About four billion people experience severe water shortages for at least one month a year, and around 1.6 billion — almost a quarter of the world's population — have problems accessing a clean, safe water supply, according to the United Nations. While the UN's sustainable development goals call for water and sanitation for all by 2030, the international organization says scarcity is increasing and more than half the people on Earth will be living in water-stressed regions by 2050.