For all their variety and variation, cities are, at their root, physical systems. That means, at some fundamental level, they are also expressions of the laws of physics. In physics size matters (or "scale" as we call it). Physicists learn different things about an object by looking at it from different scales. In our first exploration of physics and cities we stayed at the street level. At that scale we saw cities as machines: cars and elevators, pipes and plumbing. Then we went up to the roof. At that scale we saw cities as engines, vast systems for turning energy into work. Today we want to take the final step in our journey and look at cities at the largest scale. Today we get the 20,000-foot view: the city from the sky!

When we climb into jet-planes, flying high over the land, we see physics and cities from the grandest perspective: the city as a force in planetary evolution.

Take a plane, especially at night, and the view from 20,000 feet lets you see far more than the layout of any individual city. Instead you see vast, sprawling networks of interconnected human habitation. Cities are the nodes of those networks.

At night you see a spider-web pattern of light spreading across the planet. It's densest in the cities, thins out in the suburbs, finally transforming into delicate tendrils that link one metropolitan area to another. From the sky you see that entire regions, hundreds of miles across, have been colonized by aggregations of cities and suburbs. From horizon to horizon, the view from the air shows individual cities spreading until some areas become one, an almost seamless urban axis (think of the coastline from Boston down to Atlanta). It's only when we fly over landscapes that are truly far removed from cities that the surface finally becomes dark, dotted only with the occasional outlines of small, faint towns.

So what does this tell us about the physics of cities?

From 20,000 feet (or better yet from orbit) our perspective actually shifts from physics to astrophysics because now we begin to get a kind of planetary view of cities. I think its best to call this an astrobiological perspective. Astrobiology is relatively new field where researchers ask about life in a planetary context. It combines astronomy and biology, of course, but also involves lots of other fields like geochemistry and atmospheric science to create a truly global view of life and planets.

Astrobiology asks how the evolution of a planet can affect conditions for life. For example, how do changes in a planet's orbit over time make it more or less hospitable to life? Thinking this way has really helped folks look at the problem of the origin and evolution of life in some new and interesting ways. But for me what's really remarkable about this perspective is that it's also shown that, just as planets affect life, life affects planets too!

A bunch of times in Earth's history, the evolution of life has completely changed big important characteristics of the planet. Things like, say, the chemistry of the atmosphere. Where does the oxygen in our atmosphere come from? It's pretty important right? Well, in the beginning, billions of years ago, the atmosphere had almost no oxygen. Tiny microbes, breathing in other gasses and then breathing out oxygen, pumped this key gas into the atmosphere! Their "waste" gas gave us the oxygen-rich atmosphere we know and love. And, like I said, these kinds of changes have happened more than once.

Sometimes life has driven the planet into a warmer and wetter state. Sometimes it has resulted in a so-called "Snowball Earth" phase where the whole planet almost froze.

Some researchers have a name for this back and forth between life (the biosphere) and the planets other systems (the atmosphere, oceans, ice and land). They call it co-evolution. That means the planet and life are changing the course of each other's history.

So what does this have to do with physics and cities? Well those nighttime views from really high up show you something really remarkable happening and I would argue that its co-evolution again.

That vast web of geometries traced out in light shows you cities as a kind of infestation. They're like living networks colonizing the planet. Far from the network nodes – the cities – we see energy being harvested in forms as diverse as food and petrochemicals. Then through those tendrils of roads, rail and pipelines the network moves energy (and information) into, around and between the cities as they grow. And all that activity we can see as lights and land use from 20,000 feet is changing the planet in ways we can't so easily see.

Our relentless city building is really at the heart of our changing the planet in ways that include increasing atmospheric levels of CO2 and methane, in the warming of the land and the oceans and the lowering the planets total ice cover. Life – in this case us and our city building – is once again driving the planet in new directions. There is even a new word for this.

Every geological epoch gets a name. Looking back over millions of years, you have your Eocene, your Pliocene, your Holocene epoch (that was the most recent one). Now folks are suggesting that we are entering a new epoch, the Anthropocene.

The Anthropocene means it's our activity which is the main driver changing the planet. But you know what? The Anthropocene is really the epoch of the city. It's our city building — on a massive, planetary scale — that is at work here. The idea of a single world-girding city pops up a lot in science fiction. I doubt we are heading towards that anytime soon. But it is truly amazing to recognize that an organizing principle that began some 4,000 to 6,000 years ago has exploded into, perhaps, the dominant driver of planetary history. That is both pretty impressive and pretty scary at the same time.

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