枯草庵残光記「メタンの温室効果」（2014年11月30日）

　メタン（Ｍｅｔｈａｎｅ；ＣＨ４）は二酸化炭素に次ぐ地球温暖化に及ぼす影響が大きい温室効果ガスであり、メタンのC-H共有結合は太陽からの赤外線を強く吸収してそのエネルギーを外部に放出し結果として地球を温める働きがあるため、地球温暖化に大きな影響を与えている。ＩＰＣＣ（Intergovernmental Panel on Climate Change；気候変動に関する政府間パネル）の第５次評価報告書によると、その地球温暖化への寄与は同じ量の二酸化炭素の28倍で温室効果の比率はCO2が65%、CH4が16%だそうだ。但し、この数字はガスの単位重量あたりの温室効果であって、温暖化をもたらす効果は単位量あたりの温室効果と濃度増加のかけ算で決まるそうだ。 このことから、温暖化に対して最大の寄与を示す二酸化炭素（CO2）の削減を急がなくては本質的な対策にならないそうだ。
　メタンの主な自然発生源は湿地・水田や白アリ等細菌の作用によるものと油田・天然ガス田から発生するもので、人為発生源は畜産（牛、ヤギ、ヒツジなどのげっぷ）、稲作（水田の地中から発生）、埋め立て、バイオマス燃焼等多岐にわたっている。この内最近油田・天然ガス田での化石燃料採掘時における大量発生が問題視されている。
メタンは温室効果ガスの一種で、メタンが温室効果を持つ理由は、①二酸化炭素に次いで地球温暖化に影響が大きい、②同じ重量で二酸化炭素よりも強い温室効果を持つ、③大気中での平均寿命は比較的短く、約12年だが、同じ重量であれば二酸化炭素よりも強い温室効果を持つ。

　メタンの他にも温暖化ガスがあり、一酸化二窒素（N2O）、フロン類、六フッ化硫黄（SF6）などCO2、CH4、N2O、HFC、PFC、SF6　等の削減が同時に叫ばれているが、これらについては別の機会に説明したい。

 

　Methane (CH4) is a greenhouse gas that has the second largest impact on global warming after carbon dioxide, and its C-H covalent bond strongly absorbs infrared rays from the sun and releases that energy to the outside, which results in warming the earth, and therefore has a major impact on global warming. 　According to the Fifth Assessment Report of the IPCC (Intergovernmental Panel on Climate Change), methane's contribution to global warming is 28 times that of the same amount of carbon dioxide, and the ratio of greenhouse effects is 65% for CO2 and 16% for CH4.　However, this figure is the greenhouse effect per unit weight of gas, and the effect on global warming is apparently determined by multiplying the greenhouse effect per unit amount by the increase in concentration. For this reason, it seems that a fundamental measure will not be taken unless we hurry to reduce carbon dioxide (CO2), which has the greatest contribution to global warming. 　The main natural sources of methane are from the action of bacteria such as wetlands, rice paddies and termites, and from oil fields and natural gas fields, while anthropogenic sources are diverse, including livestock farming (burps from cows, goats, sheep, etc.), rice cultivation (produced underground in rice paddies), landfilling, and biomass burning.　

　Of these, the large amounts of methane generated during fossil fuel extraction in oil and natural gas fields have recently become a problem. 　Methane is a type of greenhouse gas, and the reasons it has a greenhouse effect are: 1) it has the second largest impact on global warming after carbon dioxide. 　2) it has a stronger greenhouse effect than carbon dioxide for the same weight, and 　3) its average lifespan in the atmosphere is relatively short, about 12 years, but for the same weight it has a stronger greenhouse effect than carbon dioxide.

　There are other greenhouse gases besides methane, such as nitrous oxide (N2O), fluorocarbons, and sulfur hexafluoride (SF6), and there are calls for the reduction of CO2, CH4, N2O, HFCs, PFCs, SF6, etc., but I would like to explain these on another occasion.

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