LONDON/WASHINGTON - A new generation of biofuels, meant to be a low-carbon alternative, will on average emit more carbon dioxide than burning gasoline over the next few decades, a study published in Science found on Thursday.

Governments and companies are pouring billions of research dollars into advanced fuels made from wood and grass, meant to cut carbon emissions compared with gasoline, and not compete with food as corn-based biofuels do now.

But such advanced, cellulosic biofuels will actually lead to higher carbon emissions than gasoline per unit of energy, averaged over the 2000-2030 time period, the study found.

That is because the land required to plant fast-growing poplar trees and tropical grasses would displace food crops, and so drive deforestation to create more farmland, a powerful source of carbon emissions.

Biofuel crops also require nitrogen fertilizers, a source of two greenhouse gases: carbon dioxide (CO2) and the more powerful nitrous oxide.

In the near-term I think, irrespective of how you go about the cellulosic biofuels program, you're going to have greenhouse gas emissions exacerbating the climate change problem, said lead author, Jerry Melillo, from the U.S. Marine Biological Laboratory.

U.S. ethanol industry group the Renewable Fuels Association said biofuels are by definition emissions neutral because their tailpipe carbon output is absorbed by growing plants.

Without steps to protect forests and cut fertilizer use, gasoline out-performs biofuels from 2000-2050 as well.

The paper did not mean cellulosic biofuels had no place.

It is not an obvious and easy win without thinking very carefully about the problem, said Melillo. We have to think very carefully about both short and long-term consequences.

A related study, also published in the journal Science on Thursday, said the United Nations had exaggerated carbon savings from biofuels and biomass, in a mistake copied by the European Union in its cap and trade law, by ignoring deforestation and other land use changes.

The mistake was carried into U.S. climate legislation as well, and would worsen as governments put a price on carbon, driving more biofuel use, it said.

FOOD

There will be increasing pressure to convert the biomass of the world into an energy source, said Steve Hamburg, chief scientist at green group the Environmental Defense Fund and co-author of the second Science paper.

Then it competes with agriculture, water protection, biodiversity, a whole host of things, and that doesn't provide benefits to the atmosphere, he told Reuters.

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

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