<P> Atmospheric methane is the methane present in Earth's atmosphere . Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere . The 100 - year global warming potential of methane is 28 . That is, over a 100 - year period, it traps 28 times more heat per mass unit than carbon dioxide and 32 times the effect when accounting for aerosol interactions . Global methane levels had risen from 722 parts per billion (ppb) in pre-industrial times to 1800 ppb by 2011, an increase by a factor of 2.5 and 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, for example, a minimum in the northern tropics during April − May mainly due to removal by the hydroxyl radical . </P> <P> Early in the Earth's history carbon dioxide and methane likely produced a greenhouse effect . The carbon dioxide would have been produced by volcanoes and the methane by early microbes . 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 . </P> <P> 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 . </P>

Which gas most likely was not part of earth's early atmosphere