Scientists at the University of Nebraska North Dakota have just reported that they brewed up a biofuel that's chemically almost identical to regular kerosene-based jet fuel. Unlike other biofuels, which have relatively high freezing points and therefore require planes to have special fuel heaters or fly at lower altitudes, this biofuel freezes at the same temperature as kerosene—negative 47 Celsius. So does this mean that Brad was premature in writing about the death of affordable air travel in a carbon-constrained world? Maybe not.

For starters, there's growing evidence that biofuels are anything but carbon neutral, especially when you take into account land-use change. Cutting down the Brazilian rainforest or draining Indonesian peat bogs to plant crops for biofuels releases so much carbon dioxide that it will be a long time before those biofuels have a carbon footprint smaller than the fossil fuels they're replacing. Cellusoic biofuels may someday solve this problem, by making it possible to get a lot more fuel out of each acre under cultivation—but they're not viable yet.

The bigger problem is that carbon emissions are only one of the ways that aviation contributes to global warming. It turns out that contrails—yes, those fluffy white contrails that make little boys want to grow up to be pilots—may also have a surprisingly large impact on climate. The ice crystals that form from the water vapor in jet exhaust have a classic greenhouse effect, letting most sunlight in but not letting heat radiate back to space. This effect is most pronounced during the winter—when colder air and higher relative humidity make ice crystals more likely to form—and at night, when the heat trapped by contrails doesn't get offset by the small amount of sunlight they reflect back to space. In 1999—the last time it looked at the issue directly—the Intergovernmental Panel on Climate Change estimated that air travel caused slightly more warming through contrails it did through carbon dioxide emissions. Recent studies have lowered these estimates for contrail-induced warming, but it's still a concern.

There are ways to get around the contrail effect—like limiting night flights, tweaking jet engines, or requiring planes to fly at lower altitudes, where contrail formation is less likely.  But all of them come with tradeoffs. And that means that despite some progress in biofuels, the era of cheap flights may still be coming to a close.

--Rob Inglis, High Country News