Atmospheric Methane and the Clathrate-gun Conjecture Methane, the Clathrate-gun conjecture and a disturbed Carbon Cycle A look at recent studies in climate science Phil Harris March 2012 This is a longer version of a more concise article by Phil Harris published by Ugo Bardi at his blog. It is retained in order to provide access to a wider selection of references to up-to-date climate science papers. It contains many direct quotes from and introductions to these papers. The author would value additions, comments and corrections. Introduction: a personal quest When Ugo Bardi wrote recently of atmospheric methane and the potential “clathrate gun”, I was prompted to dig out my old hard copies of papers that I had collected in the early 1990s, mostly from the journal Nature. These papers mentioned the potent greenhouse gas methane deposited as clathrates (gas hydrates), but mostly in those days with reference to climate changes involving clathrate release in remote geological ages. I asked about later updates and raised questions about the age of current methane gas hydrate deposits and the part they had played, or had not played in altering atmospheric methane concentrations, or indeed climate, during the Pleistocene. Glacial conditions increasingly predominated during the last two to three million years, but these were interspersed with short temperature excursions in addition to the longer inter-glacial responses to orbital forcing (i.e. the responses to initial forcing due to Milankovitch cycles; see the last 0.8Ma)1. How stable were these methane hydrate deposits over these glacial cycles; indeed, how old are the deposits? Methane is a reactive molecule with a relatively very short residence time in the atmosphere, so what do we know anyway of the current as well as past variation in methane (CH4) concentrations in the air? What do we know of the relative volumes of current sources and sinks, including clathrates? Ugo suggested that I attempt to collate some up-to-date information. I cannot legitimately call what follows a scientific review. I will try mostly to use direct quotes. I have a scientific background but in this case I can only be an interested observer, and I will not attempt any critique of methodology or draw any conclusions except perhaps via my last quote at the end. Are methane gas hydrates potentially more easily mobilised than other sequestered carbon deposits? Do we know, apart from the more random tectonic events, whether 1 This cooling trend has been reviewed 2011 by Hansen & Sato (NASA); submitted for publication; FULL PAPER Atmospheric Methane and the Clathrate-gun Conjecture there are climate-related positive feedback mechanisms that more frequently trigger big pulses of CH4 (‘the gun’) from clathrate deposits, and if so, from which deposits? Can such releases be demonstrated in the geological record? i [An interesting set of full papers from 2011 can be accessed from this reference , with particular focus on measurement, distribution and mixing of greenhouse gases in the atmosphere, and on mitigation policies; Greenhouse gases in the Earth system: setting the agenda to 2030 ] Opening remarks on the carbon cycle There has been a planetary carbon-cycle for a very long time. Carbon in the atmosphere is resident mostly as carbon dioxide (CO2) and this CO2 is in part exchanged over very short time scales, for example seasonally, with sinks such as the ocean, and with vegetation during photosynthesis, but for the most part remains in the atmosphere for many decades and in smaller part for centuries. In remote geological time carbon became sequestered in very large persistent sinks of carbonaceous rock and in petroleum and gas deposits. Weathering, tectonic movement and volcanic activity release carbon from rocks, and seepage occurs from trapped “fossil fuels” and buried organic material, but since the last 10s of millions of years, the earlier sequestration has had the net ongoing effect of a reduced carbon gas level maintained in the atmosphere. Thus, more recent geological ages have experienced much lower levels of free CO2 and CH4 than those remote epochs when the largest ancient carbon stores were laid down.2 Such ancient sink accumulations have of course included significant quantities of trapped CH4 gas, such as those associated with coal and oil that are now being released by human activity, and those trapped in the form of “meta-stable” gas hydrates. If the total warming effect is calculated over a century, methane is x25 more potent greenhouse gas than CO2 molecule for molecule, but is resident in air at very low concentrations for a much shorter average time of 7.9 years. In summary, carbon accumulates in living material following photosynthesis. This carbon is in turn released as CO2 by aerobic respiration or as CH4 during active anaerobic decomposition by microorganisms, including ruminant digestion, or as both CO2 and CH4 by wild fires. Ancient carbonaceous rocks release carbon mostly in the form of CO2 by weathering or other physical action, although CH4 also seeps from deposits of ancient trapped gas, or is released from clathrate (gas hydrate) deposits. Carbon is thus continually being cycled; footnote3. Human activity adds 2 Ibid 1 Hansen & Sato 3 There is no reason to question the biogenic origin of almost all of the carbon sequestered geologically on earth or of the fraction participating in the contemporary carbon cycle, but there is a reference to relatively insignificant a- biogenic CH4 reaching the earth’s surface in ref 2, and to a putative mechanism in ref 20. There are large deposits of methane elsewhere in the solar system presumably of inorganic origin. Atmospheric Methane and the Clathrate-gun Conjecture both CO2 and CH4 to the atmosphere from industrial activity, and CH4 from recent extension of agriculture. Sources and sinks of CH4; including gas hydrates (referenceii) Gas hydrates are deposits of sequestered methane associated with the ocean floor and sedimentary overburdens and with permafrost, and appear to have had a continuous but dynamic presence over geological time. Methane continually seeps from miscellaneous geological sources including gas hydrates as well as from deeper petroleum sources. Until additional sources were opened-up during the industrial period, methane in the air was a result of routine releases from a combination of natural sources, but with a majority coming from relatively recently photosynthesised material. Soils depending iii on type, location, temperature and wetness can be either sources or sinks of CH4 . Methane is cycled very rapidly and needs continuous large-scale release to maintain any significant level in the atmosphere. The chief agents acting on released methane are OH’ radicals in the atmosphere during daylight, but soil organisms also make a significant if much smaller contribution to oxidising CH4 to CO2 of perhaps 5-10% of the global sinks iv. There is an interesting relationship with atmospheric ozone: v Methane contributes to additional planetary warming through its involvement in tropospheric photochemistry leading to ozone formation, and its oxidation in the lower stratosphere generates water vapor that acts as a highly effective GHG [Green House Gas]. [This study actually focuses on ancient ‘greenhouse worlds’ and is referred to again below.] Much of the methane released from ocean floors is oxidised by bacteria within the water column and does not reach the surface. Also, contemporary sequestration is vi known to occur via bacterial action next to CH4 seepages at the ocean floor . There is thus a ‘methane cycle’ as a subset of the ‘carbon cycle’. The natural cycle includes seepages from petroleum sources but mostly is maintained by both microbial methanogenesis and methane oxidation under anaerobic/anoxic conditionsvii. There has been a very significant increase in CH4 in the atmosphere since 1850 due to massive releases from fossil fuels and the spread of agriculture. viii For example, during the last glacial period the concentration was w0.35ppm. This amount increased to w0.75ppm by 1850 at which time the CH4 concentration increased much more rapidly as a result of agricultural expansion and industrialization (Lelieveld et al., 1998). By 1998, the globally averaged atmospheric surface abundance of CH4 was 1.745 ppm, corresponding to a total burden of w4850 Tg CH4 (ICPP, 2001). Methane makes a significant contribution to modern greenhouse gas radiative forcing. Atmospheric Methane and the Clathrate-gun Conjecture ix The global atmospheric methane burden has more than doubled since pre-industrial times, and this increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse-gas emissions. The above 1998 study discussed the slowing in the 1990s of the previously rapid rise of atmospheric CH4. That rise has since resumed; see below in “Modern Methane Trends”. Snowball Earth There was, a very long time ago, a Snowball Earth; a period that ended around 635Ma. Gas hydrate releases are mentioned as one of putative positive feedback mechanisms that brought this phenomenon to an end. x Hypotheses accounting for the abruptness of deglaciation include ice albedo feedback, deep-ocean out-gassing during post-glacial oceanic overturn or methane hydrate destabilization. Scientific discussion continues about this interesting period, but for our purposes it is worth noting the reasons why we do not have a snowball earth. xi Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth's atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Non-condensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect.