Volcanic Versus Anthropogenic CO2: An Example of the Importance of Geoscience Evidence
Abstract
The climate change debate has revived the belief, widespread among climate change skeptics, that volcanoes emit more CO2 than the 35 billion metric tons per year (Gt/y) from human activities. In fact, anthropogenic CO2 emissions dwarf all global estimates of the annual present-day volcanic CO2 emission rate [Gerlach, Eos, 14 June 2011]. These estimates of CO2 output at divergent plate, intraplate, and convergent plate subaerial and submarine volcanoes range from 0.13 to 0.44 Gt/y, with preferred estimates falling between 0.15 and 0.26 Gt/y. Volcanoes emit significantly less CO2 than cars and pickup trucks, land use changes, or cement production; their output is comparable to that of about 2 dozen 1000-megawatt coal-fired power stations or of nations like Pakistan, Poland, and South Africa. The ratio of the anthropogenic CO2 emission rate to the 0.26-Gt/y maximum preferred estimate for volcanoes rises from about 18 in 1900 to approximately 135 by 2010, reflecting a 650% growth in anthropogenic emissions. Infrequent large paroxysmal volcanic explosions can cause significant increases above the 0.26-Gt/y preferred volcanic CO2 estimate. But contemporary paroxysms are unlikely to have caused breaching of the 0.44-Gt/y upper limit for global volcanic CO2 emissions; that would take more than 3 times the 0.05-Gt CO2 output of the 1991 Pinatubo paroxysm. Prorated over a 100-year recurrence interval, the 1991 Pinatubo paroxysm adds only 0.0005 Gt/y to the global volcanic CO2 emission rate. On average, humanity's ceaseless emissions release an amount of CO2 comparable to that of the 1980 Mount St. Helens paroxysm (0.01 Gt) every 2.5 hours, the 1991 Mount Pinatubo paroxysm (0.05 Gt) every 12.5 hours, and the 0.26-Gt maximum preferred estimate for annual global volcanic CO2 every 2.7 days. A global volcanic CO2 output exceeding the 35-Gt/y anthropogenic output would imply an annual mass of volcanic CO2 emissions more than 3 times the likely annual mass of erupted magma. The degassing of more than 35 gigatons of CO2 from global magma production-volcanic plus plutonic-would imply global magma averaging more than 30-weight-percent CO2. This conflicts with evidence that primary CO2 concentrations of common magmas are not greater than about 1.5 weight percent. Further, to create more than 35 Gt/y of volcanic CO2 would require global magma production (at 1.5 weight percent CO2) in excess of 850 cubic km/y. It is implausible that this much magma production-over 40 times the annual mid-ocean ridge magma supply-is going unnoticed, on land or beneath the sea. Releases of more than 35 Gt/y of volcanic CO2 into the oceans would also be inconsistent with seawater's acid-buffering capacity and its role as a major sink for atmospheric CO2. Scaling up volcanism to the hypothetical intensity required to generate CO2 emissions at anthropogenic levels suggests that humanity's CO2 emissions may already exceed the annual CO2 emissions of several continental flood basalt eruptions or the CO2 output of 1-10 supereruptions every year. Present-day global volcanic CO2 emissions are clearly not an excuse for disregarding concerns about humanity's CO2 emissions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPA21A..06G
- Keywords:
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- 0800 EDUCATION;
- 6300 POLICY SCIENCES;
- 6304 POLICY SCIENCES / Benefit-cost analysis;
- 6600 PUBLIC ISSUES