First, we can calculate the amount of carbon in the atmosphere in the form of CO2.

Earth's surface area = 3.1416 * diameter^2
                     = 3.1416 * (7900 miles)^2
                     = 1.96 * 10^8 sq mi = 5.47 * 10^15 sq ft = 7.87 * 10^17 sq in

The weight of the atmosphere equals the surface area times the pressure
                     = (7.87 * 10^17 sq in) * (14.7 lb/sq in)
                     = 1.157 * 10^19 lb

The volume fraction of CO2 in the atmosphere is 380 ppm = 0.00038

The density ratio between CO2 and air = 44/28.97 = 1.519

So the weight fraction of CO2 in the atmosphere = 1.519 * 0.00038 = 0.000577

And the weight of CO2 = 0.000577 * 1.157 * 10^19 lb = 6.68 * 10^15 lb
                      = 3,036,000 million metric tons (MMT)

Of that amount, carbon is 12/44 * 3,036,000 = 827,000 MMT

Now we can estimate the amount of carbon available in known reserves of fossil fuels.

Here are the data we'll use:

The data for determing carbon content comes from http://bioenergy.ornl.gov/papers/misc/energy_conv.html

    * coal (average) = 25.4 metric tonnes carbon per terajoule (TJ)
            1.0 metric tonne coal = 746 kg carbon
    * oil (average) = 19.9 metric tonnes carbon / TJ
    * 1.0 cubic meter natural gas (methane) = 0.49 kg carbon

Carbon content of coal = .746 MT/MT * 910,000 million MT = 679,000 million MT = 679,000 MMT

Carbon content of oil = 2.1757 * 10^10 barrels * 6.1 * 10^9 J/barrel * 19.9 MT(carbon)/ 10^12 J

   = 2.64 * 10^9 MT(carbon) = 2640 MMT

Carbon content of gas = 2.044 * 10^14 CF / (35.31 CF/CM) * .49 * 10^-3 MT(carbon)/CM

   = 2.84 * 10^9 MT(carbon) = 2840 MMT

So, the total amount of carbon in the known reserves is 684,000 MMT

Now we will assume that all the known reserves of these fuels are burned and all the resulting CO2 remains in the atmosphere; none of it dissolves in the ocean or is stored anywhere else. That means an increase by a factor of

                    F = (827,000 MMT + 684,000 MMT) / 827,000 MMT = 1.82

Atmospheric scientists point to a saturation effect, in which temperature rise has a logarithmic relationship to the concentration of greenhouse gases.[31]

This ignores other influences, such as solar activity, local temperatures, time lags, clouds, increased rainfall, the effects of other greenhouse gases, etc. But to illustrate the effects of increased greenhouse-gas concentration, we'll compare conditions in 2005 with those in 1900, and calculate what the effects would be for higher concentrations.

In 1910, referring to the charts shown below, the global-average temperature was 0.5 deg C below the average for 1961-1990, and the CO2 concentration was 300 parts per million. In 2005, the corresponding data were 0.45 deg C above the average and 380 PPM.

Because the concentration rose to 380 ppmv, the temperature rose 0.95 deg C.

                    DT = K * ln(P2 / P1)
                    where DT = temperature rise, deg C
                          K  = constant for this case
                          P1 = initial concentration of CO2 = 300 ppmv
                          P2 = endpoint concentration of CO2, ppmv

                     K = DT / ln(P2 / P1) = 0.95 / ln (380 / 300) = 4.02

So, if we raise the concentration by a factor of 1.82, the new concentration will be

                     P2 = 1.82 * 380 = 694 ppmv

                     DT = 4.02 * ln(694 / 300) = 3.37

which is 2.4 degrees higher than the present temperature. Keep in mind that this calculation is overly simplified, and makes two erroneous assumptions: that no more reserves will be found, and that none of the CO2 will leave the atmosphere. The only point of the calculation is to show that a runaway temperature rise isn't one of the things we have to worry about.

REFERENCE

31. forecast.uchicago.edu/archer.ch4.greenhouse_gases.pdf