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 seabeds and permafrost which, because the methane itself is a powerful greenhouse gas, leads to 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.
Methane clathrate, also known commonly as methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Potentially large deposits of methane clathrate have been found under sediments on the ocean floors of the Earth, although there are many orders of magnitudes in between the estimates of various experts. In fact, the existence of vast oceanic methane clathrate formation is uncertain and usually only based on reflective seismology and pieces larger than 10 cm have only been recovered from three sites.
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 72 over 20 years and 25 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 not thought 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 kind of exception is recent changes to the Gulf Stream causing widespread gas hydrate destabilization: "It is unlikely that the western North Atlantic margin is the only area experiencing changing ocean currents; our estimate of 2.5 gigatonnes of destabilizing methane hydrate may therefore represent only a fraction of the methane hydrate currently destabilizing globally."
Another kind of exception is 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. 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 [is] 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.
According to data released by the EPA atmospheric methane (CH4) concentrations (ppb) remained between 400–800ppb (between years 600,000 BC to 1900) and since 1900 have risen to levels between 1600–1800ppb.
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 an alternating series of extra cold and extra warm years, arguably more devastating 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 deplete the oxygen dissolved in the water, leading to ocean anoxia. In addition to the scrubbing effect, gaseous methane will react with dissolved oxygen generating carbon dioxide and further depressing the oxygen content of the water. This process acts as a buffer towards undersea methane reaching the surface but causes further carbon dioxide emissions.
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 science fiction novel 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.
- The novel The Swarm by Frank Schätzing features what first appear to be freak events related to the world's oceans.
See also 
- Kennett, James P.; Cannariato, Kevin G.; Hendy, Ingrid L.; Behl, Richard J. (2003). Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis. Washington DC: American Geophysical Union. ISBN 0-87590-296-0.
- Behl, Richard J. (7 April 2000). "Carbon Isotopic Evidence for Methane Hydrate Instability During Quaternary Interstadials". Science 288 (5463): 128–133. doi:10.1126/science.288.5463.128.
- Sowers, Todd (10 February 2006). "Late Quaternary Atmospheric CH4 Isotope Record Suggests Marine Clathrates Are Stable". Science 311 (5762): 838–840. doi:10.1126/science.1121235. PMID 16469923.
- Severinghaus, Jeffrey P.; Whiticar, MJ; Brook, EJ; Petrenko, VV; Ferretti, DF; Severinghaus, JP (25 August 2006). "Ice Record of 13C for Atmospheric CH4 Across the Younger Dryas-Preboreal Transition". Science 313 (5790): 1109–12. doi:10.1126/science.1126562. PMID 16931759.
- "The Day The Earth Nearly Died". Horizon. 2002. BBC. http://www.bbc.co.uk/science/horizon/2002/dayearthdied.shtml.
- Collet, Timothy S.; Kuuskraa, Vello A. (1998). "Hydrates contain vast store of world gas resources". Oil and Gas Journal 96 (19): 90–95.
- Laherrere, Jean (May 03, 2000). "Oceanic Hydrates: More Questions Than Answers". Energy Exploration & Exploitation 18 (4): 349–383. doi:10.1260/0144598001492175. ISSN 0144-5987.
- Benton, Michael J.; Twitchet, Richard J. (July 2003). "How to kill (almost) all life: the end-Permian extinction event". Trends in Ecology & Evolution 18 (7): 358–365. doi:10.1016/S0169-5347(03)00093-4.
- Kennedy, Martin; Mrofka, David; Von Der Borch, Chris (2008). "Snowball Earth termination by destabilization of equatorial permafrost methane clathrate" (PDF). Nature 453 (7195): 642–645. Bibcode:2008Natur.453..642K. doi:10.1038/nature06961. PMID 18509441.
- Ryskin 2003
- Archer, D. (2007). "Methane hydrate stability and anthropogenic climate change" (PDF). Biogeosciences 4 (4): 521–544. doi:10.5194/bg-4-521-2007. See also blog summary.
- Phrampus, Benjamin J.; Hornbach, Matthew J. (25 October 2012). "Recent changes to the Gulf Stream causing widespread gas hydrate destabilization". Nature 490 (7421): 527–530. doi:10.1038/nature11528. PMID 23099408.
- Compare: "Methane bubbling through seafloor creates undersea hills". Monterey Bay Aquarium Research Institute. 5 February 2007.
- Connor, Steve (September 23, 2008). "Exclusive: The methane time bomb". The Independent. Retrieved 2008-10-03.
- Connor, Steve (September 25, 2008). "Hundreds of methane 'plumes' discovered". The Independent. Retrieved 2008-10-03.
- Translation of a blog entry by Örjan Gustafsson, expedition research leader, 2 September 2008
- Shakhova, N.; Semiletov, I.; Salyuk, A.; Kosmach, D.; Bel'cheva, N. (2007). "Methane release on the Arctic East Siberian shelf". Geophysical Research Abstracts 9: 01071.
- Shakhova, N.; Semiletov, I.; Salyuk, A.; Kosmach, D. (2008). "Anomalies of methane in the atmosphere over the East Siberian shelf: Is there any sign of methane leakage from shallow shelf hydrates?" (PDF). Geophysical Research Abstracts 10: 01526.
- Mrasek, Volker (17 April 2008). "A Storehouse of Greenhouse Gases Is Opening in Siberia". Spiegel International Online.
- IMPACTS: On the Threshold of Abrupt Climate Changes, Lawrence Berkeley National Laboratory News Center, 17 September 2008
- CCSP 2008
- "Atmosphere Changes". US Environmental Protection Agency. Retrieved 18 February 2012.
Further reading 
- Benton, Michael J.; Richard J. Twitchett (July 2003). "How to kill (almost) all life: the end-Permian extinction event" (PDF). TRENDS in Ecology and Evolution 18 (7): 358–365. doi:10.1016/S0169-5347(03)00093-4., cited by 21 other articles.
- Svensen, Henrik; Planke, Sverre; Malthe-Sørenssen, Anders; Jamtveit, Bjørn; Myklebust, Reidun; Eidem, Torfinn Rasmussen; Rey, Sebastian S. (3 June 2004). "Release of methane from a volcanic basin as a mechanism for initial Eocene global warming". Nature 429 (6991): 542–5. Bibcode:2004Natur.429..542S. doi:10.1038/nature02566. PMID 15175747.
- Thomas, Deborah J.; James C. Zachos, Timothy J. Bralower, Ellen Thomas and Steven Bohaty (December 2002). "Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum" (PDF). Geology 30 (12): 1067–70. Bibcode:2002Geo....30.1067T. doi:10.1130/0091-7613(2002)030<1067:WTFFTF>2.0.CO;2. ISSN 0091-7613.
- Ryskin, Gregory (September 2003). "Methane-driven oceanic eruptions and mass extinctions" (PDF). Geology 31 (9): 741–4. Bibcode:2003Geo....31..741R. doi:10.1130/G19518.1.
- Archer, D.; Buffett, B. (2004). "Temperature sensitivity and time dependence of the global ocean clathrate reservoir". American Geophysical Union, Fall Meeting. http://adsabs.harvard.edu/abs/2004AGUFMGC51D1069A.
- "'Fountains' of methane 1,000m across erupt from Arctic ice — a greenhouse gas 30 times more potent than carbon dioxide". Daily Mail. 13 December 2011.
- "Homepage of Dr. Paul Wignal, Reader in Palaeoenvironments". University of Leeds.
- James P. Kennett (20 May 2005). "Abstract: Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis". Paul A. Witherspoon Distinguished Seminar Series. Earth Sciences Division, Lawrence Berkeley National Laboratory.
- Hudson, Geoff (24 May 2009). "The trigger for the clathrate gun". Ockham's Razor. Radio National, Australian Broadcasting Corporation.
- Chakoumakos, Bryan C. (2004). "Preface to the Clathrate Hydrates special issue" (PDF). American Mineralogist 89: 1153–4.
- CCSP (2008). Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Clark, P.U., A.J. Weaver (coordinating lead authors), E. Brook, E.R. Cook, T.L. Delworth, and K. Steffen (chapter lead authors). Reston VA: U.S. Geological Survey.
- Adam, David (14 January 2010). "Arctic permafrost leaking methane at record levels, figures show". Guardian.
- Harris, Richard (26 January 2010). "Methane Causes Vicious Cycle In Global Warming". NPR.
- Methane: A Scientific Journey from Obscurity to Climate Super-Stardom Good Sept. 2004 background report from NASA GISS
- Historical events
- Biology News: Gassy emissions no longer in suspect dock for melting the last ice age
- Maslin, M.A.; Owen, M.; Day, S.; Long, D. (2004). "Linking continental slope failure to climate change: Testing the Clathrate Gun Hypothesis". Geology 32 (1): 53–56. doi:10.1130/G20114.1.