Image via MorgueFile
As you may have heard, the CO2 concentration in Earth’s atmosphere has now risen to 400 parts per million (ppm). Although the media has covered this story and pointed out that “this is the first time in human history that atmospheric CO2 has reached this level,” they have in general failed to explain how we know that the extra atmospheric CO2 is from fossil fuel burning rather than from natural causes.
Without such an explanation, the door stands open for climate science doubters to ask, what proof do we have that the CO2 actually came from fossil fuels? All kinds of noxious gases erupt from below ground. Maybe the extra CO2 came from the Earth’s mantle by way of volcanic eruptions. Sounds reasonable . . .
But if you want a ready answer to this plausible sounding objection, you can find it in my recent book, Climate Myths: The Campaign Against Climate Science(Northbrae Books, 2013), in which this and other spurious arguments by climate science deniers are dispelled. As Myth #3 states (from Climate Myths):
Carbon in the form of carbon dioxide is released from the Earth’s mantle into the atmosphere through volcanic hot spots, volcanic subduction zones (places where the great plates of the Earth’s crust dive beneath each other), and through midocean ridges.
The element carbon has three naturally occurring isotopes. Isotopes are related forms of the same element. They behave the same chemically and differ only in mass. The isotopes of carbon are carbon-12, carbon-13, and carbon-14. They occur in nature in known ratios.
Carbon-12 is by far the most common, accounting for almost 99 percent of all the carbon on Earth. Most of the other 1 percent of the carbon is carbon-13. Unlike carbon-12 and carbon-13, which are stable, carbon-14 is radioactive and extremely rare in nature.
Plants preferentially utilize the lighter carbon-12, and so carbon derived from fossil fuels—which are largely formed from dead, decayed, and compressed ancient plant matter—have a higher carbon-12 to carbon-13 ratio than carbon from other sources. Fossil fuels—being derived from the decayed remains of ancient plants—have the same carbon isotope ratio as plants. Thus the increase in this ratio that has been detected in atmospheric carbon dioxide samples over time confirms that the carbon dioxide increases of concern are coming from the burning of fossil fuels and other plant material, rather than from an increase in outgassing from the Earth’s mantle.
Although the outgassing of the Earth’s mantle has released vast quantities of carbon dioxide since the planet’s formation billions of years ago, the outgassed carbon dioxide does not exhibit an enrichment in carbon-12. It therefore cannot account for the enrichment of carbon-12 found in the atmosphere.
The outgassing of the Earth’s mantle as it relates to climate and atmospheric chemistry is explained much more fully in the Intergovernmental Panel on Climate Change’s Working Group I Report and is also well explained on the award-winning science website, http://www.Realclimate.org.
Yet another type of information supports the conclusion that the recent increase in the atmosphere’s carbon dioxide content is being caused by the burning of fossil fuels and the destruction of forests. It is corroborated by data on how much fossil fuel has been burned and how many tons of carbon-containing forests have been destroyed.
The tonnage of carbon dioxide in the atmosphere corresponds well to the quantity of carbon dioxide generated by the burning of fossil fuels and forests (some 500 billion metric tons) minus the Earth’s well-understood natural carbon dioxide removal processes, just as it clearly does not correspond to any outgassing of carbon dioxide from the Earth’s mantle.
To sum up, based on reasonable scientific approximations of the quantity of carbon dioxide added and removed from the atmosphere, it’s possible to calculate the concentration of carbon that would be expected in the atmosphere if the source of the increase were the burning of fossil fuels and clearance of forest. Sure enough, these estimates are in close agreement with the actual concentration of carbon dioxide in the atmosphere, which as of mid-2012 was 394 ppm [And is now 400ppm].