In the case of biodiesel, these indirect emissions will probably make the fuel worse for the environment than conventional fossil fuels. Both biodiesel and vegetable oil use the same source crops. As demand for biodiesel has risen, vegetable oil producers have turned to oil palm, grown in Southeast Asia. Consequently greater areas of rainforest have been chopped down, and large expanses of carbon-rich peat-lands drained, to make space for palm oil plantations.

Currently estimates indicate that up to 2% of global carbon dioxide emissions result from the on-going loss of the carbon in these peat-lands; more biodiesel demand fed by vegetable oil will make the problem worse.

Indirect land-use effects have now become central to the debate about the value of biofuels. But because estimating these effects requires the use of uncertain economic models, some biofuel researchers, public officials and biofuel companies say that they should be ignored.

Searchinger's new paper, published in ERL, contends that this argument stems from a fundamental misunderstanding about the way in which biofuels can reduce emissions. "Biofuels do not reduce the carbon dioxide released by cars, but they may offset these releases when the plants used for the fuel absorb additional carbon," he says. "That occurs if biofuels use new crops grown on otherwise highly unproductive land. But diverting crops that would have grown anyway does not absorb additional carbon and therefore does not directly offset anything."

Using crops farmers already produce for fuel instead of food is like moving money from your left pocket to your right Timothy Searchinger

Using existing crops can still reduce emissions, but not necessarily in desirable ways, says Searchinger. For example, if the crops are not replaced, people and livestock eat less and breathe out less carbon, but that is not a good way to solve global warming. "Using crops farmers already produce for fuel instead of food is like moving money from your left pocket to your right," explains Searchinger. "It does not produce more carbon or make you richer."

Searchinger's paper explores the significance of this need for biofuels to adsorb "additional" carbon to generate greenhouse-gas reductions. It argues that most of the global estimates of bioenergy potential make a similar mistake in counting carbon that people either already use or that the Earth would store in plants and soils, and keep away from the atmosphere anyway.

Currently US and California law requires lifecycle analysis of greenhouse-gas emissions from biofuels, including both direct and indirect emissions from land-use change. However, a bill passed in July 2009 bars administrators from estimating international indirect land-use change because of the high uncertainty of the estimates. Meanwhile the EU is debating its stance on this issue right now and it must decide by the end of 2010 whether it will include indirect land-use change emissions from biofuels.

Searchinger presents two approaches to addressing the uncertainties of indirect land-use change. One would combine the results of multiple models and take a precautionary approach. Another approach is to use feedstocks that are inherently additional, for example by growing bioenergy crops on unproductive land or using timber or crop residues that would otherwise decompose and send their carbon into the air.

If indirect emissions are taken into account Searchinger believes that it will be very hard for most countries to reach the biofuels targets that they have set themselves, in the timescales proposed. However, he believes that there is still some room for environmentally sustainable growth in the biofuel market if it is carried out in a moderate and considered way.