Alex Steffen

The planet's biggest problems have to do with sustainability, environmental decline, global poverty, disease, conflict and so forth. Really, they're all interconnected - it's one big problem, which is that the way we're doing things can't go on.

Cities offer us powerful leverage on our most stubborn, wasteful practices. Long commutes in our cars, big power bills from our energy-hogging buildings, shopping trips to buy stuff that'll spend a few short months in our homes and long centuries in our landfills.

For carbon-neutral cities, there are things worth talking about in how our consumption patterns can change - sharing goods, etc. - but those are a fraction of the impacts of transportation and building energy use. If we need to choose priority actions, the most important things are to densify, provide transit, and green the buildings.

There are a lot of different ways of building a prosperous society, and some of them use much less energy than others. And it is possible and more practical to talk about rebuilding systems to use much less energy than it is to think about trying to meet greater demands of energy through clean energy alone.

The biggest thing growing cities need to do is minimize barriers to development so that as long as someone is doing good urbanism, they can get permitted quickly and get building quickly.

Copenhagen has done a remarkable job creating streets that are focused on bicycles and pedestrians.

In tough times, some of us see protecting the climate as a luxury, but that's an outdated 20th-century worldview from a time when we thought industrialization was the end goal, waste was growth, and wealth meant a thick haze of air pollution.

Clean air and water, a diversity of animal and plant species, soil and mineral resources, and predictable weather are annuities that will pay dividends for as long as the human race survives - and may even extend our stay on Earth.

Cabbies in particular seem to like discussing the fate the Earth.

The denser places get, the lower the amount of energy people use to get around it.

I think we're going to start to see a new model of civic advocacy where people get together once in a while to protest, but it's more about an ongoing, sustained engagement in issues, networks and communities about which people care.

We don't know yet how to build a society which is environmentally sustainable, which is shareable with everybody on the planet, which promotes stability and democracy and human rights, and which is achievable in the time-frame necessary to make it through the challenges we face.