KEY POINTS:
Buy a packet of Walkers cheese and onion crisps in Britain and you're munching on a carbon footprint weighing 75g. Wear a pair of Timberland's Greenscapes Mountain sneakers in the United States and you can leave a 3.5 green index in your real footprint.
Quaff a 2006 Sanctuary pinot gris in New Zealand and you'll be happy in the knowledge it's a CarboNZero wine.
Save the carbon, save the planet - by shopping. That's what customers are being encouraged to do through an increasing number of carbon labels appearing on products.
As far-fetched as it sounds, it's selling our products over there.
This is just the beginning. British supermarket chain Tesco plans to spend £500 million ($1.3 billion) over the next five years to lead "a revolution in green consumption"
Marks & Spencer has similar goals - a £200m eco-plan which includes becoming carbon neutral within five years.
Earlier this year both supermarkets introduced labels with an aeroplane logo on some foods to show they are imported by air.
And that's not all. The British Government-funded Carbon Trust is under way with a grand scheme that could see every product or service carrying a label showing its carbon footprint.
Our Government is also looking at ways its departments can procure eco-friendly products in line with its climate-change policy. And Green Tick, a New Zealand certification organisation, is ready for the coming trend with a climate-friendly addition to its eco-label line-up. To look behind the labels and the label-making process is to find a tangle of political and business ideologies.
Some see carbon labelling as a genuine attempt to save the planet from global warming. Others say it's a calculated marketing ploy to sell more, possibly at a premium. Then there's the thought that the labels are trade protectionism in an environmental protection disguise.
But sooner or later all arguments come face to face with an inescapable truth - mathematics.
"You can't manage what you don't measure," says Andrew Barber of AgriLink NZ, an agricultural engineering consultancy. Barber measures carbon emissions for a living, using complex calculations and analysis to determine the environmental impacts of products over their lifecycle.
Called life-cycle assessment, such research attempts to provide a "cradle to grave" view of all the inputs and outputs of a product - from the raw materials used until it's finally disposed of.
The research has played an essential part in countering the more simplistic food-miles measurements - showing that many goods, even if they are shipped from the other side of the world (New Zealand), can have less carbon dioxide emissions than the same foodstuffs produced in Britain. But deciding how much of the lifecycle to measure and how to do it is problematic. "It's fraught with assumptions and rules that make it complicated," says Barber. "You can hide a lot behind your approach. There needs to be a standardised methodology."
Although much of the work is governed by the International Organisation for Standardisation (ISO), there is plenty of room for movement within the definitions.
What is needed, says Barber, is more transparency in the numbers and computer models employed.
The British Carbon Trust trials show how difficult measuring carbon can be. So far they have looked at the carbon "embedded" in a product by the time it reaches the shelves in the shop.
In the Walkers cheese and onion crisps example, the 75g of carbon in a packet includes all the carbon dioxide emitted while growing the potatoes and making the vegetable oil - including that from pesticides, fertilisers, tractor fuel - as well as manufacturing, packaging and transporting, plus the disposal of the packet.
But it doesn't cover the carbon dioxide generated by the shop that sells the crisps (electricity and transport), nor the emissions from the consumption of the crisps, such as those created by buyers driving to and from the supermarket.
Similarly, while the New Zealand Wine Company can account for all its emissions in making of its Sanctuary and Grove Mill wines - from the growing of the grapes through fermentation, bottling and shipping - its measurements stop at the British port and don't include transport to outlets. And while the winemaker counts its business-related air-travel emissions, it has yet to include those of contract grape-growers.
Then there's the thorny issue of greenhouse gas emissions other than carbon dioxide, such as methane and nitrous oxide which are particularly relevant to animal produce such as wool, milk and meat.
Studies show that New Zealand primary produce is farmed efficiently and can foot it with European producers in terms of carbon dioxide emissions. But there is yet to be agreement on how to include the other greenhouse gases in the calculation. Debate abounds, too, on how far assessments should go. Cradle to grave, cradle to gate, cradle to plate or cradle to supermarket trolley?
At this point the bewildered consumer seems justified in asking whether life-cycle assessment is a fool's errand, an impossible task.
"It almost seems that way," says Barber. "Because behind the numbers there is so much people don't see and there is so much debate about what those numbers should be."
He suggests a return to the first principles. "It's about resource and energy use, a holistic view, not just transport inputs. It's about finding where the best efficiency can be achieved in a production system."
There's debate also about what to show on the label that is meaningful for a buyer. Rather than show a number like 78g, Barber suggests what needs to be conveyed is improvement. "To keep your certification you should have to prove you are reducing your carbon footprint, say using a five-year rolling average."
But while carbon labels struggle for meaning, the most vexing aspect for a consumer to understand is just how a "carbon neutral" rating is achieved.
Invariably it's through buying offsets - shares in a carbon sink, such as a forest which absorbs carbon dioxide, or some other emissions mitigation programme.
Carbon neutrality is an accounting ledger where bad emissions are cancelled out by trees and good deeds. Offsetting is the most inconvenient truth behind the label.
Just ask Rob White, chief executive of the New Zealand Wine Company which, after an extensive project to reduce energy use and become more efficient, still produced 312 tonnes of carbon dioxide emissions last year.
"You can never get to a zero-carbon footprint, that's impossible," White says. "So the final step is mitigation."
For his company that meant buying carbon credits, at $15 a tonne, in about 100ha of regenerating native forest. They were bought from Landcare, a crown research institute. This year, the company will buy a similar amount, but the market rate is now $25 a tonne.
Landcare Research administers the carbon credit scheme and pays landowners $15 a tonne of carbon dioxide sequestered in land set aside for reforestation.
Although the Landcare scheme appears reputable and is subject to monitoring and auditing, other offset schemes - involving everything from planting trees in Uganda to buying low-energy lightbulbs for distribution in developing countries - don't fare so well.
Planting trees, for example, to offset air travel emissions will work, but it takes about 100 years for the tree to grow enough to recapture the carbon emitted by a flight. Offsets that invest in renewable energy, such as windfarms, have problems too.
"New Zealand windfarms haven't reduced the amount of gas and coal that we burn to generate power by one iota," says University of Canterbury mechanical engineering lecturer Susan Krumdieck. "It actually requires fossil fuel consumption in the long run," she says, pointing out that for every 600 megawatts of wind generation power, engineers know they have to have 600 megawatts of thermal power generation from gas. That's because of the continuously increasing demand for electricity coupled with the thermodynamic reality of running thermal power stations which are not easily turned off when the wind is blowing.
It's easy to see why many say the logic of offsetting is fundamentally flawed. Rather than reducing emissions, offsets simply defer the problem. Carbon trading is an economically expedient game to salve guilty consciences.
Ann Smith of Landcare Research agrees that offset schemes that aren't tied to emission reductions are little more than throwing money at problem to balance the books.
"If you measure and then offset, that's the ambulance at the bottom of the cliff. But if you measure, reduce, then offset, the offsetting is the icing on the cake."
To counter "greenwash" - including the forward buying of carbon credits in schemes which fail or don't materialise - Smith says there must be science and third party auditing on both sides of the carbon balance sheet.
But is there another way? Krumdieck believes there is - a more straightforward measurement of the fossil fuel content of products.
She says a carbon footprint tells you what happened as a consequence of fossil fuel use, but not how to fix it. "What you really need to know is how much fuel you're using and what you can do to use less."
Krumdieck advocates a label that uses megajoules to show the fossil fuel energy used in making a product, in a manner similar to the calorie content information shown on food labels. She says it's a relatively uncomplicated energy audit calculation. "Fossil fuels are directly and transparently accountable as they already incur tax and are part of the cost of business."
Such a measure, Krumdieck says, would allow consumers to make informed choices - such as deciding to buy products that use less fossil fuel in their making.
It would also bring consumers face to face with the reality of global warming.
"If we're really going to start doing something about this, we're going to have to start dealing with our consumption,"Krumdieck says.
But as elegant as fossil fuel accounting sounds, it gets more complicated when choosing between products such as lightbulbs.
A carbon label covering manufacture would show that energy-efficient lightbulbs have a worse carbon footprint and fossil fuel content than ordinary lightbulbs.
That gets cancelled out by a label that takes use into account - because energy-saving bulbs last longer and use about a fifth of the power of incandescent bulbs.
But the footprint changes again when including how to get rid of the product at the end of its life. That's because energy-saving bulbs contain a small amount of mercury, which is toxic to health and which bioaccumulates in the ecosystem, making it more costly to dispose of. Buyer beware - but of which bulb?
Lightbulbs aren't the only obstacles in the green consumer's shopping path. Where "air-freighted" once meant fresh or up to date, it now means harbinger of global warming.
The word "sustainable" on eco-labels like Green Tick also requires redefinition. Products such as Ecostore's household cleaners have been independently certified as "contributing to improvements in environmental quality due to their low toxicity, low environmental impact, and sourcing from renewable resources".
But they don't yet have a "climate friendly" Green Tick for their greenhouse gas emissions.
That seems a contradiction in terms. To be sustainable, surely you have to be climate-friendly.
Meanwhile, the New Zealand Wine Company is finding out just how well carbon labelling works - with an expected doubling of sales in Britain, not just with existing customer Sainsbury's taking more, but from Tesco. "We weren't in their stores," says chief executive Rob White, "but on the back of the CarboNZero programme they've now listed two of our products."
