Smil is convinced that a transition to renewable energy is inevitable in the long run. But in the short term, he thinks it will take far longer and be much more disruptive than most green-energy proponents would like to admit.

On the optimistic side, you have advocates like Al Gore. In a 2008 speech, Gore said that “enough solar energy falls on the surface of the earth every 40 minutes to meet 100 percent of the entire world's energy needs for a full year,” and called for the U.S. to generate all of its electricity from renewables by 2018. Smil argues that these kinds of pronouncements ultimately do us a great disservice by drastically overstating the clean energy that's actually available to harness productively, while understating the work, time, and investment that will be needed to transform our energy infrastructure. Into this debate, steps Gates, Smil's number-one fan, with a $2 billion pledge to invest in research and development to accelerate the transition to renewables.

Reading Smil is a wild ride. He dances up and down scales, moving from broad global trends to very specific local examples to illustrate his arguments. In Energy: A Beginner's Guide, a discussion of the energy costs of a human picking an orange up off the floor and putting it on a counter is separated by mere pages from an estimate of the energy expended by an erupting volcano. Soon after, you are learning about the (hilariously inefficient) photosynthesis of plants and comparing the productive capacity of the biosphere with likely constraints on our ability to harness solar power at planetary scale.

To keep up with Smil, let me introduce a few of his basic concepts: To begin, we need to understand what he means when he talks about resources and “prime movers.”

First, the resources. Pretty much all available energy on the earth comes from energy radiated by the sun. Straight sunlight offers heat and light. Some of that is captured by plants, which can in turn be eaten or burned for light and heat. The plant eaters (human or otherwise) can be put to work or eaten. Some of the sunlight evaporates water, which rises into the atmosphere and then falls as rain. Some of this rain lands on very high places and converges as gravity pulls it into streams and rivers that run towards lakes and oceans. The mechanical energy offered by running water is essentially stored sunlight. Meanwhile, differences in temperature because of the uneven distribution of sunlight (owing in part to the shape, tilt, and rotation of the earth) causes air to circulate, meaning winds are stored sunlight too. As are waves (which are driven by winds). And, of course, fossil fuels are reserves of stored sunlight, derived from biological depositions which have been accumulating for millions upon millions of years.

The exceptions to the sunlight rule are: geothermal energy, which comes from the very hot core of the earth (often in the form of volcanoes); tidal energy, which is the result of water interacting with the gravity of the earth, moon, and sun; and nuclear energy, which comes from either forcing together or breaking apart atoms. Some of these resources are renewable, but at the moment, the dominant suppliers of energy to human civilization (the fossil fuels) are not.