But, as leaders prepare for the UN climate change summit in Paris (COP21) next week, we must grasp the fact that cities do not have to exacerbate social and environmental difficulties. Well-designed, well-governed cities will in fact improve the lives of billions of people who will be newly urban by 2050.
London in the 19th century provides interesting insights. As it exploded with migrants attracted to urban jobs, it became the world's first megacity. And, like some megacities of today, it became over-congested, dirty, polluted and dangerous. Life expectancy for a man was less than 30 years. It was, as the late planner Peter Hall described it, the "City of Dreadful Night".
But by the late 1880s London had turned itself round. It invented the first form of metropolitan government in the world and initiated a programme of investment in a wide range of infrastructure: sewers, housing, parks, public transport and much more. Despite all the problems in the city today, London is a reminder that big cities can be tamed, humanised and improved. But to do this, they require leadership, strong design and focused investment.
In an industrialised society, buildings and the movement of goods and people between them account for two-thirds of energy consumption. As a result, cities contribute to about 70 per cent of global CO2 emissions. But while this sounds like bad news, there is plenty of evidence to suggest that cities can be designed to be both productive and clean. Stockholm reduced emissions by 35 per cent from 1993 to 2010 but grew its economy by 41 per cent, one of the highest growth rates in Europe. Copenhagen has done much the same. If Beijing, Mexico City or Sao Paulo were to take their cue from Stockholm, Copenhagen or even London, the impact would be startling.
Good examples are not only to be found in affluent Northern Europe. The Colombian city of Medellin - the homicide capital of the world in the 90s - has embarked on a series of social and transport projects recently, including cable cars to service the poorest slums, which have contributed to a more stable and sustainable urban society. Today, Medellin and Colombia's capital, Bogota, are seen as examples of how to turn cities around through investment in infrastructure such as cycleways, bus rapid transit systems, libraries and schools - all driven by charismatic and committed mayors.
Design plays a huge part. Cities that are consistently rated highly by the public in terms of quality of life are relatively compact and pedestrian-friendly, with good public transport and generous parks and civic spaces.
These more desirable cities are comparatively dense and have evolved historically from a traditional European concept. They consume less energy than the more recent suburban model of cities - like Los Angeles with its low-density housing and a dependence on car travel. A new study suggests that urban sprawl costs the United States economy more than US$1 trillion ($1.5 trillion) annually.
Across the globe, people are likely to live longer and healthier lives in cities.
In most countries in the world, cities provide better access to education and health services. The longest life expectancies today can be found in high-density, highly developed cities like Hong Kong or Singapore.
Unlike cumbersome national governments and international organisations, cities can act quickly and decisively.
When it comes to the future of life on Earth, cities are not the problem - they are the solution.
Lord Norman Foster is a British architect whose company, Foster + Partners, maintains an international design practice famous for high-tech architecture.
