This is another contentious matter. How much fossil fuel do we have to burn to achieve the doubling of the pre-industrial concentration of carbon dioxide of 285 ppmv? How much fossil fuel is left?

Fuel matters are very contentious and we need to distinguish between resources and reserves. The figures presented here are from the 2008 BP Review. Resources are the total amounts of the fossil fuels as far as we know them, whether they are available for use is of no matter. Reserves are not the amounts of the fuels that have been mined or extracted from the Earth. They represent the amounts that we know are extractable under present economic conditions.

The current estimates of the reserves are given in the Table.

Fuel Reserves/Gt Carbon content/Gt
Oil 169 144
Natural Gas 129 97
Coal [Hard] 431 371 
Coal [Lignite] 417 221


The present concentration of CO2 in the atmosphere is 387 ppmv. The pre-industrial value was 285 ppmv and to increase that to twice its value, 570 ppmv we need to burn some fuel.

If all the oil reserves were burned the extra carbon content would be 144 x 0.47 = 68 ppmv. As extra carbon dioxide enters the atmosphere more photosynthesis is possible and more of the gas dissolves in the oceans so that an amount of CO2 equivalent to ~50% of the injected carbon remains in the atmosphere in any one year. This means that the atmospheric concentration of CO2 would increase to only 387 + 34 = 421 ppmv if we were to burn all the oil reserves.

Let's now burn all the natural gas reserves. This would produce an injection of 97 Gt of carbon into the atmosphere, equivalent to an increase of concentration of 97 x 0.47 = 46 ppmv and if an amount equivalent to 50% of the increase remained, the additional concentration would be 23 ppmv.

Added to the concentration produced by burning to oil this would give a total concentration of 421 + 23 = 444 ppmv.

To reach the doubling value of 570 ppmv we would have to burn some coal, to produce an increase of the concentration of 570 - 444 = 126 ppmv. This could be achieved by burning enough coal to produce an injection of CO2 that would raise the concentration by 2 x 126 = 252 ppmv. This would be provided by the injection of 252/0.47 = 536 Gt of carbon equivalent to 536, almost all of the coal reserves!

Providing the data are reasonably accurate it seems that the attainment of the doubling of the pre-industrial value of the CO2 concentration will be very expensive in terms of the present-day reserves.

These figures underline the importance of the development of alternative sources of energy. The main alternative source is the nuclear reactor using uranium or thorium fission processes and the seemingly still faraway harnessing of the hydrogen fusion reaction. In the mean time there are developments of wind and water power which are 'drops in the ocean' in terms of providing the necessary energy for the world.

Equivalence of Oil, Gas and Coal

The energy content of one tonne of oil when burned in an electricity generating station is 41.6 GJ.

One tonne of natural gas [1380 cubic metres] is equal to 1.23 tonnes oil equivalent. Methane [natural gas] possesses a high percentage of hydrogen [25%] and gives more energy per tonne than oil does.

One tonne of hard coal [anthracite & bituminous coal] is equivalent to only 0.67 tonnes of oil. This is because of the relative absence of hydrogen.

One tonne of lignite is equivalent to only 0.33 tonnes of oil.

A note about reserves and the possibility that we might run out of fossil fuel sometime in the future

From the latest consumption figures [again from the BP Statistical Review 2008] and the amounts of reserves given above it is possible to calculate the number of years it would take, at the present rate of usage to exhaust the reserves. For natural gas we get 61 years, for oil 43 years and coal 165 years. This will not come to pass, however, because we would by these times have found that the fuels are so expensive that alternatives would be in use hopefully. The figures are a warning to us to really sort out the energy requirements for future times with the further development of nuclear fission energy and the commercial realization of solar energy as the better options at the present time.