used. There are two major problems with this tremendous reliance on fossil fuels. One of
these problems is that fossil fuel sources are running out. There is a lot of debate about
how fast these resources are being depleted and when they will be gone (or become so
costly that they are impossible to afford). Indications are that world petroleum production
has peaked around the year 2000, give or take several years. Resources of coal are
considerably more abundant, but this gets into the second major problem with fossil
fuels, their contribution to greenhouse gas atmospheric carbon dioxide. As discussed in
Chapter 8, atmospheric carbon dioxide levels have grown by about 50% during the last
150 years. Now at a level of about 380 parts per million by volume in the atmosphere,
these levels are increasing by about 1 part per million per year. Most reputable models for
the effect this will have upon global temperatures project an increase of several degrees
C within the next several decades as atmospheric CO 2 levels increase. Although such a
temperature increase seems small, its effects upon global climate would be profound and
perhaps even catastrophic.
Fossil fuels vary in their contribution to greenhouse gas carbon dioxide; the lower
the content of H, the greater the contribution to atmospheric carbon dioxide. Consider
the combustion of an atom of carbon in natural gas, liquid petroleum, and coal. Natural
gas consists of molecules of methane, CH 4 , so the combustion of an atom of carbon in
methane can be represented by the chemical reaction
CH 4 + 2O 2 → CO 2 + 2H 2 O + energy (6.4.1)This reaction produces 1 molecule of CO 2 , but it also involves the combustion of 2
hydrocarbon-bound H atoms, which also produces a lot of energy. In the case of liquid
petroleum, such as gasoline or diesel fuel, the approximate ratio of C to H atoms is 1:2,
rather than 1:4 as in methane. So the combustion of a petroleum hydrocarbon containing
1 C atom can be represented as
CH 2 +^3 /2O 2 → CO 2 + H 2 O + energy (6.4.2)
In this case only half as much hydrocarbon-bound H is burned, so significantly less
energy is produced per C atom than in the combustion of natural gas. Coal is even worse.
Despite its being a black solid that would lead one to believe coal is pure carbon, it is in
fact a hydrocarbon with an approximate simple formula of CH0.8. So the combustion of
an atom of carbon in coal can be represented in a somewhat cumbersome manner as
CH0.8 + 1.2O 2 → CO 2 + 0.4H 2 O + energy (6.4.3)Less hydrocarbon-bound hydrogen is available to burn in coal than in petroleum and
much less is available than in methane, so the amount of carbon dioxide emitted to the
atmosphere per unit energy produced from coal is higher than with petroleum and much
higher than with natural gas.
The problem with world energy supplies is clear; the solution is not so obvious.
Industrialized societies have become dependent upon fossil carbon sources for energy
Chap. 6. Energy Relationships 143