Six gases have generally been considered the major greenhouse gas concern, and methane is the second most abundant after carbon dioxide and accounts for around 8% of the total greenhouse gas emissions in the USA. But, in when compared to the effect of carbon dioxide methane is calculated to be 20 times more potent over a period of 20 years and up to 100 times more potent as a greenhouse gas over 100 years. The reason for this is that methane remains a relatively short time in the atmosphere. Scientists estimate that methane is responsible for 20% of the current global warming trend.
Methane is released into the air from the ground in the process of production of coal, oil and natural gas. It also escapes from landfill sites when food rots, from wetlands and marshes and also from cultivated plots such as rice paddy fields. Large amounts of methane are released into the air as a result of cellulose digestion by herbivores such as cattle, sheep and goats. It is estimated that, for example, in the US, cattle are the highest source of methane released into the atmosphere. In addition, there are large deposits of methane stored on the ocean floor captured in chemical compounds called clathrates in lumps of ice. If warming of the oceans leads to changes in currents and temperature rise then methane could be released. Some scientists believe that past releases of methane from clathrates were implicated in previous periods of warming.