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
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,