Full marks to those who keep a tight rein on their carbon footprint, but don't relax just yet: water is the new carbon, and our engorged water footprints need to be scrutinised before the rivers really do run dry.

At the World Economic Forum in January, the UN secretary general, Ban Ki-moon, warned that water and food shortages would be the crises of 2008.

Last week we watched the escalating food crisis reverberate around the globe. Conflicts fuelled by water shortages may well be next, triggered by climate change, population growth and poor water management.

The phrase "water footprint" was coined to describe the embedded or "virtual" water in a food or industrial product – the real volume of water used to create that product. It is difficult to avoid using products which have not been involved in a water-intensive process somewhere along the line, and the figures are staggering: it takes 1,760 litres to get one pint of milk out of a cow and into your fridge; a kilogram of cheddar swallows up 5,000 litres.