Rise in Pre-Industrial Methane Gas Caused by Humans and the Environment

For years scientists have argued whether increases of potent methane gas concentrations in the atmosphere were triggered by natural causes or human activities during a time period that started about 5,000 years ago and stretched to the start of the industrial revolution. Now, they have their answers. It turns out that the increase in methane during this time was likely caused by both rather than one or the other.

Although many papers are dedicated to proving the fact that humans might have caused a rise in greenhouse gases, this alone doesn't fully explain the rising levels of atmospheric methane during the past 5,000 years. Instead, it's very likely that human-caused methane was supplemented by environmental factors.

In order to determine methane levels in the past, the researchers examined ice cores from polar regions. Gas bubbles containing ancient air trapped within the ice can be analyzed and correlated with chronological data. This can then tell scientists what the atmosphere was like over a series of decades.

The researchers then used previous models that hypothesized reasons for the methane increase that began 5,000 years ago. They then compared these models to the ice core data.

So what did they find? Ice cores from Greenland had higher methane levels than those from Antarctica. This is due to the fact that there are great methane emissions in the Northern Hemisphere. The difference of methane levels, called the Inter-Polar Difference, though, did not change appreciably over time.

"If the methane increase was solely natural or solely anthropogenic, it likely would have tilted the Inter-Polar Difference out of its pattern of relative stability over time," said Logan Mitchell, one of the researchers, in a news release.

The findings reveal that it's likely that both the environment and humans played a role when it comes to increasing methane levels during this time period.

"We think that both played a role," said Mitchell in a news release. "The increase in methane emissions during the late Holocene came primarily from the tropics, with some contribution from the extratropical Northern Hemisphere. Neither modeled natural emissions alone, nor hypothesized anthropogenic emissions alone, are able to account for the full increase in methane concentrations. Combined, however, they could account for the full increase."

The findings are important for understanding the increase of methane in our warming world. This, in turn, could inform future climate models.

The findings are published in the journal Science.