"Liquid coal, for example, can produce 80 percent more global warming pollution than gasoline," said the Union of Concerned Scientists, a non-profit environmental group, in a statement introducing its study.

Liquid coal is viewed as a potential replacement to the oil on which countries rely heavily to fuel vehicles.

"Corn ethanol, conversely, could be either more polluting or less than gasoline depending on how the corn is grown and the ethanol is produced," the report said.

The analysis was based on replacing a fifth of all gasoline consumed in the United States with alternative fuels by 2030.

If most of these alternatives consist of liquid coal, the change could pump pollution into the atmosphere equivalent to 34 million more cars on the road. Favoring cleaner "advanced biofuels," on the other hand, could cut harmful gases by a similar amount.

The cleanest alternative, the report said, is cellulosic ethanol, made from grass or wood chips -- it could cut emissions by more than 85 percent.

"We need to wean ourselves off oil, but we should replace it with the cleanest alternatives possible," said the author of the study, Patricia Monahan, in the report. "Let's not trade one bad habit for another."

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