Doing the analysis above reminded me that a wholesale transition to alternatives seems unlikely to deliver energy at current levels of consumption/production. I'd like to briefly consider a few possible trajectories / scenarios here, which I'll explore in more depth another time.
- Business-as-usual: we'll just keep on going with fossil fuels until we can no longer do so; that is we'll follow the oil depletion curve down and try to substitute with coal, tar sands, and other dirty fuels. We may not build new coal plants in the United States, but we probably won't decommission them as fast as we should to deal with climate issues, and China, India, and other countries will continue using coal at breakneck rates. This scenario might, in the short term, maximize global economic output (though it will likely still be decreasing on a long-term basis given the economic impact of oil depletion). It will however cause us to overshoot 450 ppm of CO2, taking us to perhaps 500 or 550 ppm, which is probably past the point of no return in terms of warming - natural feedbacks are likely to take over. (I think we probably are unlikely to go much further than that, since we'll start running out of cheap coal at that point.)
- Unmanaged descent: we'll keep using fossil fuels, but the economic contraction due to oil depletion will hit hard enough that we'll end up using less energy overall. In this way, we'll haphazardly decrease our energy use at the expense of global human hardship. In this scenario, we'd probably avoid exceeding 450 ppm of CO2 simply due to a non-functioning economy, though we also won't be able to build alternatives at anything near the rate I describe above.
- Managed descent: there are a lot of things that need to be done just right to manage our descent. First, we'd need policy-based solutions, either in the form of a carbon tax (or the equivalent) or energy quotas. Second, we'd need to stabilize swings in oil prices as I discussed before. Third, we'd need to invest in alternatives that have the highest capacity yield per unit time. From what I can tell, solar thermal is one of the best options, as it doesn't require particularly advanced technology and therefore could probably be ramped up quickly. It is however only viable in desert regions. My preference would be to target solar thermal, wind, and algae fuel as the three main alternative sources; solar PV can help at the household scale.