I have dished out a healthy share of criticism about the paths we are taking into the energy future, so perhaps it’s time I offered some paths of my own. I will outline them as simply as possible, since the data and thinking behind them could fill a book.

First we must know where we’re going.

Credible models show that by the end of this century, essentially all of the fossil fuels on earth will be consumed—oil, natural gas, and coal. Presumably, whatever fuels do remain at that point will be reserved for their highest and most valuable purposes like making crude oil into plastics and pharmaceuticals, not burning it in 15% efficient internal combustion engines.

Consider the following world model for all fossil fuels:

Source: “Olduvai Revisited 2008,” The Oil Drum, by Luís de Sousa and Euan Mearns. Cumulative peak of fossil fuel energy is 2018. Data sources: Jean Laherrère for natural gas, Energy Watch Group for coal and The Oil Drum for oil. [This is an exceptional study and I recommend it to my readers!]

By the end of this century then, a mere 90 years from now, we’ll need to have an infrastructure that runs exclusively on renewably generated electricity, biofuels, and possibly nuclear energy. That’s where we’re going.

Fortunately, there is more than enough available renewable energy to meet all of our needs, if we can harness it. Unfortunately, we’re starting from a point at which less than 2% of the world’s energy comes from renewables like wind, solar and geothermal.

Hydro provides about 6%, and nuclear about 6%, but for reasons too numerous to get into here, some of which my longtime readers have already heard, I don’t believe either source will increase much in the future, and both could actually decline.

Our challenge then is to make that 2% fraction grow to replace about 86% of the world’s current primary energy, in 90 years or less.

We are currently at peak oil, a short, roughly 5-year plateau which goes into terminal decline around 2012. All fossil fuel energy combined peaks around 2018, less than a decade from now.

All strategies for accommodating the fossil fuel decline require decades to have any significant effect. The now-iconic study “Peaking of World Oil Production: Impacts, Mitigation, & Risk Management” (Hirsch et al., 2005) demonstrated that it would take at least 20 years of intensive, crash-program mitigation efforts to meet the peak oil challenge gracefully. Another study, “Primary Energy Substitution Models: On the Interaction between Energy and Society,” (C. Marchetti, 1977) showed that it generally takes decades to substitute one form of primary energy for another, and 100 years for a given source of energy to achieve 50% market penetration.

Therefore, we are going to have to accomplish most of the renewable energy revolution in a scenario of ever-declining fuel supply. In just 50 years, we’ll be working with about half our current energy budget. So in fact we may only have about 50 years to build most of the new renewable energy and efficiency capacity we will need to get us through the end of the century.

Another important factor is that exports will fall off much faster than total supply. (See my article on the oil export crisis from last year.) Foucher and Brown (2008) have shown that the world’s top five oil exporters could approach zero net oil exports by around 2031. Net energy importers like the US could be increasingly starved for fuel as decline sets in and accelerates, and net energy exporters could wind up shouldering much of the burden of new manufacturing. This factor means that we will have to front-load as much of our development as possible.

The final and most important factor is population. The few population models that actually take fossil fuel depletion into account assume that global population increases roughly out to the global fuel peak, and then stabilizes at that level or declines naturally while economic development promotes lower fertility rates and renewables and energy efficiency increase to fill the gap of declining fossil energy. I understand why this assumption is made—because the alternative is too ghastly to contemplate—and for the immediate purpose of this article I will go along with it. I will note however that history and scientific observation of populations suggest some sharp episodes of decline are more likely, and in my estimation we will end this century with a considerably smaller population than anyone forecasts, at some level well below today’s.

How, then, can we replace or offset through efficiency at least 40% of our current energy supply with renewables in the next 50 years, while fuel prices are rising and the global economy is flat or shrinking due to a lack of fuel?

A proper model for achieving this goal would be a very large undertaking, the sort of thing that should be done by a team of experts with a budget. (Is anybody at the Department of Energy listening?) But I can identify some key pathways that are, in my estimation, no-brainers. Because the solutions going forward will be quite different for each country, I will limit my recommendations to the US.

1: Rail. Rail should be Priority 1, and should be granted the largest portion of public funding.

2: Rooftop Solar PV. Utility scale projects like giant solar farms in the desert and giant wind farms in the Midwest (or offshore) all face serious hurdles in siting, permitting, environmental impact, and transmission capability. Rooftop photovoltaic (PV) solar systems face no such issues and can be deployed right now, building capacity incrementally over time.

3: Alternative Vehicles. Since reconfiguring our urban topology around transit and deploying light rail will take decades, we will need some transitional solutions that still allow us to get around in cars for a good many years.

4: Efficiency. Most of the efficiency gains we can make are thermal: reducing the energy it takes to heat and cool buildings.

5: Utility Scale Renewables. We’ll need large solar plants across the Southwest, and huge wind farms in the Midwest and offshore.

6: A Beefier, Smarter Grid. The good news is that we already have most of the technologies we need in this area. All that we lack is the will and the funding to put it in place.

7: Keep Drilling. If we back off too much too soon from oil and gas production, it could leave us without adequate or reasonably priced fuel to accomplish this transformation, and sink the entire effort.

This post originally appeared on GetRealList, and has been reprinted with permission.