Michael McCarthy: Are carbon offsets an excuse to carry on polluting?

Why change the Chelsea tractor if you feel you've cancelled out its malign effects?

It's not a new idea, in essence. In Anglo-Saxon England it was known as Danegeld: you pay money to somebody else in order to have an easy life. In Aethelred the Unready's time, you paid marauding Vikings so that your land might not be laid waste; today you pay a carbon offset company so that your conscience about helping to cause climate change may be similarly undamaged.

The principle is the same. You don't have to do anything else; you just pay the money and sleep easy, be you an Anglo-Saxon farmer, or the 21st century driver of a Chelsea tractor whose exhaust emissions are pouring into the atmosphere daily. And my goodness, is it proving popular. As the concern about global warming steadily increases, carbon offsetting is everywhere.

Carbon offsetting means buying reductions in emissions of carbon dioxide, the principle greenhouse gas, to neutralise, the CO2 emissions you have been responsible for, in driving your car for a year, or in taking long-haul (or indeed, short-haul) flights. These reductions can, theoretically, come from a number of sources: planting trees, which absorb carbon dioxide as they grow; renewable energy projects such as wind farms, which do not produce CO2 at all (and so would effect a great reduction if they replaced a coal-fired power station, for example); or installing energy-saving technology.

The beauty of the idea is that you don't have to find any of these projects yourself, or go through the tiresome business of checking whether they really are effecting reductions in greenhouse gases: a company does that for you. You just hand over a quickly calculated amount of cash to the firm concerned, often with a click of a mouse, and hey presto, your carbon emissions have been neutralised. As the late Tommy Cooper would have said: Just like that! Carry on driving and flying as much as you like, and feel you're not damaging the planet. You feel great, you feel worthy, your environmental sins have been absolved.

The simplicity of the act of offsetting and the instant moral benefits have sparked a huge offsetting boom (and a market worth enormous sums): many of the biggest British companies, keen to exhibit their corporate social responsibility, are now offseting as much of their activities as possible, with the aim of going "carbon-neutral". In August, BP launched its "targetneutral" offsetting website aimed at the general motorist, while more and more holidaymakers are choosing to offset their flights.

To cavil at such evidence of environmental responsibility might seem churlish, but increasingly, there are concerns about the emphasis on offsetting. The initial worry has been about the initial idea, planting trees, which has been used extensively by such offsetting firms as Britain's The CarbonNeutral Company (formerly Future Forests).

The difficulty is that it is not easy to calculate just how much CO2 a tree may absorb in the course of its lifetime (and remember that it is the lifetime that counts, although the flight you have just neutralised may have lasted only three hours). What happens if the tree concerned becomes diseased, or is logged, or is burnt in a forest fire? Then the CO2 it has absorbed will be released back into the atmosphere. As global warming takes hold in the 21st century, much forest may die back anyway - this has already been predicted for the Amazon in the 2040s - and trees may go from being a carbon "sink" (soaking the stuff up) to a carbon source.

Secondly, who polices the offsetting companies? Who makes sure they are doing what they say they are doing, and that their projects are worthwhile? Anyone can start one up, and there are increasing stories of cowboy behaviour. There is a well-regarded international benchmark for offsetting, known as The Gold Standard, but not all companies adhere to it (it inconveniently excludes planting trees).

Thirdly - and this is the principal objection - offsetting does nothing to change people's behaviour, in fact, it gives them a licence to carry on behaving as before. Why change the Chelsea tractor for something not quite so egregious in emissions terms, if you feel you've cancelled out its malign effects? But you haven't, really. The CO2 you are putting into the atmosphere has been lessened not a jot by the notional carbon reduction elsewhere that you may have paid for. You're still polluting just as much.

Behaviour will have to change if societies are to get to grips with cutting CO2. There is a value in offsetting, and that is that it starts people thinking about their own contribution to the enormous challenge which climate change presents. But it also can have the unfortunate effect of making people feel that dealing with the climate threat is a free lunch. And it's anything but that.