Clathrate gun hypothesis
The clathrate gun hypothesis is the popular name given to the idea that increases in sea temperatures or drops in sea levels can trigger a strong positive feedback effect on climate: first, warming causes a sudden release of methane from methane clathrate compounds buried in seabeds and seabed permafrost; second, because methane itself is a powerful greenhouse gas, temperatures rise further, and the cycle repeats. This runaway process, once started, could be as irreversible as the firing of a gun.
In its original form, the hypothesis proposed that the "clathrate gun" could cause abrupt runaway warming on a time scale less than a human lifetime. A 2018 published review concluded that the clathrate gun hypothesis remains controversial, but that better understanding is vital.
Studies published in 2000 attributed the hypothesis to be responsible for warming events in and at the end of the Last Glacial Maximum, but this is now thought to be unlikely. At one point there seemed to be stronger evidence that runaway methane clathrate breakdown may have caused drastic alteration of the ocean environment (such as ocean acidification and ocean stratification) and of the atmosphere over 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. 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. Yet, it is still argued that a potential positive feedback mechanism from clathrate dissociation would amplify future global warming. However, past hydrate dissociation at Svalbard eight thousand years ago has been attributed to isostatic rebound (continental uplift following deglaciation).
The SWIPA 2017 report notes, "Arctic sources and sinks of greenhouse gases are still hampered by data and knowledge gaps."
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. A similar event is the methane hydrate releases, following ice-sheet retreat during the last glacial period, around 12,000 years ago, in response to the Bølling-Allerød warming.
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 the estimates of total resource size given by various experts differ by many orders of magnitude, leaving doubt as to the size of methane clathrate deposits (particularly in the viability of extracting them as a fuel resource). Indeed, cores of greater than 10 centimeters' contiguous depth had only been found in three sites as of 2000, and some resource reserve size estimates for specific deposits/locations have been based primarily on seismology.
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; some have suggested that this was a main factor in the planet warming 6 °C, which 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, 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 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.
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., 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.
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. Global averaged monthly mean atmospheric methane is currently at ~1860 ppb CH4, increases between 8.8 ± 2.6 through 2017 compare to an average annual increase of 5.7 ± 1.1 ppb between 2007 and 2013.
“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.”
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 CO2.
This is what led 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.
Hong et al. 2017 studied methane seepage in the shallow arctic seas at the Barents Sea close to Svalbard. Temperature at the seabed 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 at the sediment-water interface. Hydrates can be stable through the top 60 meters of the sediments and the current observed releases originate from deeper below the sea floor. They conclude that the increased methane flux started hundreds to thousands of years ago, noted about it, "..episodic ventilation of deep reservoirs rather than warming-induced gas hydrate dissociation." 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,"
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. However, the ESAS region averages 45 meters in depth, and it is assumed that below the seafloor, sealed by sub-sea permafrost layers, hydrates deposits are located.
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." 
Bill McGuire notes, "There may be a threat of submarine landslides around the margins of Greenland, which are less well explored. Greenland is already uplifting, reducing the pressure on the crust beneath and also on submarine methane hydrates in the sediment around its margins, and increased seismic activity may be apparent within decades as active faults beneath the ice sheet are unloaded. This could provide the potential for the earthquake or methane hydrate destabilisation of submarine sediment, leading to the formation of submarine slides and, perhaps, tsunamis in the North Atlantic."
Research by Klaus Wallmann et al. 2018 concluded that hydrate dissociation at Svalbard 8,000 years ago was due to the rebound of the seabed following ice-sheet retreat. As a result the water depth got shallower with less hydrostatic pressure, without further warming. The study, also found that today's deposits at the site become unstable at a depth of ~ 400 meters, due to seasonal bottom water warming, and it remains unclear if this is due to natural variability or anthropogenic warming.
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.
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