Clathrate gun hypothesis

The clathrate gun hypothesis (now largely debunked) is the name given to the idea that as sea temperatures rise in the Arctic, this can trigger a strong positive feedback effect on climate. The hypthesis is that this warming would cause a sudden release of methane from methane clathrate compounds buried in seabeds and seabed permafrost, and then, because methane itself is a powerful greenhouse gas, temperatures rise further, and the cycle repeats. The original idea was that this runaway process, once started, could be as irreversible as the firing of a gun. It originates from a paper by Kennett et al published in 2003, whch proposed that the "clathrate gun" could cause abrupt runaway warming on a time scale less than a human lifetime.

However in 2017, a major literature review by the 2107 USGS Hydrates project concluded that evidence is lacking for the hypothesis. In the same year, the Royal Society review found that there is a relatively limited role for cilmate feedback from dissociation of the methane clathrates. In 2018, the CAGE research group (Centre for Arctic Gas Hydrate, Environment and Climate) published evidence that the methane clathrates formed over 6 million years ago and have been slowly releasing methane for 2 million years throughout the ice ages, rather than releasing methane only recently as had previously been thought.

At one time this hypothesis was thought to be responsible for warming events in and at the end of the Last Glacial Maximum,, but this is now thought to be unlikely. . The clathrates are kept stable by pressure of the depth of sea above the deposits, as well as by the low temperatures at the sea bed, so the clathrates can also decompose as the reseult of lowering sea levels during ice ages, or by the sea floor rising along continental shelves when the ice resting on the land melts. This needs to be distinguished from clathrate dissociation through a warming sea, which is needed for the clathrate gun hypothesis.

At one point it was thought that runaway methane clathrate breakdown might also have acted over longer timescales of tens of thousands of years during 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. It was thought to have caused drastic alteration of the ocean environment (such as ocean acidification and ocean stratification) and of the atmosphere. However, the pattern of isotope shifts expected to result from a massive release of methane does not match the patterns seen there. First, the isotope shift is too large for this hypothesis, as it would require five times as much methane as is postulated for the PETM, and then, it would have to be reburied at an unrealistically high rate to account for the rapid increases in the 13C/12C ratio throughout the early Triassic before it was released again several times. Possibly the temperature increase of the PETM was due to roasting carbonate sediments including coal beds by volcanism. Potentially this may have released more than 3 trillion tons of carbon See Permian–Triassic extinction event.

Mechanism
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, which is stable under pressure, and remains stable under higher temperatures than ice, up to a few degrees above 0 °C depending on the pressure. The methane is trapped inside dodecahderal clustres of water molecules surrounded by tetrahedra in a lattice.

Methane forms a structure I hydrate. It's built up of dodecahedral cages made up of water molecules which are kept stable by a methane molecule inside each one. These form part of a larger lattice with tetrakeidecahedral cavities which also contain methane molecules. Potentially large deposits of methane clathrate have been found under sediments on the ocean floors of the Earth

Methane is much more powerful as a greenhouse gas than carbon dioxide, although it has a short atmospheric lifetime of around 12 years. Shindell et al(2009) calculated that it has a global warming potential, the ratio of its warming potential to that of CO2. of between 79 and 105 over 20 years, and between 25 and 40 over 100 years, after acciybtubg for aerosol interactions.

As the oceans warm then methane can be released as the methane dissociates. The deposits extend to a depth of many meters and how much of an effect this is depends on how far down into the deposits the dissociation proceeds. The 2017 and 2018 studies   have suggested only the topmost layers would be affected, while the original hypothesis was based on the supposition that deep layers would dissociate. The amount of the effect also depends on what happens to the methane as it rises in the water column above it after it is released from the clathrates.

At a density of around 0.9 g/cm3, methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment. So the clathrate deposits all consist of clathrates firmly bound within the ocean sediments.

Subsea 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 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
Euxinic (i.e. sulfidic) and anoxic 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.,, ) 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
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 shallowest, 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 in parts per billion (ppb) remained between 400–800ppb in the years 600,000 BC to 1900 AD, and since 1900 AD have risen to levels between 1600–1800ppb.

A USGS metastudy in 2017 by the USGS Gas Hydrates Project concluded

""“Our review is the culmination of nearly a decade of original research by the USGS, my coauthor Professor John Kessler at the University of Rochester, and many other groups in the community,” said USGS geophysicist Carolyn Ruppel, who is the paper’s lead author and oversees the USGS Gas Hydrates Project. “After so many years spent determining where gas hydrates are breaking down and measuring methane flux at the sea-air interface, we suggest that conclusive evidence for release of hydrate-related methane to the atmosphere is lacking.”"

From the Royal Society report:Climate updates What have we learnt since the IPCC 5th Assessment Report? Royal Society update on questions from IPCC review of methane clathrate research from 2017 ""Clathrates: Some economic assessments continue to emphasise the potential damage from very strong and rapid methane hydrate release (Hope and Schaefer, 2016), although AR5 did not consider this likely. Recent measurements of methane fluxes from the Siberian Shelf Seas (Thornton et al., 2016) are much lower than those inferred previously (Shakhova et al., 2014). A range of other studies have suggested a much smaller influence of clathrate release on the Arctic atmosphere than had been suggested (Berchet et al., 2016; Myhre et al., 2016). New modelling work confirms (Kretschmer et al., 2015) that the Arctic is the region where methane release from clathrates is likely to be most important in the next century, but still estimates methane release to the water column to be negligible compared to anthropogenic releases to the atmosphere. A recent review (Ruppel and Kessler, 2017) emphasises that there remains little evidence that clathrate methane is reaching the atmosphere at present. Although methane that is oxidised in the water column will not reach the atmosphere, it will have the effect of further lowering the pH of the ocean (Boudreau et al., 2015). A recent modelling study joined earlier papers in assigning a relatively limited role to dissociation of methane hydrates as a climate feedback (Mestdagh et al., 2017). Methane concentrations are rising globally, raising interesting questions (see section on methane) about what the cause is (Nisbet et al., 2016; Rigby et al., 2017; Schaefer et al., 2016; Turner et al., 2017). finally new measurements of the 14C content of methane across the warming out of the last glacial period (Petrenko et al., 2017) show that the release of old carbon reservoirs (including methane hydrates) played only a small role in the methane concentration increase that occurred then"" ""Clathrates: Some economic assessments continue to emphasise the potential damage from very strong and rapid methane hydrate release, although AR5 did not consider this likely. Recent measurements of methane fluxes from the Siberian Shelf Seas are much lower than those inferred previously. A range of other studies have suggested a much smaller influence of clathrate release on the Arctic atmosphere than had been suggested. …. A recent modelling study joined earlier papers in assigning a relatively limited role to dissociation of methane hydrates as a climate feedback. Methane concentrations are rising globally, raising interesting questions (see section on methane) about what the cause is, finally new measurements of the 14C content of methane across the warming out of the last glacial period show that the release of old carbon reservoirs (including methane hydrates) played only a small role in the methane concentration increase that occurred then.""

Arctic Ocean
Research carried out in 2008 in the Siberian Arctic showed 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. At the time, some of the melting was thought to be the result of geological heating, but more thawing was believed to be due to the greatly increased volumes of meltwater being discharged from the Siberian rivers flowing north. The current methane release had previously been estimated at 0.5 megatonnes per year. Shakhova et al. (2008) estimate that not less than 1,400 gigatonnes 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 gigatonnes 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.

This is what lead to the original Clathrate gun hypothesis, and 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.

However, later research cast doubt on this picture. Hong et al (2017) studied the seepage from large mounds of hydrates in the shallow arctic seas at Storfjordrenna, in the Barents Sea close to Svalbard. They showed that though the temperature of the sea bed has fluctuated seasonally over the last century, between 1.8 and 4.8 °C, it has only affected release of methane to a depth of about 1.6 meters. Hydrates can be stable through the top 60 meters of the sediments and the current rapid releases came from deeper below the sea floor. They concluded that the increase in flux started hundreds to thousands of years ago well before the onset of warming that others speculated as its cause, and that these seepages are not increasing due to momentary warming. Summarizing his research, Hong stated: ""The results of our study indicate that the immense seeping found in this area is a result of natural state of the system. Understanding how methane interacts with other important geological, chemical and biological processes in the Earth system is essential and should be the emphasis of our scientific community,""

Further research by Klaus Wallmann et al (2018) found that the hydrate release is due to the rebound of the sea bed after the ice melted. The methane dissociation began around 8,000 years ago when the land began to rise faster than the sea level, and the water as a result sarted to get shallower with less hydrostatic pressure. This dissociation therefore was a result of the uplift of the sea bed rather than anthropogenic warming. The amount of methane released by the hydrate dissociation was small. They found that the methane seeps originate not from the hydrates but from deep geological gas reservoirs (seepage from these formed the hydrates originally).They concluded that the hydrates acted as a dynamic seal regulating the methane emissions from the deep geological gas reservoirs and when they were dissociated 8,000 years ago, weakening the seal, this led to the higher methane release still observed today.

They now know that the methane there has been escaping at the same rate for millions of years! "Recent observations of extensive methane release from the seafloor into the ocean and atmosphere cause concern as to whether increasing air temperatures across the Arctic are causing rapid melting of natural methane hydrates. Other studies, however, indicate that methane flares released in the Arctic today were created by processes that began way back in time – during the last Ice Age."

"Newest research from the Center for Arctic Gas Hydrate, Climate and Environment (CAGE) shows that methane has been leaking in the Arctic for millions of years, independent of warm or cold climate. Methane has been forming in organic carbon rich sediments below the leakage spots off the coast of western Svalbard for a period of about 6 million years (since the late Miocene). According to our models, methane flares occurred at the seafloor for the first time at around 2 million years ago; at the exact time when ice sheets started to expand in the Arctic."

"The acceleration of leakage occurred when the ice sheets were big enough to erode and deliver huge amounts of sediments towards the continental slope. Methane leakage was promoted due to formation of natural gas in organic-rich sediments under heavy loads of glacial sediments. Faults and fractures opened within the Earth’s crust as a consequence of growth and decay of the massive ice masses. This brought up the gases from deeper sediments higher up towards the seafloor. These gases then fueled the gas hydrate system off the Svalbard coast for the past 2 million years. It is, to this day, controlling the leakage of methane from the seabed."

So, the methane deposits formed in the late Miocene starting 6 million years ago, and the methane leaks have been going on for two million years through multiple ice ages.

Continental slopes
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 (in 2012) 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."

Model simulations
A study of the effects for the original hypothesis, based on a coupled climate–carbon cycle model (GCM) assessed a 1000-fold (from <1 to 1000 ppmv) methane increase—within a single pulse, from methane hydrates (based on carbon amount estimates for the PETM, with ~2000 GtC), and concluded it would increase atmospheric temperatures by more than 6 °C within 80 years. Further, carbon stored in the land biosphere would decrease by less than 25%, suggesting a critical situation for ecosystems and farming, especially in the tropics.

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.