Cities are part of the system 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 or so years ago.

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.

There are nearly eight billion Humans on the planet now, more than twice as many as 50 years ago.

It is an accidental experiment with enormous stakes, as it is not clear that the Earth’s 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 use our annual supply of renewable resources by August every year, after we are cutting into non-renewable supplies – in effect stealing from future generations.

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 creates a great opportunity. If we would manage urbanisation properly, we could remove ourselves from a considerable percentage of the the planet’s surface.

That would be good for 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, by the way; 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.

So emptying half the Earth of its Humans would not have to be imposed: it is happening anyway.

This vision is one possible format for our survival on this planet. Cities will have to be green cities.

We will have to have decarbonised transport and energy production, white roofs, gardens in every empty lot, full-capture recycling, and all the rest of the technologies of sustainability we are already developing.

That includes technologies we call law and justice, yes, justice. Robust women’s rights stabilise families and population.

Income adequacy and progressive taxation keep the poorest and richest from damaging the biosphere in the ways that extreme poverty or wealth do. Peace, justice, equality and the rule of law are all necessary survival strategies.

All this can be done. All this needs to be done.