Geoscience Reference
In-Depth Information
carbon). The direct consequences of this evolution are a massive
destocking of organic fossil carbon and a degradation of the ecological
status of the continental and marine environments.
Through combustion, we combine organic fossil matter with the
oxygen from the atmosphere to form CO
2
. This is in some way a
gigantic biological respiration, even if it is not metabolic, with a result
strictly inverse to that of photosynthesis. But this respiration uses the
resource at a speed greatly exceeding that of its renewal. We consume
in a few decades matter that accumulated in deposits (e.g. coal,
petroleum and natural gas) over the course of tens of millions of years.
We observe that while coal is a sedimentary rock initially rich in
organic matter (mother rock), petroleum and natural gas accumulate in
porous deposits (sandy or carbonated rocks, the so-called reservoir
rocks) topped by impermeable layers of clay or salt deposits. These
deposits can only be formed as the result of a long process in the
course of which the mother rock reaches, by burying layer after layer
over several kilometers, the conditions in temperature and pressure
necessary for the formation of petroleum and natural gas (by natural
thermal cracking), to their very slow expulsion from the mother rock
(primary migration), and to their transfer and trapping in reservoir
rocks (secondary migration). The use of shale gas implies a
retrieval of gaseous hydrocarbons that have still not been expelled
from the mother rock. It requires artificially creating porousness in the
mother rock and a draining toward the surface to accelerate the
expulsion.
The imbalance between the speed of exploitation and natural
reconstitution of these resources does not affect the Earth system, but
will oblige the human population itself to modify its way of life. It
concerns the transition of energy, which is a major subject but which is
outside the scope of this chapter. According to some thinkers and
economists, such as Bourg and Jancovici [BOU 11, JAN 07],
a
lasting human civilization will oblige mankind to consume five times
less energy than today.
The second impact is, of course, the increase in the concentration
of greenhouse gases in the atmosphere, mainly CO
2
and CH
4
