So, do you really have to get on board that plane?

Planning an Easter trip to Europe or further afield this weekend, or have you just returned from a holiday in time for the end of the school holidays? If you're flying, do you know what impact the journey had on the environment, or how much that damage will cost to put right? Air travel is the fastest-growing source of the greenhouse gas emissions responsible for climate change. Around the world, 16,000 commercial jet aircraft create more than 600m tonnes of carbon dioxide every year. Each time you take a flight, you contribute to that total.

The dilemma is increasingly difficult to avoid. There is a growing consensus among scientists that carbon emissions need to fall by at least 60 per cent over the next 50 years to stabilise climate change. Yet emissions are increasing at a faster rate than ever. The message is that people must think harder about flying. But if you are going to use air travel, you might at least consider carbon offsetting schemes.

HOW DO THESE SCHEMES WORK?

The two best-known carbon offsetting scheme providers, the Carbon Neutral Company and Climate Care, work on the same principle. This is that while everyone produces carbon dioxide emissions there are ways to reduce the earth's total emissions.

It is possible to quantify how much carbon dioxide you are responsible for producing when taking a plane journey. Similarly, it is possible to quantify how much you would have to invest in projects designed to reduce emissions to balance out the damage your flight did.

A return flight to New York, according to Climate Care, would produce 1.56 tonnes of carbon dioxide emissions. The company calculates that a donation of £11.67 to its projects would reduce global emissions by that amount.

In fact, the donations required to render your air travel carbon neutral are relatively modest.

Even far-flung destinations, such as Sydney, cost less than £40 to offset - in the vast majority of cases, you're talking about less than 10 per cent of the ticket price.

Last month, the Government agreed to offset emissions produced by official flights taken by ministers or civil servants.

The Carbon Neutral Company has won endorsements from celebrities such as the singer KT Tunstall, while Climate Care has the backing of a joint initiative from the arch rivals of the travel guide industry, Lonely Planet and Rough Guides.

WHERE DOES MY MONEY GO?

Offsetting schemes invest in projects designed to reduce carbon emissions. The most obvious example is forestry - trees absorb carbon dioxide and produce oxygen - so some of your cash is likely to end up in reforestation programmes around the world.

However, last year, the Carbon Neutral Company changed its name - it was previously known as Future Forests - to reflect the growing range of projects in which it invests. All the offsetting schemes back renewable and sustainable energy initiatives, as well as themes such as recycling and waste management.

Climate Change, for example, is involved in a scheme in India that is replacing stoves in school. The old stoves were run on liquid petroleum gas; the replacements will be powered by more eco-friendly bio-mass fuel.

Generally, you can't choose which projects your money supports, though very large donors - offsetting schemes also accept donations from businesses and other organisations - may be able to. You could consider using the carbon offsetting scheme run by Friends of Conservation, a wildlife charity that uses the money to back reforestation, particularly where animal and plant species are threatened.

HOW CAN COMPANIES BE SO EXACT?

Offsetting schemes publish the methodologies they use to calculate the pollution generated by your flight and the emission reductions paid for by your money. They also have independent committees staffed by representations of organisations such as the World Wildlife Fund. These committees monitor the activities of the schemes to ensure they're producing the reductions they claim.

However, some projects have less predictable results than others. It may be that certain investments made by offsetting schemes do not produce the savings expected. Part of the role of the independent scrutineers is to ensure that such shortfalls are made up elsewhere.

Offsetting schemes almost always invest in developing world projects, rather than in countries that have signed up to the Kyoto Protocol, an international agreement to cut emissions in developed economies. They say helping richer countries achieve promises already made would be counter-productive.

SO CAN I NOW FLY WITH A CLEAN CONSCIENCE?

It's not that simple. Carbon offsetting schemes have their critics - of the principles and the practices. The most obvious problem is that anyone who can afford to offset their emissions may feel entitled to go on flying as they always have, rather than obliged to change their behaviour. People might even take flights they would not otherwise have booked.

There are also arguments about the true benefits of the projects your money funds. For example, some scientists argue that trees are not an effective way to mop up carbon emissions produced by burning fossil fuels.

"We also have concerns about tree-planting, because there are no guarantees these trees won't be felled, or, worse, burnt, soon after they have been planted," adds Richard Dyer, an aviation campaigner at Friends of the Earth. "And we're concerned about some of the other projects backed, because many of them might have been funded anyway."

Above all, there is only so much good work carbon offsetting can do. To soak up Britain's annual greenhouse gas emissions, you would need to plant a forest the size of Devon and Cornwall each year. And unlike with other industries, there is little prospect of new technology that will clean up aviation in the short or medium term. In that context, passenger air miles must fall, or at least stop rising.

I NEED TO CUT DOWN ON FLYING AFTER ALL?

Climate Care and the Carbon Neutral Company insist that people should try to take fewer flights. Friends of the Earth is particularly scathing about shorter flights.

"Easter breaks for many people will include short-haul trips, where alternative modes of transport are much more practical," Dyer says. "Short-haul travel is particularly polluting because the take-off and landing segments, when most fuel is burnt, represent a large proportion of the flight plan."

A flight from London to Edinburgh, for example, would produce seven times the carbon emissions generated by the equivalent journey taken by rail.

The cost of short-haul journeys is also reflected in the offsets required - while the sums needed are smaller, they represent a much larger proportion of the ticket price.

BUT AT LEAST OFFSETTING MEANS I'LL BE DOING SOMETHING?

Absolutely. "We need to reduce the fossil fuels we use and travel we do, but simply saying to people 'don't do that' is not effective," says Tom Morton, a director of Climate Care. "We see this as part of a wider picture - at some stage we will all have individual carbon allowances and our schemes are helping people to get used to the idea that they produce a personal carbon footprint."

Jonathan Shopley, chairman of the Carbon Neutral Company, believes that once you start offsetting carbon emissions, you'll think twice about making them. "This is a pricing signal that will work," he says.

But sustainable travel is not only about finding greener forms of transport. David Weston, of the Travel Foundation, which promotes sustainable tourism, argues: "Many developing communities are dependent on tourism. If everyone stopped flying it would cause great hardship."

Developing countries are likely to be hit hard by climate change, but local people rarely want travellers to stay at home.

While you're away

* The carbon footprint of your trip does not begin and end with the journey there and back. You also have an impact on the environment while you're on holiday.

* Turn off heating or air conditioning when you don't feel it's needed, or at least turn it down.

* Switch off the lights when you leave your accommodation and never leave televisions or other appliances on standby.

* Be careful with your use of water. Take showers rather than baths and tell staff that you are happy to reuse towels and bed linen, rather than requiring freshly washed replacements every day.

* Consider making a donation to the Travel Foundation, which funds carbon reduction programmes in resorts around the world. It is currently working with operators in the Caribbean, for example, on an energy-efficiency scheme for tourist accommodation that will be the equivalent of taking 2,800 cars off the road.

* Useful contacts : Carbon Neutral Company: 08701 99 99 88, www.carbonneutral.com; Climate Care: 01865 207 000, www.climatecare.co.uk; Friends of Conservation: 020-7603 5024, www.foc-uk.com; the Travel Foundation: 0117 927 3049, www.thetravelfoundation.org.uk.