Inclusion and tolerance are just as important. Does the city welcome strangers and help them to fit in? Does it see difference and diversity as an opportunity or a threat? Does it seek to attract not just the wealthiest, but all those seeking a better life? Does it invite guests to be part of its story? This has always been the spirit of New York, as Sinatra encapsulated so well when he sang “I want to be a part of it”. More than 80 per cent of those choosing to live in Abu Dhabi are foreigners, surely one of the highest proportions anywhere in the world. And it is no coincidence that the British capital chose the slogan "London is open" for an advertising campaign last year.

 

Youth is, of course, where a city gets its energy, dynamism and sense of constant renewal. It is about that sense of being a place that understands the need to earmark and preserve spaces to meet, create and relax. Singapore is a great example of this, not least through its investment in the modern skills that its young people need to thrive. Dubai’s Museum of the Future signals a similar level of dynamism and ambition.

 

Getting this right is going to matter more in the 21st century. The rebirth of the city-state will be a key feature of the new power landscape. More than half the world's population already live in cities and the population of megacities has increased tenfold in 40 years. By 2030 that figure will be five billion, or 60 per cent of the population, and 40 cities will have a population of more than 10 million. By 2050 there will be more city dwellers than the entire population of the world in the 1990s.

 

Many mayors have argued that the city is already the most effective government unit, close enough to citizens to engage but with increasing powers to shape lives. With trust in national politics falling in much of the world, the ambitious politician will, in the coming decades, often choose municipal over state politics. Many cities are already ahead of national governments on urban regeneration, citizen engagement, energy and transportation. Cities can succeed, even in failed states. But no state succeeds without successful cities.

 

As they gain power at the expense of states, cities will increasingly compete in the way that countries do. Look at the already intense contests to host major sports events. Cities might become less willing to cede decisions over global governance to nation states. You can already see how cities are flexing their muscles in standing up to the Trump administration’s attempt to withdraw the US from efforts to tackle climate change.

 

As a result of these changes, we might also find that our identities become shaped to an even greater extent by our city of residence more than the country of origin — look at New Yorkers, Londoners, Beirutis or Parisians.

 

Cities have often thrived when they are close to the energy source of the era. But history is full of the ruins of cities that thought they would last forever. In many cases they were destroyed by war and the belief that building walls rather than bridges was the best way to survive. In some cases they were destroyed by geography, levelled dramatically by earthquakes or slowly corroded by the failure to adapt to technological change.

 

Tomorrow’s cities of opportunity will have learnt these lessons. They will act as talismanic examples of tolerance and co-existence. They will become beacons for progress in the countries that surround them. They will establish new partnerships with each other to promote understanding and perhaps ambassadors will once again represent cities, as they did in the early days of European diplomacy. While some countries compete to show that they are closed, protectionist and nativist, they will compete to show that they are open, dynamic and diverse.

 

Augustus said that he found Rome a city of bricks and left it a city of marble. We need a similar ambition for tomorrow’s cities. Those that do best in the 21st century will be those that are magnetic.