Methane (CH4), a molecule of carbon and hydrogen, is the main component of natural gas. It's often known as marsh gas since it's commonly seen bubbling up from marshlands, the product of slowly decaying organic materials. There are known to be huge reservoirs of natural methane stored under the surface of the oceans. Ruminating cattle, rice cultivation and the burning of biomass and fossil fuels generate the majority of man-made methane currently in the atmosphere.
While the amount of methane in the atmosphere is far less than that of carbon dioxide, its potential to trap long-wave heat energy radiated from Earth is much greater. The greenhouse effect caused by a molecule of methane is around 7.5 times greater than that caused by a molecule of carbon dioxide. And although methane's lifespan in the atmosphere is much shorter than carbon dioxide (around 12 years in comparison to up to 200 years), its concentration has more than doubled since pre-industrial times, shown by concentrations extracted from ice cores.
In total, the amount of methane generated by human activities outweighs that generated by natural sources, and its abundance in the atmosphere can be seen to closely follow the growth in world population since the Industrial Revolution. This trend is alarming since the world population is expected to double during the 21st century. So unless strict measures are employed to curb its release into the atmosphere, the contribution to global warming from methane is likely to double.