You may have missed this story but the Stockholm (Sweden) International Water Institute last year (August 2012) said "there will not be enough water available on current croplands to produce food for the expected 9 billion population in 2050 if we follow current trends and changes toward diets common in western nations." The report said that humans would have to cut from getting 20% of their daily protein from animal-based products to 5% by 2050 to accomodate water deficits.
The scientists speculated that a shift toward vegetarian diets could help free up large portions of arable land to human food production. Statistics show that a whole years worth of showers use about 5,200 gallons of water while it takes 5,214 gallons of water just to produce a single pound of beef.
If this were to happen - it would be a dramatic shift in dietary habits for most people - but (as the research discussed in the book The China Study and the documentary Forks Over Knives proves) average lifepans would likely dramatically increase and the occurrence of major diseases such as Diabetes, Heart Disease and even Cancer would dramatically decrease. Obesity percentages would plummet.