This story is part of Headwaters, a series on the future of our most critical resource
Conflict over scarce water resources is as old as human history, but in the 21st century, rapidly rising demand for water and threats to supply pose nightmarish risks for people around the globe.
The U.S. Defence Department, United Nations' agencies and the business-dominated World Economic Forum all point to the proliferation of water crises as a major threat to human security and economic well-being next year and in the years to come.
For many leaders gathering for the climate-change summit in Paris on Monday, global warming is a "clear and present danger" that will exacerbate existing water-related stresses and undermine efforts to lift the world's poorest people out of poverty.
Desertification in sub-Saharan Africa and drought in the Middle East have contributed to conflict and a flood of refugees. Extreme storms and rising water levels threaten to inundate low-lying states and coastlines from the South Pacific islands to heavily populated countries such as Bangladesh. United Nations scientists warn that drought and climate-related natural disasters will cause major crop losses and reduced yields for farmers in developing countries.
Negotiators in Paris will grapple with Third World demands for rich countries to provide billions of dollars to help them adapt to climate change.
But for the people who face the most devastating effects, the obvious strategy – although heart-wrenching and dangerous – will be to leave their homelands.
"The cause of migration is incredibly complex," said Michael Werz, a senior fellow with the Center for American Progress, a Washington-based think tank. "But it is clear that deteriorating environmental conditions contribute to decisions to migrate. It's an ancient human adaptive mechanism."
Europe is already reeling from the influx of hundreds of thousands of refugees from the Middle East and Africa, while many Asian countries also have growing migrant populations. But the number of displaced people is expected to soar in the coming decades due to conflicts fuelled by water scarcity, natural disasters related to climate change and the loss of livelihood from drought-stricken crops, Mr. Werz said.
Drought also fuels conflict. And climate scientists say some of the most arid regions in the middle latitudes will get considerably drier as climate change disrupts weather patterns.
Peter Gleick at San Francisco's Pacific Institute has chronicled water-related conflict, starting with the border war between two Sumerian cities in ancient Mesopotamia 4,500 years ago, when the king of Lagash attacked the city of Umma to gain land and water.
Included on Mr. Gleick's list is the current war in Syria, which has caused the death of more than 220,000 civilians and a flood of refugees seeking to resettle in Canada and elsewhere.
In the years before Syria erupted in civil war, a searing drought withered 75 per cent of the crop in northern regions of the country and forced an estimated 50,000 farm families to flee to the cities, where they found little relief.
The displaced farmers were among the disgruntled Syrian people who demanded democratic reform in the country in 2011, sparking a crackdown by President Bashar al-Assad that ignited the ferocious conflict that has engulfed the country and gave rise to the brutal Islamic State faction.
Mr. Gleick is not claiming drought caused the civil war, but he said it contributed to the conflict, and added that climate change was clearly a factor in the extended dry spell.
Scientists and UN officials worry that climate change will simply compound existing water crises across the world.
Some 750 million people lack access to even minimally treated drinking water, while billions rely on unsafe sources, according to the United Nations water agency. Nearly 1.2 billion people – almost one-fifth of the world's population – live in areas where water is now scarce, and many of those regions are likely to become even more arid with the onslaught of climate change.
Meanwhile, rapidly melting glaciers in the Himalayas and areas of western North America threaten key sources of water for major population centres.
At the same time, demand for water is soaring with population growth, industrialization and rising incomes that increase consumption of water-intensive food such as meat and fresh fruit. If current trends continue, the world will be demanding 40 per cent more water than will be available by 2030, the UN agency forecast in a recent report.
"Water is really embedded at the core of socioeconomic growth," said Paul Reig, associate at the Washington-based World Resources Institute, which has created a database of water stress in regions across the globe.
"When you have industrialization and increasing standards of living, that is associated with higher demands for natural resources, including water."
But strategies exist to reduce the threat of water shortages.
The impact of the drought in Syria was worsened by the regime's failure to invest in modern irrigation, its hostility toward the rural, northern population, and the history of conflict between ethnic and religious factions.
Governments and water experts are beginning to examine the complex nature of "virtual water trade" to determine the impact of industrial and agricultural imports and exports.
Farmers – who account for 75 per cent of global water use – will have to become more efficient.
California, for example, is a net water importer.
Mr. Gleick said the state needs to rethink whether it makes sense to produce low-value, water-intensive commodities such as almonds.
"We've taken water for granted far too long, and we can't do that any more," he said.