New research by the Scripps Institution of Oceanography and the University of Rochester eases long-standing fears that thawing Arctic permafrost could belch out mass amounts of climate-warming methane gas.
Scientists had warned of a scenario in which the thawing tundra could release enough methane to sharply accelerate global warming. However, the study published Friday in the journal Science found that permafrost that melted during a previous warm period between 18,000 and 8,000 years ago released little of the greenhouse gas. So it's likely that the permafrost won't contribute much methane during the ongoing warming period, they said.
That conclusion offers a rare glimmer of hope on the typically gloomy climate change front, the study authors said.
"Because so much of the climate stories are bad news, I'm very glad to be able to share some good news for a change," said Jeff Severinghaus, a geoscientist at Scripps and an author on the study.
Permafrost consists of soil that has been continuously frozen, even during the summer, for more than a year. It's mostly found in Siberia, Alaska, and Northern Canada, and in some cases has been iced-in for millennia.
When the permafrost thaws with rising temperatures, the ice melts and the soil below becomes waterlogged, creating low-oxygen conditions, according to Scripps Oceanography. That allows microbes in the soil to consume the carbon and produce methane.
Methane gas consists of four hydrogen atoms bonded to a carbon atom, and is a potent greenhouse gas - 80 times more powerful than carbon dioxide, Severinghaus said. Scientists had projected that an eruption of methane from thawing permafrost could add an additional 2 degrees Fahrenheit of warming to the ongoing temperature rise over the next century, he said.
"Burning fossil fuels is heating the planet, that's for sure," he said. "The worry was that would cause an extra warming because of the release of methane from tundra."
To test that premise, Severinghaus worked with other researchers at Scripps and the University of Rochester to test ancient air trapped in polar ice. University of Rochester graduate student Michael Dyonisius led the study with his adviser, Vasilii Petrenko, a professor of earth and environmental sciences, and one of Severinghaus' former students at Scripps Oceanography.
The team drilled and collected ice cores from Taylor Glacier in Antarctica, where they could find bubbles with small caches of pure, ancient air trapped inside.
Each tiny air sample contained minute amounts of methane, a gas found in the atmosphere at only one part per million. Then they examined the proportion of carbon isotopes in the methane molecules to determine the age of the gas. Ordinary carbon has 12 neutrons, but carbon-14 is a radioactive form of the chemical with two extra neutrons and a short lifespan. It decays within 5,000 years on average, so it acts as a reliable age indicator, Severinghaus said.
"Think of it like a dye," he said. "It doesn't change the behavior of the carbon, but it allows you to track it, as if you put some red dye in a stream."
By looking at the proportion of carbon-12 and carbon-14 in the gas samples, they could tell whether it was "young methane" - generated recently - or "ancient methane." That allowed them to calculate how much the thawing tundra contributed to atmospheric methane levels during the last period of glacial melt.
"The concentration of methane told us how much was released, and the isotopes told us which kind - was it young or old methane?" Severinghaus said. "We can say with great confidence now that tundra methane did not contribute to atmospheric methane at all, during the last time that the world warmed a lot."
That's not to say that the thawing permafrost didn't release methane. Instead, he and his co-authors believe that other soil bacteria break down the gas before it reaches the atmosphere, buffering a surge of the greenhouse gas.
"Think of the bacteria as being our friends," he said. "They eat methane before it can get out of the soil."