Natural gas plays a more complex role in global climate change. This threat arises from increasing emissions of greenhouse gases, a class of atmospheric substances that allow sunlight to reach Earth's surface but partially block the radiation of Earth's heat back into space. Carbon dioxide (CO₂) is the principal greenhouse gas, and combustion of fossil fuels is the primary source of CO₂ emissions due to human activity. From a pre-industrial level of 280 parts per million by volume (ppmv), atmospheric concentration of the gas has risen to 350 ppmv. Under existing energy policies, experts expect it to increase throughout the 21st century.

When burned, natural gas emits about 14 kilograms of CO₂ per gigajoule of energy produced (kg/GJ); oil and coal emit about 20 and 25 kg/GJ respectively.³⁰ The higher efficiency of gas-burning generators enhances the fuel's natural advantage; a gas-fired combined-cycle system with 48% efficiency releases only half the CO₂ of a conventional coal plant of the same capacity and 38% efficiency. Gas cogeneration, which captures waste heat for residential or industrial uses, performs even better. (See Tables 2 and 3.)

Methane leakage from the natural gas industry presents a less serious greenhouse problem than CO2 emissions from the combustion of fossil fuels.

There is a complicating wrinkle, however. Natural gas consists largely of methane (CH₄), itself a greenhouse gas-indeed by weight or per molecule, a much more powerful one than CO₂. The World Bank reckons that if a gas plant loses 3% of its supply to the atmosphere, it relinquishes its greenhouse advantage over coal as well.³¹ While experts disagree about leakage from the U.S. pipeline system, it certainly amounts to less than 3%.³² However, poorly maintained systems leak more, perhaps much more. Recent unpublished data from Russian researchers suggests that the Russian gas system leaks 35 million metric tons of methane annually, implying leakage of 9% of throughput.³³

In two respects, however, methane leakage from the natural gas industry presents a less serious greenhouse problem than CO₂ emissions. First, there is no technical mystery regarding the measures necessary to reduce gas leakage, at least to the level that the United States has achieved. Implementing such controls on the vast Russian pipeline system might be expensive, but a tighter system would also capture a large and immediate commercial benefit. At a value of only $1.00 per mcf, each 1% reduction in the rate of leakage would be worth nearly $200 million annually,³⁴ entirely apart from the value of forestalling catastrophic climate change.

Second, methane remains in the atmosphere for a relatively short time-perhaps a dozen years-compared with 50 to 200 years for CO₂.³⁵ Even under generous supply assumptions, dwindling stocks will reduce world natural gas usage by the latter decades of the next century³⁶ and the greenhouse effect of methane leakage during the period of high gas consumption will decline soon thereafter. The leakage therefore will have little impact on the long-run global temperature. The chief danger, then, is that methane leakage would cause a more rapid rise in temperature in the near future, reducing the time available to humans and other species for adapting to a warmer globe.

Scientists, policymakers and industrialists disagree over what limit on the atmospheric concentration of CO₂ should anchor a reasonable climate policy; a few still deny the need for any such limit (and any such policy). However, discussion frequently centers on a ceiling of 550 ppmv, about twice the pre-industrial level. That figure represents a political compromise between many conflicting concerns, most notably the protests of some government economists and industrial interests that a more ambitious goal would precipitate economic havoc. While we do not defend the 550 ppmv figure on substantive grounds, we use it for the sake of discussion in the following pages.

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