However, short-term steps can be taken to start combating the problem, provided there is concerted international effort to implement policy effectively.

Climate change: Rising global temperatures and less reliable rainfall are key drivers in accelerating the looming water crisis. In particular, melting of glacial ice could be devastating, as almost 20 per cent of the world's population rely on glaciers and seasonal snow packs for water supply. Glaciers are in retreat in most parts of the world and once they melt there is no replacement.

Spiralling demand. While global warming will affect water supplies, much of the future shortage will be due to rising demands from a growing global population with increasingly affluent lifestyles and energy/water consumption.

· As countries like China and India grow more prosperous, their citizens are switching to more protein-rich Western diets. These are far more water-intensive -- it takes over 15,000 litres of water to produce a kilogram of industrial-produced beef; ten times that required to produce a kilogram of wheat.

· Energy generation, which is increasing rapidly in the developing world, requires significant amounts of water for cooling power plants. The United States already uses over 500 billion litres of fresh water per day for this purpose.

Short-term solutions. In the long term, cutting-edge technologies for water purification will be an essential component for tackling the looming crisis. In the short term, however, the key in food-insecure regions is to manage 'green water' -- moisture that infiltrates soil from rainfall and can be taken up into plant roots. Agricultural solutions for more effective 'green water' usage are low-tech -- harvesting rainwater, planting roots deeper, better terracing and switching from ploughing to tilling -- but the potential gains are significant.

Policy challenges. Nevertheless, even simple short-term solutions create policy challenges relating to who bears costs of changing agricultural and energy methods and who is responsible for educating farmers: · While countries such as Israel (where water is a precious resource) and the Netherlands (where there is overabundance of water) have national strategies to control the resource, the situation generally is poorly managed and lacks cohesion. · For example, in the United States over 20 federal agencies deal with water, from flood control to coastal commissions. Policy is rarely coordinated, making concerted solutions difficult. The size of the international crisis arising from lack of adequate clean water and sanitation means significant scientific and political work is needed to address such challenges -- particularly in developing countries. A broad approach would incorporate sustainable energy sources, and implement educational and capacity-building strategies. In the short term, better policy coordination and resource management could moderate demand significantly and improve availability, with improved water purification technology following over a longer time frame.