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 concentrations 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 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.