The principle of carbon offsetting is to redeem the amount of carbon dioxide (CO2)generated by a particular activity, from a flight to Paris to a rock concert, by paying a sum to be invested in a project that will remove or prevent the equivalent amount of CO2 from being released into the atmosphere.

A number of companies have been set up to provide off-setting services, by presenting consumers with a calculator  to work out their CO2 debt and a range of options to neutralise their emissions. The majority of these companies operate for profit and are largely unregulated. They then invest their customers' cash in projects that range from building wind turbines in India, planting trees in Scotland, and providing low-energy light bulbs to people in developing nations.

Offsetting is one of the fastest growing areas in a global carbon-trading industry worth £15billion. About 1.5 million Britons were estimated to have offset one of more flights last year.

The Association of British Travel Agents (ABTA) and Lastminute.com recently become the latest travel companies to offer carbon offsetting. It is not just flying that can be offset - it can be anything from a baby's nappies to a listed building. The offsetting is done by activities which will absorb CO2 or reduce its production, for example by planting trees and offering investment in renewable energy and development projects in developing companies. Projects include building bio-gas digesters in India, installing energy-efficient light bulbs in Kazakhstan, or buying energy efficient stoves for villagers in Honduras.

The case in favour of carbon offsetting-

• Offsetting is funding diverse projects in the developing world that make real reductions in greenhouse gases.
• Offsetting is a "gateway" to encourage consumers to take fewer or shorter flights, and raising awareness of carbon emissions.
• The aviation and travel industries are increasingly ready to offer offsets to passengers and engage in the climate change debate.

The case against carbon offsetting

• Offsetting does not cut total CO2 emissions and is a distraction from deciding how to reduce CO2 at the inter-governmental level.
• Offsetting flights leads air travellers to forget that their flights are still emitting CO2 rather than considering not flying at all.
• In Britain alone it would be necessary to plant a forest the size of Dorset each year to offset the UK's CO2 emissions.

Some offset companies have been accused of buying carbon rights for trees that have already been planted or would have been planted anyway. In any case the CO2 is released when a tree dies and rots away - there is no guaranteed carbon sequestration. Recent research in America has suggested that planting trees in northern climates can contribute to climate changing by the dark surface of trees absorbing the sun's heat, rather than allowing the earth's surface to reflect it. Leading offset company Climate Care, based in Oxford, says it is slashing its reliance on forestry projects to 5% of its total offset. Carbon Neutral Company, another big offset company is cutting its forestry portfolio from 80% to 20%. Charlie Kronick, climate campaigner for Greenpeace has commented: "There is nothing wrong with sustainable forestry per se, but what it won't do is solve climate change, and it is disingenuous to claim otherwise."

Careful who you pay

If you are thinking of paying for carbon off-setting please be aware - Only PURE the Clean Planet Trust, Global Cool, Equiclimate and Carbon Offsets already meet the Government's standard by offering Certified Emission Reduction credits for their offsetting products.

Below are some of the carbon off-setting companies and the projects they are supporting:

Atmosfair (Germany) - Solar energy, India; Sewage treatment, Thailand

Carbon Balanced (UK)  - Tree-planting, Belize

Carbon Clear (UK) - Tree-planting, India and Tanzania

Carbon Counter (US) - Coal-use reduction, Duluth, Minnesota

Carbon Footprint (UK) - Tree-planting, Kenya

Carbonfreelife (UK) - tree planting

Carbon Fund (US) - Wind farm, Nebraska

Carbon Neutral (UK)- Energy-saving bulbs, Jamaia; Solar-lighting, India; Tree-planting, Devon

Carbon Offsets (UK) - Energy efficiency and tree-planting

Carbon Zero (Canada) - Wind farm, Alberta

Climate Care (UK) - Energy-saving bulbs, South Africa

CO2 Balance (UK) - Tree-planting, Cumbria

Global Cool (UK)- Wind farms, UN-approved

EasyBeingGreen (Australia) - Energy-saving bulbs, Australia

Equiclimate (UK) - EU emissions trading scheme

Greenseat (Dutch) - Tree-planting, Holland and Czech Republic

Myclimate (Swiss)- Solar energy, Eritrea

Pure (UK) - Cancels "licences to pollute"

Zerofootprint (Canada) - Tree-planting, Canada and US

(Reference sources - The Independent and The Guardian)