Editing Methane clathrate (section)

====Reservoir size====
[[File:Gas_hydrate_under_carbonate_rock.jpg|thumb|Gas hydrate under carbonate rock on the seafloor of the northern Gulf of Mexico]]
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size decreased by roughly an [[order of magnitude]] per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 1970s.<ref name="Milkov 2004">{{Cite journal |last=Milkov |first=AV |year=2004 |title=Global estimates of hydrate-bound gas in marine sediments: how much is really out there? |journal=Earth-Science Reviews |volume=66 |issue=3–4 |pages=183–197 |doi=10.1016/j.earscirev.2003.11.002 |bibcode=2004ESRv...66..183M}}</ref>  The highest estimates (e.g. 3{{e|18}} m<sup>3</sup>)<ref name="Trofimuk 1973">{{Cite journal |last=Trofimuk |first=A. A. |author2=N. V. Cherskiy |author3=V. P. Tsarev |year=1973 |title=[Accumulation of natural gases in zones of hydrate—formation in the hydrosphere] |journal=[[Doklady Akademii Nauk SSSR]] |volume=212 |pages=931–934 |language=ru}}</ref> were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. Improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates form in only a narrow range of depths ([[continental shelves]]), at only some locations in the range of depths where they could occur (10-30% of the [[Gas hydrate stability zone]]), and typically are found at low concentrations (0.9–1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory occupies between {{convert|1e15|and|5e15|m3|e6mi3|sigfig=2|abbr=off}}.<ref name="Milkov 2004"/>  This estimate, corresponding to 500–2500 gigatonnes carbon (Gt&nbsp;C), is smaller than the 5000 Gt&nbsp;C estimated for all other geo-organic fuel reserves but substantially larger than the ~230 Gt&nbsp;C estimated for other natural gas sources.<ref name="Milkov 2004"/><ref name="USGS 2000">USGS World Energy Assessment Team, 2000. US Geological Survey world petroleum assessment 2000––description and results. USGS Digital Data Series DDS-60.</ref> The permafrost reservoir has been estimated at about 400&nbsp;Gt&nbsp;C in the Arctic,<ref>{{cite journal |last1=MacDonald |first1=G. J. |year=1990 |title=Role of methane clathrates in past and future climates |journal=Climatic Change |volume=16 |issue=3 |pages=247–281 |doi=10.1007/bf00144504 |bibcode=1990ClCh...16..247M|s2cid=153361540 }}</ref>{{Citation needed|date=October 2011}} but no estimates have been made of possible Antarctic reservoirs. These are large amounts. In comparison, the total carbon in the atmosphere is around 800&nbsp;gigatons (see [[Carbon#Occurrence|Carbon: Occurrence]]).

These modern estimates are notably smaller than the 10,000 to 11,000&nbsp;Gt&nbsp;C (2{{e|16}} m<sup>3</sup>) proposed<ref>{{Cite journal |last=Buffett |first=Bruce |author2=David Archer |date=15 November 2004 |title=Global inventory of methane clathrate: sensitivity to changes in the deep ocean |journal=Earth and Planetary Science Letters |volume=227 |issue=3–4 |pages=185–199 |url=http://geosci.uchicago.edu/~archer/reprints/buffett.2004.clathrates.pdf |quote=Preferred ... global estimate of 3<sup>18</sup> g ... Estimates of the global inventory of methane clathrate may exceed 10<sup>19</sup> g of carbon |doi=10.1016/j.epsl.2004.09.005 |bibcode=2004E&PSL.227..185B}}</ref> by previous researchers as a reason to consider clathrates to be a geo-organic fuel resource (MacDonald 1990, Kvenvolden 1998). Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites<ref name="Milkov 2004"/> does suggest that only a limited percentage of clathrates deposits may provide an economically viable resource.