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Methane: the Closet Greenhouse Gas

Thursday, October 13, 2011

Sheep on New Zealand landscape It is flatulated, it is belched, it emanates from excrement and urine, and there is an awful lot of it being produced every day by over six billion humans and about as many farm animals – not counting fowl. While the world is pointing its wavering finger at carbon dioxide as the chief culprit, an even bigger potential perpetrator of global warming is methane, a greenhouse gas that is 25 times more effective per molecule than CO2 in heating up the atmosphere.

While the actual contribution of bodily methane generated by people has not been estimated, a 1999 figure for methane produced by livestock puts it at some 35 per cent of the total contributed by human activity, world-wide. Of course that varies with location. In New Zealand, where there are 30 sheep for each person, a recent estimate set the contribution of sheep methane at over 50 per cent of the country’s entire greenhouse gas emissions – including CO2. There and in other countries where people share relatively small territories with farm animals, the concern with methane emissions from livestock digestive functions has been serious. A 2007 in The Guardian stated that, “Burping cows and sheep are being targeted by UK scientists to help bring down Britain’s soaring levels of greenhouse gas pollution.” Such studies have concluded that growing clover and various legumes, and substituting them as feed that will replace grass and hay, could cut animal methane emissions by as much as 50 per cent.

landfill In the U.S., the primary source of methane is currently from landfills, which contribute up to 50 per cent of the gas emitted nationwide. Another significant source was methane released from underground coal mines. The decayed organic matter that constitutes coal has retained its methane content because it has been under pressure, deep in the earth. As the coal in an underground mine is dug up and exposed, the methane is released. Until fairly recently, methane in coal mines was treated as a danger and a nuisance, something to be burned and vented, because it might explode or asphyxiate miners in significant concentrations. Before miners had methane sensors, crews would bring canaries in a cage down the mine with them. If the small birds keeled over, they knew it was time to leave.  That’s why the term, the “canary in the coal mine” entered the language as an early sign of trouble.

However, in recent years, modern mines are designed to capture as much of the methane as possible, both to use as a fuel and to run machinery, like the ventilation systems used to collect the methane and bring in fresh air. It is also collected and sold to nearby power plants to generate electricity. While still a danger, it is more a source of profit than a liability – except in some of the older mines, where it is still primarily a danger.

Like livestock-related and landfill-related emissions and coal mine gas, most of the world’s atmospheric methane comes from sources that are connected to the activities of humans across the planet. Some is also generated by natural sources, primarily wetlands, where the gas is released from decaying organic matter. For the time being, atmospheric methane constitutes only 20 percent of the earth’s greenhouse gas total.

But this balance may change – and it may change radically.

Meeting Because of the increased global warming created primarily by the CO2 we have generated, a new and primary source of atmospheric methane is being unleashed. At several periods in its history, the earth has been significantly warmer, and in each case the amount of methane in the atmosphere was radically higher than it has been throughout human history. As the earth cooled again, this methane was sequestered in “clathrates,” compounds in which a methane atom is captured in an ice crystal. These compounds lie in vast deposits of permafrost under the arctic tundra and in even more vast deposits deep in the sea floor, at the borders of continents where the waters’ near-freezing temperatures keep them stable.

As the land around our poles thaws and the permafrost melts, it is starting to release the methane captured there. Methane is very volatile, so it burns easily. The peat fires that engulfed Moscow and thousands of square miles of northern Russia in smoke in the summer of 2010 were fed by methane.

An even bigger concern are the clathrates under the Arctic seas. As they warm, will they release their massive store of ancient methane and change the world we know into a different, and possibly uninhabitable planet?

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