A lot is written about the greenhouse gas carbon dioxide, but there is a considerably more potent gas that has climate scientists worried - methane - and it is pouring out of underground storage in the States, sickening local residents. Find out more about it...

A natural gas storage well about 30 miles northwest of Los Angeles has leaked about 800,000 metric tons of greenhouse gases into the atmosphere, sickening local people and increasing California's emissions by about a quarter. While not visibly polluting like the Deepwater Horizon oil spill, it will take just as long - 3 months - to drill down to the 8,700-foot-deep leak and block it, and the owners are unable to estimate how much gas will have been released by then. This is bad news for the locals who have to be evacuated from their homes.

California is not the only area in the United States which is leaking methane. There have been leaks in New Mexico's San Juan Basin for years...

Natural gas is mostly methane, a naturally occurring greenhouse gas which makes up only a tiny fraction of the atmosphere, but contributes about 20% to global warming. It is given off by wetlands, forest fires, plants and animals from bacteria to termites to cows. The warmer it gets, the more methane is given off by plants such as rice paddies, and it is also released from entrapment in permafrost and ice. Humans add to the natural methane with the fossil fuel industry, landfills, waste water treatment facilities, and our increased burning of forests and raising of animals such as cows. While all these add gradually to the level of atmospheric methane, as recorded by Mauna Loa, there is an outside chance that if the planet warms enough that there will be a sudden irreversible release of methane hydrates or clathrates in the seas, leading to mass extinction. There are already signs that a warmer ocean may be releasing frozen methane.