The rivalry between Tesco and Wal-Mart is well known. Tesco's imminent entry to the US heartland of the world's largest retailer may have created an extra edge to the battle. And, unsurprisingly, the two giants are squaring up over carbon issues as well as over such things as employee conditions and global sourcing policies. Tesco said earlier this year that it would eventually put carbon labels on all its 70,000 food lines. It has been trying to find way of doing this using Life Cycle Analysis, putting a greenhouse gas cost on every element of a product's move from farm to plate. This was always a hugely over-ambitious project and recent weeks have seen the company drift back from its early optimism. Now Wal-Mart has come up with a similarly impossible dream – to use the Carbon Disclosure Project (CDP) to assess and manage the energy footprint of its suppliers. These big retailers know that they have to be seen to be doing something about greenhouse gases, so they have both launched incomplete schemes that will achieve little.