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(詳細はmethane present in Earth's atmosphere. Atmospheric methane concentrations are of interest due to methane's impact on climate change, as it is one of the most potent greenhouse gases in Earth's atmosphere. The 100-year global warming potential of methane is 29 (i.e., over a 100-year period, it traps 29 times more heat per mass unit than carbon dioxide and 32 times the effect when accounted for aerosol interactions.〔 (Online )〕) Global methane levels, have risen to 1800 parts per billion (ppb) by 2011, an increase by a factor of 2.5 since pre-industrial times, from 722 ppb, the highest value in at least 800,000 years. Its concentration is higher in the Northern Hemisphere since most sources (both natural and human) are located on land and the Northern Hemisphere has more land mass. The concentrations vary seasonally, with a minimum in the late summer mainly due to removal by the hydroxyl radical. Early in the Earth's history—about 3.5 billion years ago—there was 1,000 times as much methane in the atmosphere as there is now, released into the atmosphere by volcanic activity. During this time, Earth's earliest life appeared. These first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today. Methane is created near the surface, and it is carried into the stratosphere by rising air in the tropics. Uncontrolled build-up of methane in Earth's atmosphere is naturally checked—although human influence can upset this natural regulation—by methane's reaction with hydroxyl radicals formed from singlet oxygen atoms and with water vapor. ==Methane as a greenhouse gas== Methane in the Earth's atmosphere is a strong greenhouse gas with a global warming potential of 29 over a 100-year period. This means that a methane emission will have 29 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect (100 times as strong as carbon dioxide) for a brief period (having a half-life of 7 years in the atmosphere〔(Phys.org - methane )〕), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20-year time period is 86. The concentration of methane in Earth's atmosphere has increased by about 150% since 1750, and it now accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases.〔(【引用サイトリンク】title=Technical summary )〕 According to NOAA the atmospheric methane concentration was measured at 1890 ppb in Ireland (Northern Hempishere) and 1760 ppb (Tasmania, Southern Hemisphere) in 2013 - arctic methane release from melting methane clathrates has played a role, but other recent emissions include peat fires in Southeast Asia. Atmospheric methane concentration has not been as high as this for over 420,000 years and correlates to an increase in average Earth temperature of 9 °C (see graph, which shows the close correlation between CH4 concentrations, concentrations, and Earth's temperature variations). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atmospheric methane」の詳細全文を読む スポンサード リンク
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