Entirely replacing coal with natural gas would reduce current international, energy-related CO₂ emissions by about 20%.³⁷ A total displacement is scarcely plausible, however. Apart from the political ability of the world coal industry to forestall such measures,³⁸ there appears to be insufficient natural gas to support such a change.

The substitution of natural gas for coal can only reduce CO₂ emissions below their projected path if growth in gas supply substantially exceeds the availability that most experts predict (the "mainline" projections). Such a bonanza is unlikely in the United States. After decades of exploration and drilling, geologists have defined the American resource base reasonably well: uncertainty regarding future price and supply principally reflects the impossibility of predicting the rate of reductions in exploration and production costs.

Faster supply growth in other countries appears more plausible. In many regions, petroleum firms have had little incentive to explore for natural gas because no market exists for the gas already produced in association with oil. It is impossible to know how much gas a systematic exploration would reveal, but one study contends that new supplies could support consumption about 48% above mainline projections between 2020 and 2040.³⁹ Although this would not suffice to displace all coal, it could delay the date when a larger contribution would be required from increased energy efficiency and non-fossil energy to offset global climate change.

Given a compromise goal of 550 ppmv of CO2, no plausible amount of natural gas could delay past 2050 the need to rely principally on increased energy efficiency and non-fossil fuels to meet the growing demand for energy services.

How long could natural gas forestall the time when nations would have to wean themselves from fossil energy? Given the compromise goal of 550 ppmv of CO₂ discussed above, no plausible amount of natural gas could delay past 2050 the need to rely principally on increased energy efficiency and non-fossil fuels to meet the growing demand for energy services. Substituting natural gas for coal alone would fall far short of meeting the 550 ppmv goal.

The 2050 date assumes first a supply of natural gas that is well above the mainline projections and second, that all the additional gas would displace coal-the best way to use gas to lower CO₂. An earlier shift away from fossil fuels might be required by a smaller gas supply, by using some of the gas to displace oil in transportation, or by a lower atmospheric concentration goal.

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