Water sustainability could be the greatest test that urban environments face. How cities respond to the dual challenges of providing protection from the elements and ensuring a freshwater supply will shape urban development in the foreseeable future.

In the past, the surest way an army could bring a besieged city to its knees was to cut off its water supply. Today, global warming is delivering a similar effect. It brings an increased frequency of violent storms and searing droughts that assail population centres, threatening their sustainability.

In the past few years, cities such as Atlanta in the USA and Barcelona in Spain have come within 90 days of running out of water, while floods have ransacked places like Brisbane, Rio de Janeiro and Jeddah. In some places, droughts and floods come almost sequentially. In 2011, China experienced a record-breaking drought that was ended by violent storms that left 178 dead and caused economic losses of US$3.3 billion.

These phenomena are linked to global warming through the precipitation-evaporation cycle. This requires and releases huge amounts of energy to vaporise and condense water. "It works like a giant steam furnace to redistribute heat from the equator, where it's hotter, to the poles, where it's colder", explains Professor Jay Famiglietti of the University of California's Centre for Hydrologic Modelling: "The need to redistribute more heat in a warming climate is driving increasingly intense and prolonged El Nino weather systems, changes in flood and drought magnitude and duration, and the increasing numbers of hurricanes and typhoons. Due to the Earth's increasingly intense water cycle, 'the steam furnace' must work harder and harder, and is doing so in variable bursts in space and time rather than as a steady global increase."

Climate change has become the one truly global challenge facing the world today. Singapore taps into the ideas of leading leaders in diverse fields to look at innovative solutions to how we can tackle the imminent global water crisis.

The need for innovation is acute. Famiglietti has been looking at the minute changes in Earth's gravitational pull that occur as a result of changes in the distribution of water around the globe. His studies have confirmed that the water cycle is becoming more exaggerated with greater peaks and troughs of evaporation and precipitation, and the ground water supplies on which two billion people depend are also being depleted at an alarming rate.

In some areas, water that has been held in deep aquifers for millennia is being pumped out, and the rain is not replenishing them. Residents of Sana'a in Yemen, for example, currently have to drill up to a kilometre beneath the ground to find potable water. If that groundwater gives out, there will be no alternative but to abandon the city.

The island city-state of Singapore has limited natural water resources. In an increasingly urbanised world, stresses on governments' ability to deliver drinking water have led to public-private partnerships to match public need with private capabilities. In 'The three Ps for a blue planet', we see how the lessons learnt from Singapore's experience of public and private sector cooperation - where local companies and international organisations have worked together to develop new technologies and processes - can ensure that the outsourcing of water services is a seamless process.

In the old days the first duty of a ruler would be to protect the city by building up the walls against attackers. Now, besieged by global warming, modern day rulers must protect against the challenges of the water cycle. It means investment in water resource development, efficiency, drainage and wastewater management. It is difficult to do both politically, financially and in terms of engineering, because essentially authorities must protect against too much water and too little water at the same time, with no certainty that either will happen.

It is an important issue for governments, businesses and citizens to engage in. And that is the purpose of Singapore International Water Week 2011 this month. It brings together everyone concerned with water management from across the world to share experience, develop new strategies and inspire each other to take the action required to withstand the coming siege

Christoper is the publisher of market analysis publication Global Water Intelligence. He has published specialist reports on the water sector, and is a regular speaker at water-related conferences.