Ancient methane in thawing permafrost or beneath Arctic ice may not pose a major climate threat, according to a new study.
Methane is a potent greenhouse gas with a global warming potential many times more powerful than carbon dioxide.
However, US scientists say even if old methane is released from these large natural stores in response to warming, very little actually reaches the atmosphere.
A team from the University of Rochester came to this conclusion – presented in a paper in the journal Science – after studying emissions from a period in Earth’s history partly analogous to the warming of Earth today.
“One of our take-home points is that we need to be more concerned about the anthropogenic emissions – those originating from human activities – than the natural feedbacks,” says co-author Michael Dyonisius.
With colleagues, Dyonisius drilled the Taylor Glacier in Antarctica and collected ice cores that contain tiny air bubbles with small quantities of ancient air trapped inside. They then used a melting chamber to extract the air and study its chemical composition.
Their focus was on air from the time of Earth’s last deglaciation, 8000 to 15,000 years ago.
“The time period is a partial analogue to today, when Earth went from a cold state to a warmer state,” Dyonisius says. “But during the last deglaciation, the change was natural. Now the change is driven by human activity, and we’re going from a warm state to an even warmer state.”
Analysing the carbon-14 isotope of methane in the samples, the researchers found that methane emissions from the ancient carbon reservoirs were small, leading Dyonisius to conclude that “the likelihood of these old carbon reservoirs destabilising and creating a large positive warming feedback in the present day is also low”.
They also believe the methane released does not reach the atmosphere in large quantities due to several natural buffers.
In the case of methane hydrates, they say, if the methane is released in the deep ocean, most of it is dissolved and oxidised by ocean microbes before it ever reaches the atmosphere.
If the methane in permafrost forms deep enough in the soil, it may be oxidised by bacteria that eat the methane, or the carbon in the permafrost may never turn into methane and may instead be released as carbon dioxide.
“It seems like whatever natural buffers are in place are ensuring there’s not much methane that gets released,” says co-author Vasilii Petrenko.
The data also shows that methane emissions from wetlands increased in response to climate change during the last deglaciation, and it is likely wetland emissions will increase as the world continues to warm today.
Even so, Petrenko says, anthropogenic methane emissions currently are larger than wetland emissions “by a factor of about two”, and that is what we should be concerned about.
In a related commentary, Joshua Dean from the University of Liverpool, UK, notes that old methane release may have been important in the geological past because of the long time scales involved.
“Hundreds or even thousands of years of warming may be required to generate sustained methane release from old carbon stores that can then outpace methane consumption,” he writes.
“Old methane release therefore does not occur fast enough nor at magnitudes that will be important in the immediate future when compared with methane release from contemporary sources such as wetlands and human activity, which release a third and half of all methane currently in the atmosphere, respectively.”