What is carbon offsetting?

You like to think you do your bit for the environment - you recycle, walk or catch the bus to work and run an energy-efficient home. But that overseas flight you've booked is threatening to undo all your efforts. One return flight to the UK could cause the equivalent carbon dioxide emissions per person of driving from Cape Reinga to Invercargill 20 times.

For a few extra dollars, you could buy some "carbon offsets". The idea is that you can cancel out the emissions from your flight by paying to fund environmental projects - such as a wind farm or a tree-planting scheme.

Buyer beware

There's little regulation and no single defined standard for offset schemes. A recent UK review by The Carbon Rating Agency concluded the majority are likely to be delivering 30 percent less than they promise.

Worst-case scenario: you could be handing over money for nothing. Critics say there's little to stop a company claiming to be running a scheme that doesn't exist. Even if the scheme is legitimate, there are problems in calculating how much carbon is offset.

Take a tree-planting scheme. Trees take a long time to soak up emissions: the carbon from a flight taken today could take up to 100 years to be absorbed. Exactly how much carbon is taken up will depend on factors like tree species, soil and climate. And there's no guarantee the trees will survive - a fire could effectively wipe out any claimed offsets.

Offset projects also have to be in addition to "business as usual". They can't be projects that would have happened anyway.

Does it work?

Carbon offsetting has its critics. International watchdog group Carbon Trade Watch has panned offsetting for placing too much emphasis on individual lifestyles and distracting attention from the "collective political action that needs to be taken to tackle climate change".

Even those involved in the industry agree offsetting should be something you do after you've done as much as you can to reduce your carbon emissions. Melissa Munro, director of New Zealand company Offset the Rest, says offsetting is not a solution to climate change. But she believes it can contribute towards a solution by promoting the flow of funds to renewable-energy and energy-efficiency projects.

Carbon certification

There are about a dozen certification standards for voluntary offset projects. The two most well known are the Gold Standard and the Voluntary Carbon Standard (VCS).

The Gold Standard is regarded as the most demanding. Only renewable-energy and energy-efficiency projects can qualify for Gold Standard certification. Tree planting is ruled out because the scheme wants to promote a shift away from fossil fuels towards renewable-energy sources. Gold Standard certification has been awarded to an estimated nine percent of voluntary offset projects.

Less rigorous, the VCS offers certification for renewable-energy, energy-efficiency and forestry projects. Around 29 percent of voluntary offset projects have VCS certification.