Researchers writing in the journal Nature have come up with two widely differing theories as to the cause.
One suggests the decline was caused by greater commercial use of natural gas, the other that increased use in Asia of artificial fertiliser was responsible.
Both studies agree that human activities are the key element.
And there are suggestions that methane levels are now on the rise again.
Methane is regarded as one of the most potent greenhouse gases, trapping over 20 times more atmospheric heat than carbon dioxide.