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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.”

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