The hope with biofuels is that they can offer a carbon-neutral energy source, because the crops that are grown for fuel will remove as much carbon from the atmosphere as will be released when they are eventually burned. This is basically the same thing that happens when we burn coal or oil. The difference is that in the case of the latter, the carbon was absorbed hundreds of millions of years ago, and this is part of the problem. Humans will take just a few hundred years to burn through tens of millions of years of oil deposits. To grow enough crops to keep up with our current demand will require lots of additional lands to be cleared for agriculture. If rainforests are cleared to plant sugar cane, all the carbon that is currently locked in the trees will be released. The other problem is that adding nitrogen fertilizer to these crops releases nitrogen oxide, which is another greenhouse gas. A recent study showed that burning maize biofuels actually increases greenhouse gas emissions for this reason. However, this is more the fault of the choice of biofuel crop and the production method rather than a flaw in the biofuel concept as a whole. The shrub “jatropha”, for instance, can be grown on land too poor for trees or other crops to grow on, and is already used for biodiesel in India, Cambodia, and some African countries.