Humans drilling for oil and gas are to blame for rocketing levels of the greenhouse gas methane in our atmosphere, experts have warned.

New research found that Earth's last bout of global warming which occurred at the end of the last ice age 11,500 years ago, did not lead to a spike in methane levels.

Instead, the likely source of rising levels of atmospheric methane was from tropical wetlands, the authors of the new study say.

The study points to humans triggering the recent rise in methane in our atmosphere, rather than the gas escaping from Earth's permafrost as it is melted by climate change. Atmospheric methane has increased from 750 parts per billion in the year 1750 to more than 1,800 parts per billion today - mostly from human sources.

Fossil fuel production, the creation of rice paddies, and cattle ranching are the main offenders, the researchers say. The study suggests that human emissions of geologic methane may be as much as 25 per cent higher than previous estimates.

Although not abundant as carbon dioxide, methane is a much more powerful greenhouse gas, and so the rising levels are an important contributor to global warming.

Study coauthor Professor Edward Brook, from Oregon State University in Corvallis, Oregon, said: 'All of the natural gas that we mine is very old and leaking inevitably occurs during that process.

'Natural gas is considered a cleaner energy source than coal, but it can be a significant problem depending on how much of the methane is leaking out.'

Human methane emissions are the second largest contributor to global warming after carbon dioxide.

But there has been uncertainty as to the source of that methane and whether it has changed over time.

Professor Brook said: 'Our findings show that natural geologic emissions of methane - for example, leakage from oil seeps or gas deposits in the ground - are much smaller than previously thought.