Clathrate gun hypothesis

The clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures (and/or falls in sea level) can trigger the sudden release of methane from methane clathrate compounds buried in the seabeds and their permafrost, which, because the methane itself is a powerful greenhouse gas, in turn causes further temperature rise and further methane clathrate destabilization – in effect initiating a runaway process, as irreversible once started as the firing of a gun.

In its original form, the hypothesis proposed that the "clathrate gun" could cause abrupt runaway warming in a timescale less than a human lifetime, and might be responsible for warming events in and at the end of the last ice age. This is now thought unlikely.

However there is stronger evidence that runaway methane clathrate breakdown may have caused drastic alteration of the ocean environment and the atmosphere of earth on a number of occasions in the past, over timescales of tens of thousands of years; most notably in connection with the Permian extinction event, when 96% of all marine species became extinct 251 million years ago.

Mechanism

Methane clathrate, also known incorrectly as methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Extremely large deposits of methane clathrate have been found under sediments on the ocean floors of the Earth. The sudden release of large amounts of natural gas from methane clathrate deposits in runaway climate change could be a cause of past, future, and present climate changes. The release of this trapped methane is a potential major outcome of a rise in temperature; it is thought that this is a main factor in the global warming of 6°C that happened during the end-Permian extinction, as methane is much more powerful as a greenhouse gas than carbon dioxide (despite its atmospheric lifetime of around 12 years, it has a global warming potential of 62 over 20 years and 23 over 100 years). The theory also predicts this will greatly affect available oxygen content of the atmosphere.

Possible release events

Two events possibly linked in this way are the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. It may also have had a role in the sudden warm-up of "Snowball Earth", 630 million years ago. However, warming at the end of the last ice age is thought not to be due to methane release.

Related mechanism: dissolved methane release

Focusing on the Permian-Triassic boundary, Gregory Ryskin explores the possibility that mass extinction can be caused by an extremely fast, explosive release of dissolved methane (and other dissolved gases such as carbon dioxide and hydrogen sulfide) that accumulated in the oceanic water masses prone to stagnation and anoxia (e.g., in silled basins).

Current outlook

Most deposits of methane clathrate are in sediments too deep to respond rapidly, and modelling by Archer (2007) suggests the methane forcing should remain a minor component of the overall greenhouse effect. Clathrate deposits destabilize from the deepest part of their stability zone, which is typically hundreds of metres below the seabed. A sustained increase in sea temperature will warm its way through the sediment eventually, and cause the deepest, most marginal clathrate to start to break down; but it will typically take of the order of a thousand years or more for the temperature signal to get through.

One exception, however, may be in clathrates associated with the Arctic ocean, where clathrates can exist in shallower water stabilized by lower temperatures rather than higher pressures; these may potentially be marginally stable much closer to the surface of the sea-bed, stabilized by a frozen 'lid' of permafrost preventing methane escape. Recent research carried out in 2008 in the Siberian Arctic has shown millions of tons of methane being released, apparently through perforations in the seabed permafrost, with concentrations in some regions reaching up to 100 times normal. The excess methane has been detected in localized hotspots in the outfall of the Lena River and the border between the Laptev Sea and the East Siberian Sea. Some melting may be the result of geological heating, but more thawing is believed to be due to the greatly increased volumes of meltwater being discharged from the Siberian rivers flowing north. Current methane release has previously been estimated at 0.5 Mt per year. Shakhova et al. (2008) estimate that not less than 1,400 Gt of Carbon is presently locked up as methane and methane hydrates under the Arctic submarine permafrost, and 5-10% of that area is subject to puncturing by open taliks. They conclude that "release of up to 50 Gt of predicted amount of hydrate storage highly possible for abrupt release at any time". That would increase the methane content of the planet's atmosphere by a factor of twelve, equivalent in greenhouse effect to a doubling in the current level of CO2.

In 2008 the United States Department of Energy National Laboratory system and the United States Geological Survey's Climate Change Science Program both identified potential clathrate destabilization in the Arctic as one of four most serious scenarios for abrupt climate change, which have been singled out for priority research. The USCCSP released a report in late December 2008 estimating the gravity of this risk.

Possible outcomes

According to Gregory Ryskin, a sudden release of methane from the ocean may lead to either global cooling or global warming. The explosions and burning of methane would produce lots of smoke and dust, which would lead to global cooling. The methane and carbon dioxide would "create the greenhouse effect, which may lead to global warming". Professor Ryskin writes that it is "difficult to predict" whether global cooling or warming would result.

The evolution of dust and smoke, if it caused global cooling would likely only last a short time before the particulates washed out of the atmosphere. Then the raised levels of Methane and the derivative Carbon Dioxide would take over. The likely result would be a flick-flack of extra cold and extra warm years, arguably more devestating to crop production than a trend in one direction or the other.

It may be possible to explain past marine extinctions by the scrubbing effect. If an inert gas is bubbled through water, the surface of each bubble acts as a semi permeable membrane. Gases diffuse across this membrane according to their concentration inside and outside the bubble. The result of bubbling methane through the ocean is to scrub out the oxygen, leading to ocean anoxia.

The consequences of a methane-driven oceanic eruption for marine and terrestrial life are likely to be catastrophic. Figuratively speaking, the erupting region "boils over," ejecting a large amount of methane and other gases (e.g., CO2, H2S) into the atmosphere, and flooding large areas of land. Whereas pure methane is lighter than air, methane loaded with water droplets is much heavier, and thus spreads over the land, mixing with air in the process (and losing water as rain). The air-methane mixture is explosive at methane concentrations between 5% and 15%; as such mixtures form in different locations near the ground and are ignited by lightning, explosions and conflagrations destroy most of the terrestrial life, and also produce great amounts of smoke and of carbon dioxide. Firestorms carry smoke and dust into the upper atmosphere, where they may remain for several years; the resulting darkness and global cooling may provide an additional kill mechanism. Conversely, carbon dioxide and the remaining methane create the greenhouse effect, which may lead to global warming. The outcome of the competition between the cooling and the warming tendencies is difficult to predict.

In fiction

* The book Mother of Storms by John Barnes offers a fictional example of catastrophic climate change caused by methane clathrate release.
* In The Life Lottery by Ian Irvine unprecedented seismic activity triggers a release of methane hydrate, reversing global cooling.
* The hypothesis is the basis of an experiment in the Playstation 2 game Death By Degrees.
* In Transcendent by Stephen Baxter, averting such a crisis is a major plotline.
* The novel The Black Silent by author David Dun features this idea as a key scientific point.
* In the anime Ergo Proxy, a string of explosions in the methane hydrate reserves wipes out 85% of human life on Earth.
* The novel The Far Shore of Time by Frederik Pohl features an alien race attempting to destroy humanity by bombing the methane clathrate reserves, thus releasing the gas into the atmosphere.

From http://en.wikipedia.org/