Ask Leo: what is radiative forcing?

Carbon Footprint

Scientists measuring the impact emissions have on the climate often talk about 'radiative forcing', and say that aviation's emissions have a radiative forcing impact of around 2.7. But what is radiative forcing, and do all scientists agree on aviation's impact?

The warming impact of aviation emissions is notoriously difficult to quantify. Because aircraft exhaust contains other greenhouse gases whose impact is less well understood than carbon dioxide (such as nitrogen oxide [NOx] and the water vapour that makes up condensation trails), and because this whole bundle of gases is deposited right where we don't want it – into the upper atmosphere – trying to ascertain exactly how much warming will result from a given journey is riven with uncertainty.

The precise height a plane cruises at makes a difference to these calculations, and it even appears that night flights have a greater warming impact than day ones, trapping heat radiating from the earth whilst not deflecting any incoming solar radiation. The only thing there is much agreement about in the scientific community is that aeroplane emissions have a much higher 'radiative forcing' than would arise from the same amount of CO2 emitted at ground level.

As if all these fairly baffling factors weren't complicated enough, scientists have just flagged up yet another variable that could make a significant difference to a plane journey's radiative forcing. It seems that flights through the tropical skies could be dramatically worse than those taken at mid-latitudes, because of the way the brighter sunshine there interacts with the NOX emissions. In the skies above Britain, NOX emissions both create ozone, a greenhouse gas, and destroy methane, another greenhouse gas, with these two interactions effectively cancelling each other out.

But in the tropics, the brighter sunshine generates ozone five times faster, whereas methane destruction increases only slightly – the net result being a much higher radiative forcing for a tropical flight than for a temperate one*. This news shouldn't just make us more worried about flying on holiday in the tropics – it has extremely worrying implications for the massive growth in domestic air traffic in developing tropical nations too. India has the fastest growing airline fleet in the world.

All this complexity is enough to make anyone's head hurt. But as noted in the United Nations Framework Convention on Climate Change, scientific uncertainty is no excuse for inaction. Aviation remains exempt from all national and international controls on greenhouse gas emissions. So long as it does, one thing we can be sure of is that the goal of these agreements – climate stabilization at a 'safe' temperature – is in serious jeopardy from air travel.