Saturday, March 19, 2011

Atmospheric CO2 levels due to fossil fuels

In my last post, I provided a simple model that predicted when the Earth’s fossil fuels will be exhausted.  The model, or perhaps I should say scenario, was based on current reserves and current rates of consumption, with switching of energy supply from oil to gas when the oil is depleted, and from gas to coal when the oil and gas are depleted.

Today’s post is concerned with the atmospheric CO2 levels that will result from combustion of these remaining fossil fuels.  I’ll take as given the data and methodology in my last post and I’ll keep any further assumptions as simple as possible.  With an exercise like this, I’m trying to make broad inferences rather than dwelling on fine details.  I’m only going to consider CO2 emissions from consumption of fossil fuels; emissions from other sources (such as fugitive emissions, leakage, agriculture, cement production, …) are not included here.  Also ignored will be feedback effects such as CO2 emissions from melting of permafrost.

The starting point is a few facts about the Earth’s atmosphere:

Mass of the atmosphere                      5.144×1018 kg [1]
Molar mass of dry air                          0.0290 kg/mol [2]
Mols of air in atmosphere                   1.77×1020       
Molar mass of CO2                             0.044 kg/mol
Atmospheric concentration of CO2    386 ppm by volume (for 2008)
Mass of CO2 in atmosphere               1.77×1020×386×10-6×0.044 = 3.01×1015 kg   

That estimate for the atmospheric mass of CO2 agrees reasonably well with other estimates, e.g. it’s a 5% discrepancy from the figure given by [3] (3.16×1015 kg).  Perhaps the neglect of water vapour explains some of the discrepancy?  In any case, for the purpose of this post, my estimate is accurate enough.  I’ll need the above estimates to convert the mass of CO2 in the atmosphere into parts per million by volume.

Next, I need a result for how much CO2 is produced when a barrel of oil is consumed (note consumed, not burnt).  Fortunately Jim Bliss [4] has done this calculation very nicely.  He points out that when oil is consumed, not all of it is burned, thereby releasing CO2.  Some goes to bitumen for roads, some goes to the petrochemical industry, and he estimates that of the oil in a barrel (169 litres) only 101 kg is used as a fuel.  Bliss continues:
 
“When fuel oil is burned, it is converted to carbon dioxide and water vapour.  Combustion of one kilogram of fuel oil yields 3.15 kilograms of carbon dioxide gas.  an average barrel of crude oil will produce a minimum of 317kg of CO2 when consumed.”

We know that 2008 world oil consumption was 84.5 million barrels per day, with each barrel producing 317 kg of CO2 as Bliss has shown.

Next we need similar estimates for natural gas.  If natural gas is assumed to consist only of methane that combusts according to
CH4 + 2O2 → CO2 + 2H2O,
then since the molecular masses of CH4 and CO2 are 0.016 kg and 0.044 kg respectively, each kg of methane will produce 0.044/0.016 = 2.75 kg CO2.   Now, 1 m^3 of natural gas contains 1000/22.4 = 44.6 mols with mass 0.714 kg (under standard conditions, 0°C and 1.00 atm pressure).  So each m^3 of natural gas produces 0.714×2.75 = 1.96 kg CO2, which is what I need since I know the natural gas consumption, currently 3,018.7 billion m^3/yr. [5]

Coal has a wide variation in water and ash content, which affects the carbon content by weight.  I’ll make the assumption that on average commercial coal contains 70% Carbon by weight.  One mol of CO2 (molecular weight 0.044 kg) is produced for each mol of C (molecular weight 0.012 kg) used.  Therefore each kg of Carbon produces 0.044/0.012 = 3.7 kg of CO2, so each kg of coal will produce 0.7×3.7 = 2.6 kg CO2.  According to [6], world coal production in 2008 was 6,781 million tonnes (and increasing by several per cent per year, although that won’t be taken into account here), so the CO2 released would be 17.6 gigatonnes/yr.

In summary, the following are my estimates for CO2 currently released from the various fossil fuels (in gigatonnes/yr): oil 9.8, natural gas 5.9, coal 17.6.   

By way of comparison, reference [6] cites the following CO2 releases as provided by the Energy Information Agency: oil 11.0 Gt/yr, natural gas 5.8 Gt/yr, coal 11.4 Gt/yr.  Reference [8], directly sourced to the EIA gives CO2 emissions from coal as 12.5 Gt/yr in 2007.  Another figure comes from reference [9], which states

“In the entire world, annual CO2 emissions approach 30 billion tons per year, and it is safe to say about half of these emissions come from coal”

Perhaps my CO2-from-coal estimates are a bit high, but I think my overall CO2 release figures should be sound enough for me to proceed with my scenario, particularly since I am assuming no increase in emissions in the future.

Next, we need to know the airborne fraction, AF, or how much of the CO2 remains in the atmosphere after CO2 absorption by ocean and land.  Following [7], take AF = 0.45.

And finally we need to accommodate the assumptions in the scenario that once the oil has gone, the energy demand is switched to natural gas, and once both oil and gas are gone the energy demand is switched to coal.  This issue relates to the relative Carbon intensities of oil, natural gas and coal, and is required only after 46 years, which is how long my previous post predicted that oil will last.  Simple proportionality arguments were used here.

Figure 1 shows the results of these calculations, with my scenario forecasting the atmospheric CO2 levels for the next 75 years, by which stage the model says all the fossil fuels have been consumed.  The end value at 2084 is 560 ppm, which climate science would say will lead to significant warming and raised sea levels.  Also shown on Figure 1 is the historical CO2 record since 1960, as measured at Mauna Loa [10].


Figure 1: Atmospheric CO2 levels at Mauna Loa over the
past 60 years and as predicted by this scenario.

The projected CO2 levels in Figure 1 look quite plausible, especially for the near-term future.  The slight kink in the scenario curve at year 2062 is when the oil and gas have both been depleted and all the fossil fuel demand is met by coal, which has a higher CO2 intensity than oil or gas.  In my judgement, the scenario is likely to underestimate atmospheric CO2 levels, especially under a business-as-usual scenario over several decades. 

I think these results confirm important messages from the professional climate science community, of which I’m not a member.  The estimates vividly illustrate the enormity, perhaps impossibility, of the task to stabilise CO2 levels at a level like 450 ppm.  They also show the gravity of the situation facing humankind if we don’t break our Carbon habit.

In doing this post, I came across several excellent web sites devoted to atmospheric CO2.  Reference [11] is a good exemplar.

Acknowledgement: thanks to Robin Johnson’s Economics Web Page for suggesting I prepare these estimates.

References:

[1] K.E. Trenberth & C.J. Guillemot, J Geophysical Research, 99 (1994), 23079-23088
[5] BP Statistical Review of World Energy.

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