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"Green" fuels raise emissions by destroying land, two papers argue.

Almost all biofuels used today cause more greenhouse gas emissions than conventional fuels if the full emissions costs of producing these "green" fuels are taken into account, two studies published Thursday conclude.

The benefits of biofuels have come under increasing attack in recent months, as scientists have taken a closer look at the global environmental cost of their production. These latest studies, published in the prestigious journal Science, are likely to add to the controversy.

These studies for the first time take a detailed, comprehensive look at the emissions effects of the huge amount of land that is being converted to cropland globally to support biofuels development.

The destruction of natural ecosystems - whether rain forest in the tropics or grasslands in South America - not only releases greenhouse gases into the atmosphere when they are burned and plowed, but also deprives the planet of natural sponges to absorb carbon emissions.

Together the two studies offer sweeping conclusions: It does not matter if it is rain forest or scrubland that is cleared, the greenhouse gas contribution is significant.

More important, they discovered that, globally, the production of almost all biofuels resulted, directly or indirectly and intentionally or not, in new lands being cleared for food or fuel.

The plant-based fuels were originally billed as better than fossil fuels because the carbon released when they were burned was balanced by the carbon absorbed when the plants grew. But even that equation proved overly simplistic because the process of turning plants into fuels causes its own emissions - for refining and transport, for example.

The Renewable Fuels Association, which represents ethanol producers, called the researchers' view of land-use changes "simplistic."