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Monday, June 12, 2017

Volcanic CO2 Emissions

An internet meme was recently posted in the Facebook group March For Science, by a frustrated scientist looking for ways to counter nonsense.  The meme claims that Mt. Etna has already put 10,000 times more CO2 into the atmosphere than all of the man-made emissions in history.  That claim is not remotely true.  Somebody just made it up, and put it on a photo of a volcano, and it has been shared thousands of times by people who don’t want to believe in science.

As readers of my blog know, I have been looking at data on global CO2 for a number of years.  I recently researched natural CO2 emissions, and added those emissions to my chart of annual CO2 emissions from fossil fuels, cement manufacturing, and deforestation.  Natural CO2 emissions are shown as the small purple bar at the top of the stacked-bar graph.  Shown on the graph are CO2 emissions from natural gas, oil, coal, cement manufacturing, flaring, deforestation, and natural volcanism.   Industrial emissions are from Boden et al, 2013, Deforestation is from Houghton, 2008, and volcanic emissions from Burton et al, 2013 and Lee et al, 2016.
Measurement of Volcanic CO2 Emissions
Measurement of CO2 emissions from volcanos is accomplished by surface observations, aerial surveys (including manned flights and drones), satellite observations and soil-gas surveys.  A variety of methods are used, as described by Burton (2013).  Direct measurements of CO2 concentrations are supplemented by measurements of SO2 or tracer gases, when the relative concentrations of CO2 and the other gases is accurately known.  This process allows greater precision in CO2 determinations.

Volcanic sources of CO2 include eruptive events, point-source passive degassing from active volcanoes, diffuse emissions from active volcanoes, emissions from tectonic, hydrothermal, or inactive volcanic areas, volcanic lakes, and mid-oceanic ridges.  Eruptive events are popularly believed to contribute greatly to atmospheric CO2, but in fact, these events are completely trivial.

Volumes of Volcanic CO2 Emissions from Eruptive Events
The largest eruptive event of the past 100 years was the eruption of Mt. Pinatubo in Indonesia in 1991.   That eruption was estimated to have released 50 million tonnes of CO2 into the atmosphere (Gerlach et al. 2011, cited by Burton).  The eruption of Mt. Pinatubo released only one-ten of one percent of the man-made CO2 emissions of 37 gigatonnes in the single year of 2009.

The volume of CO2 emitted by the four largest eruptions of the past 200 years is about 600 Mt of CO2,  based on volumes of ejecta and the CO2 content of magma (Burton, 2013).  An earlier estimate of the average volume of all eruptions of the past 300 years gives an annual CO2 volume of only about 1 Mt per year (Crisp, 1984, cited by Burton).

Total Volumes of Volcanic CO2 Emissions
Estimates of natural CO2 emissions have increased markedly over the past 25 years, from about 70 million tonnes per year to about 700 million tonnes per year.  Newer work has recognized passive and diffuse CO2 emissions from inactive volcanoes and tectonically active terranes, and measured emissions from these sources.  The current best estimate is 708 million tonnes per year, after adding in estimates for emissions from mid-oceanic ridges and the East African rift (Lee et al, 2016).  By comparison, man-made emissions of CO2 (including deforestation) were about 37 gigatonnes (37,000 million tonnes) in 2009.

The Carbon Cycle
Natural processes which add and subtract CO2 from the atmosphere and oceans necessarily become balanced over geologic time.   Natural processes which add CO2 include eruptive volcanism, passive volcanic emissions, diffuse volcanic sources, volcanic lakes, mid-oceanic rifts, onshore rifts and metamorphism of carbonate rocks.  Natural processes which remove CO2 include the formation of limestone, by biologic and chemical processes, weathering of silicate rocks, deposition of land plants in bogs forming coal, and deposition of algae in anoxic marine environments, forming black shales.   There is debate about the importance of tectonic subduction in permanently removing carbon from surface environments, and the volumes of carbon which might be permanently removed by that process.  Although some of these processes are not well quantified, the volumes proposed are typically in the range of hundreds of million tonnes, far less than the gigatonnes of man-made carbon emissons.

Volcanic processes add carbon dioxide to the atmosphere, but far less than human activities.  Eruptive events, such as the frequent eruptions at Mt. Etna in Italy, or the giant 1991 eruption of Mt. Pinatubo in Indonesia, add a surprisingly trivial amount of carbon to the atmosphere.  On average, all modern eruptive volcanic events add an average of 1 to 3 million tonnes of CO2 to the atmosphere each year.  By contrast, quiet, passive outgassing and diffuse volcanic sources add about 540 million tonnes of CO2 to the atmosphere each year.  Mid-oceanic ridges and the East African rift add approximately 160 million tonnes more CO2.  In all, volcanic sources add about 1.9% of the CO2 emissions from human sources, including deforestation. 

Since this investigation began with an Internet meme, I decided to make my own, with a quantitative truthful statement and scientific references.  Here it is.

Boden, et al, 2013, Global and National Fossil-fuel CO2 Emissions, in Global Carbon Atlas

Burton et al, 2013,  Deep Carbon Emissions from Volcanoes
Discussion of CO2 flux from subaerial volcanic eruptions on page 332.
Total CO2 flux from volcanic sources:  637 mT per year, p. 341, table 6.
The eruption of Mt. Pinatubo in 1991 was the largest volcanic eruption since 1912.   That eruption produced ~50 Mt of CO2 (Gerlach et al. 2011).  Individual eruptions are dwarfed by the time-averaged continuous CO2 emissions from global volcanism.  The eruption of Mt. Pinatubo was equivalent to only 5 weeks of global subaerial volcanic emissions. 
The average volume of eruptive CO2 emissions over the past 300 years was only 0.1 cubic kilometers, which suggests an annual rate of about 1 million tonnes of CO2 annually (Crisp, 1984, cited in Burton).
CO2 consumption from continental silicate weathering was 515 Mt/yr, (Gaillardet et al., 1999, cited in Burton).
Metamorphism accounts for the release of about 300 million tonnes of CO2 annually.  (Mörner and Etiope, 2002, Carbon degassing from the lithosphere. Global Planet Change 33:185-203, cited in Burton). 

Lee et al, 2016, Massive and prolonged deep carbon emissions associated with continental rifting,  Nature Geoscience Letters, Jan.18, 2016. 
Paper accounts for additional CO2 emissions from East African Rift, potentially bringing natural world CO2 emissions to 708 mT, an increase of 11% from previous estimates.

Houghton, R.A. 2008. Carbon Flux to the Atmosphere from Land-Use Changes: 1850-2005. In TRENDS: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

Marland, G., T.A. Boden, and R.J. Andres. 2008. Global, Regional, and National Fossil Fuel CO2 Emissions. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

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