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Dragon's den - CO2: volcanic or anthropogenic?


Colin Summerhayes* considers the evidence and struggles to find substance in the explosive political dispute over the real source of atmospheric CO2

Geoscientist 21.08 September 2011

According to the Intergovernmental Panel on Climate Change1, CO2 is warming the atmosphere. The Panel is convinced that most of it comes from human activities, not from volcanoes; but many people, including some Earth scientists working in fields outside volcanology, think that volcanoes emit more CO2 than human activities do2.

Although this belief is widespread among ‘climate sceptics’, the geological facts prove otherwise. The latest review from the US Geological Survey shows that present day volcanoes emit rather modest amounts of CO2 globally - about as much annually as human activities in Florida3. This finding confirms the position described in the Society’s recent statement on climate change4.
  • Image: Explosive eruption takes place on Vanuatu. Photographed by Anna Coulbeck.  © Anna Coulbeck. Reproduced with permission.
volc vanuatu


Terrence Gerlach of the Cascades Volcano Observatory (USGS) explains that volcanoes do, of course, emit CO2 from the degassing of magma. In the absence of humans, they are the major source for restoring CO2 lost from the atmosphere and ocean by silicate weathering, and the deposition and burial of carbonates and organic carbon. Subaerial and submarine volcanoes are estimated to emit between 0.18 and 0.44 Gt CO2/year (1 Gigaton = 1 billion tonnes). The preferred average is 0.26 Gt CO2/year, which is dwarfed by the 35 Gt CO2/year now emitted by human activities.

These volcanic emissions are comparable to the 0.2 Gt CO2/year emitted by the flaring of waste gases, the 0.22 Gt CO2/year emitted from a couple of dozen 1000Mw coal-fired power stations (equal to c. 2% of the world’s coal-fired electricity generating capacity), or the annual CO2 emissions from individual countries like Pakistan or Poland. So, the human-induced CO2 emissions estimated for 2010 - 35 Gt - were 135 times greater than the 0.26 Gt CO2/year estimated from volcanoes of all sorts.

Gerlach calls this ratio the ‘Anthropogenic CO2 Multiplier’ (ACM) – an index of the dominance of anthropogenic over volcanic CO2 emissions. The ACM rose gradually from around 18 in 1900 to 38 by 1950, then rapidly to its present level of 135 (Gerlach supplies error bars for those interested in a higher level of detail). This change reflects the exponential rise in documented CO2 emissions from the burning of fossil fuels.


Most volcanoes do not erupt steadily. They tend to be paroxysmal. Large, infrequent eruptions can therefore cause significant short-term divergence from the global estimate of 0.26 Gt CO2/year. But as Gerlach notes, modern volcanic paroxysms are unlikely to have breached the upper limit of 0.44 Gt CO2/year.

When Mt St Helens erupted in May 1980 it contributed a mere 0.01 Gt CO2. Even the stupendous eruption of Mt Pinatubo in June 1991 only released around 0.05 Gt CO2. Over a 100-year recurrence interval, the 1991 Pinatubo eruption would have added only 0.0005 Gt CO2/year to the global volcanic emission average. Although the actual rate of CO2 emission per hour during a big eruption may be about the same as anthropogenic emissions over the same period, eruptions don’t last very long. As a result, big eruptions have small effects when averaged over the long term. Gerlach writes: “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”.

Exploring further, Gerlach notes that if global volcanic output equalled or exceeded 35 Gt CO2/year, that would imply that the annual mass of volcanic CO2 emissions was more than three times the known annual mass of erupted magma. Such a supply would also suggest that CO2 makes up more than 30 wt % of global magma supply, which, if true, would make all volcanoes tremendously explosive.

In reality, CO2 concentrations are more like 1.5 wt % in magmas from ocean ridges, plumes and subduction zones. It is highly implausible geologically that magma production in amounts sufficient to supply CO2 to the atmosphere at present rates (i.e., over 40 times the rate of current mid-ocean ridge magma supply) can be occurring, in some undiscovered magma source, on the sea bed. Indeed, Gerlach points out that the release of such vast amounts of volcanic CO2 into the ocean would very quickly turn it acid. To produce the CO2 volumes required would call for magma output rates that exceed the known outputs of historic Large Igneous Province eruptions, and over much shorter time spans..

No justification can therefore be found in the rock record for the volumes of magma production, or high concentrations of magmatic CO2, that would be required to support the notion that volcanoes are behind the increases in atmospheric CO2 that we have seen since the industrial revolution began. Volcanic CO2 is dwarfed by anthropogenic outputs. Those who propose otherwise should know better.

As our President is fond of saying – “It is unwise to argue with a rock”.

* Scott Polar Research Institute, Cambridge. Colin Summerhayes is Vice President of the Society


  1. IPCC, 2007: Climate Change 2007: The Physical Science Basis Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S, D Qin, M Manning, Z Chen, M Marquis, K B Averyt, M Tignor and H L Miller (eds)] Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp
  2. Plimer, I, 2009, Heaven and Earth: Global Warming – the Missing Science Taylor Trade Publ, Lanham, Md 504pp
  3. Gerlach, T, 2011, Volcanic versus anthropogenic carbon dioxide EOS, v 92 (24), 201-2