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Saturday, December 21, 2019

Understanding the Source of Rising Atmospheric CO2


For the last time, increasing atmospheric CO2 is coming from fossil fuels, and not from volcanoes.

The concentration of CO2 in the atmosphere is rising rapidly.  Before widespread burning of coal, circa 1750, atmospheric CO2 was about 280 parts per million (ppm).  By 1955, global CO2 concentration had risen to 314 ppm.  Average global CO2 levels are now about 412 ppm, and are still rising at about 3 ppm per year. 

Industrial processes are able to change the composition of the earth’s atmosphere because there really isn’t very much atmosphere, and there isn’t very much CO2.  The atmosphere thins rapidly with altitude, so that about half of the atmosphere is less than 3 miles above the earth, and breathable atmosphere extends only about 6 miles above the earth.  Further, there isn’t very much CO2 in the atmosphere – about 400 ppm, or 0.04%.  Nevertheless, that small amount of CO2 is very effective at blocking thermal infrared radiation, which is why changing the CO2 concentration of the atmosphere has already had a significant impact on global climate. 

Figure 1.  There isn’t very much atmosphere, and there really isn't very much CO2.  The pie-slice of CO2 in the second figure is exaggerated three-fold for visibility.

A common myth that circulates on social media is that rising CO2 in the atmosphere is coming from volcanoes.   It isn't.  I already wrote one blog post about the origin of atmospheric CO2.  (https://dougrobbins.blogspot.com/2017/06/volcanic-co2-emissions.html). 

This blog post will present additional evidence that rising CO2 is of human origin.  The evidence is:
  • Declining oxygen concentration of the atmosphere
  • The quantity of missing oxygen
  • The location of declining oxygen concentration by hemisphere
  • Volumetric data for fossil fuel emissions, deforestation, and volcanism, compared to volumes of CO2 appearing in the atmosphere
  • The location of rising CO2 by hemisphere
  • Changing carbon isotopic composition of the atmosphere
  • The location of the declining carbon isotope measure (del C13) by hemisphere
  • The steady rise of atmospheric CO2, whereas volcanic eruptions are intermittent (although slow emissions from non-eruptive events, mid-ocean ridges and rifts also occur). 
Atmospheric CO2 is now also monitored by two orbiting carbon observatories (OCO), which directly measure CO2 concentrations in the atmosphere and connect rising atmospheric CO2 with points of origin.

The myth that volcanoes are responsible for human-caused atmospheric disruption has been propagated since the 1990s.  The book “Merchants of Doubt” provides a history of claims that volcanoes were responsible for destruction of stratospheric ozone, or for acid rain in the US and Canada.  Those claims were thoroughly debunked long ago.  Nevertheless, articles attributing rising CO2 to volcanoes still appear on climate-change denying websites, (e.g. James Edward Kamis’ 2018 post on ClimateChangeDispatch).

Let’s look at the evidence.

Fossil-Fuel Combustion
When fossil fuels are burned, atmospheric oxygen is converted to CO2.  Consequently, the oxygen concentration in the atmosphere falls.  If we quantify oxygen depletion in the atmosphere, we find that it validates the volumes of fossil fuel consumption reported by inventory methods (BP Statistical Review, CDIAC, EIA, etc.).  The volumes of CO2 determined by either method are approximately twice what is necessary to account for the observed rise in atmospheric CO2.  The remaining CO2 is dispersed into CO2 reservoirs in the oceans and biosphere.  A full accounting of the CO2 flows on earth can be found in the Global Carbon Project or Berkeley Earth websites. 

Simply stated, the depletion of atmospheric oxygen quantifies CO2 emissions from fossil fuels.  This volume of CO2 emssions more than accounts for the rise in atmospheric CO2.  There isn’t any room for a significant contribution from volcanoes without somehow getting rid of the CO2 from the combustion of fossil fuels in some as-of-yet unidentified carbon sink (which is unlikely to exist). 

Depletion of Atmospheric Oxygen
The amount of oxygen in the atmosphere is falling (although not enough to cause trouble for breathing).   Atmospheric oxygen is falling because oxygen is consumed by burning fossil fuels.  This would not occur if the source of rising CO2 was from volcanoes (Figure 2).  As seen in the bulk CO2 and Del C13 charts, there is a strong seasonal signal in the concentration of atmospheric oxygen, related to the growing season in each hemisphere.  The amplitude of the seasonal cycle is somewhat stronger in the Northern Hemisphere, due to the preponderance of temperate land-mass and agriculture. 
Figure 2.  The concentration of atmospheric oxygen is falling, due to combustion of fossil fuels.  The Per Meg (del O2/N2) can be roughly converted to ppm by multiplying by 0.2095, the fractional concentration of oxygen in the atmosphere.  A discussion of the Per Meg (del O2/N2) measure can be found on the Scripps Institute CO2 website FAQs.  The loss of 700 ppm of oxygen is a relatively small change because of the greater abundance of oxygen in the atmosphere as compared to CO2.  The percentage of oxygen in the atmosphere is about 20.95%; the percentage of CO2 in the atmosphere is about 0.04%.

Oxygen Depletion by Hemisphere
Falling oxygen concentrations in the Northern Hemisphere lead falling oxygen in the Southern Hemisphere (Figure 3).  This is because 90% of fossil fuels are being burned in the Northern Hemisphere, consuming oxygen in the Northern Hemisphere.  Atmospheric mixing works to equilibrate oxygen concentrations, but continuing combustion of fossil fuels in the Northern Hemisphere keeps oxygen lower than in the Southern Hemisphere.

Figure 3.  Atmospheric oxygen recorded by Scripps Institute network of atmospheric observatories.   The seasonal cycle at each station was filtered with a 12-month rolling average.  The Northern Hemisphere leads the Southern Hemisphere in falling oxygen.

Oxygen – Carbon Stoichiometry
The number of molecules of oxygen disappearing from the atmosphere is a very close match to the number of carbon atoms burned in fossil fuels and deforestation (Figure 4).  There is a quantitative match, showing that for each atom of carbon burned, one molecule of oxygen disappears from the atmosphere, as C + 02 -> CO2.  The depletion of atmospheric oxygen, in stochiometric balance with human carbon combustion, validates the volume of CO2 released into the atmosphere by burning fossil fuels.  
Figure 4.  Moles of carbon burned by fossil fuels and deforestation annually, compared to atmospheric oxygen depletion in moles.   The close match confirms that volumes of CO2 released from fossil fuels and deforestation are responsible for rising atmospheric CO2.  
Moles of oxygen depletion can be calculated by converting “per meg” to ppm (oxygen/atmosphere) and assuming an initial volume of the total atmosphere of 1.81E+20 moles (various Internet sources).  Notes on the calculation are given in the Appendix, following References.

Volumetric Evidence
CO2 emissions from gas, oil, coal, cement, flaring, and deforestation are now about 40 gt per year, and forecast to go higher.   Estimates and measurements of volcanic CO2 emissions are far smaller than known volumes of CO2 from fossil fuels and deforestation (Figure 5).  Estimated volumes of volcanic CO2 include deep carbon emissions, and passive emissions from continental rifts and mid-ocean ridges.  Volcanic CO2 emissions are only about 1.8% of human CO2 emissions by volume.

Figure 5.  Annual Human CO2 Emissions by type and Volcanic CO2 Emissions, with EIA forecast to 2040.  Estimates of CO2 emissions from volcanic activity have been revised significantly higher since the 1990s, as CO2 emissions from deep volcanic source, continental rifts and mid-ocean ridges have been recognized and quantified.  Still, volcanic CO2 emissions are now estimated at about 700 million tonnes, compared to about 40 gigatonnes of CO2 from fossil fuels and deforestation.

Fraction of CO2 Emissions Which Remain in the Atmosphere
Only about 44% of human CO2 emissions remain in the atmosphere; the rest of the CO2 is absorbed by the oceans or taken up by plants.  Human CO2 emissions are more than twice what is necessary to account for rising atmospheric CO2.  Since volcanic CO2 emissions represent only 1.8% of human CO2 emissions, it is impossible for volcanoes to account for the large volume of CO2 now appearing in the atmosphere.  (Figure 6). 

Figure 6.  If all human CO2 emissions remained in the atmosphere, atmospheric CO2 concentrations would rise about twice as fast as what is observed (red line).  Actual average global CO2 is rising at a rate of about 44% of cumulative human CO2 emissions.  If only volcanic CO2 was entering the atmosphere, atmospheric CO2 would rise only negligibly, offset by the removal of carbon by natural processes.

Difference in CO2 between Northern and Southern Hemispheres, and
Comparison to Net CO2 Emissions from the Northern Hemisphere
About 90% of humans live in the northern hemisphere, and 90% of human CO2 emissions originate in the northern hemisphere.  Atmospheric CO2 concentrations in the northern hemisphere are consistently higher than CO2 concentrations in the southern hemisphere.   The amount of the difference is very close to the net CO2 emissions from fossil fuels in the northern hemisphere (Figure 7).  The close correspondence of net Northern Hemisphere CO2 emissions and the difference between Northern and Southern CO2 concentration is partly a coincidence between the mixing rate between the hemispheres and the reporting period for CO2 emissions.  However, the consistent fit is a clear proof that fossil fuel emissions in the Northern Hemisphere are principally responsible for rising CO2.  The largest volcanic eruptions of the past 60 years have been in the Southern Hemisphere, but these have made no impact on the record of atmospheric CO2.
Figure 7.  Northern and Southern Hemisphere CO2 concentrations, and net fossil-fuel emissions from the Northern Hemisphere.  Major volcanic eruptions, such as Mt. Pinatubo and Mt. Hunter in the Southern Hemisphere in 1991, are not observed as a difference in CO2 observations between the hemispheres. 

Carbon Isotope Ratios in Atmospheric CO2
Natural carbon mostly occurs in two isotopes: C12 and C13.  Plants and all fossil fuels (which derive from plants) are enriched in C12 by biological processes, giving fossil fuel emissions and deforestation a “lighter” isotopic signature (more C12) than the atmosphere.  The measure of carbon isotopic ratios is d C13/C12, typically called “del C13”.  Samples which are relatively enriched in “light” C12 have a negative del C13, while samples that are enriched in “heavy” C13 have a positive del C13.  A technical definition of del C13 is given at the bottom of the article, below the references.

In the 1950s, the atmosphere had a del C13 value of about -7.5, reflecting a higher concentration of C12 than the oceans, which has a del C13 of about zero.  As mentioned above, fossil fuels are enriched in C12 with typical values in the range of -20 to -30.   Biogenic natural gas has been fractionated twice, and may have del C13 values ranging from -40 to -70.  Volcanic emissions have a heavier isotopic signature than the atmosphere, with a del C13 value of about -1 to -4.  

The isotopic composition of the atmosphere is steadily becoming lighter as CO2 concentrations rise.  This is only possible if the additional CO2 is from a source isotopically lighter than the atmosphere, not heavier.  Thus, fossil fuels, and not volcanoes or the oceans, are the source of rising CO2 (Figure 8).
Figure 8.  The Del C13 Carbon Isotopic Record of Atmospheric CO2, recorded by the Scripps Institute.  The record is marked by a strong seasonal cycle in the Northern Hemisphere.  During the Northern Hemisphere growing season C12 is preferentially taken out of the atmosphere by plants, and released back to the atmosphere in winter causing the seasonal cycle in the air.  Overall, the Del C13 index has fallen from -7.5 to -8.5 since 1977, showing an increasing prevalence of light C12 (characteristic of fossil fuels and deforestation) in the atmosphere.

After filtering the seasonal cycle, we see that the Northern Hemisphere leads the Southern Hemisphere in falling Del C13 isotope ratio (Figure 10).  This is because 90% of fossil fuel emissions occur in the Northern Hemisphere. The remaining wavy signal in the Del C13 record correlates to El Nino cycles (Figure 9), with a rapidly falling Del C13 ratio during El Nino events, and a slightly rising Del C13 ratio during La Nina events.  It is unclear whether the El Nino signal in the data is due to fractionation between atmosphere and ocean, changes in the uptake of carbon in the ocean, or related climate events. 
Figure 9.   The Del C13 carbon isotope record in atmospheric CO2, from Scripps Institute atmospheric observatories; the seasonal cycle was filtered with a 12-month rolling average.  Broadly, the Northern Hemisphere leads the Southern Hemisphere in falling Del C13, because 90% of CO2 emissions from fossil fuels occur in the Northern Hemisphere.  The residual long-wavelength signal relates to the El Nino/La Nina cycle. [See an earlier post:
https://dougrobbins.blogspot.com/2013/11/carbon-isotopes-in-atmosphere-part-ii.html ]

Rate of Increase in Global Atmospheric CO2
Atmospheric CO2 is steadily rising around the globe.  Volcanic eruptions are intermittent; the largest eruption (Mt. Pinatubo) and the third-largest eruption (Mt. Hudson) of the past century both occurred in 1991, but there is no perceptible change in the rate of rising atmospheric CO2 (Figure 10).  Likewise, other large volcanic eruptions produced no perceptible impact over the period of detailed CO2 observations [Mount Agung (1963), Mt. St. Helens (1980), El Chichon (1982). Puyehue-Cordón Caulle (2011)].  Slow, *quiet*, emissions of CO2 also occur from non-eruptive events, mid-ocean ridges and onshore rifts, but these have been well quantified over the past 20 years, and do not contribute significant volumes of CO2 to the atmosphere.
Figure 10. Global atmospheric CO2.   This is my version of the Keeling Curve.  Data is from the Scripps Institute network of atmospheric observatories.  Cool colors indicate stations in the Northern Hemisphere. Warm colors show stations in the Southern Hemisphere. Atmospheric CO2 falls in the Northern Hemisphere summer, as carbon is taken up by plants, and rises in winter as the plants decay.  There is a strong seasonal cycle dominated by the Northern Hemisphere due to predominant location of temperate landmass and agriculture in the Northern Hemisphere.  Apart from the seasonal cycle, CO2 has risen steadily from about 314 ppm in 1955 to about 412 ppm today.

Orbiting Carbon Observatories and NASA CO2 Modeling
Two new NASA satellites, OCO2 & OCO3, now provide worldwide continuous CO2 monitoring.  Data gathered by these satellites will provide a detailed identification of the specific sources of CO2 across the entire globe. 

NASA also prepared a supercomputer simulation of atmospheric CO2, based on ground-based and aerial CO2 observations.  A video of the simulation can be seen on YouTube:
The simulation highlights major CO2 sources, in the eastern US, eastern China, industrial centers of central & eastern Europe, oilfields of western Siberia, and wildfires in the Amazon rainforests (Figures 11A and 11B).

Figures 11A and 11B.  Atmospheric CO2 Concentrations from NASA supercomputer simulations for the year 2006.  The location of CO2 sources is apparent from cities, industrial centers, and wildfires.

Conclusion
Rising CO2 concentrations are unquestionably from human sources, as a result of combustion of fossil fuels and deforestation.  There is volumetric, temporal, isotopic, geographic, stoichiometric evidence supporting human sources of rising atmospheric CO2.  There is no evidence that volcanoes make a significant contribution to rising CO2.

References:
Previous Posts on Volcanic and Atmospheric CO2

Posts on Twitter on Volcanic CO2

External References:
Scripps Institute CO2 Home
Scripps Institute O2 Program

Boden, et al, 2013, Global and National Fossil-fuel CO2 Emissions, in Global Carbon Atlas
http://www.globalcarbonatlas.org/en/content/fossil-fuel-emissions
http://www.globalcarbonatlas.org/docs/Public_Presentation_of_the_GCA_Paris_EN.pdf

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.

Erik Klemetti, 2015, Volcanic versus Anthropogenic Carbon Dioxide: An Addendum, WIRED website.

Pre-industrial atmospheric del C13 was about -6.5, and declined following industrialization, in correlation with rising atmospheric CO2.

Representative del C13 values from volcanism:
Del C13 :  -3.2
Del C13: -4.9 to -6.3
Del C13: Currently  -0.9 to -1.4; 1970s and 1980s ~ -4
Del C13: -6 to -10.  N.B.: These are samples of soil gases in a rift zone with known petroleum generation, and may be contaminated by thermogenic or biogenic CO2 deriving from petroleum sources.  
Faure, 1984, Principles of Isotope Geochemistry
Del C13: -2 to -6. 

Volcanoes and CO2
A good discussion of former and current estimates of CO2 emissions from volcanoes and fossil fuels.  Also, a good recap of errors made by certain commentators in creating and propagating the volcanic CO2 myth. 

Naomi Oreskes and Erik Conway, 2010, Merchants of Doubt.
A deeply researched book about right-wing scientists, funded by industry, working outside of their fields of expertise, tried to throw doubt on science that might result in regulations in the interest of public health or environmental protection.  The scientists involved were generally retired, had worked in military science and were given compensation or recognition in return for their efforts.  Among the false narratives they created was the idea that volcanoes were responsible for chlorine damage to stratospheric ozone, and that volcanoes were responsible for acid rain in the US and Canada.  Neither idea is correct.  The idea of blaming volcanoes for man-made atmospheric disruption has now been extended to CO2 and climate change.

Which emits more carbon dioxide: volcanoes or human activities?  Climate.gov.

The following are examples of deliberately misleading media articles about atmospheric CO2.
Volcano eruption WARNING: Intense volcanic CO2 activity 'drives GLOBAL EXTINCTION'
Article omits mention of fossil fuels entirely.

J.E. Kamis, 2018, Discovery Of Massive Volcanic CO2 Emissions Puts Damper On Global Warming Theory
This article contains false claims.  Notably, the article claims that:
“Natural volcanic and man-made CO2 emissions have the exact same and very distinctive carbon isotopic fingerprint.  It is therefore scientifically impossible to distinguish the difference between volcanic CO2 and human-induced CO2 from the burning of fossil fuels (see here).”
The reference provided (https://skepticalscience.com/anthrocarbon-brief.html) directly contradicts the claim!  “In fact the global C13/C12 ratio has declined, which is very strong evidence the source of the CO2 increase has was C12 enriched, ie, derived from photosynthesis.  Therefore it is very strong evidence that it comes from the biosphere or fossil fuels, rather than from volcanoes or oceanic outgassing.”

Appendix
Stoichiometry Calculations
Annual fossil fuel emissions are reported in tonnes of CO2 by CDIAC, the BP Annual Statistical Review of World Energy, and the EIA.  One tonnes of CO2 (1000 kg) contains 22,722 moles of CO2. 

Calculation Notes for atmosphere stoichiometry.  The Scripps pages on units and FAQs are helpful in understanding the use of the “per meg” unit, and conversion to ppm. 

“Per meg” units of oxygen reported by Scripps can be converted to ppm (oxygen/atmosphere) over small ranges by multiplying by 20.95%, the current oxygen fraction in air.  Parts per million (ppm) of oxygen can then be converted to moles by multiplying by the number of moles in the atmosphere (1.81E+20), from various Internet sources.  https://www.theweatherprediction.com/habyhints3/976/

A Few Words about CO2 Carbon Isotopes
There are two stable isotopes of carbon, C13 and C12.  C12 is the more abundant isotope; the natural ratio of C12 to C13 is about 99 to 1.  The standard measure of carbon isotopes compares the C12/C13 isotope ratio of the sample in question to the C13/C12 ratio of a standard limestone, according to the expression:

d C13/C12 = ((C13/C12 sample/C13/C12 standard) – 1)*1000.

This expression, commonly termed “del 13”, amplifies small but meaningful differences in the isotopes, which are diagnostic of certain processes and occurrences of carbon.  The standard is a uniform Cretaceous limestone with a d 13 value defined as zero.   Positive values indicate a heavier composition, i.e., a greater concentration of C13 than the standard.  Negative values indicate a lighter composition, i.e., a smaller concentration of C13 than the standard.

Plants fractionate carbon, favoring the lighter isotope C12.  Anything derived from plants, including oil, gas, and coal (and algae, animals and people) carries a light (negative) d C13/C12 signature.  Limestone carries a d C13/C12 ratio near zero.  The atmosphere, in 1977, had a d C13/C12 ratio of about -7.5; it is currently about -8.3, reflecting the influence of fossil fuels.  Oceans have a slightly positive d C13/C12 ratio of dissolved inorganic carbon, although Northern Hemisphere waters show a negative ratio due to the greater use of fossil fuels in the Northern Hemisphere.  Fossil fuel CO2 emissions and CO2 emissions from deforestation carry a very light d C13/C12, often in the range of -25 to -28 (although biogenic natural gas, which is fractionated twice, can have del 13 value in the range of -40 to -70).  The distinctive isotopic signature of CO2 from fossil fuels and deforestation is useful in tracking the movement of carbon through the atmosphere and oceans.