Figure 2: A piece of methane hydrate dredged from the
seafloor. As the hydrate breaks up, it releases methane gas which
can be set alight as shown here. (Photo courtesy Gary Klinkhammer,
OSU-COAS) |
Volcanos are another source of light carbon as
carbon dioxide gas within eruptions. But this source would also
imply an enormous, and highly unlikely, amount of volcanism to
match the observations. In fact, only one source of carbon that
is isotopically light and available in large enough quantities
has been pinpointed so far, this is the reservoir of methane hydrate
deposits (Figure 2) buried on the continental shelves of the oceans
(Figure 3).
Bacteria produce methane as they decompose organic matter in
the ocean sediments, and in cold, high-pressure environments,
methane hydrates will form. This is an ice-like solid that consists
of methane surrounded by water molecules in a lattice structure.
However, if the temperature warms, or the pressure is reduced
(for instance if local sea level decreases), the hydrate will
break up and release the methane as gas which can bubble up through
the ocean and enter the atmosphere.
What would be the consequences of such a large emission of methane
into the atmosphere? At present, methane has a residence time
of about 10 years before it is oxidized to carbon dioxide. However,
the chemistry of this process is highly non-linear, and as emissions
increase, the capacity of the atmosphere to deal with the excess
methane decreases and the residence time lengthens. This can lead
to quite large increases in the methane concentration. This matters
because molecule for molecule, methane is a more powerful greenhouse
gas than carbon dioxide. The climate consequences depend very
strongly on exactly how long the extra methane hangs around.
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