Climate scientists have long fretted about the hundreds of billions of tons of methane frozen under the floor of the Arctic Ocean. If the water warms enough, some of that methane could escape. Nobody knows how soon or how quickly such a release might happen, but since methane is a far more potent heat-trapping gas than the more familiar carbon dioxide, it could add to the temperature increase already under way thanks largely to human emissions from fossil fuel burning.
But frozen Arctic methane turns out to be just the tip of the iceberg, so to speak. According to a paper released Wednesday in Nature, there could be just as much methane trapped on the opposite side of the planet, under Antarctica’s vast ice sheets.
“It’s very hard to say what the effect would be if it were released,” said lead author Jemma Wadham, of the University of Bristol, in an interview. “And it’s hard to say when it might happen, and where. But there is potential for a release, for sure.”
Wadham and her colleagues didn’t actually detect the Antarctic methane directly. What they did do was to prove that the frozen continent has all the right conditions in place to make methane deposits a very likely bet.
The first link in their chain of reasoning is the fact that Antarctica was largely ice-free millions of years ago, sporting lush forests that eventually decomposed to form soil rich in organic matter. Parts of West Antarctica were open water, where marine life created similarly rich sediments at the bottom of the sea — sediments that are up to eight miles thick in some places, with thousands of feet of ice on top of them.
In principle, bacteria should have decomposed some of these vast storehouses of organic matter into methane. The methane should bubble up to the undersides of the ice sheets. Once there, the freezing cold and high pressure of the overlying ice should have transformed them into deposits known as methane hydrates, the same formations scientists know are trapped on the continental shelves of the Arctic Ocean.
What scientists didn’t know was whether this bacterial decomposition actually happened, even if the textbooks said it should. So Wadham and her co-authors took soil samples from the margins of glaciers in both Antarctica and Greenland. “We spent a couple of years chain-sawing out sediments frozen into the bottom of the ice,” she said, adding with a connoisseur’s judgment, “They were very nicely preserved.”
They hauled the samples back to the lab and allowed them to thaw under carefully controlled, oxygen-deprived conditions where oxygen-hating, methane-belching bacteria known as methanogens could do their work — assuming they were there. And sure enough, the soil began producing methane.
The same thing should presumably be happening underneath Antarctica’s ice, where heat percolating from the depths of the Earth have prevented sediments from ever having frozen.
“You have perfect conditions, and the ice has been there for 30 million years,” Wadham said, “so there’s been plenty of time for methane to build up.”
It takes a combination of cold and pressure to turn methane into methane hydrates, and if either one of these conditions is relaxed, the methane can escape. In the Arctic, it’s the loss of cold scientists worry about as the seawater overlying methane-rich sediments continues to warm.
In the Antarctic, Wadham said, “it’s the drop in pressure that would come if the ice sheets get thinner.” It’s not happening yet, but that could change if the temperature of waters surrounding the Antarctic continent continues to rise.
Wadham emphasized that the threat remains theoretical at this point. Not only is it unclear whether Antarctica’s ice sheets might get substantially thinner, or when it could happen, but nobody has proven that methane hydrate deposits lie below them. The new study simply provides one strong link in a chain that is still being forged.
It’s clear, however, that the forging process needs to continue. “We need people to drill to the bottom of the ice sheets to find out what’s actually going on,” Wadham said.