Unconventional sources of oil and gas that are set for further exploitation include Canada's tar sands, shale and coal-bed methane deposits around the world.

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Even if new finds elsewhere have been rarer and less accessible than in the past, vast quantities of oil could now be profitably stripped from tar sands and shale.

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Oil shale and tar sands are other sources of liquid fuel that are harder on the environment than petroleum.

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Crude oil, or its substitutes, will be produced for decades, but at an increasingly expensive tariff, both in terms of cost and energy, as it is sourced in the form of a heavy, sour (high-sulphur) material, or in synthetic form from tar-sands, shale and coal.

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There must be solutions with less severe tradeoffs than shale gas and tar sands oil.

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Meanwhile, back in North America, new techniques were being developed to extract oil and natural gas from shale formations deep underground, from tar sands in Canada, and from deep water in the Gulf of Mexico.

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For their calculations, the two assumed that where there was a gap between demand and conventional supply it would be filled with synthetic fuels, first with tar-sands oil and later with oil from coal and shale. (According to high-end estimates, coal and oil shale could together yield some ten trillion barrels of unconventional crude.) They then calculated what the impact would be on global carbon-dioxide levels.

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