Geology Reference
In-Depth Information
ance and survival of these flimsy, delicate molecular cages. Where temperatures are low,
pressure is less important to their survival, and methane hydrates form in cold, water-
logged terrestrial environments such as in the permafrost. But at higher temperatures,
great pressures are needed to keep them intact, and the hydrates can only be found under
a great weight of water such as along continental margins where there are enough dead
bodies to feed the hungry methanogenic bacteria. Here, methane hydrates can reach a
thickness of up to 500 metres.
Huge amounts of methane have been stored away in hydrates over millions of years.
The total store in the hydrates and in free methane gas trapped beneath them is in excess
of 10 million million (10,000 billion) tonnes, by far the biggest reservoir of organic car-
bon on the planet and about 13 times as much carbon as is held in today's atmosphere.
Huge amounts of carbon are stored in the permafrost hydrates alone—almost as much as
is held in the sum total of all the world's terrestrial biotic communities. If burnt, meth-
ane hydrates would yield more than twice the energy held in all the world's combined
reserves of oil, coal and gas.
Herein lies the danger, for methane hydrates easily fall apart when they experience
slight changes in temperature or pressure. When this happens, methane is released to
the atmosphere, warming the planet. It is almost certain that the breakdown of meth-
ane hydrates helped to warm the Earth at the end of the last ice age, and they may even
have contributed to the Permian mass extinction, when most marine creatures died, per-
haps because of suffocation as huge amounts of free methane reacted with oxygen. We
would be wise to anticipate similarly drastic methane releases in our times, due to our
frenzied emissions of planet-warming gases. We don't know quite where the tipping
points for catastrophic methane emissions are, but there are serious suggestions that a
warming of 3°C would lead to the release of 85% of the methane after a few thousand
years—effectively an irreversible change, as far as humans are concerned. The melting
is happening as you read these words. Reports just in from the permafrost of western
Siberia, one of the fastest-warming regions of the planet, reveal that millions of hectares
of frozen bog are melting, suggesting that a vast release of methane will most likely
shortly follow. A quarter of the total methane stored on land, some 70 million tonnes,
could be released from the region if its permafrost melts completely.
There is another terror associated with the dissolution of methane hydrates: tsunamis.
Stable hydrates are as solid as rock, but as soon as they fall apart, previously solid sub-
strate turns to liquid mud that creates a sub-sea slump that can whiplash the water into
coherent killer waves. The third giant Storegga slide on the Norwegian continental shelf
is an example—7,000 years ago the tsunami it created dumped sediments four metres
above the high tide line in parts of Scotland.
