-based life (just as it would be misleading to show surface temperatures with a temperature axis starting at zero Kelvin). It would also reduce the resolution available to show any spikes due to volcanic eruptions... –  Mar 04 '14 at 12:51
  • ...which would *inevitably* lead to the objection that the graph had been manipulated to bias it against volcanic origin argument by not scaling the graph to the variation in the data! If there is a good scientific reason to display a graph in a particular way, that is the way it should be done, and if "unwary or unsophisticated readers" have concerns then be willing to address them. –  Mar 04 '14 at 12:55