It takes a lot of water to make a cow.
It takes other stuff, too, but scientists issuing warnings about our global diet are specifically concerned about the water. By 2050, there are likely to be about 9 billion of us humans, and if we're going to avoid catastrophic water shortages, a report from the Stockholm International Water Institute (hosts of this week's Stockholm World Water Week) says we might have to eat a lot less meat:
The analysis showed that there will not be enough water available on current croplands to produce food for the expected population in 2050 if we follow current trends and changes towards diets common in Western nations (3,000 kcal produced per capita, including 20 per cent of calories produced coming from animal proteins). There will, however, be just enough water, if the proportion of animal based foods is limited to 5 per cent of total calories and considerable regional water deficits can be met by a well organised and reliable system of food trade.
For an in-depth investigation of the global population boom and its environmental impact—the largest generation in history is just now entering its childbearing years—spend some time reading and watching the Los Angeles Times' Beyond 7 Billion project.