Ancient clues to modern methane sources

Methane is the second most important anthropogenic greenhouse gas, responsible for as much as 30% of global warming. The atmospheric concentration of the gas has more than doubled since the Industrial Revolution, and it continues to rise at a rate close to that of the worst-case projections. But CH₄ has many sources—wetlands, agriculture, landfills, leaks from the production and use of fossil fuels, and natural seepage from fossil sources—and disentangling their respective contributions has proven stubbornly tricky.

Isotopic measurements can help. Biologically sourced CH₄ contains some of the radioactive isotope carbon-14, whereas fossil CH₄, trapped underground for millions of years, contains none. But ¹⁴C measurements of today’s atmospheric CH₄ can’t distinguish between natural and anthropogenic fossil emissions. To do that, the University of Rochester’s Vasilii Petrenko took a look back in time, to CH₄ that was trapped in Antarctic ice between 11 000 and 12 000 years ago.

The advantages of that time period are many. It’s well before the rise of anthropogenic emissions, but not so long ago that biogenic ¹⁴C has completely decayed away. And it was a period of rapid change, both in global CH₄ levels and in many local climates. Though the changes in climate were mostly confined to the Northern Hemisphere, the CH₄ signature is well reflected in the Antarctic: The mixing time of the global atmosphere is about a year, and the atmospheric lifetime of CH₄ is an order of magnitude longer.

Because CH₄ is a trace atmospheric gas and ¹⁴C is a trace isotope, an accurate measurement requires about 1000 kg of ancient ice, as shown here. That’s too much to be feasibly obtained by the traditional method of drilling deep into the ice sheet. Fortunately, Antarctica’s Taylor Glacier exposes plenty of ice with a large range of ages at its surface.

The results of the isotopic analysis were surprising: Across the entire period of study, essentially all of the ancient CH₄ was biogenic, with no more than 15 teragrams per year globally (about 7% of the total) coming from fossil sources. That figure is substantially lower than estimates of today’s natural fossil CH₄ emissions, which average 52 Tg/yr. It’s not impossible that geological CH₄ seepage could have increased threefold over 11 000 years, but it would be a puzzle of its own: If anything, the emissions should have decreased over time, as sea levels rose and oil fields were drained of natural gas.

Petrenko and colleagues plan to make similar measurements on ice from 200–250 years ago—much more recent, but still largely preindustrial. If they get a similarly low number for natural fossil CH₄, that implies that today’s anthropogenic fossil emissions are higher than previously thought. That’s actually good news: It means we humans have more power than we’d realized to reduce our influence on climate. (V. V. Petrenko et al., Nature 548, 443, 2017 .)