The green airline is an oxymoron; air travel will be the fastest-growing source of carbon emissions by 2050. By 2020 we'll take half a billion flights annually (up from 189m in 2002), and aviation does far more damage than its perceived 2 per cent of UK emissions when you factor in radiative forcing: at altitude, the negative effects of burning kerosene are amplified to the power of three. Despite this it has a rather charmed life: no VAT, no fuel duty and is exempt from the climate-change levy.

The only thing to spoil the in-flight party is whingeing by eco types - typically described by Ryanair's Michael O'Leary as 'hairy environmentalists'. But even Ryanair wants to express its eco credentials, claiming it has halved its carbon emissions per passenger in five years thanks to a new €10bn fleet of Boeing 737-800s. But easyJet says it is the greenest, emitting 27 per cent fewer greenhouse gases per passenger kilometre than traditional airlines, having ordered 100 new Airbus A319s and retired 22 old aircraft last year. And you won't notice airlines voluntarily retiring routes rather than planes, even the half-empty ones which led Plane Stupid (planestupid.com) to expose the way some airlines enjoy running empty craft between regional airports to keep hold of landing slots.

The battle for greenest airline focuses on pimping planes, eco style. According to Ian Poll, professor of aerospace engineering at Cranfield University, bigger planes like the Airbus 350 will reduce fuel consumption per passenger by 25-30 per cent. Boeing claims its 787 Dreamliner is 27 per cent more efficient in terms of fuel consumption. Virgin Atlantic has ordered 15 Dreamliners to be delivered by 2011. Then there's Aer Arran's 'eco plane'. The new ATR 72-500s boast 'fuel consumption per passenger up to 15 per cent lower than a typical European car on a 200-mile sector'. Again, no mention of that radiative forcing. Further fuel economy is gained by 'packing in the passengers on all the flights' (Ryanair's terminology), so going business means a heavy carbon burden for everyone else. Not that the no-frills carriers have much more of a halo - they created the problem of rising passenger numbers: according to Civil Aviation Authority figures, 3.1m travelled from the UK to Europe on a 'no-frills' airline in 1996. By 2005, it was 51.5m.

What you need is a league table, preferably colour coded, where you could have some idea of impact at the time of booking. Flybe labels aircraft from A (good and fuel efficient) to F (very poor). Forty-six Flybe jets rate well and 34 rather badly. But this is an in-house initiative, so how objective can it be? If you have to fly, ask as many questions as you can when booking. Make a nuisance of yourself. Airlines have scant regulatory pressure on them; it's time they felt the heat of the consumer spotlight.

lucy.siegle@observer.co.uk