Why Urban Density and Renewables Don’t Add Up to a Climate Solution

Alex Steffen’s Faulty Arithmetic

The argument that increased urban density has very significant climate benefits has been well made by Edward Glaeser, David Owen and others. The US writer Alex Steffen has joined the ranks of those with books out arguing for promoting density, with the view that we simply cannot reduce emissions enough through low carbon energy alone. Urban density will do the trick. He appears to believe that cities can reduce energy use by 90%, but only seems to provide hand waving explanations of how this is possible.

However, a statement he made in a recent interview to The Atlantic is reflective of a common problem with solutions to climate change: the unwillingness to do basic arithmetic. He says:

For example if you have a more distributed energy system, you can have the energy system in one neighborhood go down, and energy systems in other neighborhoods remain unaffected. By distributing things, you make it possible for disaster to strike, and not have everything go down if something fails.

Now, presumably Steffen doesn’t have neighborhoods being powered by small modular reactors or gas plants with CCS in mind. So, he must somehow believe that neighborhoods can be powered entirely by local renewables, with perhaps some yet to be invented storage technology providing back up. A fundamental problem is that his vision of high urban density and localized energy production are in conflict.

Consider New York City. This city certainly fits the category of high urban density. However, think about what would happen if New York tried to power itself entirely from renewables within city limits: constant blackouts. This is a simple consequence of its high population density and the laws of physics. An author such as Steffen who claims to have thought deeply about climate change, urbanization and energy really ought to be aware of this. So, why can New York not power itself from local renewables?

A little back of the envelope arithmetic. Now, from memory, I know that UK energy use works out at about 1.25 Watts per square meter. The United States as a whole has about double the per capita energy use of the UK, however New York is about 30% of the US average. Factoring in the difference between UK and New York population densities would give us a ball park figure of something like 30 Watts per square meter in New York.

That’s the demand side. Can we possibly get this from renewables? Wind power is clearly off the cards in New York. But what if we covered every inch of New York in solar panels? We need 33 Watts per square meter. This is about double the energy density you could get from solar thermal in the North African desert. New York is clearly not getting this. A ball park maximum (and someone correct me, if I am wrong here) for ideally placed New York solar panels would be 10 Watts per square meter, already just one third of total demand. Now, how much of New York could you actually cover in solar panels? Let’s go with one-third as a rather optimistic top estimate. All of a sudden you are down to around 10% of energy needs coming from solar, with the real figure likely to be a great deal lower.

So, it seems impossible to argue that a dense urban area could power itself from distributed power, and Steffen’s imagined urban areas of the future are nothing but the imaginings of a wishful thinker.

Densely populated cities will have to be powered largely by energy pumped in from outside, be it from wind farms or nuclear power plants. This much ought to be axiomatic. Yet, here I am writing this.

Robert Wilson is a PhD candidate in Mathematical Ecology at the University of Strathclyde in Glasgow. He blogs at Carbon Counter, where this article first appeared. You can follow him on Twitter @planktonmath.

Photo credit: Flickr user davidyuweb