It was a past president of the US Conference of Mayors who said at the turn of the millennium: “The 19th Century was a century of empires, the 20th Century, a century of nation states. The 21st Century will be a century of cities.”

This is proving to be far more prophetic than many would have imagined.  The rise of cities, the co-operation and connections that exist between us, and how we can unlock social and economic progress, is undoubtedly an exciting and defining feature of our times.  

According to the UN, more than half of the world’s population now lives in urban centres, a proportion that is projected to grow to 67% by 2050.  In the US, the largest 100 metropolitan areas account for two thirds of the country’s population and the majority of overall economic output.   

As the Mayor of London, I have seen how cities like my own, as well as New York, Mumbai, Paris and Chicago, now link together like never before to form the beating heart of our global economy, how they account for the vast majority of innovation and how they are at the forefront of everything from science to culture.

At a time when we are seeing a rise in populism and a pull across many countries in the West towards looking inwards, nativism and isolationism, our global cities can act as an antidote to this regressive trend, demonstrating the folly of these actions. 

In cities, we see the highest levels of immigration and the most striking examples of rapid cultural change.  Yet it is also in cities like London and New York where we celebrate our diversity as a great source of strength, rather than viewing it as a weakness. 

It is also major cities that can lead the charge when it comes to solving pressing issues that national governments are either unwilling or unable to confront.  This includes tackling some of the biggest problems facing the world – from air pollution and climate change, to the necessity of moving to clean energy sources and skilling up workers for the jobs of tomorrow. 

During these unprecedented, uncertain times, this alternative outlook should provide us with some optimism about what can still be achieved. The dynamism of our progressive cities stands in stark contrast to the ever more dysfunctional character of many national governments, which are increasingly incompetent and gripped by partisanship, paralysis and gridlock.  

In the UK, we have Brexit on the horizon following the referendum to leave the European Union.   This is not something I supported as it could lead to great damage to our economy and prosperity.  But in London, we are already using all the local powers at our disposal to mitigate against the potential impact, which could fall the hardest on the most disadvantaged communities.    

We are also getting the message out loud and clear to the rest of the world that, despite Brexit and the negative approach of the current British government, London remains open to the world – open to people, trade and investment. 

The truth is that in many cases, nation-states have proved ill-equipped to deal with the big economic challenges of our time. This has led to greater economic inequality and exacerbated some of the divisions that have started to dominate our national politics.  Governments appear powerless in the face of globalisation – unable to help those communities who feel left behind by deindustrialisation and the changing nature of our economies.

Worryingly, the same could prove to be true for one of the biggest challenges facing Britain, American and countries across Europe today: how we can ensure the advancement of technology is utilised for the benefit of everyone and all communities.  

Like previous industrial revolutions, the current tech revolution poses potential risks as well as great opportunities.  I genuinely believe our global cities have the power like never before to help shape the future by harnessing new technology to enable good growth, improve lives and reduce inequality. 

Our cities are nimbler, more attuned to the concerns of citizens and able to respond quicker to their needs.  In this world of fast-paced, relentless change, cities can cope better with digital disruption, turning technological upheavals to our collective advantage.  This is a topic that will dominate debates in the years ahead, and one I plan to contribute to in more detail in a keynote speech and a Q&A with the editor-in-chief of HuffPost, Lydia Polgreen, at the upcoming South by Southwest conference in Austin, Texas. 

We often hear of people talking about the ‘special relationship’ between the UK and the US.  This is born from our incredible shared history and shared values, and it’s something we cherish and hold dear in my country.  But I believe the way to build on this special relationship is not just to rely on the old bonds that bind us together, such as the unique way we share security intelligence.  It must also be about coming together to tackle new challenges, and I believe greater city-to-city collaboration will be one of the key ways we can do this and to strengthen our relationship in future.

Through cities working together, I am confident we can act as a powerful counterweight to the lethargy and lacklustre character of nation states and help to shape the century ahead for the better. 

Sadiq Khan is the Mayor of London, and will be joining HuffPost editor-in-chief Lydia Polgreen for a Q&A session following his keynote speech at SXSW in Austin, Texas on Monday 12 March at 8pm GMT