Swamp gas trapped under miles of Antarctic ice, a chemical souvenir of that continent's warmer days, may someday escape to warm the planet again, an international team of researchers report in Nature this week.
The researchers suggest that microbes isolated from the rest of the world since the ice closed over them, some 35 million years ago, have kept busy digesting organic matter and making methane—a much more effective greenhouse gas than carbon dioxide.
If global warming causes the ice sheets to retreat in the coming decades or centuries, the researchers warn, some of the methane could belch into the atmosphere, amplifying the warming.
Jemma Wadham of the University of Bristol, England, and her colleagues have not actually detected methane-producing microbes under the Antarctic ice sheet. They haven't detected methane either—though they are participating in drilling projects that could do so later this year. Yet a top journal has now published their analysis of the potential climate impact of those undiscovered microbes. That says a lot about the paradigm shift in microbiology in recent decades.
The presumption now is: Microbes are everywhere. In the seething water of an undersea volcano? Obviously. In the crushing pressure half a mile (0.8 kilometer) under the pitch-dark seafloor? Demonstrably. Under a mile or two of Antarctic ice? Why not?—there've been a few unconfirmed reports already—and why wouldn't some of those bugs be producing methane?
"You've got bugs, you've got organic carbon in sediments, and there's no oxygen because it's so far from the atmosphere," Wadham said. "When you put all those things together, it's perfect for the production of methane. It's like a huge wetland."
Antarctic Microbes Busy Under Ice
While waiting for a drill that could take her there, Wadham has done her best with a chain saw. For years she has marched up to the leading edge of glaciers in Antarctica, Greenland, and Canada and sawed off cubic-foot (0.03 cubic-meter) blocks from the base of the ice—blocks that include sediments picked up by the glaciers as they advanced. Wadham shoves the blocks into sterile bags, stows them in trunks full of Styrofoam, cheerfully pays extreme excess baggage fees, and prays she and her cargo can make it to her sub-zero freezer in Bristol in 24 hours.
In the lab she incubates small vials of melted ice and sediment for as long as two years, scrupulously avoiding contamination. The result: "Every glacier where we look," she said, "we find microbes in the sediments beneath the ice"—including microbes that are producing methane, albeit at slow rates.
Those measured rates are what Wadham and her colleagues used to estimate how much methane might have been produced on the scale of the Antarctic continent. (See Antarctica pictures.)
Antarctica has been at or near the South Pole for more than a hundred million years, but for most of that time the planet was much warmer than today—because the amount of carbon dioxide in the atmosphere was much greater. Plant and pollen fossils confirm that the continent was covered by forests and tundra rather than ice—around 52 million years ago there were even palm trees. Fjords and large bays cut deep into its interior.
Deep stacks of sediment would have accumulated in those marine basins, as they do in coastal water today. Inevitably, methane-producing microbes would have been hard at work in that mud, digesting the organic matter—around 21 trillion tons of it, the researchers estimate. The microbes are still at it.
"Imagine being a microbe living in a sediment basin 35 million years ago," said Slawek Tulaczyk, a glaciologist at the University of California, Santa Cruz, who worked with Wadham. "Do you care if you get covered by a mile of ice? Nothing really changes for you."
"Really Rapid Change" Coming to Antarctica?
Except that the methane you're making can no longer escape. Thousands of feet down in the sediment, geothermal heat keeps things warm enough for the microbes to keep producing methane. As the gas diffuses upward, however, it enters a zone where it feels not only the pressure but also the cold of the overlying ice sheet. The combination transforms it into methane hydrate: a solid, ice-like substance in which each methane molecule is trapped in a cage of water.
Hydrate is strange, fragile stuff. If the pressure drops or the temperature rises enough to take it out of its comfort zone—for instance, because the ice above it melts—it falls apart. The methane escapes to the atmosphere.
That's the worry for the future. Climate scientists have long been concerned about the positive feedback that would result if global warming were to destabilize huge reservoirs of methane hydrate in the Arctic.
Now they have the Antarctic to think about too. Wadham and her colleagues calculate there could be anywhere from 70 to 390 billion tons of carbon in hydrates under the East Antarctic ice sheet, and a few tens of billions of tons under West Antarctica. (The methane there may have been made by geothermal heating of sediments rather than microbes.) That's less than estimates for the Arctic but in the same ballpark.
You might think the Antarctic methane would be secure under such a thick ice cap. But the Antarctic has been losing a lot of ice lately. (Related: pictures of modern Antarctic warming.)
And it's precisely the glaciers covering former marine basins that are receding the fastest because their leading edges are being eaten away by a warming sea. It's conceivable that before the century is out those glaciers could recede enough to release whatever hydrates they've been covering.
"The longer I'm in this glaciology business," said Tulaczyk, "the more I'm willing to accept scenarios for really rapid change."