A new study concludes that the much-debated Keystone XL pipeline could produce four times more global warming pollution than the State Department calculated this year.
The proposed pipeline would carry oil from tar sands in western Canada to Texas.
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The researchers estimate it would increase world carbon by as much as 121 million tons a year, compared with the department's estimate of 30 million tons.
Study lead author Peter Erickson says the government calculations didn't take into account that the added oil from the pipeline would drop prices. That would spur consumption, which in turn would mean more pollution.
The study was published Sunday by the journal Nature Climate Change.