Clathrate gun hypothesis

Marine extinction intensity during Phanerozoic

%

Millions of years ago

(H)

K–Pg

Tr–J

P–Tr

Cap

Late D

O–S

The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the traditional "Big Five" extinction events and the more recently recognised Capitanian mass extinction event are clickable links; see Extinction event for more details. (source and image info)

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 on a timescale less than a human lifetime, and was responsible for warming events in and at the end of the last glacial maximum. This is now thought unlikely.

However, there is stronger evidence that runaway methane clathrate breakdown may have caused drastic alteration of the ocean environment (such as ocean acidification and ocean stratification) and the atmosphere of earth on a number of occasions in the past, over timescales of tens of thousands of years; these events include the Paleocene–Eocene Thermal Maximum 56 million years ago, and most notably the Permian–Triassic extinction event, when up to 96% of all marine species became extinct, 252 million years ago.

Mechanism

Specific structure of a gas hydrate piece, from the subduction zone off Oregon

Gas hydrate-bearing sediment, from the subduction zone off Oregon

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) and 33 when accounted for aerosol interactions. The theory also predicts this will greatly affect available oxygen and hydroxyl radical content of the atmosphere.

Subsea permafrost

Main article: Permafrost

Subsea permafrost occurs beneath the seabed and exists in the continental shelves of the polar regions. This source of methane is different from methane clathrates, but contributes to the overall outcome and feedbacks.

From sonar measurements in recent years researchers quantified the density of bubbles emanating from subsea permafrost into the ocean (a process called ebullition), and found that 100–630 mg methane per square meter is emitted daily along the East Siberian Shelf, into the water column. They also found that during storms, when wind accelerates air-sea gas exchange, methane levels in the water column drop dramatically. Observations suggest that methane release from seabed permafrost will progress slowly, rather than abruptly. However, Arctic cyclones, fueled by global warming, and further accumulation of greenhouse gases in the atmosphere could contribute to more rapid methane release from this source.

Metastable methane clathrates

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.

The so-called self-preservation phenomenon has been intensively studied by Russian geologists starting in the late 1980s. This metastable clathrate state can be a basis for release events of methane excursions, such as during the interval of the last glacial maximum. A study from 2010 concluded with the possibility for a trigger of abrupt climate warming based on metastable methane clathrates in the East Siberian Arctic Shelf (ESAS) region.

Ocean anoxia

Main article: Anoxic event

Anoxic and euxinic events happened in the past on different time scales ranging from decades to centuries (from impact events) or through climate change within tens of thousands of years or a few million years. According to Gregory Ryskin, such a scenario could lead to the release of methane and other gases (e.g., CO₂, H
₂S) into the atmosphere, from the ocean. Following atmospheric methane excursions he postulates explosions and burning of methane would produce lots of smoke and dust, which would first lead to global cooling. And likely after a relatively short geological period following stratospheric cooling, global warming would take over.

Possible release events

Two events possibly linked to methane excursions are the Permian–Triassic extinction event and the Paleocene–Eocene Thermal Maximum (PETM). Equatorial permafrost methane clathrate may 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.

Current outlook

Main article: Climate change in the Arctic

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 on the order of a thousand years or more for the temperature signal to get through. However, there is also a possibility for the formation of gas migration pathways within fault zones in the East Siberian Arctic Shelf, through the process of talik formation, or pingo-like features.

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.

Arctic Ocean

Main article: Arctic methane release

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 levels. 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. A 2012 assessment of the literature identifies methane hydrates on the Shelf of East Arctic Seas as a potential trigger.

Continental slopes

Profile illustrating the continental shelf, slope and rise

A trapped gas deposit on the continental slope off Canada in the Beaufort Sea, located in an area of small conical hills on the ocean floor is just 290 meters below sea level and considered the shallowest known deposit of methane hydrate.

Seismic observation of destabilizing methane hydrate along the continental slope of the eastern United States, following the intrusion of warmer ocean currents, suggests that underwater landslides could release methane. The estimated amount of methane hydrate in this slope is 2.5 gigatonnes (about 0.2% of the amount required to cause the PETM), and it is unclear if the methane could reach the atmosphere. However, the authors of the study caution: "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."

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.
• In Charles Stross' Laundry Files universe, an intentionally triggered clathrate gun scenario is viewed as a possible retaliatory strategy that could be utilized by Blue Hades in response to terminal violation of the Benthic Treaty.

See also

• Abiogenic petroleum origin
• Clathrate compound
• Long-term effects of global warming
• Limnic eruption
• Mass extinction
• Methane chimney
• Ocean acidification
• Physical impacts of climate change
• Shutdown of thermohaline circulation
• Storegga Slide

Further reading

• Benton; et al. (2003). "How to kill (almost) all life: the end-Permian extinction event" (PDF). Ecology and Evolution. 18 (7): 358–365. doi:10.1016/S0169-5347(03)00093-4. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Explicit use of et al. in: |last= (help), cited by 21 other articles.
• Svensen; et al. (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. {{cite journal}}: Explicit use of et al. in: |last= (help)
• Thomas; et al. (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. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Explicit use of et al. in: |last= (help)
• Archer; et al. (2004). "Temperature sensitivity and time dependence of the global ocean clathrate reservoir". American Geophysical Union, Fall Meeting. {{cite conference}}: Explicit use of et al. in: |last= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help)

External links

• 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 et al. Reston VA: U.S. Geological Survey. {{cite book}}: Invalid |ref=harv (help)
• 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
• 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. Bibcode:2004Geo....32...53M. doi:10.1130/G20114.1.