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.”

Source:  Dave Demerjian