What we found

Emissions offset

There were big differences in the amount of carbon emissions that the schemes claimed to offset (see Schemes compared). Qantas calculated 331kg of emissions would be offset; Air New Zealand worked it out as 433.9kg; the carboNZero programme (run by Landcare Research) had a figure of 958.09kg.

One reason the carboNZero programme's estimate is more than twice the airlines' figures is because it uses a "multiplier" to take account of the higher global-warming potential of air travel. The airlines don't use a multiplier at all.

Programme manager Ann Smith says there's debate about the size of the multiplier that should be used. The Intergovernmental Panel on Climate Change recommends a multiplier of 2.7; but the carboNZero programme has opted for a multiplier of 1.9 (advocated by the UK Department for the Environment, Food and Rural Affairs).

Costs

We also found wide variation in prices. Qantas had the cheapest offset of the three airlines ($3.87), followed by Pacific Blue ($9.54) and then Air New Zealand ($14.80).

The most expensive was carbon.org.nz's offset ($156). This $156 price tag applies to any international travel - regardless of whether you fly from Auckland to Brisbane or from Auckland to Barcelona. And carbon.org.nz doesn't use an emissions calculator, so you don't know exactly what quantity of emissions you're offsetting. All the website tells you is that four trees will be planted on your behalf. That's not enough detail to make an informed choice before you buy.

Carbon.org.nz and two other schemes also include administration costs in the offset price (though information on how these costs are calculated is sketchy). This means not all of what you pay goes directly towards offset projects.

The airlines say no administration costs are charged. But Pacific Blue does include a "small" brokerage fee and also adds a 1.2 percent credit-card fee to your purchase (and this is on top of the $4 fee the airline charges when you book an international flight by credit card).

Projects funded

Five of the six schemes purchase offsets from projects that have been subject to some kind of independent assessment. These projects mostly involve renewable-energy or energy-efficiency initiatives (see Schemes compared).

The exception was carbon.org.nz, which uses revenue for tree planting on private land. Carbon.org.nz is registered to a web-design company. One of the company's directors is Olmec Sinclair whose family also owns the North Canterbury property where the trees are planted. He says over 600 trees (mostly oaks) have been planted so far.

What happens to the trees? There are no rules governing this kind of private offset venture. Carbon.org.nz's trees, for example, won't be there forever. The intention is to log the trees and use the wood for "community housing", though replanting is planned. Olmec Sinclair may be well-meaning, but there's no independent assessment of the scheme and it's very difficult to know just what you're getting for your money - if anything.

Landcare Research ecologist Larry Burrows is highly sceptical about the merits of this kind of scheme. He says the amount of carbon a small tree absorbs is tiny and it's not credible to sell credits from many years in the future for a flight made now. If the trees are harvested, then the scheme needs "to show how it will retain the offsets it has sold". You'd also have to factor in the carbon emissions from soil disturbance and the machinery used to log and mill the trees. Once these are included, Larry Burrows says, there may actually be a loss of carbon.