The Island of Doubt

As if the developing world didn't have a enough to worry about when it comes to joining the industrialized 21st century without following in the developed world's polluting footprints. A new study by British researchers finds that where you fly makes a difference to your impact on the climate. The take-home message is jet flights near the equator do more damage than at high latitudes.

"Impact of perturbations to nitrogen oxide emissions from global aviation" by Mark Köhler et al in the Journal of Geophysical Research (Vol. 113, D11305) studied the effects of all that nitrogen oxide spewing from jet engines on the levels of ozone and methane in the atmosphere. Both ozone and methane are potent greenhouse gases, and nitrogen oxide triggers the generation of ozone and the destruction of methane.

Averaged over all the flight paths everywhere on the planet, there is no net effect on radiative forcing (the technical term for drivers of global warming). The ozone produced results in a radiative increase of 30 mW per square metre, while destroyed methane resulted in a decrease of 19 mW. There's also a kind of reverse feedback in which methane generates ozone, so you have to subtract the effect of methane destruction again. In this case, that another −11 mW. Add it all up and "the sum of the forcing from the three mechanisms considered here is, at least in the global mean, close to zero."

But that's the global mean. Down in the equatorial region, things are a little different. Turns out that the bright sunlight associated with the deserts and other features common to that part of the planet enhances the process by which nitrogen oxide generates ozone, by upwards of a factor of five. Methane destruction, however, remains pretty much the same. So flying a jet at 35,000 feet over Nairobi is significantly more effective at creating greenhouse gases than flying one over New York or London.

The paper's authors stick to the science, rather than take on the role of international aviation policy advisers. The closest they get to that would be:

This has important consequences for future air traffic growth, together with changes to flight routing as a result of Air Traffic Control or operational procedures.

The research raises the question of whether future attempts to control aircraft emissions should consider extra penalties for flights in tropical countries where air travel is booming. India, for instance, has the fastest growing airline fleet in the world.

For now aircraft emissions are excluded from international treaties on curbing greenhouse gas emissions. But the European Union has plans to control aircraft emissions from 2011.