Environment

Want to salve your conscience and offset your holiday carbon emissions? You might want to rethink that trip to the tropics. A typical flight there has a greater impact on global warming than a flight in temperate latitudes.

As well as producing carbon dioxide and contrails, planes also produce nitrogen oxide, which triggers both the creation of the warming gas ozone, and the destruction of another greenhouse gas, methane (Journal of Geophysical Research, DOI: 10.1029/2007/JD009140).

In mid-latitudes, these ozone and methane reactions cancel each other out and you get zero net warming from nitrogen oxide emissions, says Keith Shine of the University of Reading, UK. But the brighter sunlight in the tropics is very efficient at converting nitrogen oxide to ozone - in fact it creates ozone five times faster than in the air of mid-latitudes, according to Shine's calculations - whereas methane destruction only increases marginally. Worryingly, the warming effects of ozone are particularly strong at a plane's typical cruising altitude of 35,000 feet, he adds.

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.

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