Sustainable, green, renewable, organic—the words come up so often in energy and climate debates that they tend to sound as if they mean the same thing. But of course they don't. Nuclear reactors emit no carbon and are therefore in a sense green, but uranium is nonrenewable; hydropower is green and renewable but may not always be sustainable, because the ecological consequences can be bad and reservoirs are not limitless; coal is organic, but its carbon emissions make it the very opposite of green. All that is obvious enough. But even so, it may be jarring to hear—as we have found and will describe—that organic biofuels can't possibly fuel a growing world economy in a sustainable manner, whereas, in principle, inorganic fuels could.
That inorganic might beat organic contradicts fashionable prejudice, which like all fashion changes with the season. Take the case of the United States: First came the enthusiasm for corn ethanol, its extravagant subsidization, and a farm-industrial miniboom. Then, when corn's limits started to become better known and its costs more glaringly obvious, we started to hear about the promise of switchgrass, a native species of the North American prairie that promises high energy-conversion efficiencies. President George W. Bush first mentioned it in a 2006 speech to the nation. Before long, Al Gore was chiming in too, promising that with adequate government support for research, grass-based fuels could free us from the dual specters of energy shortage and runaway climate change.
In Germany rapeseed has been all the rage; in India, jatropha; and in Brazil, sugarcane ethanol. Yet the plain fact is that nobody really knows when or whether organic fuels will be competitive. Engineering breakthroughs, by their nature, are unpredictable—that's what makes them exciting. So to evaluate whether organic fuels could ever be in a position to power the world, we looked at them purely in terms of physical resource availability, assuming that the costs would eventually become competitive. We asked how much land and water would be needed to make the quantities of biofuel that a prosperous world would need. We also asked whether there were other sources of fuel and energy that might put less strain on resources while adding less greenhouse gas to the atmosphere.
To our own surprise, the model we constructed showed that there is simply not enough land and water to support a prosperous biofueled world. At the same time, it suggested that inorganic sources, such as photovoltaic cells, can in principle do the job.
Our simple model, developed at the school of electrical and computer engineering at Georgia Tech, is designed to evaluate alternative energy scenarios. It simulates the consumption of energy, land, water, and carbon both globally and on regional scales; it projects emissions of carbon and waste heat; and it takes all obvious interdependencies into account. It assumes that electricity and water can move throughout the world without loss, that all land and freshwater are available for human use, and that in 20 to 25 years all the countries of the world will have access to sufficient resources to live as well as the United States does today.