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 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.) Global methane levels, had 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 seasonall