Just as wars over oil played a major role in 20th-century history, a new book makes a convincing case that many 21st century conflicts will be fought over water. China, Egypt and Pakistan are just a few countries facing critical water issues in the 21st century.

In Water: The Epic Struggle for Wealth, Power and Civilization, journalist Steven Solomon argues that water is surpassing oil as the world's scarcest critical resource. "We've now reached the limit where that trajectory can no longer continue," Solomon tells NPR's Mary Louise Kelly (listen to the audio below).

"Suddenly we're going to have to find a way to use the existing water resources in a far, far more productive manner than we ever did before, because there's simply not enough."

While Solomon points out many of the hurdles that we must cross to help this issue, he point out that The United States has made significant progress in curbing its water use, thanks to market forces and legislation such as the Clean Water Act.

Click here to read the NPR interview and book excerpt.