As affordable petroleum becomes scarcer, it's easy to imagine switching to electric cars or outfitting hydrogen-powered ships with supplementary sails. But it's harder to picture what we might run jumbo jets on in the future. Perhaps passengers will help to drive propellers as whip-wielding stewardesses urge them to pedal faster, or reach under their seats to hook fresh batteries to electric engines in mid-flight. This hardly seems likely, though. So what might really keep air travel viable in a post-petroleum world?
One of the main candidates is butanol, a close chemical relative of the fuel in butane cigarette lighters. It's more suitable for aviation than ethanol, which offers only modest energy outputs, isn't fully compatible with existing internal combustion engines, and corrodes the pipelines it travels through. Butanol is made from the fibrous cellulose in pulverized wood, which is fed to yeasts that digest it into a flammable liquid that yields more energy per gallon than ethanol does, works better in standard engines, and doesn't eat its own transport pipes.
As with most emerging technologies, it's not yet economically viable in comparison to gasoline (for cars) or kerosene (for jets). But that's not saying much. In the coming decades, gasoline and kerosene will become less affordable as petroleum sources dwindle, and $100-a-barrel oil has already made alternative fuels more competitive than they were a few years ago. With that in mind, inventors and investors are developing biobutanol as a possible savior for the airline industry.
In central Maine, for instance, a fiber and fuel mill is setting its sights on butanol production. The goal is to sell pulp to paper mills at first, but eventually the company hopes to send most of its ground-up trees to fermentation tanks.
If all goes according to plan, biobutanol operations such as this could give a much-needed boost to Maine's struggling timber industry. The heat-trapping carbon dioxide emitted from biofuel combustion eventually recycles back into vegetation, so the effect on climate is minimal. Unlike coal, butanol emits no sulfurous crud that might produce acid rain. And wood-farming doesn't encourage deforestation in favor of cornfields, nor does it inflate the price of food crops like ethanol production does; this alone is encouraging some ethanol plants to switch to butanol production.
On the other hand, a global butanol boom might replace slow-growing natural forests with monocultures of fast-growing pines or eucalyptus. But maybe there's a way to keep from turning the world's woodlands into sterile rows of pulp popsicles while also conserving biodiversity.
Cellulose is cellulose regardless of what kind of tree it comes from, so perhaps enlightened butanol farming could create varied woodlands that support more plants and animals than today's plantations do. That could keep more of the planet sustainably forested, with long-term harvest rotations clearing small spaces that would grow over with brushy browse that feeds and shelters wildlife before eventually reverting back to trees.
There's another good thing about butanol: you can use it to make rubber and plastics. That's an important point to consider as we enter the post-petroleum age. Only about half of each barrel of oil pumped from the ground is burned by automobiles. Much of the rest goes into the building blocks of modern life, from the plastics in your laptop or cell phone to the pesticides and pharmaceuticals that keep us fed and healthy. Running out of affordable petroleum will involve much more than sticker shock at the gas pump; it will strike at the heart of modern civilization. All the more reason to take serious stock of our options now, before the real crunch hits.
Even if you don't believe in human-driven climate change, a timely switch to alternative energy sources is still an idea that most of us can agree on and support. And if biobutanol is all that its proponents claim it is, air travel may still be an option for our descendants far into the future, too.
Curt Stager is an ecologist, paleoclimatologist, and science journalist with a Ph.D. in biology and geology from Duke University. His new book is DEEP FUTURE: The Next 100,000 Years of Life on Earth (St. Martin's Press, March 2011).