Say what you will about James Howard "The End of Suburbia" Kunstler, he's got a vision and he's certainly true to it. But is it a true vision?
The conclusion that the end of cheap petroleum will cause the abandonment of suburban and exurban communities is based on the assumption that these areas require large amounts of energy from outside themselves. This claim would have surprised the yeoman freeholders of 200 years ago who grew all their own food, produced their own energy using relatively inefficient plants, and traded still more for tools and other finished goods. Both houses and people lived off what grew on the land.
Time and ingenuity have rolled on, and McCormick's reaper, steam power and then the internal combustion engine have both increased the amount of land one person can farm and also the energy invested in it. In so doing, they have also reduced the price of farm products to levels so low, the agricultural inputs to foodstuffs are but a small fraction of the retail cost. The economics of the small freehold have long since gone negative; nobody farms anything so small except as a hobby. But economics are one thing; energetics are another.
An exurban house may sit on one or several acres, or may be part of a cluster with large areas reserved for greenspace. Some owners prefer acres of grass (up to golf courses), some have woods, others like landscaped gardens; the style is largely dependent upon taste and budget. Kunstler sees these homes as dinosaurs, about to be wiped out by the looming meteor of peak oil. But what he doesn't see is that land is more than an extravagance; it is also a resource.
Considerable effort has gone into re-establishment of ecosystems once common in the USA, but largely pushed aside by forest clearance and conversion of land to agriculture. One of these ecosystems making a comeback is the tallgrass prairie, consisting largely of fast-growing perennials like switchgrass. Switchgrass has attracted considerable attention lately as a biomass crop, as has elephant grass or Miscanthus. Being useful to humans is a strong selective advantage for a species; humans will go to great efforts to spread them.
What if they were deliberately spread to exurbia?
Reliable productivity of the grass species appears to be on the order of 4-5 tons/acre/year; one sterile Miscanthus hybrid is claimed to have yielded as much as 27 tons/acre. If a dry (short) ton of biomass yields 15.8 million BTU, the owner who devotes half of a one-acre plot to energy might be able to depend on getting 39.5 million BTU of fuel every year, perhaps as much as 213 million BTU at the extreme. Dried clippings from the rest of the lawn, trimmings from the shrubs and other stuff would add something to this total. Compressed and turned into pellets, this fuel crop would easily fit into a bin for later use for heat or other purposes. Harvest would take something more than the standard rotary riding mower, but the required equipment is certainly within the reach of small consortia if not individuals acting on their own or as contractors.
The big issue is, can the land produce enough fuel to allow its owners to continue to live in exurbia? That answer depends in turn on how much fuel they need, and for what purposes. If I recall correctly, the average gas-heated home uses 50 million BTU of fuel for space heat, another 15 million BTU for heating water, plus some for cooking. This looks like it could require ¾ acre or more to satisfy. But that assumes no improvements elsewhere.
Rather than trying to produce all the way to sufficiency, conservation can make up the difference. An Energy Star-rated house uses only 70% as much energy as the average, cutting the 65 million BTU to about 46. Houses entered into the Solar Decathlon competition do far better; they use around 30% as much energy as the average even before their solar features are considered. If the average gas-heated home was built or brought up to Solar Decathlon standards, its total heating needs would shrink from 65 million BTU/year to 19.5 million BTU. Excess heat from a solar DHW system could take a further bite out of heating needs on cold sunny days, but the extent depends upon the microclimate and the weather.
Compared to ~20 million BTU of need, 39.5 million BTU of fuel looks like a healthy surplus; 200 million BTU looks like enormous riches (go ahead, leave the cover off the hot tub!). Irony of ironies, the exurbs could be self-sufficient while more compact (thus land-starved) suburbs go begging.
But keeping warm isn't the end of the matter. Exurbia is not self-contained; its residents must travel to work, to appointments and to shopping. This means more energy.
The typical exurbanite drives something like an SUV, and drives considerable distances. These vehicles average less than 20 MPG; a driver covering 18,000 miles per year would burn 900 gallons or more, consuming 114+ million BTU of energy. Only large estates or the best harvests of Miscanthus are going to be able to meet such demands. But this is where the plug-in hybrid comes in.
With a plug-in hybrid, anything that generates electricity makes "motor fuel". If we assume a plug-in hybrid SUV consuming 500 watt-hours per mile, 18000 annual miles could be satisfied with 9000 kWh of electricity (about 30.7 million BTU). Burning 39.5 million BTU of grass in a cogenerator at 20% efficiency makes 7.9 million BTU of electricity, about 26% of transportation needs. 20 square meters of 15%-efficient PV panels receiving 1000 kWh/m^2/yr yields another 3000 kWh (10.2 million BTU), or about 33% of transportation needs. The remaining 41% could be met by wind or other purchased power; at 10¢/kWh, the ~12.6 million BTU (3690 kWh) would cost under $400/year. Compared to $4500/year for 900 gallons of $5 gasoline, this is nothing.
These figures could be slashed if the exurbanite drives something greener when roads are clear and not hauling compost. Attention to efficiency (or the willingness to sacrifice more lawn) could turn the exurban homestead into a net energy supplier. Hello, yeoman hobby energy farmer!
So, is Kunstler's vision of a countryside emptied of commuters a likely outcome? It does depend; people might not be willing to make the lifestyle adjustments to satisfy their energy needs from the land they're living on, and might prefer a balcony on a high-rise to a back yard they can't see over between June and December. But I don't give this much chance of happening; too many people are going to like the feeling of independence whether or not it gives them a chance to hunt rabbit and pheasant in their energy acreage. Even if it goes down, exurbia could rise again.
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