Move over Carbon. Make way for Water. It's not just carbon emissions that matter. The amount of water used to make things, especially food, is already a big issue and getting bigger. BBC has a neat illustration (without citing the source of the numbers) of water consumption for common household items.
It turns out that a cup of tea (250ml?) requires an amazing 30 litres of water when you add up the whole life cycle from growing the plant to consumption. But astounding as that is, coffee requires more than four times as much - 140 litres. Do you feel environmentally bloated?
Beer checks in at 150 litres on BBC but usually beer bottles contain more than 250 ml so one might argue that drinking beer is more environmentally friendly than coffee. The Waterfootprint.org number for beer is only 75 litres per 250 ml glass.
Of course, total environmental impact goes beyond one dimension and therein lies a serious problem for those who want or are willing to alter their consumption patterns but are wary of jumping onto the latest fad advice that on closer examination turns out to be poorly analyzed. The tea & coffee example seems to have some substance - this credible looking study at Waterfootprint.org corroborates the BBC numbers.
And what about the carbon footprint of tea and coffee? Is tea better there too? My googling was unsuccessful in finding numbers. This discussion on Earth.org.uk about trying to figure it out for tea alone suggests the answer isn't easy to determine.
Water has become such a valuable commodity in such short supply that countries with deficits are turning to renting land in other countries to grow their food, since agriculture usually loses out in competition with industry (see BBC's The pending scramble for water). Maybe the Chinese will want to rent out Saskatchewan?
Good sites: World Water Council, Waterfootprint.org