Do cheap flights cost the Earth? To examine the price the environment pays for the aviation industry, Kat Arney spoke with Professor Douglas Crawford-Brown, Director of the Cambridge Centre for Climate Change Mitigation Research, to find out how much flying contributes to climate change...

Douglas - At the moment, it's only a couple of percent, 2 to 3 percent, of the carbon dioxide that's emitted around the world on average. But if you look at somebody like myself, and we'll come to that in just a bit, it's probably 50 percent, 60 percent of my carbon footprint. But also, as we start to reduce carbon dioxide from other things like our electricity system and so forth, air transport will start to become an even larger percentage than it is today.

Kat - Air transport does seem to have a reputation as being a baddie of the climate world. How does it stack up against other forms of transport?

Douglas - It's not actually so bad. So, we'll take my case for example. The problem with it is not that I ride on an aeroplane. In fact, in the places that I go to, I would emit more carbon if I were to get into a car or to get onto a bus and go to those places. The problem is that I go tens of thousands of kilometres flying around the world to teach people to reduce their carbon footprint. Peter and I can discuss that at some point.

Kat - There's got to be an irony in that. Is it just that this is what we have to put up with to live in a global world? Is there any way that we could actually improve planes to reduce this kind of impact on the planet?

Douglas - We could, but first of all, I would say that the greenest travel is the travel that you don't make at all because you didn't need to in the first place. And so, things like telecommuting are starting to replace the need to be getting onto aeroplanes or into cars. But also, what really matters is how many people you're packing into an aeroplane or a car or a bus, or what have you. Pack enough people onto an aeroplane, put them into economy, don't let them go into first class, and they will be emitting less than they would have if they'd gotten into a car or onto a bus. Not less than if they'd gotten onto a train though.

Kat - That may make some people in first class unhappy, but I'm sure the rest of us in cattle class will feel a bit more virtuous. I fly to Canada to visit my sister sometimes and I think, maybe I should offset my carbon. What should we do? Do these kind of schemes make any difference?

Douglas - Well, the only thing that really makes a difference is not getting onto the plane to begin with, but offsetting, I'm really torn on offsetting. There's no doubt that one can plant trees and this will pull some of the carbon dioxide out. The problem that we have is that the CO2 that you're emitting from your aeroplane travel is being emitted immediately, but the CO2 that's going to come out as a result of the tree is going to come out over the next 50 years or so. What matters is not just the amount of CO2 that we take out of the atmosphere but how quickly we do it. So, I'm not a big fan of offsetting.

Kat - Are there any realistic strategies that could be used in the immediate or the very near term future to reduce the carbon impacts of flying?

Douglas - Well, engineers look a lot at the efficiency of the engines, the drag on the surface of the aeroplane and so forth. But that we've got to the point now where we've pretty much done all of those improvements and therefore, what really is the next step is moving towards fuels that don't emit as much CO2. So, something like biofuels moving into the aviation industry. We're not there yet for biofuels. We can't produce enough of them at the moment, but that certainly is a solution.

Kat - Do you think we will see carbon neutral aviation, or do you think that people will seriously stop flying or reduce their flights anytime soon?

Douglas - I don't think certainly in my lifetime, which doesn't say much given my age. In my lifetime, I don't think we'll ever see carbon neutral air travel. But certainly, in my son's lifetime, there's a possibility that at least we'll have air travel that's down by a factor of two or three lower in emissions than is currently the case today. Again, I don't think we'll ever hit the zero carbon.

Kat - In terms of things like policy, we're talking about maybe expanding one of the two big airports in the south of England. Should we be looking to have more flights but they're more efficient or should we be looking just to curb our love for the skies?

Douglas - Yes, well I mean, there are reasons that people fly around. I have reasons that I fly around. I like it quite a bit, but there's no doubt that we're going to be reaching a limit to the number of people who can be traveling anymore in the future by air and therefore, I think we'll be cutting back on at least the rate of expansion of airports. But it's an important economic component of the UK for example. So, that's going to be a tough battle to fight with the treasury.