Today the world population will officially reach seven billion according to the UN. Reaching that landmark reminds us of the massive challenges, including in the Middle East, created by an ever-increasing number of humans on the planet.
Growing populations are also driving another mega-trend - urbanisation through migration. In 1800, less than 3 per cent of the world's population lived in cities, yet by the end of 2008, this had risen to more than 50 per cent, and there were 26 "mega-cities" (cities of 10 million or more inhabitants) worldwide, including Istanbul, Cairo and Lagos.
Despite the economic success of these huge cities, governments at every level are preparing for the risks that these urban centres pose. For instance, will it be possible to meet the everyday needs of food, water and health, and also deal with the growing vulnerability of mega-cities to environmental stresses exacerbated by the effects of climate change?
There is already cause for some alarm. For instance, the tsunami in Japan this year forced Tokyo to reconsider its approach to nuclear power and to protecting its cities. The 2003 heatwave in Paris was so devastating because both the public and authorities were unprepared for dealing with such extreme weather conditions, which were worsened by building practices, especially the lack of air-conditioning.
During the 21st century, cities across the world will continue to grow. Energy demands will increase, particularly for transport of food, water and resources for industry and infrastructure.
The associated increased carbon emissions are contributing to global warming and pose their own climate risks. In China, where people are being subsidised to move from the countryside, cities have grown by a factor of two in only five years. The "heat island" effect in urban centres means temperatures are increasing about three times faster than over other land areas.
The main risk for cities on coastal plains is their increasing vulnerability to rising sea levels and river flooding, seen in the Bangkok floods now. There will be further episodes such as the one in New Orleans six years ago when it was hit by Hurricane Katrina.
In at-risk countries, such as the Netherlands, researchers are preparing for these type of problems. For instance, Delft University's hydraulic engineering department has been developing an early warning and monitoring system, which considers the effects of subsidence, to protect coastal communities.
The larger the urban area, the greater the damage that natural hazards can inflict; and increasingly it will be more difficult to protect life and property even if there is an effective warning system. As a recent hurricane in Houston, Texas showed, despite the known dangers from combined hazards such as winds and floods, there is now insufficient time to evacuate some cities safely, even highly developed ones.
So there is a pressing need for cities to develop emergency refuge areas. In most cases, refuges will need to be built from scratch. Engineers and planners are considering how to identify and design such emergency centres and how they should be connected to the wider urban system, including transport.
Training populations to use the centres effectively is also essential. Refuges have successfully withstood cyclones and floods in Bangladesh and, unlike those in some other developing countries, have been used by vulnerable communities, because they could take their farm animals with them - without the animals they would be destitute.
Emergency energy supplies for communities, essential for medical emergencies, should improve. This especially applies to advanced solar power that is effective even in cloudy conditions.
Research teams are collaborating in construction of "system dynamics" models for the operation of infrastructure, environment and socio-economic aspects of large cities. These models resemble computer programmes for global climate change and its connections to economic developments.
What these models need is more relevant environmental and socio-economic data. International agencies such as the World Health Organisation and the World Meteorological Organisation, as well as national governments, need to collaborate and make maximum use of new media. This will gather data showing how people experience both rapidly occurring hazards such as tornadoes, and slower, but still deadly, phenomena such as loss of crops from rising sea levels and salt penetration.
These mega-cities have begun to collaborate on information exchange through an initiative called C40 Cities, which coordinates policies for dealing with hazards, and to put more pressure on national governments to provide assistance, especially with finance and data.
Lord Julian Hunt is a visiting professor at Delft University. Yuguo Li is a professor of mechanical engineering at the University of Hong Kong