According to the EPA:

Methane (CH₄) is a principal component of natural gas. It is also formed and released to the atmosphere by biological processes occurring in anaerobic environments. Once in the atmosphere, methane absorbs terrestrial infrared radiation that would otherwise escape to space. This property can contribute to the warming of the atmosphere, which is why methane is a greenhouse gas.

Methane has more than 20 times the heat-trapping capacity of CO2.
The largest human source of U.S. methane emissions are landfills, with 34% of the total.

Other key human sources of methane are natural gas systems, cattle and other animals, wastewater treatment facilities, rice paddies, and coal mines.

Methane represents 7.9% of U.S. global warming gas emissions--these percentages are given in CO2 equivalents, and have already factored in methane's higher Global Warming Potential (GWP).

We reduced emissions of methane by about 10% between 1990-2004; during that period CO2 emissions grew by 20%.

Comment: methane reduction is the ultimate low-hanging fruit. Unlike CO2, which is produced by many necessary and beneficial human activities, a lot of human generated methane comes from poorly managed waste facilities such as landfills, farms, and sewage plants, or from things like leaky gas systems and old coal mines. Often, reduction strategies turn a waste or nuisance gas into an economic asset--as when manure or landfill gas is used to generate electricity. Also, it is much easier to target a few dozen natural gas companies, a few thousand landfills, or even the nation's dairy farmers, than to change the behavior of hundreds of millions of drivers or light bulb users.