Water.

[lonestarnot]

http://www.washingtonpost.com/blogs/wonkblog/wp/2015/06/16/new-nasa-studies-show-how-the-world-is-running-out-of-water/

New NASA data shows how the world is running out of water

The aquifers under the most stress are in poor, densely populated regions, such as northwest India, Pakistan and North Africa, where alternatives are limited and water shortages could quickly lead to instability.