Pick up a packet of Walkers potato crisps in the UK and you might notice something unusual on the back. Alongside the normal information on how much fat, salt and calories they contain, there is another label. It informs the consumer that the bag contains only 34.5g of crisps, but 75g of carbon dioxide.

That carbon dioxide is not to be found within the packet. It is "virtual" CO2 - a calculation of the amount of the gas expelled into the atmosphere as a result of the processes that went into the packet's production, from growing the potato to packaging and distributing the finished crisps.