Previous studies estimated that between 1.7 billion and 3.1 billion people are affected by severe water scarcity, but those findings were based on annual data that failed to capture variations in consumption and supply throughout the year. In the paper published today, researchers used monthly data on freshwater consumption and supply to evaluate water scarcity from 1996 to 2005, accounting for agriculture, municipal use, and environmental needs. Of the 4 billion people facing severe water scarcity at least one month a year 130 million live in the US — mostly in California, Texas, and Florida. Other affected countries include Bangladesh (130 million people), Pakistan (120 million), and Nigeria (110 million).

"Water scarcity has become a global problem affecting us all," says Arjen Hoekstra, professor of water management at the University of Twente in the Netherlands, and co-author of the study. "We’re all connected to it."

Demand for water has increased in parallel with the world's population, with agriculture accounting for the majority of global consumption. Climate change is expected to exacerbate the crisis in coming years, experts have warned, contributing to severe droughts and water shortages in some parts of the world.

The paper's authors recommend implementing caps on water consumption and improving the efficiency of water use. Hoeksra believes labeling products according to their water use could raise awareness around the issue, as well. At last month's World Economic Forum, UN Secretary General Ban-Ki Moon announced the formation of an emergency panel charged with developing solutions to mitigate water scarcity, but Hoeksra says more concrete actions are needed.

"We have seen, over the past five years, increasing interest, and we expect it to continue over the coming five years," Hoeksra says. "But of course, we would also hope that something is going to happen at the global level... In terms of actual action, we don’t see so much yet."