When it comes to forecasting global warming, methane is an unpredictable, menacing figure. The greenhouse gas is 28 times more powerful at trapping heat than carbon dioxide over a 100-year span. And as the planet warms, scientists fear vast stores of the gas will be released from Arctic permafrost and the deep ocean, warming the planet even further.
Evidence from two new studies offers hope: First, a swift release of massive quantities of ancient methane is unlikely. Second, humans seem to be a bigger source of modern methane emissions than previously thought—meaning people have more control over how much winds up in the atmosphere. “It’s generally encouraging news,” says Michael Dyonisius, a geochemist and graduate student at the University of Rochester (U of R) who led the study of ancient methane.
Methane comes from two main sources: biological and geological. Biological methane can be released from rotting plants and burping cows, whereas geological sources include natural seeps from fossil fuels and leaks from natural gas and coal mining operations. The methane molecules in older fossil fuel sources contain almost no carbon-14, a radioactive isotope of carbon created by cosmic ray bombardment.
To find out whether a spike in old methane could have triggered ancient bouts of warming, the researchers looked to ice sheets in Antarctica that trapped air bubbles, including small amounts of methane, over tens of thousands of years. Getting the samples took a lot of ice: Dyonisius and his colleagues drilled 11 tons from Antarctica’s Taylor Glacier. They melted the ice cores, siphoned off the gas, and measured levels of carbon-14 methane at intervals from 15,000 to 8000 years ago, an era when Earth switched from an ice age to a climate up to 0.5°C warmer than today.
That warming didn’t coincide with a big jump in carbon-14-depleted methane, the researchers report today in Science. That suggests the warmer temperatures didn’t trigger a big release of methane from permafrost or the ocean. “That's one climate catastrophe we can check off,” Dyonisius says of the ocean scenario.
In a second study, the same team harvested ice from Greenland to estimate how much modern atmospheric methane comes from leaks in extraction operations and pipelines, versus natural geologic seeps from the earth. Because both types lack carbon-14, the scientists compared levels of carbon-14-depleted methane from the 1870s with levels from when the fossil fuel era was in full swing—in the decades leading up to the 1940s. (Later years were avoided because they are skewed by nuclear weapons testing, which boosts carbon-14 levels.)
The data reveal that levels of carbon-14–depleted methane were much lower in the 1870s. That means modern geologic sources of methane are much smaller than previously estimated, and that the big jump came from humans, they report this week in Nature. They estimate annual geologic methane emissions at about 1.6 million tons, dramatically lower than recent estimates of between 30 million and 60 million tons per year. (Methane released from all sources totals approximately 570 million tons a year.)
The new findings are meeting some resistance. Giuseppe Etiope, a geochemist whose calculations are challenged in the new paper, questions how the geologic emissions could be so low. One recent study, for instance, suggests that 3 million tons of methane rise every year from one part of the Arctic Ocean alone. “This a scientific conundrum,” says Etiope, of Italy’s National Institute of Geophysics and Volcanology. “If they are right, many other people are wrong.”
Katey Walter Anthony, an aquatic ecologist at the University of Alaska, Fairbanks, who studies methane from lakes created by melting permafrost, has questions about the Greenland study, but she also hasn’t found flaws in the U of R team’s method. “What I think needs to happen is we all need to get together and be very vulnerable and say, ‘Where could I be going wrong?’” she says.
The study of ancient methane emissions, on the other hand, is consistent with her research, which shows that permafrost lakes didn’t release vast quantities of methane as the planet left its last ice age. The danger now, she says, is that temperatures by the end of this century could rise several degrees higher than during that previous warming event. If that unleashes still more carbon trapped in permafrost, some of it might be converted into greenhouse gases including carbon dioxide or methane. “The carbon has to go somewhere,” she says.