The Other Greenhouse Gases
Is methane really worse for the environment than carbon dioxide?
Brendan I. Koerner was online on Nov. 29 to chat with readers about this article. Read the transcript.
You're always going on and on about carbon dioxide emissions and their role in cooking the planet. But CO 2 isn't the only greenhouse gas worth worrying about, right? I've heard methane is a lot more toxic to the environment than carbon dioxide.
CO2 certainly gets most of the doomsday ink, and for good reason. In terms of sheer weight, it accounts for around 85 percent of America's greenhouse gas emissions, which amounted to 7.074 billion metric tons in 2004; methane accounts for just 8 percent of that frightening total. On top of that, carbon dioxide is often spotlighted because it's so closely linked to the appalling fossil-fuel dependence decried by treehuggers and politicians alike: Ninety-four percent of the nation's anthropogenic CO2 emissions are due to fossil-fuel combustion. The No. 1 source of our nation's anthropogenic methane emissions, by contrast, is the decomposition of garbage in landfills—a situation you can't help ameliorate by buying a Prius or installing solar panels on your roof.
Yet methane deserves more attention than it's received so far because, as you note, it's arguably more deleterious to the environment than the widely feared CO2. The Environmental Protection Agency uses a statistic called Global Warming Potential (GWP) to assess the threat posed by various greenhouse gases. GWP measures how much heat one molecule of a gas will trap relative to a molecule of carbon dioxide. Methane has a GWP of 21, which means it's 21 times more effective at preventing infrared radiation from escaping the planet. So, although methane emissions may be relatively piddling, they're definitely a cause for concern. (Their one saving grace is an atmospheric lifetime of just 12 years, versus between 50 and 200 years for carbon dioxide.)
As with many of its fellow greenhouse gases, methane has become far more prevalent in the Earth's atmosphere since the dawn of the Industrial Revolution. By analyzing the chemical composition of air bubbles trapped in ice sheets, scientists have estimated that the atmospheric concentration of methane has increased by 150 percent since the mid-1700s; over that same time period, the atmospheric concentration of carbon dioxide has risen by "just" 35 percent. About 60 percent of global methane emissions stem from human activity—aside from landfills, the chief anthropogenic culprits are natural gas production and use, coal mines, and "enteric fermentation" (the polite term for the burps of livestock).
The one sliver of good news is that methane emissions seem to be leveling off. According to Environment Canada, atmospheric methane concentrations should permanently stabilize if we cut our current methane output by a seemingly manageable 8 percent. As a consumer, you can help a minuscule amount by reducing the amount of waste you send to landfills. But the most promising solutions aren't on the end-user level. The Lantern mentioned one such remedy a few weeks back: capturing methane from landfills and then using it to generate electricity or to supply gas-hungry industrial operations. In the agricultural realm, those cow burps can be made less methane-rich by fiddling with the animals' diets; Australian scientists contend, for example, that adding cottonseed oil to livestock feed can reduce each cow's methane emissions by up to 30 percent. (The typical cow belches forth about a third of a pound of methane per day.)
But some environmentalists worry that such ingenious technological solutions will come to naught, given the consequences of rising temperatures on the world's cold spots. There's lots of methane stored in the permafrost that covers much of northern Canada and Siberia, and that gas would be released should appreciable melting occur.
If we somehow manage to lick our methane problem—or at least keep it in check—perhaps we can then move on to the third most prevalent greenhouse gas: nitrous oxide, commonly known as laughing gas (or, to sybarites, hippie crack). Nitrous oxide currently accounts for 5.5 percent of America's greenhouse gas emissions; only about 40 percent of those emissions are anthropogenic, with agricultural fertilizers being the main source. The gas's GWP is 310 and it has an atmospheric lifetime of 120 years—10 times longer than that of methane.
But nitrous oxide's GWP is dwarfed by that of sulfur hexafluoride. Chiefly used for a range of esoteric applications—such as preventing molten magnesium from oxidizing and for etching semiconductor wafers—SF6 has a GWP of 23,900, making it the most brutally effective greenhouse gas known to man. And sulfur hexafluoride's atmospheric lifetime? A depressing 3,200 years.
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