Clean natural gas? Not so fast, study says

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Switching from burning coal to natural gas won’t have an appreciable effect on global warming, at least not in the next few decades, a study suggests.

In fact, cutting worldwide coal burning by half and using natural gas instead would increase global temperatures over the next four decades by about one-tenth of a degree Fahrenheit, according to Tom Wigley, a senior research associate at the National Center for Atmospheric Research.

Strictly speaking, coal produces more global-warming gas per unit of energy than natural gas. But the tradeoff is complicated by the types of greenhouse gases and other pollutants associated with each of these carbon-based fossil fuels.

‘From the CO2 perspective, gas is cleaner, but from the climate perspective, it gets complicated,’ said Wigley.

Coal burning is notoriously dirty, producing sulfur dioxide, nitrogen oxides, carbon dioxide, soot and ash, as well as other pollutants. None are too good for humans or the planet, but the sulfates can act to block incoming solar radiation, with a slight cooling effect. (Before anyone proposes burning more high-sulfur coal, the net effect of burning coal is still warming).

Meanwhile, ‘clean’ natural gas, touted by the industry and T. Boone Pickens, can be a mess to produce. An unknown amount of methane -- a potent greenhouse gas with far more heat-trapping potential than carbon dioxide -- leaks in the process of producing natural gas.

Even assuming there is no leakage -- unlikely, most would agree -- the switch analyzed by Wigley would still add to Earth’s overall average temperature through about 2050. After that, temperatures would fall, but only by a few tenths of a degree Fahrenheit. If a substantial amount of methane leaks, the warming trend will last until 2140, he found.

Bear in mind, the most widely reviewed studies predict a global average temperature rise of 5.4 degrees Fahrenheit by 2100 under current fossil-fuel consumption rates.

“Relying more on natural gas would reduce emissions of carbon dioxide, but it would do little to help solve the climate problem,” said Wigley, who is also an adjunct professor at the University of Adelaide in Australia. “It would be many decades before it would slow down global warming at all, and even then it would just be making a difference around the edges.”

Let’s say the rate of methane leakage could be held to 2% during this great switch to natural gas, a figure Wigley describes as somewhat optimistic. Wigley’s model shows a net reduction in the global average temperature of about two-tenths of a degree by 2100. But if the leakage is in the neighborhood of 10% -- an upper bound, according to Wigley -- the warming trend will not abate until 2140.

“Whatever the methane leakage rate, you can’t get away from the additional warming that will occur initially because, by not burning coal, you’re not having the cooling effect of sulfates and other particles,” Wigley says. “This particle effect is a double-edged sword because reducing them is a good thing in terms of lessening air pollution and acid rain. But the paradox is when we clean up these particles, it slows down efforts to reduce global warming.”

When the natural gas industry touts the cleanliness of natural gas, it largely relies on old data that the Environmental Protection Agency has acknowledged are flawed.

Peer-reviewed studies place the methane leakage rate at about 3.6% to 7.9% for shale gas, particularly when hydraulic fracturing methods are used. (About two thirds of the country’s gas consumption is expected to come from shale gas by 2010, according to the American Petroleum Institute. It’s now around half.)

Wigley’s study is particularly timely as more attention is turned toward shale gas in the sprawling Marcellus Shale formation underlying New York, Pennsylvania, Maryland, Virginia, West Virginia and Tennessee. There’s also a lot of interest in an arch-shaped set of shale formations from northern Alabama through Mississippi, Arkansas and Texas. And there is extensive production of shale gas in Wyoming’s Powder River basin.

The effects of the shale gas industry were recently the focus of an Oscar-nominated film, Gasland.

-- Geoff Mohan

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