According to the World Economic Forum's Global Risks Report 2016, water crises are one of the three biggest challenges we face. A new study has backed up these findings, estimating that 4 billion people – two-thirds of the global population – are facing water scarcity.

This figure is bigger than previously thought. In the past, studies looked at water scarcity on an annual basis. They concluded that between 1.7 billion and 3.1 billion people were affected. This latest study instead examined water scarcity on a monthly level, which gives a more accurate reflection of just how severe the problem has become: "Water scarcity has become a global problem affecting us all," the authors point out.

Nearly half of the people suffering from water scarcity live in India and China; around the world, close to half a billion people face water scarcity throughout the year.

The study also focuses on other regions, such as parts of the US, Australia, Europe and the Middle East.

Figure 1: Quarterly averaged monthly blue water scarcity

Source: Science Advances

Already, 500 million people live in places where water consumption is double the amount replenished by rain for the whole year, creating very difficult living conditions as underground aquifers are drained. But according to the researchers, the problem will become even more acute as populations grow and water use continues to rise.

So what are we to do? According to the study, we need to make better – and fairer – use of what we already have. But it won't be easy, they admit: "It will be one of the most difficult and important challenges of this century."