Aircraft Emissions Are Bad, But How Bad?

It's become almost a reflex to bash aviation's role in climate change. Passenger planes are big, they emit stuff into the atmosphere, so they must be bad. It's true, but it's not quite that simple.

Last week, climate and atmospheric scientists from the US and Europe presented research to a group of aviation types gathered at the Royal Society of London. Much of their focus was on nitrogen oxide (NOx), and what they found is that simply dismissing NOx as just another nasty greenhouse gas is not only bad science, but could also be unwise policy.

Keith Shine, a professor from Reading University, says that while there's a great deal of research being conducted on the role of NOx in climate change, nothing is conclusive at this point, in part because the gas seems to both harm and help. "NOx has some particularly slippery issues we have to grapple with," he says. "It is a very reactive gas in the atmosphere and has different impacts." 

One of those impacts is the formation of ozone, which facilitates
growth in CO2 by preventing its absorption by plants. But NOx only
leads to ozone formation in the troposphere (the lower part of the
atmosphere). Once you get above 20km, into the stratosphere, NOx
actually causes ozone depletion. As an added bonus, NOx
leads to creation of hydroxyl, which acts as a sort of detergent that
cleans many pollutants out of the atmosphere and and
helps destroy methane, another bad news greenhouse gas. “The discovery in recent years of the effect of the OH molecule as a
detergent chemical has been a pleasant surprise to the scientific
community,” Shine says.

He says that at this point it's hard to know if there is a net impact one way or the other. "It turns out
that by emitting on greenhouse gas into the atmosphere we are also
partly destroying another. To add complexity to the subject, methane is
one of the most important molecules that leads to ozone formation. So
NOx is creating more ozone but also destroying methane, leading to less
ozone."

And it's even trickier than that. The effects of NOx depend not
just on the altitude at which they are released, but also on the geographical region. "With COx it doesn't matter where the emissions
take place," Shine says. Whether it be the North Pole or the tropics,
the impact is the same." But this isn't the case for NOx, which seems
to have a bigger impact at the equator.

It's pretty complicated stuff, and Shine is the first to admit that
although the science is getting better there's still a lot to figure
out. When asked what advice he'd give policy makers with regards to
aviation and the climate based on what we know so far, he urges them to stay focused on one just one thing.
"If the worry is about climate change over a 50 or 100 year
period....in my view the only thing we should be concerned with is CO2
emissions."

Photo by Flickr user Jasmic