The ultimate limits to carbon burning: an order of magnitude calculation

Total amount of fossil carbon on the Earth, from Vanderbroucke and Largeau (1)

During the past few years, the development of "shale gas" and "shale oil" in the US, generated a wave of optimism that spread widely in the mediasphere. It was common to hear of "a century of abundance" or even of "centuries" provided by these new sources. However, with the recent collapse of the oil market, these claims seem to have gone the same way as those of the sightings of the Loch Ness monster. But there remains a point to be made: what is exactly the limit to what we can burn? Could we really keep burning for centuries? Or, maybe, even for millennia or more?

Let's see if we can make a calculation, at least in terms of order of magnitudes. The first question is how much fossil carbon do we have on this planet. The total is reported to be about 1.5x10⁺¹⁶ t (metric tons), mainly in the form of kerogen, a product of the decomposition of organic matter which is a precursor to the formation of fossil fuels (gas, oil, and coal) (2) .

It looks like a lot of carbon, especially if we compare this number with the amount we are burning nowadays. The data reported by CDIAC (Carbon Dioxide Information Analysis Center) report 9.2x10⁺⁹ t of carbon transformed into CO2 as the result of fossil fuel burning (gas+oil+coal) in 2013. As an order of magnitude estimate, at this rate, we could go on burning for more than a million years before truly running out of fossil carbon.


But, obviously, that's not possible. Simply, there is not enough oxygen in the atmosphere to burn all the existing fossil carbon. The total amount of free oxygen is estimated to be about 1.2x10⁺¹⁵ t or 3.7x10⁺¹⁹ mol O₂⁠ (a "mole" is a unit used in chemistry to compare the amount of reactants in chemical reactions). One mole of molecular oxygen will react with exactly one mole of carbon to form carbon dioxide and, since 1.5x10⁺¹⁶ t of carbon correspond 1.25x10 ⁺²¹ mol, there follows that we cannot possibly burn more than about 1% of the existing fossil carbon. Instead of a million years, we are down to about 10,000 years.

Of course, then, burning that 1% of carbon would mean to use up all the oxygen of the atmosphere and that would be bad for us, no matter how much we need fossil fuels. In practice, we can't use up more than a few percent of the atmospheric oxygen; otherwise the effect on human health and on the whole ecosphere would be likely disastrous. Let's say that we are willing to bet that a 5% loss is still safe, even though nobody could be sure about that. It means that we only have 500 years or so to keep on burning before we start feeling symptoms of suffocation. But the story doesn't end here. 

So far, we have been reasoning in terms of the total amount of fossil carbon as if it were all burnable, but is it? Kerogen, the main component of this carbon, can be combined with oxygen producing a certain amount of heat (3) but it can hardly be considered as a fuel, because it would be very expensive to extract and the net energy yield would be modest or even negative. In 1997, Rogner (4) carried out an extensive survey of the carbon resources potentially usable as fuel. At page 149 of this link, we can find an aggregate estimate of 9.8x10⁺¹¹ t of carbon as "reserves" and up to 5.5x10⁺¹² t of "resources", the latter defined as not economically exploitable at the current prices. "Additional occurrences" are reported to a possible amount of 1.5x10⁺¹³ t of carbon, but that is a rather wild estimation. If we limit ourselves to proven reserves, we see that at the present rate of about 1x10⁺¹⁰ t/year we would have about a century of carbon to go.

We are not finished, yet. We now need to consider how much carbon we can combine with oxygen before the increased greenhouse effect caused by the resulting carbon dioxide generates irreversible changes in the Earth's climate. The "tipping point" of the climate catastrophe is often estimated as that corresponding to a temperature increase of 2 deg C and, in order not to exceed it, we should not release more than about 10⁺¹² t of CO2 in the atmosphere. That corresponds to 3.7x10⁺¹¹ t of carbon (5). This is about one third of Rogner's global reserve estimate. So, at this point, we don't have a century any more, but only about three-four decades (and note that the estimation of what we can burn and still avoid catastrophe may have been optimistic. See also here for a more detailed estimate that takes into account different kinds of fuels).

You see how misleading it can be to list carbon resources as if they were soldiers lined up for battle. Not everything that exists inside the Earth's crust can be extracted and burned and we can't afford to extract and burn everything that could be extracted without wrecking the atmosphere. Taking into account the various factors involved, we went down from more than a million years of supply to just a few decades.

But, of course, calculating the number of remaining years at constant production rates is also misleading. In practice, fuel production rates have never been constant over history; rather, the production tends to follow a "bell shaped" curve that peaks and then declines. Today, we may be close to the peak (See e.g. here). Will the impending decline save us from catastrophic climate change? At present, we cannot say; too many are the uncertainties involved in these estimates. What we can say is that we are not facing centuries of abundance, but a decline which might even very rapid, considering the possibility of a "Seneca collapse." 

In short, the age of fossil fuels is ending. It is time to take note of that and move to something else.




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(1) M. Vandenbroucke, C.     Largeau, Kerogen origin, evolution and structure, Organic Geochemistry, Volume 38, Issue 5, May 2007, Pages 719-833, ISSN 0146-6380, http://dx.doi.org/10.1016/j.orggeochem.2007.01.001.  

2. Falkowski, P., R.J. Scholes, E. Boyle, J. Canadell, D. Canfield, J. Elser, N. Gruber, et al. 2000. “The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System.” Science 290 (5490) (October 13): 291–296. doi:10.1126/science.290.5490.291. http://www.sciencemag.org/content/290/5490/291.abstract.

(3) Muehlbauer, Michael J., and Alan K. Burnham. 1984. “Heat of Combustion of Green River Oil Shale.” Industrial & Engineering Chemistry Process Design and Development 23 (2) (April): 234–236. doi:10.1021/i200025a007. http://dx.doi.org/10.1021/i200025a007.

(4) Rogner, H-H. 1997. “AN ASSESSMENT OF WORLD HYDROCARBON RESOURCES.” 
Annual Review of Energy and the Environment 22 (1) (November 28): 217–262. doi:10.1146/annurev.energy.22.1.217. http://www.annualreviews.org/doi/abs/10.1146/annurev.energy.22.1.217?journalCode=energy.2.

(5) IPCC. Climate Change 2013: The Physical Science Basis. (Cambridge University Press, 2014).