Transforming tar sands into crude is costly as well as dirty: the process only becomes profitable with oil prices in the $60-85 range or higher.

把焦油沙转换成原油不仅费用昂贵而且对环境造成污染。 当油价在60-85美元范围波动或允许再高一点,这种提炼过程才有利可图。

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But whether Canadian tar sands or Venezuelan heavy crude, more thick oil means more CO2 in the atmosphere. And that means a climate ever more hellish.

然而无论是加拿大沥青砂还是委内瑞拉的重油，成分越稠就意味着大气中的二氧化碳会变得更多，也意味着气候会变得更加恶劣。

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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.

FORBES: Dr Richard Pike (1950-2011) and Peak Oil

Over the past year or so, opposition to new tar-sands projects has been steadily growing.

NEWYORKER: Unconventional Crude

Both Mr Obama and Mr Harper made particular mention of carbon capture and sequestration, an unproven and expensive technology that could eventually be used on the coal plants that produce about 70% of American electricity, or on the upgraders and refineries in the tar sands.

ECONOMIST: Stephen Harper looks to Washington, not Alberta