When I am thinking about how to re-order our society for the coming twin crises of energy and climate, I often find it useful to think in terms of several "budgets" that represent the resources we have to work with when performing said re-organization. This thought process, for example, is why I think that there is no alternative-fuel wondercar coming to save us. Let's take a look at a few of these "budgets" and how they point us towards a world that is increasingly walkable, bikeable, and transit-dependent.
1. Carbon Budget
So this is probably the big one. We will not be able to reduce our emissions of carbon dioxide to 0-- for one, to survive we are going to have to keep breathing. What we need to do is to reduce our carbon footprint, not only to the point where our emissions match the planet's carrying capacity, but to a point that is actually below that carrying capacity, in order to reduce the concentration of atmospheric carbon. 350.org has more, but basically, right now the concentration of CO2 is at ~400ppm and rising at 2ppm per year, meaning that we are adding carbon to the atmosphere. We need to reduce our emissions enough so that we return to a level below 350ppm no later than end of the century, so we need to get to the point where CO2 concentrations are falling by at least .57ppm per year, if we were able to re-shape our society overnight. More likely, we'll need to get to a point where, by mid-century, concentrations are falling by 1-1.5ppm per year or so.
So we have some absolute ceiling on the amount of carbon dioxide that can be emitted into the atmosphere between now and 2100. Every ton of CO2 is "spending" that budget. I think it goes without saying that we aren't currently spending it wisely. But what would actually prioritizing our needs according to that budget look like? That is, we have limited CO2 that we can emit-- what things are worth spending that budget on?
First off, there are likely industrial processes that emit CO2, and that can't be feasibly switched off of the stuff. We should try to find replacements where we can, but I suspect that there are plenty of cases where it is chemically infeasible not to give off some CO2 during the process of producing modern conveniences that we'd like to keep around.
Second, and this is something a lot of people don't think about, but aviation is currently pretty much impossible without some form of liquid hydrocarbon fuel. (The Solar Impulse is certainly an impressive engineering feat, but the single-seat 40-knot gossamer craft is not a practical transportation machine, and likely won't be for some time.) The sheer power-to-weight ratios enabled by fuel-burning piston and jet engines make powered flight possible, and electric propulsion isn't likely to catch up for some time (although ion wind thrusters may be the next big thing). If we want to continue to live in a world where everything from traffic helicopters to intercontinental jet travel are still with us, we're not likely to drastically reduce the carbon footprint of the aviation sector.
There are also a myriad of other applications where CO2 emisisons are going to be necessary, from military operations to diesel-fueled generators at remote outposts and during power outages to oceanic shipping. While we should work at making these sectors more efficient, the energy density of hydrocarbon fuels are likely going to make carbon-generating combustion the only practical option for a great number of applications, all of which we'd like to keep around in our civilization.
Beyond that, there's also the CO2 emitted by our more biological operations-- livestock farming, and our own and our pets' respiration. Gotta keep that in mind.
And then there's our day-to-day transportation needs. We can certainly spend our carbon budget on in-city transportation, sure... but why should we? We know that putting people on electrified, zero-emission buses and trains, and on bicycles and their own two feet, can move them about without emitting any appreciable carbon dioxide. We know that, with proper land-use planning, huge majorities of our population could move around their daily lives in this manner. Why should we spend our carbon budget when we don't have to?
2. Energy budget
Sure, you say, but electric cars surely must be the answer! They spend very little of our carbon budget! Right, but they spend a lot of our energy budget, so they're not exactly a panacea. And they allow sprawl to continue, which further taxes our energy budget.
We know that we're going to have to start generating the overwhelming majority of our energy needs from renewable energy sources-- solar, wind, tidal, and geothermal power. (There is mixed opinion on whether next-generation nuclear power plants could meet some of these needs, but that's not important to this argument.) Given that, we are going to have to deploy a lot of solar panels and wind and tide turbines and geothermal boreholes in the next few decades. We are going to have to meet our current energy needs plus some appreciable fraction of powering our transportation system. So every inefficiency in the system results in having to deploy more renewable power generating capacity-- or, more likely, results in a lower probability that we'll be able to generate our power entirely off of renewables.
Single-occupancy motor vehicles, no matter their powerplant, are hugely energy inefficient-- you're spending a lot of energy moving the car itself, and very little moving the passenger. So if we go to electric cars, rather than electric transit, we are going to have to put up a lot more solar panels to power the cars.
Beyond that, electric cars would still enable sprawl. Beyond being land-inefficient and soul-crushing, sprawl is also really energy-inefficient. Urban apartments and rowhouses, dwellings that share walls, require significantly less energy to heat and cool. Small-lot urban single-family homes tend to be smaller and therefore more energy-efficient in terms of climate control. Toss in the energy required to extend and maintain utility infrastructure to a comparatively small number of people on a comparatively large amount of land, and you see that sprawl is a poor choice to spend our energy budget on.
3. Social Budget
The last budget is probably the closest to an actual, financial, budget in this list. Our society has a finite amount of resources in it-- physical, financial, and political. Each dollar spent, each political speech, each gram of silicon is an opportunity to devote resources towards the transformation of our civilization to meet the climate crisis. We can only afford so much revolutionary change. We could choose, for example, to heavily subsidize research into new battery technologies, to develop quick-charge electric car filling stations around the country, to give people massive tax credits for buying an electric car, to spend our resources effecting a transformation to a whole new way of getting around. But doing so has an opportunity cost-- after developing that new system, it is unlikely that we will be able to turn around once more and gain the political and financial support to once again transform how we get around.
I am of the opinion (completely unsubstantiated, but reasonable) that our society has the political will and physical resources for maybe one big transformation this century, before the oil starts running out and the oceans start rising. When we decide to finally embark on the path of that transformation, we had better be sure it's the right one, because if it isn't, we will find ourselves facing these crises unprepared.
For the many reasons that I've mentioned on this blog, I think that a move towards more urban living, enabled by active transportation and electric public transit, linked by electric freight and high-speed passenger rail, is the form that will make our civilization most resilient in the face of the coming future. I can only hope that we are not so distracted by promises that we can keep on doing what we are doing-- packaged in the form of electric or hydrogen or other wondercars-- to preclude this transition.