Despite this litany of problems, the world's water managers and experts did not initially give them very high priority. In 2003, the U.N. World Water Development Report concluded that "climate change will account for about 20 percent of the increase in global water scarcity."

Anyone familiar with the 80/20 rule of management will get the message: Tackling climate change was not top of the water managers' "to-do lists." The reason was that they faced many more immediate problems. 

In much of the world, water use increases at faster-than-sustainable levels, driven by population growth and economic development. Meanwhile, water pollution limits the usability of what we have. 

Where water is taken direct from rivers, shortages quickly become evident as downstream users complain that they no longer have enough. But where water is drawn from underground, users may not realize that they are drawing down their water "bank balance." From the western United States to India and the Middle East, water tables are falling far faster than they can be recharged.

Urbanization is another threat, and not just because residents of rapidly growing cities want more water for their households. Often, particularly in developing countries, urban wastes pollute water resources at the expense of downstream users who must either spend large amounts of money to treat it, find other sources, or risk using it.

Economic growth brings further challenges. Prosperity changes diets, requiring both more food, and more water to produce it. And new technologies generate new kinds of pollution. 

Water managers have reconsidered their priorities. One reason was that some actions to mitigate climate change were themselves threatening the water cycle and water users. 

The expansion of biofuel production was a good example. Already water experts had identified finding enough "water for food" as a key 21st century challenge. Now, suddenly, food would have to compete with energy crops for scarce water.

Hydropower similarly offers real benefits but increases competition for water resources. As an invaluable source of renewable energy, channeling water through turbines and then back to the sea can help in the fight against climate change. 

But uncertain river flows threaten hydropower's reliability even as the premium prices renewable energy commands diverts limited water resources away from other uses.

But the most serious challenge is the loss of climate predictability. Water managers can no longer use the past to forecast the future. This is crucial, because water management historically requires long-term decisions about major infrastructure typically built for a life of 50 or even 100 years. 

Past designs assumed that climates were constant, that storms would occur with similar frequencies and intensities and cause the same kinds of floods. Droughts were projected to follow similar patterns. 

All sorts of infrastructure - from large dams, hydropower plants, and water supply systems to airport runways and the gutters in city streets -were designed on these assumptions.

The IPCC has predicted with high confidence that "many semi-arid and arid areas (e.g., the Mediterranean basin, western USA, southern Africa and northeastern Brazil) ... are projected to suffer a decrease of water resources due to climate change."

Maarten de Wit and Jacek Stankiewicz, scientists at the Africa Earth Observatory Network in Cape Town, explain that if the expected rainfall reductions occur, perennial rivers will stop flowing for part of the year across 25 percent of Africa.

v UN-Water, a consortium of 26 international agencies involved with water issues, recently concluded that "... already, water-related climate change impacts are being experienced in the form of more severe and more frequent droughts and floods."   

Water resources and water use differ greatly between and even within countries.  No single quick-fix approach can help each adapt to the future. However, an encouraging pattern of local and regional innovation is emerging.

v Industries from food and beverages to power and mining are finding ways to reduce water use and vulnerability to climate change. South Africa's new thermal power stations are dry cooled, using only 0.1 litres per kilowatt-hour of electricity, compared with 1.9 litres for wet-cooled stations.

The emerging consensus among water managers and experts is that while we cannot predict today the effects climate change will have over the next century, we are sure that many will flow through the water cycle. So water must be better managed and water systems made more resilient.

The Global Water Partnership, an international network of people and organizations working to improve water resource management sums it up: "Better water management today will build a more resilient world tomorrow. Today's investments in water security should be seen as an explicit part of a longer-term strategy for adaptation."

Mike Muller co-chairs the UN Water World Water Assessment Programme's Expert Group on Indicators, Monitoring and Databases and is a member of the Global Water Partnership's Technical Advisory Committee.

(This is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://www.america.gov)