Changes in the Earth's orbit, not human activity, may be behind a subtle increase in methane levels 5000 years ago, according to new research.
Over the last four glacial cycles (420,000 years) methane levels have bounced between 300 and 700 parts per billion volume (ppbv). Since the start of the industrial era they have nearly tripled to 1780 ppbv, attributed by most to fossil fuels and agriculture.
But scientists have been unable to adequately explain why methane levels were steadily rising in the 5000 years before the industrial era.
In 2003, Professor Emeritus William Ruddiman, a palaeoclimatologist at the University of Virginia, suggested early agricultural activity - rice growing, farming of cattle and sheep, and burning biomass - were responsible for the increase.
In a paper published in the journal Nature, Dr Joy Singarayer of the University of Bristol, and colleagues, suggest changes in the Earth's orbit and precession, not agriculture, played a major part.
Study co-author Professor Paul Valdes, also of the University of Bristol, says the Earth is in the middle of a "warm period, geologically speaking", similar to one that occurred more than 110,000 years ago. But unlike today, methane levels back then were declining.
The researchers used computer models, used to estimate future climatic conditions, to trace back over the past 130,000 years. They then used this data to estimate vegetation types, and hence methane production, across the planet.
The models took into account periodic changes in the Earth's orbit around the Sun, as well as the tilt and precession of its axis of rotation.
Singarayer and colleagues found the output of their models matched with measurements of methane from ice cores taken in Greenland.
"What we found was that our models were able to reproduce both the decrease that was seen around 100,000 years ago and the increase we've seen in the last 5000 years, without having to invoke human induced emissions through agriculture," says Valdes.
Valdes say the computer models showed wetlands in the Southern Hemisphere, in particular South America, were the source of the additional methane.
"This was based on the fact that this had some of the biggest changes in monsoon and because the land area is so large," he says.
Valdes says given the models "did a good job of simulating past changes in methane ... [it] will maybe give us a little more confidence that the computer models are working correctly and therefore more believable for future predictions."
The methane mystery
CSIRO Marine and Atmospheric Research scientist, Dr David Etheridge is impressed by the findings.
"[They] did a reasonably good global climate simulation across those periods," he says.
"They don't rule out early anthropogenic causes, but they say that it can still be explained from wetlands."
Etheridge says incorporating the orbital change into the models is quite useful.
"If you can explain those subtle changes in methane emission due to the warming of certain parts of the land surface in the past, and your modelling is robust in simulating those changes, then you've got some tools to go forward with."
Etheridge says the increases in question are quite subtle compared to the methane increase in the last 200 years, in the industrial period.
"But that's worthwhile exploring," he says, "because you want to know what's controlling these key [greenhouse] gases."
He says one of the concerns for future climate is how potential feedbacks of atmospheric greenhouse gases may amplify global warming, and that studies like this will help us better understand the releases of methane that can cause them.