Methane bubbles trapped in lake ice in early autumn Credit: Katey Walter; courtesy of Nature
Scientists have long been uncertain about the cumulative effects of so-called feedback loops on global warming. As the Earth warms, some phenomena, such as polar ice melt, will exacerbate the warming problem in a positive feedback loop. But other consequences of warming, such as increased water vapor, can actually cool the earth, potentially creating a negative feedback loop. Which way the balance of these effects would tip has been anyone's guess.
Now, a research team returning from Siberia adds more weight to the idea that the process of global warming will itself accelerate further warming. In a study published in the Sept. 7 issue of Nature, scientists report that greenhouse gases trapped in thawing permafrost are escaping into the atmosphere at a frightening rate.
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"As the permafrost thaws, these large quantities of methane are being released," said Jeff Chanton, an oceanographer at Florida State University and co-author of the study. "The methane gets out and that makes it warmer, so that means more methane gets out and that makes it even warmer, and so you get stuck in this positive feedback loop."
The researchers studied a unique type of permafrost in Siberia called yedoma. This frozen tundric dust, deposited during the last glacial age, is rich in plant roots and animal bones, resulting in a carbon content 10 to 30 times higher than average deep soils.
When organic matter decomposes under air, it escapes as carbon dioxide. But much of the yedoma in Siberia lies at the bottom of lakes. When the carbon trapped in the permafrost decomposes under water, it provides microbes with "a banquet from which they burp out methane as a byproduct," said Katey Walter, a biologist at the University of Alaska and lead author of the study.
Methane is a greenhouse gas 23 times more potent, though shorter lived, than carbon dioxide. Ultimately methane oxidizes and becomes carbon dioxide, which lingers for hundreds of years.
It has long been known that a huge amount of organic carbon remains buried in the Siberian tundra. But unlike the oceans, which are spatially uniform, soils vary dramatically from point to point, making the study of greenhouse gas released from land exceedingly complex.
Using a combination of remote sensing, aerial surveys, and year-round measurements, the scientists followed trails of frozen methane bubbles and placed traps over point sources in order to measure escaping methane. Their new calculations are 10 to 63 percent higher than the present estimate of methane emissions from northern wetlands.
"What is important about this study is that it suggests another mechanism that may be important in the release of methane," said Dan Schrag, a geochemist at Harvard University. "By better constraining what the mechanisms are, we might be able to predict how they will function in a warming world."
A study released earlier this year in Science foreshadowed these latest findings. The yedoma, it said, represent a vast reservoir of carbon neglected in most analyses of climate change. An estimated 500 gigatons of carbon have been flash frozen in yedoma regions and 900 tons in permafrost worldwide. If released, this store would more than double the amount of carbon in the atmosphere today.
The fear is that the thawing lake region, which comprises 90 percent of the Russian permafrost zone, will dump methane into the atmosphere at a rate that will dwarf any human attempts to curtail carbon dioxide emissions.
"The Earth hasn't been this warm for millions of years," said Schrag. "All that carbon that's stored in Siberia is going to get released, we just don't know whether it will be in a thousand years or a hundred years. And if it's the shorter time scale, then it's real trouble. There's enough carbon there to essentially lose control of the system."