Although fossil natural gas is an important source of greenhouse
gases, emitting 40−50 million tonnes per year, virtually all
anthropogenic, it is not the largest source of methane, either natural
or man-made. The following table are estimates of annual emissions of
methane from various sources, in millions of tonnes.
| Source |
Natural |
Man-made |
| Natural Wetlands |
100-200 |
|
| Rice paddies |
|
25-170 |
| Ruminants |
45-80 |
25-40 |
| Fossil NG |
|
40-50 |
| Biomass burning |
10-30 |
10-50 |
| Termites |
10-100 |
|
| Landfills & compost |
|
20-70 |
Obviously, the ranges for some of these sources are very
high, but if we add up the means for each, we get the following
situation:
| |
Natural |
Man-made |
| Mean |
288 |
250 |
It is therefore clear that man is contributing nearly as
much as nature. In fact, he is emitting more than nature because he has
reduced the emissions from natural wetlands by marsh drainage. This is
evidenced by the fact that the methane content of the atmosphere has
increased 225 per cent, from 800 ppbv (1850) to 1800 ppbv (2000). It is
evident that this increase is due to human activities, such as increased
areas of rice farming, increased cattle farming and, of course, using
fossil-fuel NG. This makes a significant difference to
climate-change.
A recent worry has been that, over the centuries,
natural gas has been generated in vast quantities in peat bogs in tundra
areas. This has been prevented from escaping by a thick layer of ice or
permafrost on the surface. The permafrost thickness varies from a few
tens of cm to over 1,000 m. In the same was as the polar ice caps are
melting because of climate change, so the permafrost is also melting
and, in doing so, releases the imprisoned natural gas. The quantity thus
released can be very significant and can be responsible for an important
increase in atmospheric greenhouse gases. This is obviously a very
serious positive feedback effect, because this will cause a further
increase in temperature, melting even more permafrost. The quantity of
methane captured in the permafrost is not known with accuracy.
Another potential problem may be with methane hydrates
or clathrates. Under given temperature and pressure conditions, methane
(and carbon dioxide) will form a clathrate which consists of a lattice
of water molecules capturing a gas molecule to form a crystalline solid,
in an unstable state. Under suitable conditions, decomposing seaweed or
other vegetable matter, falling to the bottom , produces methane and
carbon dioxide that can be captured. It is also surmised that "leaking"
gases from fossil sources may produce clathrates. These conditions are
optimised in medium-depth seas or lakes with an abundance of living
matter, such as the Gulf of Mexico or some tropical lakes. The quantity
of methane held in clathrates is unknown but accumulation over millennia
means that it is in millions of tonnes. As long as it remains at the
bottom, there is no problem. The problem starts when the conditions
cause the clathrate crystal to disintegrate, releasing the gas. This may
be caused by a change of temperature or pressure or even a mechanical
shock; the drama can occur because the shock of disintegration of one
crystal can cause a neighbouring crystal to disintegrate, in turn, and a
physical chain reaction can result, releasing enormous quantities of
gases. This has happened on rare occasions and is fairly speculative. It
should be noted that the Lake Nyos disaster in 1986 was probably not due
to a massive clathrate disintegration; the current theory is that large
volumes of carbon dioxide, from volcanic activity, super-saturated in
the water, was suddenly released.