"Liquid coal, for example, can produce 80% more global warming pollution than [petrol]," says the US non-profit environmental group, the Union of Concerned Scientists.

Some people have billed liquid coal, the name for petrol or diesel derived from coal, as a potential replacement to the oil on which countries rely heavily to fuel vehicles.

Likewise another alternative fuel, petrol from tar sands, is estimated to have 14% more global warming potential compared with standard petrol, says the union in its latest report Biofuels: an important part of a low-carbon diet.

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

The analysis is based on replacing a fifth of all petrol used in the US 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, the report says.

But favouring cleaner "advanced biofuels" could cut harmful gases by a similar amount.

The cleanest alternative, the report says, is cellulosic ethanol, made from grass or wood chips. It could cut greenhouse emissions compared with petrol by more than 85%.

"We need to wean ourselves off oil, but we should replace it with the cleanest alternatives possible," says study author Patricia Monahan.

"Let's not trade one bad habit for another."
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