Atmospheric methane is the 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.) 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.