Methane in the Earth's atmosphere is a strong greenhouse gas with a global warming potential (GWP) 84 times greater than CO2 in a 20-year time frame; methane is not as persistent a gas as CO2 (assuming no change in carbon sequestration rates) and tails off to about GWP of 28 for a 100-year time frame. This means that a methane emission is projected to have 28 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years assuming no change in the rates of carbon sequestration. Methane has a large effect but for a relatively brief period, having an estimated mean half-life of 9.1 years in the atmosphere, whereas carbon dioxide is currently given an estimated mean lifetime of over 100 years.
The balance between sources and sinks of methane is not yet fully understood. The IPCC Working Group I stated in chapter 2 of the Fourth Assessment Report that there are large uncertainties in the current bottom-up estimates of components of the global source, and the balance between sources and sinks is not yet well known. The most important sink in the methane cycle is reaction with the hydroxyl radical, which is produced photochemically in the atmosphere. Production of this radical is not fully understood and has a large effect on atmospheric concentrations. This uncertainty is exemplified by observations that have shown between the year 2000 and 2006 increases in atmospheric concentration of methane ceased, for reasons still being investigated.
Any process that results in the production of methane and its release into the atmosphere can be considered a 'source'. The two main processes that are responsible for methane production occur as a result of microorganisms anaerobically converting organic compounds into methane. Slightly over half of the total emission is due to human activity. Since the Industrial Revolution humans have had a major impact on concentrations of atmospheric methane, increasing atmospheric concentrations roughly 250%. According to the 2021 IPCC report, 30 - 50 percent of the current rise in temperatures is caused by emissions of methane, and reducing methane is a fast way of climate change mitigation.
