Talk to any ecowarrior about jetting off to sunnier climes and chances are you will be met with a look of horror.

Flying only accounts for three to five per cent of all our greenhouse gas emissions, yet it has come to be seen as one of the big baddies of the environmental movement.

The Bishop of London has even declared the act of getting on an aeroplane to be a sin.

So what is all the fuss about?

Flying is a rapidly growing and largely untaxed industry. Many predictions suggest this growth could cancel out any savings in emissions we make elsewhere.

Flying also allows us to use a vast quantity of fuel in a very short space of time.

In the UK, each person is responsible for an average of 9.5 tonnes of carbon dioxide every year. A passenger on a return flight to New York will be responsible for creating about 1.2 tonnes of CO2 in just eight hours.

Richard Dyer is the aviation campaigner for Brighton and Hove Friends of the Earth. He says: "It is one of the most polluting things we can do legally in our day-to-day existence"

It isn't all down to carbon dioxide, though. Other toxins are emitted during flight, including carbon monoxide and nitrogen dioxide, and all at a high altitude, which further damages the delicate upper atmosphere.

Taking all these different factors into account, a study by the Intergovernmental Panel on Climate Change suggested the climatic impact of flying is more than double the effect from just the carbon dioxide alone.

However, there is another byproduct of flying.

One which, remarkably, had awardwinning environmentalist Dr James Lovelock claiming, at a talk in Brighton last year, we should all keep flying to stave off the effects of global warming.

He was referring to a phenomenon called "global dimming", which in part is thought to be caused by the wispy white vapour trails left as flights zoom through the skies.

Global dimming could be seen as the antithesis of global warming, the ice to its fire.

It describes pollutants which reflect light away from the Earth's surface, meaning less heat is getting in.

Dr Tao Wang is a research fellow at the Tyndall Centre for Climate Change Research and the Sussex Energy Group at the University of Sussex.

He says: "Global dimming creates a cooling effect which may have partially masked the effect of greenhouse gases on global warming.

"Some scientists are now concerned the unseen effects of global warming may lead to a greater increase in temperature than previously predicted."

When flights were grounded across America after September 11, scientists were able to see what happens when there are no contrails in the sky.

They found the difference in temperature between the warmest and coolest part of the day rose by one degree centigrade, suggesting the trails were reflecting light away during the day, helping keep us cool.

Whether or not this can actually counteract global warming, however, is debatable.

Dr Wang says: "The point made by James Lovelock looks very irresponsible to me. Relying on global dimming as mitigation of global warming is implausible because the result will just be an increase in pollution and health costs from both adverse phenomena."

He continues by saying the relationship between dimming and warming is far too complex to simply say one cools and the other warms.

But all this still leaves us with the decision of whether to fly or not.

Friend of the Earth's Richard Dyer says it is not asking people never to fly again, just to consider carefully how they travel.

He says: "The industry try to paint us as saying no one should fly. We are saying it is the future growth that is the problem and needs some moderating."

He goes on to point out there is little justification for flying within the UK and a good rail service is available across much of Europe.

"Taking check-in into account, train journey times can often be the same," he says.

"If it is longer, there are other advantages. Get the train to Spain and you get to see France on the way.

It can be a lot of fun."