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The clathrate gun hypothesis is a proposed explanation for the periods of rapid warming during the Quaternary. The idea is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulated and occasionally released methane clathrate on upper continental slopes. This would have had an immediate impact on the global temperature, as methane is a much more powerful greenhouse gas than carbon dioxide. Despite its atmospheric lifetime of around 12 years, methane's global warming potential is 72 times greater than that of carbon dioxide over 20 years, and 25 times over 100 years (33 when accounting for aerosol interactions). ^[2]These warming events would have caused the Bond Cycles and individual interstadial events, such as the Dansgaard–Oeschger interstadials.^[3]

The hypothesis was supported for the Bølling-Allerød and Preboreal period, but not for Dansgaard–Oeschger interstadials,^[4] although there are still debates on the topic.^[5] While it may be important on the millennial timescales, it is no longer considered relevant for the near future climate change: the IPCC Sixth Assessment Report states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century.^[6]

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. 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.^[7]^[8]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.

In the Arctic ocean, 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.^[9] This metastable clathrate state can be a basis for release events of methane excursions, such as during the interval of the Last Glacial Maximum.^[10] 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.^[11]

Possible past releases

Studies published in 2000 considered this hypothetical effect to be responsible for warming events in and at the end of the Last Glacial Maximum,^[13]. Although periods of increased atmospheric methane match periods of continental-slope failure.^[4]^[5], later work found that the distinct deuterium/hydrogen (D/H) isotope ratio indicated wetland methane emissions as the main contributor to atmospheric methane concentrations.^[14]^[15] While there were major dissociation events during the last deglaciation, with Bølling-Allerød warming triggering the disappearance of the entire methane hydrate deposit in the Barents Sea within 5000 years, those events failed to counteract the onset of a major Younger Dryas cooling period, suggesting that most of the methane stayed within the seawater after being liberated from the seafloor deposits, with very little entering the atmosphere. ^[16]^[17]

In 2008, it was suggested that equatorial permafrost methane clathrate may have had a role in the sudden warm-up of "Snowball Earth", 630 million years ago.^[18]

Other events potentially linked to methane hydrate excursions are the Permian–Triassic extinction event and the Paleocene–Eocene Thermal Maximum.

Paleocene–Eocene Thermal Maximum

The Paleocene–Eocene thermal maximum (PETM), alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene thermal maximum", was a geologically brief time interval characterized by a 5–8 °C global average temperature rise and massive input of carbon into the ocean and atmosphere.^[19]^[20] The event began, now formally, at the time boundary between the Paleocene and Eocene geological epochs.^[21] The exact age and duration of the PETM remain uncertain, but it occurred around 55.8 million years ago (Ma) and lasted about 200 thousand years (Ka).^[22]^[23] The entire warm period lasted for about 200,000 years. Global temperatures increased by 5–8 °C.^[20]

The onset of the Paleocene–Eocene thermal maximum has been linked to volcanism^[19] and uplift associated with the North Atlantic Igneous Province, causing extreme changes in Earth's carbon cycle and a significant temperature rise.^[20]^[24]^[25] The period is marked by a prominent negative excursion in carbon stable isotope ( $δ$ ¹³C) records from around the globe; more specifically, there was a large decrease in ¹³C/¹²C ratio of marine and terrestrial carbonates and organic carbon.^[20]^[26]^[27] Paired $δ$ ¹³C, $δ$ ¹¹B, and ratio of boron to calcium data suggest that ~14900 Gt of carbon were released into the ocean–atmosphere system,^[28] over 6,000 years.^[23]

Stratigraphic sections of rock from this period reveal numerous other changes.^[20] Fossil records for many organisms show major turnovers. For example, in the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion, planktic foraminifera and calcareous nannofossils all occurred during the beginning stages of PETM. On land, modern mammal orders (including primates) suddenly appear in Europe and in North America.^[29]

Section 'Methane release' not found

Permian–Triassic extinction event

Marine extinction intensity during Phanerozoic

Millions of years ago

Plot of extinction intensity (percentage of marine genera that are present in each interval of time but do not exist in the following interval) vs time in the past.^[30] Geological periods are annotated (by abbreviation and colour) above. The Permian–Triassic extinction event is the most significant event for marine genera, with just over 50% (according to this source) perishing. (source and image info)

Approximately 251.9 million years ago, the Permian–Triassic (P–T, P–Tr) extinction event (PTME; also known as the Late Permian extinction event,^[32] the Latest Permian extinction event,^[33] the End-Permian extinction event,^[34]^[35] and colloquially as the Great Dying)^[36]^[37] forms the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras.^[38] It is the Earth's most severe known extinction event,^[39]^[40] with the extinction of 57% of biological families, 83% of genera, 81% of marine species^[41]^[42]^[43] and 70% of terrestrial vertebrate species.^[44] It is also the greatest known mass extinction of insects.^[45] It is the greatest of the "Big Five" mass extinctions of the Phanerozoic.^[46] There is evidence for one to three distinct pulses, or phases, of extinction.^[44]^[47]

The precise causes of the Great Dying remain unknown. The scientific consensus is that the main cause of extinction was the flood basalt volcanic eruptions that created the Siberian Traps,^[48] which released sulfur dioxide and carbon dioxide, resulting in euxinia,^[49]^[50] elevating global temperatures,^[51]^[52]^[53] and acidifying the oceans.^[54]^[55]^[32] The level of atmospheric carbon dioxide rose from around 400 ppm to 2,500 ppm with approximately 3,900 to 12,000 gigatonnes of carbon being added to the ocean-atmosphere system during this period.^[51] Important proposed contributing factors include the emission of much additional carbon dioxide from the thermal decomposition of hydrocarbon deposits, including oil and coal, triggered by the eruptions,^[56]^[57] emissions of methane from the gasification of methane clathrates,^[58] emissions of methane possibly by novel methanogenic microorganisms nourished by minerals dispersed in the eruptions,^[59]^[60]^[61]

an extraterrestrial impact creating the Araguainha crater and consequent seismic release of methane,^[62]^[63]^[64] and the destruction of the ozone layer and increase in harmful solar radiation.^[65]^[66]^[67]

Section 'Methane hydrate gasification' not found

Global warming feedback

Modern deposits

Most deposits of methane clathrate are in sediments too deep to respond rapidly,^[68] and 2007 modelling by Archer suggests that the methane forcing derived from them should remain a minor component of the overall greenhouse effect.^[69] 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.^[69]

However, some methane clathrates deposits in the Arctic are much shallower than the rest, which could make them far more vulnerable to warming. 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.^[70] However, the East Siberian Arctic Shelf averages 45 meters in depth, and it is assumed that below the seafloor, sealed by sub-sea permafrost layers, hydrates deposits are located.^[71]^[72]. This would mean that when the warming potentially taliks or pingo-like features within the shelf, they would also serve as gas migration pathways for the formerly frozen methane, and a lot of attention has been paid to that possibility. ^[73]^[74]^[75] 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. Their paper initially included the line that the "release of up to 50 gigatonnes of predicted amount of hydrate storage [is] highly possible for abrupt release at any time". A release on this scale would increase the methane content of the planet's atmosphere by a factor of twelve,^[76]^[77] equivalent in greenhouse effect to a doubling in the 2008 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^[78] 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.^[79] A 2012 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.^[80] Another 2012 assessment of the literature identifies methane hydrates on the Shelf of East Arctic Seas as a potential trigger.^[81]

A risk of seismic activity being potentially responsible for mass methane releases has been considered as well. In 2012, seismic observations 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."^[82] 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."^[83]

Observed emissions

East Siberian Arctic Ice Shelf

Research carried out in 2008 in the Siberian Arctic showed methane releases on the annual scale of millions of tons, which was a substantial increase on the previous estimate of 0.5 millions of tons per year.^[84] apparently through perforations in the seabed permafrost,^[75] with concentrations in some regions reaching up to 100 times normal levels.^[85]^[86] 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.^[87]

By 2013, the same team of researchers used multiple sonar observations to quantify 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 Arctic Shelf (ESAS), 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. Altogether, their updated estimate had now amounted to 17 millions of tons per year.^[88]

However, these findings were soon questioned, as this rate of annual release would mean that the ESAS alone would account for between 28% and 75% of the observed Arctic methane emissions, which contradicts many other studies. In January 2020, it was found that the rate at which methane enters the atmosphere after it had been released from the shelf deposits into the water column had been greatly overestimated, and observations of atmospheric methane fluxes taken from multiple ship cruises in the Arctic instead indicate that only around 3.02 million tonnes of methane are emitted annually from the ESAS.^[89] A modelling study published in 2020 suggested that under the present-day conditions, annual methane release from the ESAS may be as low as 1000 tonnes, with 2.6 – 4.5 million tonnes representing the peak potential of turbulent emissions from the shelf.^[90]

Beaufort Sea continental slope

A radiocarbon dating study in 2018 found that after the 30-meter isobath, only around 10% of the methane in surface waters can be attributed to ancient permafrost or methane hydrates. The authors suggested that even a significantly accelerated methane release would still largely fail to reach the atmosphere.^[91]

Svalbard

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."^[92] 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.^[93]

Research by Klaus Wallmann et al. 2018 concluded that hydrate dissociation at Svalbard 8,000 years ago was due to isostatic rebound (continental uplift following deglaciation). 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.^[94] Moreover, another paper published in 2017 found that only 0.07% of the methane released from the gas hydrate dissociation at Svalbard appears to reach the atmosphere, and usually only when the wind speeds were low.^[95] In 2020, a subsequent study confirmed that only a small fraction of methane from the Svalbard seeps reaches the atmosphere, and that the wind speed holds a greater influence on the rate of release than dissolved methane concentration on site. ^[96]

Finally, a paper published in 2017 indicated that the methane emissions from at least one seep field at Svalbard were more than compensated for by the enhanced carbon dioxide uptake due to the greatly increased phytoplankton activity in this nutrient-rich water. The daily amount of carbon dioxide absorbed by the phytoplankton was 1,900 greater than the amount of methane emitted, and the negative (i.e. indirectly cooling) radiative forcing from the CO² uptake was up to 251 times greater than the warming from the methane relase.^[97]

Current outlook

In 2014 based on their research on the northern United States Atlantic marine continental margins from Cape Hatteras to Georges Bank, a group of scientists from the US Geological Survey, the Department of Geosciences, Mississippi State University, Department of Geological Sciences, Brown University and Earth Resources Technology, found widespread leakage of methane from the seafloor, but they did not assign specific dates, beyond suggesting that some of the seeps were more than 1000 years old. ^[98]^[99] In March 2017, a meta-analysis by the USGS Gas Hydrates Project concluded:^[100]^[12]

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.

In June 2017, scientists from the Center for Arctic Gas Hydrate (CAGE), Environment and Climate at the University of Tromsø, published a study describing over a hundred ocean sediment craters, some 300 meters wide and up to 30 meters deep, formed due to explosive eruptions, attributed to destabilizing methane hydrates, following ice-sheet retreat during the last glacial period, around 15,000 years ago, a few centuries after the Bølling-Allerød warming. These areas around the Barents Sea, still seep methane today, and still existing bulges with methane reservoirs could eventually have the same fate.^[101] Later that same year, the Arctic Council published SWIPA 2017 report, where it cautioned "Arctic sources and sinks of greenhouse gases are still hampered by data and knowledge gaps."^[102]

In 2018, a perspective piece devoted to tipping points in the climate system suggested that the climate change contribution from methane hydrates would be "negligible" by the end of the century, but could amount ot 0.4-0.5 degrees Celsius on the millennial timescales.^[103] In 2021, the IPCC Sixth Assessment Report no longer included methane hydrates in the list of potential tipping points, and says that "it is very unlikely that CH4 emissions from clathrates will substantially warm the climate system over the next few centuries."^[6] Likewise, a 2022 assessment of tipping points described methane hydrates as a "threshold-free feedback" rather than a tipping point.^[104]^[105]

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 species 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.

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External links

Kennett, James P. (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. Archived from the original on 22 July 2012. Retrieved 17 March 2013.
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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.org. NPR.
Methane: A Scientific Journey from Obscurity to Climate Super-Stardom Good Sept. 2004 background report from NASA GISS
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@@ Line 1: / Line 1: @@
 [[File:Methane Hydrate phase diagram.jpg|thumb|upright=1.6|Methane clathrate is released as gas into the surrounding water column or soils when ambient temperature increases]]
 [[File:Graph CO2 CH4 and Temperature Graph in English 15 June 2015 by Reg Morrison.jpg|thumb|upright=1.6|The impact of CH<sub>4</sub> atmospheric methane concentrations on global temperature increase may be far greater than previously estimated.<ref>{{Cite web|url=http://regmorrison.edublogs.org/2011/09/26/methane/|title=Methane {{!}} Reg Morrison|website=regmorrison.edublogs.org|language=en-US|access-date=2018-11-24}} [http://regmorrison.edublogs.org/files/2013/02/METHANE-2-1sca3tx.pdf]</ref>]]
-The '''clathrate gun hypothesis''' is a proposed explanation for the periods of rapid warming during the [[Quaternary]]. The idea is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulated and occasionally released [[methane clathrate]] on upper continental slopes. These events would have caused the [[Bond event|Bond Cycles]] and individual [[interstadial]] events, such as the [[Dansgaard–Oeschger event|Dansgaard–Oeschger interstadials]].<ref>{{cite book |first1=James P. |last1=Kennett |first2=Kevin G. |last2=Cannariato |first3=Ingrid L. |last3=Hendy |first4=Richard J. |last4=Behl |year=2003 |title=Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis |publisher=[[American Geophysical Union]] |location=Washington DC |isbn=978-0-87590-296-8 |doi=10.1029/054SP}}</ref>
+The '''clathrate gun hypothesis''' is a proposed explanation for the periods of rapid warming during the [[Quaternary]]. The idea is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulated and occasionally released [[methane clathrate]] on upper continental slopes. This would have had an immediate impact on the global temperature, as methane is a much more powerful greenhouse gas than [[carbon dioxide]]. Despite its atmospheric lifetime of around 12 years, methane's [[global warming potential]] is 72 times greater than that of carbon dioxide over 20 years, and 25 times over 100 years (33 when accounting for [[aerosol]] interactions). <ref>{{cite journal |last1=Shindell |first1=Drew T. |last2=Faluvegi |first2=Greg |last3=Koch |first3=Dorothy M. |last4=Schmidt |first4=Gavin A. |last5=Unger |first5=Nadine |author-link5=Nadine Unger |last6=Bauer |first6=Susanne E. |year=2009 |title=Improved attribution of climate forcing to emissions |journal=Science |doi=10.1126/science.1174760 |pmid=19900930 |bibcode=2009Sci...326..716S |volume=326 |issue=5953 |pages=716–718|s2cid=30881469 |url=https://zenodo.org/record/1230902 }}</ref>These warming events would have caused the [[Bond event|Bond Cycles]] and individual [[interstadial]] events, such as the [[Dansgaard–Oeschger event|Dansgaard–Oeschger interstadials]].<ref>{{cite book |first1=James P. |last1=Kennett |first2=Kevin G. |last2=Cannariato |first3=Ingrid L. |last3=Hendy |first4=Richard J. |last4=Behl |year=2003 |title=Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis |publisher=[[American Geophysical Union]] |location=Washington DC |isbn=978-0-87590-296-8 |doi=10.1029/054SP}}</ref>
-The hypothesis was supported for the [[Bølling-Allerød]] and [[Preboreal]] period, but not for [[Dansgaard–Oeschger event|Dansgaard–Oeschger interstadials]],<ref name="Maslin2004">{{Cite journal |last1=Maslin |first1=M |last2=Owen |first2=M |last3=Day |first3=S |last4=Long |first4=D |date=2004 |title=Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis |url=https://www.researchgate.net/publication/32894169 |journal=Geology |language=en |volume=32 |issue=1 |pages=53–56 |doi=10.1130/G20114.1 |bibcode=2004Geo....32...53M |issn=0091-7613}}</ref> although there are still debates on the topic.<ref name="Maslin2010">{{Cite journal |last1=Maslin |first1=M |last2=Owen |first2=M |last3=Betts |first3=R |last4=Day |first4=S |last5=Dunkley Jones |first5=T |last6=Ridgwell |first6=A |date=2010-05-28 |title=Gas hydrates: past and future geohazard? |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |language=en |volume=368 |issue=1919 |pages=2369–2393 |doi=10.1098/rsta.2010.0065 |pmid=20403833 |bibcode=2010RSPTA.368.2369M |issn=1364-503X |doi-access=free}}</ref>
+The hypothesis was supported for the [[Bølling-Allerød]] and [[Preboreal]] period, but not for [[Dansgaard–Oeschger event|Dansgaard–Oeschger interstadials]],<ref name="Maslin2004">{{Cite journal |last1=Maslin |first1=M |last2=Owen |first2=M |last3=Day |first3=S |last4=Long |first4=D |date=2004 |title=Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis |url=https://www.researchgate.net/publication/32894169 |journal=Geology |language=en |volume=32 |issue=1 |pages=53–56 |doi=10.1130/G20114.1 |bibcode=2004Geo....32...53M |issn=0091-7613}}</ref> although there are still debates on the topic.<ref name="Maslin2010">{{Cite journal |last1=Maslin |first1=M |last2=Owen |first2=M |last3=Betts |first3=R |last4=Day |first4=S |last5=Dunkley Jones |first5=T |last6=Ridgwell |first6=A |date=2010-05-28 |title=Gas hydrates: past and future geohazard? |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |language=en |volume=368 |issue=1919 |pages=2369–2393 |doi=10.1098/rsta.2010.0065 |pmid=20403833 |bibcode=2010RSPTA.368.2369M |issn=1364-503X |doi-access=free}}</ref> While it may be important on the millennial timescales, it is no longer considered relevant for the near future [[climate change]]: the [[IPCC Sixth Assessment Report]] states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century.<ref name="IPCC AR6 WG1 Ch.5">{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf |publisher=Cambridge University Press, Cambridge, UK and New York, NY, USA |pages=5, |doi=10.1017/9781009157896.011|doi-broken-date=9 July 2022 }}</ref>
-==General==
-[[File:Gas-hydrate deposits by sector.jpg|thumb|upright=2.5|Gas-hydrate deposits by sector<ref name=Ruppel2018 />]]
-Studies published in 2000 considered this hypothetical effect to be responsible for warming events in and at the end of the [[Last Glacial Maximum]],<ref>{{cite journal |last1=Kennett |first1=James P. |last2=Cannariato |first2=Kevin G. |last3=Hendy |first3=Ingrid L. |last4=Behl |first4=Richard J. |date=7 April 2000 |title=Carbon Isotopic Evidence for Methane Hydrate Instability During Quaternary Interstadials |journal=[[Science (magazine)|Science]] |doi=10.1126/science.288.5463.128 |pmid=10753115 |bibcode=2000Sci...288..128K |volume=288 |issue=5463 |pages=128–133}}</ref> but the distinct [[Hydrogen isotope biogeochemistry|deuterium/hydrogen (D/H) isotope ratio]] indicates the methane was released by [[wetland methane emissions|wetlands]] instead.<ref name="Sowers2006">{{cite journal |last=Sowers |first=Todd |s2cid=38790253 |date=10 February 2006 |title=Late Quaternary Atmospheric {{chem|CH|4}} Isotope Record Suggests Marine Clathrates Are Stable |journal=Science |doi=10.1126/science.1121235 |pmid=16469923 |bibcode=2006Sci...311..838S |volume=311 |issue=5762 |pages=838–840}}</ref><ref>{{cite journal |last1=Severinghaus |first1=Jeffrey P. |last2=Whiticar |first2=MJ |last3=Brook |first3=EJ |last4=Petrenko |first4=VV |last5=Ferretti |first5=DF |last6=Severinghaus |first6=JP |date=25 August 2006 |title=Ice Record of {{chem|13|C}} for Atmospheric {{chem|CH|4}} Across the Younger Dryas-Preboreal Transition |journal=Science |doi=10.1126/science.1126562 |pmid=16931759 |bibcode=2006Sci...313.1109S |volume=313 |issue=5790 |pages=1109–12|s2cid=23164904 }}</ref> Although periods of increased [[atmospheric methane]] match periods of [[submarine landslide|continental-slope failure]].<ref name="Maslin2004" /><ref name="Maslin2010" />
-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 [[Stratification (water)|ocean stratification]]) and of the atmosphere over timescales of tens of thousands of years during the [[Paleocene–Eocene Thermal Maximum]] (PETM) 56 million years ago,<ref>{{cite journal |last1=Gehler |first1=Alexander |last2=Gingerich |first2=Philip D. |last3=Pack |first3=Andreas |date=27 June 2016 |title=Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite |url=https://www.pnas.org/doi/10.1073/pnas.1518116113 |journal=Proceedings of the National Academy of Sciences |volume=113 |issue=28 |pages=7739-7744 |doi=10.1073/pnas.1518116113 |access-date=29 September 2022}}</ref> and most notably the [[Permian–Triassic extinction event]], when up to 96% of all marine species became extinct, 252 million years ago.<ref>{{cite episode |others=Narrated by Jack Fortune |date=5 December 2002 |title=The Day The Earth Nearly Died |series=Horizon |series-link=Horizon (BBC TV series) |network=BBC |url=http://www.bbc.co.uk/science/horizon/2002/dayearthdied.shtml}}</ref><ref name="Erwin1993">{{cite book|author=Erwin DH|year=1993|title=The great Paleozoic crisis; Life and death in the Permian|publisher=Columbia University Press|isbn=978-0-231-07467-4}}</ref> However, the pattern of isotope shifts expected to result from a massive release of methane does not match the patterns seen during the Permian-Triassic extinction. 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,<ref name=Payne2004>{{Cite journal |last=Payne |first=J. L. |date=2004-07-23 |title=Large Perturbations of the Carbon Cycle During Recovery from the End-Permian Extinction |journal=Science |language=en |volume=305 |issue=5683 |pages=506–509 |doi=10.1126/science.1097023 |pmid=15273391 |issn=0036-8075|bibcode=2004Sci...305..506P |s2cid=35498132 }}</ref><ref name=Knoll1996>{{cite journal
- | author = Knoll, A.H. |author2=Bambach, R.K. |author3=Canfield, D.E. |author4=Grotzinger, J.P.
- | year = 1996
- | title = Comparative Earth history and Late Permian mass extinction
- | journal = Science
- | volume = 273
- | issue = 5274
- | pages = 452–457
- | doi = 10.1126/science.273.5274.452
- | pmid = 8662528
-|bibcode = 1996Sci...273..452K |s2cid=35958753 }}</ref> and then, it would have to be reburied at an unrealistically high rate to account for the rapid increases in the <sup>13</sup>C/<sup>12</sup>C ratio throughout the early Triassic before it was released again several times.<ref name="Payne2004" /> Yet, it is still argued that a potential [[climate feedback|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]]).<ref name="Wallmann2018">{{Cite journal|journal=Nature Communications|year=2018|author=Wallmann|display-authors=et al|title=Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming|volume=9|issue=1|pages=83|doi=10.1038/s41467-017-02550-9|pmid=29311564|pmc=5758787|bibcode=2018NatCo...9...83W}}</ref>
-The SWIPA 2017 report notes, "Arctic sources and sinks of greenhouse gases are still hampered by data and knowledge gaps."<ref>{{cite web|url=https://www.amap.no/documents/doc/swipa-2017-press-material/1544|title=SWIPA 2017 - Press Material|year=2017|work=Arctic Council}}</ref>
-===Possible release events===
-Two events possibly linked to methane excursions are the Permian–Triassic extinction event and the Paleocene–Eocene Thermal Maximum. Equatorial permafrost methane clathrate may have had a role in the sudden warm-up of "[[Snowball Earth]]", 630 million years ago.<ref>{{cite journal |last1=Kennedy |first1=Martin |last2=Mrofka |first2=David |last3=Von Der Borch |first3=Chris |year=2008 |title=Snowball Earth termination by destabilization of equatorial permafrost methane clathrate |journal=Nature |doi=10.1038/nature06961 |pmid=18509441 |bibcode=2008Natur.453..642K |volume=453 |issue=7195 |pages=642–645 |s2cid=4416812 |url=http://faculty.ucr.edu/~martink/pdfs/Kennedy_2008_Nature.pdf }}</ref> 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]]. In this case, methane release was an effect, rather than the initial cause, of warming.<ref>{{cite web|url=https://www.washingtonpost.com/news/energy-environment/wp/2017/06/01/like-champagne-bottles-being-opened-scientists-document-an-ancient-arctic-methane-explosion|title=Like 'champagne bottles being opened': Scientists document an ancient Arctic methane explosion|work=The Washington Post|date=June 1, 2017}}</ref><ref>{{cite journal|journal=PNAS|volume=114|issue=24|pages=6215–6220|year=2017|title=Postglacial response of Arctic Ocean gas hydrates to climatic amelioration|author=Serov|display-authors=et al|doi=10.1073/pnas.1619288114|pmid=28584081|pmc=5474808|bibcode=2017PNAS..114.6215S|doi-access=free}}</ref>
 ==Mechanism==
 [[File:Gashydrat mit Struktur.jpg|thumbnail|Specific structure of a [[gas hydrate]] piece, from the subduction zone off Oregon]]
 [[File:Gashydrat im Sediment.JPG|thumbnail|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 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&nbsp;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.<ref>{{cite journal |last1=Collet |first1=Timothy S. |last2=Kuuskraa |first2=Vello A. |year=1998 |title=Hydrates contain vast store of world gas resources |journal=[[Oil and Gas Journal]] |volume=96 |issue=19 |pages=90–95 |url=http://www.ogj.com/articles/print/volume-96/issue-19/in-this-issue/exploration/hydrates-contain-vast-store-of-world-gas-resources.html}}{{subscription|yes}}</ref><ref>{{Cite journal |first=Jean |last=Laherrere |date=May 3, 2000 |title=Oceanic Hydrates: More Questions Than Answers |journal=Energy Exploration & Exploitation |volume=18 |pages=349–383 |issn=0144-5987 |doi=10.1260/0144598001492175 |issue=4|s2cid=129242950 }}</ref>
+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&nbsp;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.<ref>{{cite journal |last1=Collet |first1=Timothy S. |last2=Kuuskraa |first2=Vello A. |year=1998 |title=Hydrates contain vast store of world gas resources |journal=[[Oil and Gas Journal]] |volume=96 |issue=19 |pages=90–95 |url=http://www.ogj.com/articles/print/volume-96/issue-19/in-this-issue/exploration/hydrates-contain-vast-store-of-world-gas-resources.html}}{{subscription|yes}}</ref><ref>{{Cite journal |first=Jean |last=Laherrere |date=May 3, 2000 |title=Oceanic Hydrates: More Questions Than Answers |journal=Energy Exploration & Exploitation |volume=18 |pages=349–383 |issn=0144-5987 |doi=10.1260/0144598001492175 |issue=4|s2cid=129242950 }}</ref>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 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&nbsp;°C, which happened during the end-Permian extinction,<ref>{{cite journal |last1=Benton |first1=Michael J. |last2=Twitchet |first2=Richard J. |date=July 2003 |title=How to kill (almost) all life: the end-Permian extinction event |journal=Trends in Ecology & Evolution |doi=10.1016/S0169-5347(03)00093-4 |volume=18 |issue=7 |pages=358–365 |url=http://palaeo.gly.bris.ac.uk/Benton/reprints/2003TREEPTr.pdf |access-date=2006-11-26 |archive-url=https://wayback.archive-it.org/all/20070418023344/http://palaeo.gly.bris.ac.uk/Benton/reprints/2003TREEPTr.pdf |archive-date=2007-04-18 |url-status=dead }}</ref> 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{{What|reason=72 what? Years? Degrees? Some other metric?|date=October 2021}} over 20 years, 25 over 100 years, and 33 when accounted for [[aerosol]] interactions.<ref>{{cite journal |last1=Shindell |first1=Drew T. |last2=Faluvegi |first2=Greg |last3=Koch |first3=Dorothy M. |last4=Schmidt |first4=Gavin A. |last5=Unger |first5=Nadine |author-link5=Nadine Unger |last6=Bauer |first6=Susanne E. |year=2009 |title=Improved attribution of climate forcing to emissions |journal=Science |doi=10.1126/science.1174760 |pmid=19900930 |bibcode=2009Sci...326..716S |volume=326 |issue=5953 |pages=716–718|s2cid=30881469 |url=https://zenodo.org/record/1230902 }}</ref> The theory also predicts this will greatly affect available oxygen and [[hydroxyl radical]] content of the atmosphere.
+In the [[Arctic ocean]], 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|self-preservation phenomenon]] has been studied by Russian geologists starting in the late 1980s.<ref>{{cite journal |last1=Istomin |first1=V. A. |last2=Yakushev |first2=V. S. |last3=Makhonina |first3=N. A. |last4=Kwon |first4=V. G. |last5=Chuvilin |first5=E. M. |year=2006 |title=Self-preservation phenomenon of gas hydrates |journal=Gas Industry of Russia |number=4 |url=http://gasoilpress.com/dgir/dgir_detailed_work.php?DGIR_ELEMENT_ID=283&WORK_ELEMENT_ID=5645 |access-date=2013-08-30 |archive-url=https://web.archive.org/web/20131203003733/http://gasoilpress.com/dgir/dgir_detailed_work.php?DGIR_ELEMENT_ID=283&WORK_ELEMENT_ID=5645 |archive-date=2013-12-03 |url-status=dead }}</ref> This [[metastable]] clathrate state can be a basis for release events of methane excursions, such as during the interval of the [[Last Glacial Maximum]].<ref>{{citation |last1=Buffett |first1=Bruce A. |last2=Zatsepina |first2=Olga Y. |year=1999 |title=Metastability of gas hydrate |journal=  Geophysical Research Letters|doi=10.1029/1999GL002339 |bibcode=1999GeoRL..26.2981B |volume=26 |issue=19 |pages=2981–2984 }}</ref> A study from 2010 concluded with the possibility for a trigger of [[Abrupt climate change|abrupt climate warming]] based on metastable methane clathrates in the [[East Siberian Sea|East Siberian Arctic Shelf]] (ESAS) region.<ref>{{citation |last1=Shakhova |first1=Natalia |last2=Semiletov |first2=Igor |last3=Salyuk |first3=Anatoly |last4=Yusupov |first4=Vladimir |last5=Kosmach |first5=Denis |last6=Gustafsson |first6=Örjan |year=2010 |title=Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf |journal=Science |doi=10.1126/science.1182221 |bibcode=2010Sci...327.1246S |pmid=20203047 |volume=327 |issue=5970 |pages=1246–50 |citeseerx=10.1.1.374.5869 |s2cid=206523571 }}</ref>
+==Possible past releases==
-=== Subsea permafrost ===
+[[File:Gas-hydrate deposits by sector.jpg|thumb|upright=2.5|Gas-hydrate deposits by sector<ref name=Ruppel2018 />]]
-{{Main|Permafrost}}
+Studies published in 2000 considered this hypothetical effect to be responsible for warming events in and at the end of the [[Last Glacial Maximum]],<ref>{{cite journal |last1=Kennett |first1=James P. |last2=Cannariato |first2=Kevin G. |last3=Hendy |first3=Ingrid L. |last4=Behl |first4=Richard J. |date=7 April 2000 |title=Carbon Isotopic Evidence for Methane Hydrate Instability During Quaternary Interstadials |journal=[[Science (magazine)|Science]] |doi=10.1126/science.288.5463.128 |pmid=10753115 |bibcode=2000Sci...288..128K |volume=288 |issue=5463 |pages=128–133}}</ref>. Although periods of increased [[atmospheric methane]] match periods of [[submarine landslide|continental-slope failure]].<ref name="Maslin2004" /><ref name="Maslin2010" />, later work found that the distinct [[Hydrogen isotope biogeochemistry|deuterium/hydrogen (D/H) isotope ratio]] indicated [[wetland methane emissions]] as the main contributor to atmospheric methane concentrations.<ref name="Sowers2006">{{cite journal |last=Sowers |first=Todd |s2cid=38790253 |date=10 February 2006 |title=Late Quaternary Atmospheric {{chem|CH|4}} Isotope Record Suggests Marine Clathrates Are Stable |journal=Science |doi=10.1126/science.1121235 |pmid=16469923 |bibcode=2006Sci...311..838S |volume=311 |issue=5762 |pages=838–840}}</ref><ref>{{cite journal |last1=Severinghaus |first1=Jeffrey P. |last2=Whiticar |first2=MJ |last3=Brook |first3=EJ |last4=Petrenko |first4=VV |last5=Ferretti |first5=DF |last6=Severinghaus |first6=JP |date=25 August 2006 |title=Ice Record of {{chem|13|C}} for Atmospheric {{chem|CH|4}} Across the Younger Dryas-Preboreal Transition |journal=Science |doi=10.1126/science.1126562 |pmid=16931759 |bibcode=2006Sci...313.1109S |volume=313 |issue=5790 |pages=1109–12|s2cid=23164904 }}</ref> While there were major dissociation events during the last deglaciation, with [[Bølling-Allerød warming]] triggering the disappearance of the entire methane hydrate deposit in the [[Barents Sea]] within 5000 years, those events failed to counteract the onset of a major [[Younger Dryas]] cooling period, suggesting that most of the methane stayed within the seawater after being liberated from the seafloor deposits, with very little entering the atmosphere. <ref>{{cite web|url=https://www.washingtonpost.com/news/energy-environment/wp/2017/06/01/like-champagne-bottles-being-opened-scientists-document-an-ancient-arctic-methane-explosion|title=Like 'champagne bottles being opened': Scientists document an ancient Arctic methane explosion|work=The Washington Post|date=June 1, 2017}}</ref><ref>{{cite journal|journal=PNAS|volume=114|issue=24|pages=6215–6220|year=2017|title=Postglacial response of Arctic Ocean gas hydrates to climatic amelioration|author=Serov|display-authors=et al|doi=10.1073/pnas.1619288114|pmid=28584081|pmc=5474808|bibcode=2017PNAS..114.6215S|doi-access=free}}</ref>
-Subsea permafrost occurs beneath the seabed and exists in the [[continental shelves]] of the polar regions.<ref>{{cite web |author=IPCC AR4 |date=2007 |title=Climate Change 2007: Working Group I: The Physical Science Basis |url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch4s4-7-2-4.html |accessdate=April 12, 2014 |archive-url=https://web.archive.org/web/20140413125748/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch4s4-7-2-4.html |archive-date=April 13, 2014 |url-status=dead }}</ref> This source of methane is different from methane clathrates, but contributes to the overall outcome and feedbacks.
+In 2008, it was suggested that equatorial permafrost methane clathrate may have had a role in the sudden warm-up of "[[Snowball Earth]]", 630 million years ago.<ref>{{cite journal |last1=Kennedy |first1=Martin |last2=Mrofka |first2=David |last3=Von Der Borch |first3=Chris |year=2008 |title=Snowball Earth termination by destabilization of equatorial permafrost methane clathrate |journal=Nature |doi=10.1038/nature06961 |pmid=18509441 |bibcode=2008Natur.453..642K |volume=453 |issue=7195 |pages=642–645 |s2cid=4416812 |url=http://faculty.ucr.edu/~martink/pdfs/Kennedy_2008_Nature.pdf }}</ref>
-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&nbsp;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.<ref>{{cite journal |last1=Shakhova |first1=Natalia |last2=Semiletov |first2=Igor |last3=Leifer |first3=Ira |last4=Sergienko |first4=Valentin |last5=Salyuk |first5=Anatoly |last6=Kosmach |first6=Denis |last7=Chernykh |first7=Denis |last8=Stubbs |first8=Chris |last9=Nicolsky |first9=Dmitry |last10=Tumskoy |first10=Vladimir |last11=Gustafsson |first11=Örjan |date=November 24, 2013 |title=Ebullition and storm-induced methane release from the East Siberian Arctic Shelf |journal=Nature |doi=10.1038/ngeo2007 |bibcode=2014NatGe...7...64S |volume=7 |issue=1 |pages=64–70 }}</ref>
+Other events potentially linked to methane hydrate excursions are the [[Permian–Triassic extinction event]] and the [[Paleocene–Eocene Thermal Maximum]].
-=== 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.
+=== Paleocene–Eocene Thermal Maximum ===
-The so-called [[Self-preservation|self-preservation phenomenon]] has been studied by Russian geologists starting in the late 1980s.<ref>{{cite journal |last1=Istomin |first1=V. A. |last2=Yakushev |first2=V. S. |last3=Makhonina |first3=N. A. |last4=Kwon |first4=V. G. |last5=Chuvilin |first5=E. M. |year=2006 |title=Self-preservation phenomenon of gas hydrates |journal=Gas Industry of Russia |number=4 |url=http://gasoilpress.com/dgir/dgir_detailed_work.php?DGIR_ELEMENT_ID=283&WORK_ELEMENT_ID=5645 |access-date=2013-08-30 |archive-url=https://web.archive.org/web/20131203003733/http://gasoilpress.com/dgir/dgir_detailed_work.php?DGIR_ELEMENT_ID=283&WORK_ELEMENT_ID=5645 |archive-date=2013-12-03 |url-status=dead }}</ref> This [[metastable]] clathrate state can be a basis for release events of methane excursions, such as during the interval of the [[Last Glacial Maximum]].<ref>{{citation |last1=Buffett |first1=Bruce A. |last2=Zatsepina |first2=Olga Y. |year=1999 |title=Metastability of gas hydrate |journal=  Geophysical Research Letters|doi=10.1029/1999GL002339 |bibcode=1999GeoRL..26.2981B |volume=26 |issue=19 |pages=2981–2984 }}</ref> A study from 2010 concluded with the possibility for a trigger of [[Abrupt climate change|abrupt climate warming]] based on metastable methane clathrates in the [[East Siberian Sea|East Siberian Arctic Shelf]] (ESAS) region.<ref>{{citation |last1=Shakhova |first1=Natalia |last2=Semiletov |first2=Igor |last3=Salyuk |first3=Anatoly |last4=Yusupov |first4=Vladimir |last5=Kosmach |first5=Denis |last6=Gustafsson |first6=Örjan |year=2010 |title=Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf |journal=Science |doi=10.1126/science.1182221 |bibcode=2010Sci...327.1246S |pmid=20203047 |volume=327 |issue=5970 |pages=1246–50 |citeseerx=10.1.1.374.5869 |s2cid=206523571 }}</ref>
+{{excerpt|Paleocene–Eocene Thermal Maximum|paragraphs=1-4}}
+{{excerpt|Paleocene–Eocene Thermal Maximum#Methane release|paragraphs=2-7|hat=no}}
-=== Ocean anoxia ===
+=== Permian–Triassic extinction event ===
-{{Main|Anoxic event}}
+{{excerpt|Permian–Triassic extinction event}}
+{{excerpt|Permian–Triassic extinction event#Methane hydrate gasification|paragraphs=2-8|hat=no}}
-In the past, euxinic (i.e. sulfidic) and anoxic events have occurred either over relatively short time scales (decades to centuries) due to a disrupting event such as a [[Impact event|meteor impact]] or over tens of thousands to a few million years due to global changes in the Earth's climate. Both scenarios can result in the large scale release of methane and other [[greenhouse gas]]es from the ocean into the atmosphere. It is postulated such a release could occur in a quick, explosive event due to the complex interplay of buoyancy forces and exsolution of dissolved gases in the ocean. Initially, the increased amounts of smoke and dust in the atmosphere would cause a relatively short period of stratospheric cooling but this would quickly be overtaken by the effects of [[global warming]].<ref name="Ryskin 2003">{{cite journal |last=Ryskin |first=Gregory |date=September 2003 |title=Methane-driven oceanic eruptions and mass extinctions |journal=Geology |doi=10.1130/G19518.1 |bibcode=2003Geo....31..741R |volume=31 |issue=9 |pages=741–744 }}</ref>
-== Current outlook ==
+== Global warming feedback ==
 {{Main|Climate change in the Arctic}}
+=== Modern deposits ===
-Most deposits of methane clathrate are in sediments too deep to respond rapidly, and modelling by [[David Archer (scientist)|Archer]] (2007) suggests the methane forcing should remain a minor component of the overall [[greenhouse effect]].<ref name="Archer2007">{{cite journal |last=Archer |first=D. |year=2007 |title=Methane hydrate stability and anthropogenic climate change |journal=Biogeosciences |doi=10.5194/bg-4-521-2007 |volume=4 |issue=4 |pages=521–544 |url=http://geosci.uchicago.edu/~archer/reprints/archer.2007.hydrate_rev.pdf |bibcode=2007BGeo....4..521A |doi-access=free }} See also [http://www.realclimate.org/wp-trackback.php/227 blog summary].</ref>  Clathrate deposits destabilize from the deepest part of their [[Gas hydrate stability zone|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.<ref name="Archer2007" />
+Most deposits of methane clathrate are in sediments too deep to respond rapidly,<ref>{{Cite journal|last1=Archer|first1=D.|last2=Buffett|first2=B.|year=2005|title=Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing|url=http://geosci.uchicago.edu/~archer/reprints/archer.2005.clathrates.pdf|journal=Geochemistry, Geophysics, Geosystems|volume=6|issue=3|pages=Q03002|bibcode=2005GGG.....603002A|doi=10.1029/2004GC000854|doi-access=free}}</ref> and 2007 modelling by [[David Archer (scientist)|Archer]] suggests that the methane forcing derived from them should remain a minor component of the overall [[greenhouse effect]].<ref name="Archer2007">{{cite journal |last=Archer |first=D. |year=2007 |title=Methane hydrate stability and anthropogenic climate change |journal=Biogeosciences |doi=10.5194/bg-4-521-2007 |volume=4 |issue=4 |pages=521–544 |url=http://geosci.uchicago.edu/~archer/reprints/archer.2007.hydrate_rev.pdf |bibcode=2007BGeo....4..521A |doi-access=free }} See also [http://www.realclimate.org/wp-trackback.php/227 blog summary].</ref>  Clathrate deposits destabilize from the deepest part of their [[Gas hydrate stability zone|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.<ref name="Archer2007" />
-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.<ref name="USGS">{{cite web |date=January 14, 2013 |title=Climate-Hydrate Interactions |publisher=[[USGS]] |url=https://woodshole.er.usgs.gov/project-pages/hydrates/climate.html}}</ref><ref>{{cite web |last1=Shakhova |first1=Natalia |last2=Semiletov |first2=Igor |date=November 30, 2010 |title=Methane release from the East Siberian Arctic Shelf and the Potential for Abrupt Climate Change |url=http://symposium2010.serdp-estcp.org/content/download/8914/107496/version/3/file/1A_Shakhova_Final.pdf |accessdate=April 12, 2014}}</ref><ref name="monterey">{{cite press release |date=5 February 2007 |title=Methane bubbling through seafloor creates undersea hills |publisher=[[Monterey Bay Aquarium Research Institute]] |url=http://www.mbari.org/news/news_releases/2007/paull-plfs.html |url-status=dead |archive-url=https://web.archive.org/web/20081011033742/http://www.mbari.org/news/news_releases/2007/paull-plfs.html |archive-date=11 October 2008 }}</ref>
-According to data released by the EPA, atmospheric methane (CH<sub>4</sub>) 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.<ref>{{cite web |title=Atmosphere Changes |publisher=US Environmental Protection Agency |url=http://www.epa.gov/climatechange/science/recentac.html |accessdate=18 February 2012}}</ref> 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.<ref>{{cite web|url=https://www.esrl.noaa.gov/gmd/aggi/aggi.html|title=The Noaa Annual Greenhouse Gas Index (AGGi)|year=2018|work=NOAA}}</ref>
-A USGS metastudy in 2017 by the  USGS Gas Hydrates Project concluded:<ref name=USGSHydratesProject>[https://www.usgs.gov/news/gas-hydrate-breakdown-unlikely-cause-massive-greenhouse-gas-release Gas Hydrate Breakdown Unlikely to Cause Massive Greenhouse Gas Release],  USGS Gas Hydrates Project, 2017</ref><ref name=Ruppel2018>{{Cite journal |last1=Ruppel |first1=Carolyn D. |last2=Kessler |first2=John D. |date=2017-03-31 |title=The interaction of climate change and methane hydrates: Climate-Hydrates Interactions |journal=Reviews of Geophysics |language=en |volume=55 |issue=1 |pages=126–168 |doi=10.1002/2016RG000534|url=https://zenodo.org/record/1000665 |doi-access=free |bibcode=2017RvGeo..55..126R }}</ref>
-{{quote|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.}}
-===Arctic Ocean===
 [[File:Methane Releases - East Siberian Arctic Shelf (4416688271).jpg|thumb|upright=1.6|Potential Methane release in the Eastern Siberian Arctic Shelf]]
 {{main|Arctic methane release}}
-Research carried out in 2008 in the Siberian Arctic showed millions of tons of methane being released, apparently through perforations in the seabed permafrost,<ref name="monterey" /> with concentrations in some regions reaching up to 100 times normal levels.<ref>{{cite web |last=Connor |first=Steve |date=September 23, 2008 |title=Exclusive: The methane time bomb |work=[[The Independent]] |url=https://www.independent.co.uk/environment/climate-change/exclusive-the-methane-time-bomb-938932.html |accessdate=2008-10-03}}</ref><ref>{{cite web |last=Connor |first=Steve |date=September 25, 2008 |title=Hundreds of methane 'plumes' discovered |work=[[The Independent]] |url=https://www.independent.co.uk/news/science/hundreds-of-methane-plumes-discovered-941456.html |accessdate=2008-10-03}}</ref> 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.<ref>[http://westerstrand.blogspot.com/2008/09/methane-hot-topic.html Translation of a blog entry by Örjan Gustafsson, expedition research leader], 2 September 2008</ref> The current methane release had previously been estimated at 0.5 megatonnes per year.<ref>{{cite journal |last1=Shakhova |first1=N. |last2=Semiletov |first2=I. |last3=Salyuk |first3=A. |last4=Kosmach |first4=D. |last5=Bel'cheva |first5=N. |year=2007 |title=Methane release on the Arctic East Siberian shelf |url=http://www.cosis.net/abstracts/EGU2007/01071/EGU2007-J-01071.pdf?PHPSESSID=e |journal=Geophysical Research Abstracts |volume=9 |page=01071}}</ref> 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 [[talik]]s. 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,<ref>{{cite journal |last1=Shakhova |first1=N. |last2=Semiletov |first2=I. |last3=Salyuk |first3=A. |last4=Kosmach |first4=D. |year=2008 |title=Anomalies of methane in the atmosphere over the East Siberian shelf: Is there any sign of methane leakage from shallow shelf hydrates? |journal=Geophysical Research Abstracts |volume=10 |page=01526 |url=http://www.cosis.net/abstracts/EGU2008/01526/EGU2008-A-01526.pdf |access-date=2008-09-25 |archive-url=https://web.archive.org/web/20121222144303/http://www.cosis.net/abstracts/EGU2008/01526/EGU2008-A-01526.pdf |archive-date=2012-12-22 |url-status=dead }}</ref><ref>{{cite news |last=Mrasek |first=Volker |date=17 April 2008 |title=A Storehouse of Greenhouse Gases Is Opening in Siberia |newspaper=[[Der Spiegel|Spiegel International Online]] |url=http://www.spiegel.de/international/world/0,1518,547976,00.html |quote=The Russian scientists have estimated what might happen when this Siberian permafrost-seal thaws completely and all the stored gas escapes. They believe the methane content of the planet's atmosphere would increase twelvefold.}}</ref> equivalent in [[greenhouse effect]] to a doubling in the current level of {{CO2}}.
+However, some methane clathrates deposits in the Arctic are much shallower than the rest, which could make them far more vulnerable to warming. 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.<ref>{{cite journal |last=Corbyn |first=Zoë |date=December 7, 2012 |title=Locked greenhouse gas in Arctic sea may be 'climate canary' |journal=Nature |doi=10.1038/nature.2012.11988 |s2cid=130678063 |url=http://www.nature.com/news/locked-greenhouse-gas-in-arctic-sea-may-be-climate-canary-1.11988 |accessdate=April 12, 2014}}</ref> However, the [[East Siberian Arctic Shelf]] averages 45 meters in depth, and it is assumed that below the seafloor, sealed by sub-sea permafrost layers, hydrates deposits are located.<ref>{{Cite journal|last1=Shakhova|first1=N.|last2=Semiletov|first2=I.|last3=Panteleev|first3=G.|year=2005|title=The distribution of methane on the Siberian Arctic shelves: Implications for the marine methane cycle|journal=Geophysical Research Letters |volume=32 |issue=9 |pages=L09601 |bibcode=2005GeoRL..32.9601S |doi=10.1029/2005GL022751|doi-access=free}}</ref><ref>{{cite web|url=https://www.skepticalscience.com/arctic-methane-outgassing-e-siberian-shelf-part1.html|title=Arctic methane outgassing on the E Siberian Shelf part 1 - the background|year=2012|work=SkepticalScience}}</ref>. This would mean that when the warming potentially [[talik]]s or [[pingo]]-like features within the shelf, they would also serve as gas migration pathways for the formerly frozen methane, and a lot of attention has been paid to that possibility. <ref name="USGS">{{cite web |date=January 14, 2013 |title=Climate-Hydrate Interactions |publisher=[[USGS]] |url=https://woodshole.er.usgs.gov/project-pages/hydrates/climate.html}}</ref><ref>{{cite web |last1=Shakhova |first1=Natalia |last2=Semiletov |first2=Igor |date=November 30, 2010 |title=Methane release from the East Siberian Arctic Shelf and the Potential for Abrupt Climate Change |url=http://symposium2010.serdp-estcp.org/content/download/8914/107496/version/3/file/1A_Shakhova_Final.pdf |accessdate=April 12, 2014}}</ref><ref name="monterey">{{cite press release |date=5 February 2007 |title=Methane bubbling through seafloor creates undersea hills |publisher=[[Monterey Bay Aquarium Research Institute]] |url=http://www.mbari.org/news/news_releases/2007/paull-plfs.html |url-status=dead |archive-url=https://web.archive.org/web/20081011033742/http://www.mbari.org/news/news_releases/2007/paull-plfs.html |archive-date=11 October 2008 }}</ref> 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 [[talik]]s. Their paper initially included the line that the "release of up to 50 gigatonnes of predicted amount of hydrate storage [is] highly possible for abrupt release at any time". A release on this scale would increase the methane content of the planet's atmosphere by a factor of twelve,<ref>{{cite journal |last1=Shakhova |first1=N. |last2=Semiletov |first2=I. |last3=Salyuk |first3=A. |last4=Kosmach |first4=D. |year=2008 |title=Anomalies of methane in the atmosphere over the East Siberian shelf: Is there any sign of methane leakage from shallow shelf hydrates? |journal=Geophysical Research Abstracts |volume=10 |page=01526 |url=http://www.cosis.net/abstracts/EGU2008/01526/EGU2008-A-01526.pdf |access-date=2008-09-25 |archive-url=https://web.archive.org/web/20121222144303/http://www.cosis.net/abstracts/EGU2008/01526/EGU2008-A-01526.pdf |archive-date=2012-12-22 |url-status=dead }}</ref><ref>{{cite news |last=Mrasek |first=Volker |date=17 April 2008 |title=A Storehouse of Greenhouse Gases Is Opening in Siberia |newspaper=[[Der Spiegel|Spiegel International Online]] |url=http://www.spiegel.de/international/world/0,1518,547976,00.html |quote=The Russian scientists have estimated what might happen when this Siberian permafrost-seal thaws completely and all the stored gas escapes. They believe the methane content of the planet's atmosphere would increase twelvefold.}}</ref> equivalent in [[greenhouse effect]] to a doubling in the 2008 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<ref>{{cite web |last=Preuss |first=Paul |date=17 September 2008 |title=IMPACTS: On the Threshold of Abrupt Climate Changes |publisher=Lawrence Berkeley National Laboratory |url=http://newscenter.lbl.gov/feature-stories/2008/09/17/impacts-on-the-threshold-of-abrupt-climate-changes}}</ref> 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.<ref>{{cite book |author=CCSP |others=Clark|display-authors=et al |year=2008 |title=Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research |publisher=U.S. Geological Survey |location=Reston VA |url=http://www.climatescience.gov/Library/sap/sap3-4/final-report/default.htm |url-status=dead |archive-url=https://web.archive.org/web/20130504113820/http://www.climatescience.gov/Library/sap/sap3-4/final-report/default.htm |archive-date=2013-05-04 }}</ref> A 2012 assessment of the literature identifies methane hydrates on the Shelf of East Arctic Seas as a potential trigger.<ref>{{cite journal |last1=Sergienko |first1=V. I. |display-authors=etal |date=September 2012 |title=The Degradation of Submarine Permafrost and the Destruction of Hydrates on the Shelf of East Arctic Seas as a Potential Cause of the 'Methane Catastrophe': Some Results of Integrated Studies in 2011 |journal=Doklady Earth Sciences |issn=1028-334X |doi=10.1134/S1028334X12080144 |bibcode=2012DokES.446.1132S |volume=446 |issue=1 |pages=1132–1137 |s2cid=129638485 |url=https://www.researchgate.net/publication/257850226 |format=PDF }}</ref>
+This is what led to the original Clathrate gun hypothesis, and in 2008 the United States Department of Energy National Laboratory system<ref>{{cite web |last=Preuss |first=Paul |date=17 September 2008 |title=IMPACTS: On the Threshold of Abrupt Climate Changes |publisher=Lawrence Berkeley National Laboratory |url=http://newscenter.lbl.gov/feature-stories/2008/09/17/impacts-on-the-threshold-of-abrupt-climate-changes}}</ref> 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.<ref>{{cite book |author=CCSP |others=Clark|display-authors=et al |year=2008 |title=Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research |publisher=U.S. Geological Survey |location=Reston VA |url=http://www.climatescience.gov/Library/sap/sap3-4/final-report/default.htm |url-status=dead |archive-url=https://web.archive.org/web/20130504113820/http://www.climatescience.gov/Library/sap/sap3-4/final-report/default.htm |archive-date=2013-05-04 }}</ref> A 2012 study of the effects for the original hypothesis, based on a coupled climate–carbon cycle model ([[General Circulation 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&nbsp;°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.<ref>{{cite journal |author1=Atsushi Obata |author2=Kiyotaka Shibata |date=June 20, 2012 |title=Damage of Land Biosphere due to Intense Warming by 1000-Fold Rapid Increase in Atmospheric Methane: Estimation with a Climate–Carbon Cycle Model |journal=J. Climate |doi=10.1175/JCLI-D-11-00533.1 |bibcode = 2012JCli...25.8524O |volume=25 |issue=24 |pages=8524–8541 |doi-access=free }}</ref> Another 2012 assessment of the literature identifies methane hydrates on the Shelf of East Arctic Seas as a potential trigger.<ref>{{cite journal |last1=Sergienko |first1=V. I. |display-authors=etal |date=September 2012 |title=The Degradation of Submarine Permafrost and the Destruction of Hydrates on the Shelf of East Arctic Seas as a Potential Cause of the 'Methane Catastrophe': Some Results of Integrated Studies in 2011 |journal=Doklady Earth Sciences |issn=1028-334X |doi=10.1134/S1028334X12080144 |bibcode=2012DokES.446.1132S |volume=446 |issue=1 |pages=1132–1137 |s2cid=129638485 |url=https://www.researchgate.net/publication/257850226 |format=PDF }}</ref>
-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&nbsp;°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."<ref>{{Cite journal |last1=Hong |first1=Wei-Li |last2=Torres |first2=Marta E. |last3=Carroll |first3=JoLynn |last4=Crémière |first4=Antoine |last5=Panieri |first5=Giuliana |last6=Yao |first6=Haoyi |last7=Serov |first7=Pavel |date=2017 |title=Seepage from an arctic shallow marine gas hydrate reservoir is insensitive to momentary ocean warming |journal=Nature Communications |language=en |volume=8 |issue=1 |pages=15745 |doi=10.1038/ncomms15745 |pmid=28589962 |issn=2041-1723|bibcode=2017NatCo...815745H |pmc=5477557 }}</ref> Summarizing his research, Hong stated:
+A risk of seismic activity being potentially responsible for mass methane releases has been considered as well. In 2012, [[Seismology|seismic]] observations 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 [[Paleocene–Eocene Thermal Maximum|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."<ref>{{cite journal |last1=Phrampus |first1=B. J. |last2=Hornbach |first2=M. J. |date=December 24, 2012 |title=Recent changes to the Gulf Stream causing widespread gas hydrate destabilization |journal=Nature |doi=10.1038/nature.2012.11652 |pmid=23099408 |volume=490 |issue=7421 |pages=527–530 |s2cid=131370518 }}</ref> [[Bill McGuire (volcanologist)|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."<ref>{{cite web|url=http://climatestate.com/2014/10/16/methane-hydrate-destabilisation-is-clearly-a-real-worry-particularly-in-the-context-of-warming-ocean-waters-in-the-east-siberian-continental-shelf|title=Bill McGuire: Modelling suggests with ice cap melt, an increase in volcanic activity|year=2014|work=ClimateState.com}}</ref>
-{{quote|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.<ref>{{cite web|last1=CAGE|title=Study finds hydrate gun hypothesis unlikely|url=https://phys.org/news/2017-08-hydrate-gun-hypothesis.html|publisher=Phys.org|date=August 23, 2017}}</ref>}}
-==== Continental slopes ====
+=== Observed emissions ===
+==== East Siberian Arctic Ice Shelf ====
+Research carried out in 2008 in the Siberian Arctic showed methane releases on the annual scale of millions of tons, which was a substantial increase on the previous estimate of 0.5 millions of tons per year.<ref>{{cite journal |last1=Shakhova |first1=N. |last2=Semiletov |first2=I. |last3=Salyuk |first3=A. |last4=Kosmach |first4=D. |last5=Bel'cheva |first5=N. |year=2007 |title=Methane release on the Arctic East Siberian shelf |url=http://www.cosis.net/abstracts/EGU2007/01071/EGU2007-J-01071.pdf?PHPSESSID=e |journal=Geophysical Research Abstracts |volume=9 |page=01071}}</ref>  apparently through perforations in the seabed permafrost,<ref name="monterey" /> with concentrations in some regions reaching up to 100 times normal levels.<ref>{{cite web |last=Connor |first=Steve |date=September 23, 2008 |title=Exclusive: The methane time bomb |work=[[The Independent]] |url=https://www.independent.co.uk/environment/climate-change/exclusive-the-methane-time-bomb-938932.html |accessdate=2008-10-03}}</ref><ref>{{cite web |last=Connor |first=Steve |date=September 25, 2008 |title=Hundreds of methane 'plumes' discovered |work=[[The Independent]] |url=https://www.independent.co.uk/news/science/hundreds-of-methane-plumes-discovered-941456.html |accessdate=2008-10-03}}</ref> 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.<ref>[http://westerstrand.blogspot.com/2008/09/methane-hot-topic.html Translation of a blog entry by Örjan Gustafsson, expedition research leader], 2 September 2008</ref>
+By 2013, the same team of researchers used multiple sonar observations to quantify the density of bubbles emanating from subsea permafrost into the ocean (a process called ebullition), and found that 100–630&nbsp;mg methane per square meter is emitted daily along the East Siberian Arctic Shelf (ESAS), 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. Altogether, their updated estimate had now amounted to 17 millions of tons per year.<ref>{{cite journal |last1=Shakhova |first1=Natalia |last2=Semiletov |first2=Igor |last3=Leifer |first3=Ira |last4=Sergienko |first4=Valentin |last5=Salyuk |first5=Anatoly |last6=Kosmach |first6=Denis |last7=Chernykh |first7=Denis |last8=Stubbs |first8=Chris |last9=Nicolsky |first9=Dmitry |last10=Tumskoy |first10=Vladimir |last11=Gustafsson |first11=Örjan |date=24 November 2013 |title=Ebullition and storm-induced methane release from the East Siberian Arctic Shelf |journal=Nature |doi=10.1038/ngeo2007 |bibcode=2014NatGe...7...64S |volume=7 |issue=1 |pages=64–70 }}</ref>
+However, these findings were soon questioned, as this rate of annual release would mean that the ESAS alone would account for between 28% and 75% of the observed Arctic methane emissions, which contradicts many other studies. In January 2020, it was found that the rate at which methane enters the atmosphere after it had been released from the shelf deposits into the water column had been greatly overestimated, and observations of atmospheric methane fluxes taken from multiple ship cruises in the Arctic instead indicate that only around 3.02 million tonnes of methane are emitted annually from the ESAS.<ref>{{cite journal |last1=Thornton |first1=Brett F. |last2=Prytherch |first2=John |last3=Andersson |first3=Kristian |last4=Brooks |first4=Ian M. |last5=Salisbury |first5=Dominic |last6=Tjernström |first6=Michael |last7=Crill |first7=Patrick M. |date=29 January 2020 |title=Shipborne eddy covariance observations of methane fluxes constrain Arctic sea emissions |url=https://www.science.org/doi/10.1126/sciadv.aay7934 |journal=Science Advances |doi=10.1126/sciadv.aay7934 |volume=6 |issue=5 }}</ref> A modelling study published in 2020 suggested that under the present-day conditions, annual methane release from the ESAS may be as low as 1000 tonnes, with 2.6 – 4.5 million tonnes representing the peak potential of turbulent emissions from the shelf.<ref>{{cite journal |last1=Puglini |first1=Matteo |last2=Brovkin |first2=Victor |last3=Regnier |first3=Pierre |last4=Arndt |first4=Sandra |date=26 June 2020 |title=Assessing the potential for non-turbulent methane escape from the East Siberian Arctic Shelf |url=https://bg.copernicus.org/articles/17/3247/2020/ |journal=Biogeosciences |volume=17 |issue=12 |pages=3247–3275 |doi=10.5194/bg-17-3247-2020 }}</ref>
+==== Beaufort Sea continental slope ====
 [[File:Continental shelf.png|thumb|upright=1.2|Profile illustrating the continental shelf, slope and rise]]
-{{Carbon cycle|Methane}}
+A [[radiocarbon dating]] study in 2018 found that after the 30-meter [[isobath]], only around 10% of the methane in surface waters can be attributed to ancient permafrost or methane hydrates. The authors suggested that even a significantly accelerated methane release would still largely fail to reach the atmosphere.<ref>{{cite journal |last1=Sparrow |first1=Katy J. |last2=Kessler |first2=John D. |last3=Southon |first3=John R. |last4=Garcia-Tigreros |first4=Fenix |last5=Schreiner |first5=Kathryn M. |last6=Ruppel |first6=Carolyn D. |last7=Miller |first7=John B. |last8=Lehman |first8=Scott J. |last9=Xu |first9=Xiaomei |date=17 January 2018 |title=Limited contribution of ancient methane to surface waters of the U.S. Beaufort Sea shelf |url=https://www.science.org/doi/10.1126/sciadv.aay7934 |journal=Science Advances |volume=4 |issue=1 |doi=10.1126/sciadv.aao4842 }}</ref>
-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.<ref>{{cite journal |last=Corbyn |first=Zoë |date=December 7, 2012 |title=Locked greenhouse gas in Arctic sea may be 'climate canary' |journal=Nature |doi=10.1038/nature.2012.11988 |s2cid=130678063 |url=http://www.nature.com/news/locked-greenhouse-gas-in-arctic-sea-may-be-climate-canary-1.11988 |accessdate=April 12, 2014}}</ref> 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.<ref>{{cite web|url=https://www.skepticalscience.com/arctic-methane-outgassing-e-siberian-shelf-part1.html|title=Arctic methane outgassing on the E Siberian Shelf part 1 - the background|year=2012|work=SkepticalScience}}</ref>
+==== Svalbard ====
+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&nbsp;°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."<ref>{{Cite journal |last1=Hong |first1=Wei-Li |last2=Torres |first2=Marta E. |last3=Carroll |first3=JoLynn |last4=Crémière |first4=Antoine |last5=Panieri |first5=Giuliana |last6=Yao |first6=Haoyi |last7=Serov |first7=Pavel |date=2017 |title=Seepage from an arctic shallow marine gas hydrate reservoir is insensitive to momentary ocean warming |journal=Nature Communications |language=en |volume=8 |issue=1 |pages=15745 |doi=10.1038/ncomms15745 |pmid=28589962 |issn=2041-1723|bibcode=2017NatCo...815745H |pmc=5477557 }}</ref> Summarizing his research, Hong stated:
+{{quote|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.<ref>{{cite web|last1=CAGE|title=Study finds hydrate gun hypothesis unlikely|url=https://phys.org/news/2017-08-hydrate-gun-hypothesis.html|publisher=Phys.org|date=August 23, 2017}}</ref>}}
+Research by Klaus Wallmann et al. 2018 concluded that hydrate dissociation at Svalbard 8,000 years ago was due to [[isostatic rebound]] (continental uplift following [[deglaciation]]). 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.<ref name="Wallmann2018">{{Cite journal|journal=Nature Communications|year=2018|author=Wallmann|display-authors=et al |title=Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming |volume=9 |issue=1 |pages=83 |doi=10.1038/s41467-017-02550-9 |pmid=29311564 |pmc=5758787 |bibcode=2018NatCo...9...83W}}</ref> Moreover, another paper published in 2017 found that only 0.07% of the methane released from the gas hydrate dissociation at Svalbard appears to reach the atmosphere, and usually only when the wind speeds were low.<ref>{{cite journal |last1=Mau |first1=S. |last2=Römer |first2=M. |last3=Torres |first3=M. E. |last4=Bussmann |first4=I. |last5=Pape |first5=T. |last6=Damm |first6=E. |last7=Geprägs |first7=P. |last8=Wintersteller |first8=P. |last9=Hsu |first9=C.-W. |last10=Loher |first10=M. |last11=Bohrmann |first11=G. |date=23 February 2017 |title=Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden |journal=Scientific Reports |volume=7 |doi=10.1038/srep42997 }}</ref>  In 2020, a subsequent study confirmed that only a small fraction of methane from the Svalbard seeps reaches the atmosphere, and that the wind speed holds a greater influence on the rate of release than dissolved methane concentration on site. <ref>{{cite journal |last1=Silyakova |first1=Anna |last2=Jansson |first2=Pär |last3=Serov |first3=Pavel |last4=Ferré |first4=Benedicte |last5=Pavlov |first5=Alexey K. |last6=Hattermann |first6=Tore |last7=Graves |first7=Carolyn A. |last8=Platt |first8=Stephen M. |last9=Lund Myhre |first9=Cathrine |last10=Gründger |first10=Friederike |last11=Niemann |first11=Helge |date=1 February 2020 |title=Physical controls of dynamics of methane venting from a shallow seep area west of Svalbard |journal=Continental Shelf Research |volume=194 |doi=10.1016/j.csr.2019.104030 }}</ref>
-[[Seismology|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 [[Paleocene–Eocene Thermal Maximum|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."<ref>{{cite journal |last1=Phrampus |first1=B. J. |last2=Hornbach |first2=M. J. |date=December 24, 2012 |title=Recent changes to the Gulf Stream causing widespread gas hydrate destabilization |journal=Nature |doi=10.1038/nature.2012.11652 |pmid=23099408 |volume=490 |issue=7421 |pages=527–530 |s2cid=131370518 }}</ref>
+Finally, a paper published in 2017 indicated that the methane emissions from at least one seep field at Svalbard were more than compensated for by the enhanced carbon dioxide uptake due to the greatly increased [[phytoplankton]] activity in this nutrient-rich water. The daily amount of carbon dioxide absorbed by the phytoplankton was 1,900 greater than the amount of methane emitted, and the negative (i.e. indirectly cooling) [[radiative forcing]] from the CO<sup>2</sup> uptake was up to 251 times greater than the warming from the methane relase.<ref>{{cite journal |last1=Pohlman |first1=John W. |last2=Greinert |first2=Jens |last3=Ruppel |first3=Carolyn |last4=Silyakova |first4=Anna |last5=Vielstädte |first5=Lisa |last6=Casso |first6=Michael |last7=Mienert |first7=Jürgen |last8=Bünz |first8=Stefan |date=1 February 2020 |title=Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane |url=https://www.pnas.org/doi/10.1073/pnas.1618926114 |journal=Biological Sciences |volume=114 |issue=21 |pages=5355-5360 |doi=10.1073/pnas.1618926114 }}</ref>
-[[Bill McGuire (volcanologist)|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."<ref>{{cite web|url=http://climatestate.com/2014/10/16/methane-hydrate-destabilisation-is-clearly-a-real-worry-particularly-in-the-context-of-warming-ocean-waters-in-the-east-siberian-continental-shelf|title=Bill McGuire: Modelling suggests with ice cap melt, an increase in volcanic activity|year=2014|work=ClimateState.com}}</ref>
+=== Current outlook ===
-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.<ref name="Wallmann2018"/>
+In 2014 based on their research on the northern United States Atlantic marine continental margins from [[Cape Hatteras]] to [[Georges Bank]], a group of scientists from the US Geological Survey, the Department of Geosciences, Mississippi State University, Department of Geological Sciences, Brown University and Earth Resources Technology, found widespread leakage of methane from the seafloor, but they did not assign specific dates, beyond suggesting that some of the seeps were more than 1000 years old. <ref name="2014_US">{{cite journal |title=Widespread methane leakage from the sea floor on the northern US Atlantic margin |first1=A. |last1=Skarke |first2=C. |last2=Ruppel |first3=M. |last3=Kodis |first4=D. |last4=Brothers |first5=E. |last5=Lobecker |journal=Nature Geoscience |volume=7 |issue=9 |pages=657–661 |date=21 July 2014 |doi=10.1038/ngeo2232 |bibcode=2014NatGe...7..657S}}</ref><ref name="Widespread_methane_leakage_2014">{{cite news |last1=McGrath |first1=Matt |title=Widespread methane leakage from ocean floor off US coast |url=https://www.bbc.com/news/science-environment-28898223 |access-date=24 August 2014 |publisher=BBC |date=24 August 2014}}</ref> In March 2017, a meta-analysis by the USGS Gas Hydrates Project concluded:<ref name=USGSHydratesProject>[https://www.usgs.gov/news/gas-hydrate-breakdown-unlikely-cause-massive-greenhouse-gas-release Gas Hydrate Breakdown Unlikely to Cause Massive Greenhouse Gas Release],  USGS Gas Hydrates Project, 2017</ref><ref name=Ruppel2018>{{Cite journal |last1=Ruppel |first1=Carolyn D. |last2=Kessler |first2=John D. |date=2017-03-31 |title=The interaction of climate change and methane hydrates: Climate-Hydrates Interactions |journal=Reviews of Geophysics |language=en |volume=55 |issue=1 |pages=126–168 |doi=10.1002/2016RG000534|url=https://zenodo.org/record/1000665 |doi-access=free |bibcode=2017RvGeo..55..126R }}</ref>
+{{quote|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.}} In June 2017, scientists from the Center for Arctic Gas Hydrate (CAGE), Environment and Climate at the [[University of Tromsø]], published a study describing over a hundred [[ocean sediment]] craters, some 300 meters wide and up to 30 meters deep, formed due to explosive eruptions, attributed to destabilizing methane hydrates, following ice-sheet retreat during the [[last glacial period]], around 15,000 years ago, a few centuries after the [[Bølling-Allerød warming]]. These areas around the [[Barents Sea]], still seep methane today, and still existing bulges with [[methane reservoirs]] could eventually have the same fate.<ref>{{cite news |url=https://www.washingtonpost.com/news/energy-environment/wp/2017/06/01/like-champagne-bottles-being-opened-scientists-document-an-ancient-arctic-methane-explosion |title=Like 'champagne bottles being opened': Scientists document an ancient Arctic methane explosion |newspaper=The Washington Post |date=June 1, 2017}}</ref> Later that same year, the [[Arctic Council]] published SWIPA 2017 report, where it cautioned "Arctic sources and sinks of greenhouse gases are still hampered by data and knowledge gaps."<ref>{{cite web|url=https://www.amap.no/documents/doc/swipa-2017-press-material/1544|title=SWIPA 2017 - Press Material|year=2017|work=Arctic Council}}</ref>
+In 2018, a perspective piece devoted to [[tipping points in the climate system]] suggested that the climate change contribution from methane hydrates would be "negligible" by the end of the century, but could amount ot 0.4-0.5 degrees Celsius on the millennial timescales.<ref name="Schellnhuber2018">{{Cite journal |last1=Schellnhuber |first1=Hans Joachim |last2=Winkelmann |first2=Ricarda |last3=Scheffer |first3=Marten |last4=Lade |first4=Steven J. |last5=Fetzer |first5=Ingo |last6=Donges |first6=Jonathan F. |last7=Crucifix |first7=Michel |last8=Cornell |first8=Sarah E. |last9=Barnosky |first9=Anthony D. |author-link9=Anthony David Barnosky |date=2018 |title=Trajectories of the Earth System in the Anthropocene |journal=[[Proceedings of the National Academy of Sciences]] |volume=115 |issue=33 |pages=8252–8259 |bibcode=2018PNAS..115.8252S |doi=10.1073/pnas.1810141115 |issn=0027-8424 |pmc=6099852 |pmid=30082409 |doi-access=free}}</ref> In 2021, the [[IPCC Sixth Assessment Report]] no longer included methane hydrates in the list of potential tipping points, and says that "it is very unlikely that CH4 emissions from clathrates will substantially warm the climate system over the next few centuries."<ref name="IPCC AR6 WG1 Ch.5" /> Likewise, a 2022 assessment of tipping points described methane hydrates as a "threshold-free feedback" rather than a tipping point.<ref>{{Cite journal |last1=Armstrong McKay |first1=David|last2=Abrams |first2=Jesse |last3=Winkelmann |first3=Ricarda  |last4=Sakschewski |first4=Boris  |last5=Loriani |first5=Sina  |last6=Fetzer |first6=Ingo|last7=Cornell|first7=Sarah |last8=Rockström |first8=Johan |last9=Staal  |first9=Arie |last10=Lenton |first10=Timothy  |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points |url=https://www.science.org/doi/10.1126/science.abn7950 |journal=Science |language=en |volume=377 |issue=6611 |doi=10.1126/science.abn7950 |issn=0036-8075}}</ref><ref name="Explainer">{{Cite web |last=Armstrong McKay |first=David |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points – paper explainer |url=https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |access-date=2 October 2022 |website=climatetippingpoints.info |language=en}}</ref>
-== Model simulations ==
-A study of the effects for the original hypothesis, based on a coupled climate–carbon cycle model ([[General Circulation 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&nbsp;°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.<ref>{{cite journal |author1=Atsushi Obata |author2=Kiyotaka Shibata |date=June 20, 2012 |title=Damage of Land Biosphere due to Intense Warming by 1000-Fold Rapid Increase in Atmospheric Methane: Estimation with a Climate–Carbon Cycle Model |journal=J. Climate |doi=10.1175/JCLI-D-11-00533.1 |bibcode = 2012JCli...25.8524O |volume=25 |issue=24 |pages=8524–8541 |doi-access=free }}</ref>
 ==In fiction==