At least two-thirds of the global population, over 4 billion people, live with severe water scarcity for at least one month every year, according to a major new analysis.

The revelation shows water shortages, one of the most dangerous challenges the world faces, is far worse previously than thought.



The new research also reveals that 500m people live in places where water consumption is double the amount replenished by rain for the entire year, leaving them extremely vulnerable as underground aquifers run down.



Many of those living with fragile water resources are in India and China, but other regions highlighted are the central and western US, Australia and even the city of London.



Water scarcity map. Photograph: Mekonnen et al

These water problems are set to worsen, according to the researchers, as population growth and increasing water use – particularly through eating meat – continues to rise.

In January, water crises were rated as one of three greatest risks of harm to people and economies in the next decade by the World Economic Forum, alongside climate change and mass migration. In places, such as Syria, the three risks come together: a recent study found that climate change made the severe 2007-2010 drought much more likely and the drought led to mass migration of farming families into cities.

“If you look at environmental problems, [water scarcity] is certainly the top problem,” said Prof Arjen Hoekstra, at the University of Twente in the Netherlands and who led the new research. “One place where it is very, very acute is in Yemen.”

Yemen could run out of water within a few years, but many other places are living on borrowed time as aquifers are continuously depleted, including Pakistan, Iran, Mexico, and Saudi Arabia.

Hoekstra also highlights the Murray-Darling basin in Australia and the midwest of the US. “There you have the huge Ogallala acquifer, which is being depleted.” He said even rich cities like London in the UK were living unsustainably: “You don’t have the water in the surrounding area to sustain the water flows” to London in the long term.

The new study, published in the journal Science Advances on Friday, is the first to examine global water scarcity on a monthly basis and at a resolution of 31 miles or less. It analysed data from 1996-2005 and found severe water scarcity – defined as water use being more than twice the amount being replenished – affected 4 billion people for at least one month a year.

“The results imply the global water situation is much worse than suggested by previous studies, which estimated such scarcity impacts between 1.7 billion and 3.1 billion people,” the researchers concluded. The new work also showed 1.8 billion people suffer severe water scarcity for at least half of every year.

Farming is the biggest user of water and the growing global population requires more food. Furthermore, changing diets are having a major impact, as people with rising incomes eat more meat.

“Taking a shorter shower is not the answer” to the global problem, said Hoekstra, because just 1-4% of a person’s water footprint is in the home, while 25% is via meat consumption. It takes over 15,000 litres of water to make 1kg of beef, with almost all of that used to irrigate the crops fed to the cattle.

Another unique aspect of the new research was that it included environmental water requirements, ie the water needed to ensure that life survives in the rivers and lakes. Fish can be important sources of food for people, who also use waterways for transport.

Even just one month of severe water scarcity can have a devastating impact on the health of a river, said Hoekstra: “An empty river is not a river.” Rivers that run dry before they reach the end of their course, or come close to doing so, include the Colorado river in the western US and the Yellow river in China.

David Tickner, chief freshwater adviser at WWF-UK, said: “This paper is another pointer to the urgency of this challenge. Billions of people, and many economies, are increasingly suffering because of water-related risks which could be better managed. The same risks are causing a collapse in aquatic wildlife around the globe.”

Hoekstra said caps on water use should be put in place for all river basins, companies should be transparent about how much water is needed to make their products and look to reduce it while investors should incorporate water sustainability into their decision-making.