Humanity is facing "water bankruptcy" as a result of a crisis even greater than the financial meltdown now destabilising the global economy, two authoritative new reports show. They add that it is already beginning to take effect, and there will be no way of bailing the earth out of water scarcity.

The two reports – one by the world's foremost international economic forum and the other by 24 United Nations agencies – presage the opening tomorrow of the most important conference on the looming crisis for three years. The World Water Forum, which will be attended by 20,000 people in Istanbul, will hear stark warnings of how half the world's population will be affected by water shortages in just 20 years' time, with millions dying and increasing conflicts over dwindling resources.

A report by the World Economic Forum, which runs the annual Davos meetings of the international business and financial elite, says that lack of water, will "soon tear into various parts of the global economic system" and "start to emerge as a headline geopolitical issue".

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It adds: "The financial crisis gives us a stark warning of what can happen if known economic risks are left to fester. We are living in a water 'bubble' as unsustainable and fragile as that which precipitated the collapse in world financial markets. We are now on the verge of bankruptcy in many places with no way of paying the debt back."

The Earth – a blue-green oasis in the limitless black desert of space – has a finite stock of water. There is precisely the same amount of it on the planet as there was in the age of the dinosaurs, and the world's population of more than 6.7 billion people has to share the same quantity as the 300 million global inhabitants of Roman times.

Water use has been growing far faster than the number of people. During the 20th century the world population increased fourfold, but the amount of freshwater that it used increased nine times over. Already 2.8 billion people live in areas of high water stress, the report calculates, and this will rise to 3.9 billion – more than half the expected population of the world – by 2030. By that time, water scarcity could cut world harvests by 30 per cent – equivalent to all the grain grown in the US and India – even as human numbers and appetites increase.

Some 60 per cent of China's 669 cities are already short of water. The huge Yellow River is now left with only 10 per cent of its natural flow, sometimes failing to reach the sea altogether. And the glaciers of the Himalayas, which act as gigantic water banks supplying two billion people in Asia, are melting ever faster as global warming accelerates. Meanwhile devastating droughts are crippling Australia and Texas.

The World Water Development Report, compiled by 24 UN agencies under the auspices of Unesco, adds that shortages are already beginning to constrain economic growth in areas as diverse and California, China, Australia, India and Indonesia. The report, which will be published tomorrow, also expects water conflicts to break out in the Middle East, Haiti, Sri Lanka, Colombia and other countries.

"Conflicts about water can occur at all scales," it warns. "Hydrological shocks" brought about by climate change are likely to "increase the risk of major national and international security threats".