More than four billion people - approximately two-thirds of the world's population - face severe water scarcity for at least one month out of the year.  And that's a lot more than previous estimates of between 1.7 billion and 3.1 billion.

Population growth, intense irrigated agriculture, and climate change all contribute to growing water scarcity, according to a new study published in the journal Science Advances.  The most vulnerable regions are in Australia, the western United States, as well as the Arabian Peninsula - but also in India and China, home of half of the four billion people who experience conditions of severe water scarcity at least one month of the year.

Managing Earth's freshwater resources while keeping up with the demand of a growing population "will be one of the most difficult and important challenges of this century," write the authors, Professors Mesfin M. Mekonnen and Arjen Y. Hoekstra who specialize in water management at the University of Twente in the Netherlands.  

Severe water scarcity happens when an area's farms, industries, and households consume double the amount of water available in that area.  It can lead to crop failure and low crop yields, which could cause food price increases as well as famine and widespread starvation.

"That means that groundwater levels are falling, lakes are drying up, less water is flowing in rivers, and water supplies for industry and farmers are threatened," explains Dr. Hoekstra.