Cities are part of the system we have invented to keep people alive on Earth.

People tend to like cities, and have been congregating in them ever since the invention of agriculture, 10,000 years ago.

That is why we call it civilisation.

This origin story underlines how agriculture made cities possible, by providing enough food to feed a settled crowd on a regular basis.

Cities can not work without farms, nor without watersheds that provide their water.

So as central as cities are to modern civilisation, they are only one aspect of a system.

There are nearly eight billion Humans alive on the planet now, and that is a big number: more than twice as many as were alive 50 years ago.

It is an accidental experiment with enormous stakes, as it is not clear that the Earth biosphere can supply that many people’s needs – or absorb that many wastes and poisons – on a renewable and sustainable basis over the long haul.

We will only find out by trying it. Right now we are not succeeding.

We are using up our annual supply of renewable resources by August every year, after which we are cutting into non-renewable supplies – in effect stealing from future generations. Eating the seed corn, they used to call it.

At the same time we are pumping carbon dioxide into the atmosphere at a rate that is changing the climate in dangerous ways and will certainly damage agriculture.

This situation can not endure for long – years, perhaps, but not decades. The future is radically unknowable: it could hold anything from an age of peaceful prosperity to a horrific mass-extinction event.

The sheer breadth of possibility is disorienting and even stunning. But one thing can be said for sure: what can not happen will not happen. Since the current situation is unsustainable, things are certain to change.

The tendency of people to move to cities, either out of desire or perceived necessity, creates a great opportunity.

If we managed urbanisation properly, we could nearly remove ourselves from a considerable percentage of the the planet’s surface.

That would be good for many of the threatened species we share this planet with, which in turn would be good for us, because we are completely enmeshed in Earth’s web of life.

Leave about half the Earth’s surface mostly free of Humans, so wild plants and animals can live there unimpeded as they did for so long before Humans arrived.

Same with the oceans; about a third of our food comes from the sea, so the seas have to be healthy too.

With people already leaving countrysides all over the World to move to the cities, big regions are emptier of Humans than they were a century ago, and getting emptier still.