Created near the Earth's surface by the decay of organic matter, such as in the landfills, methane is then carried into the atmosphere by the rising air in the tropics.
A relatively potent greenhouse gas, methane has a high global warming potential 25 times greater than carbon dioxide. Recent data shows that after a slowing down of methane build up over the last ten years it may be back on the rise.
The Earth's methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radioactive forcing from all of the long-lived and globally mixed greenhouse gases. – Wikipedia
One of the possible reasons for this is the large amounts of methane being released from the ocean floor.
This in turn has been suggested to having an effect on the rapid global warming of our planet. Then as a vicious cycle, the added methane in the atmosphere heats up the Earth and results in more methane being released because of the added pressure.
A recently discovered increase in the release of methane from the melting permafrost is also concerning scientists. Measurements are showing a five times greater release than what was estimated earlier in Siberia. Because of this area heating up faster than other places on Earth, the melting permafrost will have an additional affect on global warming. Ice reflects the sun's rays and heat, while the ocean water soaks it up. The less ice we have, the more the planet heats up. This can cause more releases of methane gas.