Volcanic versus Anthropogenic Carbon Dioxide: An Addendum

Carbon dioxide concentrations (black dots) and carbon isotopic composition (tan dots) for ice core samples in Antarctica. Rubino et al. (2013) Last week I posted about how volcanic emissions of carbon dioxide can't be used to explain the increase in atmospheric carbon dioxide. That argument was based on comparing the flux of volcanic carbon dioxide, both worldwide and during single eruptions, and the know emissions from even just the passenger cars of the United States. The volcanic side of the equation was dwarfed by the Camrys and Expeditions of the US, but as usual, this didn't convince everyone. Just this week, data was released saying that the largest effusive volcanic eruption since 1783, the Holuhraun lava field in Iceland, emitted 6.5 million tonnes of carbon dioxide -- that sounds like a lot until you realize its only ~1% of the total emissions of the US passenger car fleet. Since then, I've been pondering the question some more and wanted to add an addendum to the argument that helps put this to bed. Consider this plot from Rubino et al. (2013) (top). It is data derived from samples ice cores (and the air bubbles trapped in ice) in the South Pole, dated from 1000 to 2012 AD. It shows two curves, one is atmospheric carbon dioxide concentrations (black dots) and one is the carbon isotopic composition of the carbon dioxide (tan dots), in δ^13C notation. This notation is a comparison of the ^13C/^12C in the carbon dioxide to a known standard material, all listed in per mil (per thousand - ‰). Now, the first think you notice is how strongly correlated they are: as atmospheric carbon dioxide concentrations in the ice go up after 1800 AD, the carbon isotopic composition of that same carbon dioxide goes down. The change in the isotopic composition is somewhat startling - the atmosphere is happily chugging along at around -6.5‰ and then nosedives to -8.5‰ by 2012. What could cause the change in atmospheric carbon dioxide isotopic compositions? Consider the following rough estimates of isotopic composition for sources of carbon dioxide in the atmosphere (mainly taken from Faure [1986]): Volcanic gas emissions: -2 to -6‰ (for CO2) Weathering of crustal rock: -7‰ Decay of organic matter: -25‰ Burning of coal: -25‰ Burning of oil: -18 to -34‰ First thing that jumps out is that organic matter is strongly negative -- that is, it sucks in a lot more ^12C relative to ^13C. So, when we burn organic material, we release a much higher proportion of 12C to 13C. So, if you take a bunch of coal and crude oil and burn it, you will add a lot of highly negative (light) carbon dioxide into the atmosphere, thus lowering the over carbon isotopic composition of the atmosphere's carbon dioxide. Now, for argument's sake, what if you wanted to explain this without invoking the burning of fossils fuels. What would need to happen to explain the increase of carbon dioxide with an accompanying decrease in its isotopic composition? Well, if you want it to be volcanoes, then you would need both a sharp increase in volcanic emissions since 1800 and a change in the composition of the carbon dioxide they emit, from around -3 to -5‰ to something much more negative. This has not happened in either case: volcanic activity hasn't increased since before 1800 and the composition of the gases they emit hasn't changed either. So, volcanoes, again, off the hook. You might argue that it is just increased decay. However, you would need an immense amount of new organic matter decay to explain the increase in carbon dioxide in the atmosphere. Where are we hiding all this decay? Nowhere, so that can be thrown out as well. There is really no way around it. Since the dawn of the industrial age, humans have taken carbon locked in organic material and released it into the atmosphere. That burning added huge volumes of carbon dioxide (in 2014, 44 billion tonnes) that all has highly negative carbon isotopic composition. Carbon dioxide goes up, the carbon isotopic composition goes down, all recoded in the ice at the poles. Next week, we'll return to our regularly schedule volcanic discussions, but this is a topic of too much importance to pass by. We live in a world where the study of the Earth and the Earth's climate are being attacked by anti-science crusaders who want to just ignore the facts. Don't let them fool you into believing what they want you to believe, to the detriment of you and the generations that follow us. {Special thanks to Adam Kent at Oregon State for pointing me towards this isotopic argument.}