As any other carbon-based fuel, when biofuels are burned, they release the greenhouse gas carbon dioxide into the atmosphere. But if the fuel comes from a crop, then growing the plant will absorb the same amount of carbon dioxide from the air as is eventually released by the burning. Carbon in; carbon out. A cycle is created, in which growing new plants neutralizes the emissions. The logic is impeccable, but it leaves out two things. First, there is the carbon “footprint” of growing, transporting, and processing the crop. And second, the question of what else might have happened on that land and what its carbon consequences would be.

The first can be calculated. The math makes growing corn for ethanol look dumb. The large amount of energy needed to manufacture fertilizer to grow the corn, and then to process that corn into ethanol, often means it would be more climate friendly to stick with regular fossil oil. Other ethanol crops, such as sugarcane, look better because they need less fertilizer and less processing. Most of the vegetable oils slated to replace diesel look quite good, because processing is easy. You just squeeze. If it grows well, jatropha can deliver a two-thirds emissions saving, for instance. Soy looks sensible, and oil palm even better. These are the calculations used to justify both growing biofuels and the EU laws that require mixing biofuels with regular fuel. Biofuels, the regulators say, don’t eliminate emissions, but they do reduce them.

But what about the second issue? Biofuels require land. The calculation above only works if nothing else would have grown on the land in question. Usually, that is not the situation. Not many biofuels grow in deserts. If biofuels replace something else, whether a crop or natural vegetation, that has to be taken into account. The most dramatic example is oil palm. It is often grown on land formerly occupied by rain forest and carbon-rich peat bogs. Clearing the forests and draining the peat bogs will create a huge carbon footprint. Taking that into account, the overall carbon footprint of biodiesel from palm oil is often much greater than that of fossil oil.

More often, biofuels are grown on former pastures, in which case we need to know how much carbon the grass would have absorbed. Or they might be grown on fields that once grew food. Assuming the food now has to be grown somewhere else, we then need to know where it is grown, and what the carbon footprint of the food crop is. Maybe someone somewhere chopped down a forest to keep people fed. Or added extra fertilizer to another field to increase yields. Making fertilizer is an extremely energy-intensive, carbon-producing activity.

9781905811731-largeVery often, answering this chain of questions may reveal that biofuels come at a carbon cost greater than the fossil fuels they replace. It seems rather obvious. You might presume that the carbon calculators had taken this into account. But no. Tim Searchinger of Princeton University, who has campaigned among scientists for answers to these questions, says the land issue is still not being assessed by most regulators plotting our route to a low-carbon future. And it is true. Regulators I have spoken to say they have left the land bit out because it is too hard for anyone to calculate with any accuracy. That is often true. But until this carbon accounting error is fixed, regulators often simply don’t know if, or when, biofuels are worth it.

From Fred Pearce’s, Landgrabbers The New Fight Over Who Owns The Earth, Eden Project Books, Transworld Publishers, UK, 2012, 103