Methane is a colorless, odorless gas that is composed of one carbon atom surrounded by four hydrogen atoms. It is highly flammable and is the main constituent of natural gas.
Methane is produced in two ways. One is through the biologic decomposition of organic matter at shallow depths, including in swamps, landfills, and even shallow bedrock. The other is through millions of years of high pressure and high temperature processes deep underground that also create coal, petroleum and natural gas.
Methane is the second most abundant anthropogenic (human-influenced) greenhouse gas after carbon dioxide, accounting for about 20 percent of total greenhouse gas emissions, and it is extremely efficient at trapping heat in the atmosphere, at more than 25 times greater than carbon dioxide. Over the past two centuries, methane concentrations have more than doubled, mostly due to human activity, which is estimated to account for about 70 percent of total methane emissions. The largest single anthropogenic source is agriculture, including rice farming and cattle raising, followed by fossil fuel extraction and utilization systems, wastewater treatment, and some industrial processes.