Advanced biofuels will stoke global warming: study

It was also important to take account of how the land had been managed before it was grown with biofuels, said Hamburg. A previous farming practice may have been better for the planet, he said, underlining the complexity of calculating benefits.

Advocates hope that forthcoming talks to agree a new global climate deal in Copenhagen in December will protect forests, by rewarding land owners to store carbon in their trees.

The first paper did not explicitly consider the food production impact of ramping up advanced biofuels. The U.N.'s food agency says that global food output will have to increase 70 percent by 2050 to feed a growing, more affluent population.

The world's forests, rather than farmland, would have to make way for biofuels which would consume by 2100 more land than all food crops now, the first study found.

"We think there is space on earth for both food crops and the biofuels but there are consequences of using that space," in lost forest, Melillo said. "You've got to lose something."

(Writing by Gerard Wynn; Editing by Anthony Barker)