Say what you will about coal, but at least it stays where it’s put. On its way to the user, coal doesn’t gush from the rail trucks, spreading itself through the atmosphere and warming it at about 70 times the rate of carbon dioxide.
Natural gas is different. A new draft study provides evidence that, in the US, enough natural gas leaks into the air to give gas-fired electricity, megawatt-hour for megawatt-hour, a bigger greenhouse impact than electricity from good-quality steaming coal.
This news will be unnerving for the many people who point to natural gas as a relatively clean alternative to other fossil fuels. Conventional wisdom has been that a state-of-the-art natural gas power plant can produce electricity with barely 30% of the carbon dioxide emissions of a typical brown coal plant, and less than half of those for black coal.
The modest climate impact of natural gas, however, applies only from the point where the gas is burnt. In the study mentioned above, Professor Robert Howarth of Cornell University in the US examines the fuel’s broader effects. “The most recent data I could find for the US (from 2006)”, Howarth reports, “suggest a leakage rate from the oil and gas industry of an amount of methane equal to 1.5% of the natural gas consumed.”
Methane, with the chemical formula CH4, makes up about 87% of natural gas after various contaminants have been removed.
Howarth’s figure of 1.5% is consistent with data from various sources including the US Environmental Protection Agency. He acknowledges that his estimate has a large margin for error, since leaks in the gas industry are not well monitored. But 1.5%, he contends, is a conservative figure, and US government scientists and gas industry officials quoted in the New York Times on October 14 last year agree that the real amount is almost certainly higher.
If Howarth is anywhere near correct, suggestions by some environmentalists that natural gas can act as a “bridge” for a low-carbon transition from coal-fired power to renewable power fall by the wayside.
Howarth’s premises are not uncontroversial. To calculate the warming impact of methane, he uses the figure of 72 times the global warming potential, per volume, of carbon dioxide. This is the impact over the first 20 years after the methane reaches the atmosphere. The figure usually cited for the warming potential of methane is 25 times that of carbon dioxide, measured over 100 years.
Howarth’s choice here is the correct one. The figure of 72 times is appropriate to the key danger which natural gas leaks pose to the environment: their ability, due to their short-term greenhouse potency, to help trigger quick-acting “positive climate feedbacks”.
In the atmosphere, methane reacts with oxygen to form water and carbon dioxide. After seven years, half of the methane is gone, and after 20 years little remains. But a big pulse of the gas, its effects amplified by quick-acting feedbacks, could within a few years raise temperatures to the point where they exceeded natural “tipping points”. The climate would then “flip” to a new, hotter state.
One such positive feedback is seen in the impact of greenhouse gases on Arctic permafrost. Warming in the Arctic causes permafrost to melt, and in the oxygen-poor conditions of the resulting swamps, bacteria turn organic matter into methane — which warms the environment still further.
Methane in the atmosphere is now at about two-and-a-half times the level in 1750. It now accounts for about 20% of the warming effect of all long-lived greenhouse gases. From 1998, concentrations of methane stopped rising for a period, perhaps due to the drying of tropical wetlands.
The last two years, however, have seen levels trend upward once again. As well as reflecting the amounts of methane now bubbling from Arctic swamps, this renewed rise may stem from a boom in the extraction — and leaking — of natural gas.
Over the next 20 years, a big expansion of natural gas production could help bring about the methane pulse that humanity should be dreading.
ticket? You try being on a highway when "Groove is in the Heart" by Deee-Lite comes on the radio and see if you don’t end up going a bit faster than you expected. The gash down my passenger side door? That concrete beam in the parking garage came out of nowhere!
