Why do so many people talk about carbon footprints, when what they mean is energy efficiency and power consumption?

Indeed, as 3Com rep Fiona put it during a recent conversation on energy efficient networking, which headed off at a tangent: "When did 'Carbon' become like 'the Hoover' for greens?"

If your aim were merely to reduce your carbon footprint, you could take the route of our friends across the Channel. In France, you could (in theory, at least) run the most bloated, inefficient electrical gear you like and still have one-fifth the carbon footprint of a UK company doing the same, because some 80 percent of France's power is generated by zero-carbon nuclear power stations.

OK, so that ignores the energy used - and carbon dioxide emitted - in building, maintaining and decommissioning the power stations, and in dealing with the spent fuel and radioactive waste.

But surely it's not just about greenhouse gases - we should be trying to use less power, and use it more efficiently anyway, never mind its source, right? If less of that power comes with accompanying carbon dioxide emissions, so much the better.

Or maybe the second law of thermodynamics doesn't apply once Carbon enters the frame...