Are cities the answer to the world's population crisis? I hope so given that now more than 50% of the earth's population is living in them.

While in London last week, I visited the Tate Modern Museum to see the "Global Cities" show. Curated by Ricky Burdett and Sheena Wagstaff, the exhibit explores what happens when rapidly expanding cities, the need for social justice, and our increasingly fragile environment intersect.

The theme is powerful and the show does well at illustrating the opportunities and threats created by massive cities. The curators were able to stitch together an array of notable contributors including Zaha Hadid, Nigel Coates, and Rem Koolhaas. Their work here is strong - we expect that from these practitioners - but it is the geographer's statictics that are the conceptual centre of the show.

Did you know that there are now 20 mega-city regions with more than 10 million people each? How about that there are 450 cities with more than 1 million inhabitants? Tokyo is the world's largest city with more than 34 million citizens but is also the best-practices model for how massive cities can remain livable. How about this stat: Six million cars operate on Sao Paulo Brazil's streets and a thousand new cars are registered there every day.

Toronto's problems seem insignificant by comparison.

Yet, the city is suffering through yet another funding crisis. The latest subway extension scheme is threatened with cancellation as a result. We don't know how to build bike lanes as one way to reduce the need for cars (this is an embarrassment given the strides European cities have made to promote cycling that on this point alone we don't deserve the title of world-class city).

For all its promise, Toronto is losing its place as a best-practices model for the contemporary city. We still, however, can claim to offer a degree of social justice and civility that is enviable. How long wil that last without investment in our overall infrastructure?

Modern cities will either liberate us or enslave us, save the environment or help destroy it. Toronto still has the potential to show the world how a city can be a positive force but we have to invest in that positive future or risk being swept aside by the forces of rapid urban change.