I’ve lately become intrigued by the idea of "energy slaves" a deliberately unsettling way to highlight how unusual the last 200 years have been in terms of energy and economic growth.

Suppose you were a wealthy white farmer in the antebellum South and owned 10 slaves. Assume you have no other access to energy beyond their muscle power. If you monopolize the fruits of their collective labor, you get a 10x force multiplier: the output of 11 men, harnessed to the sole benefit of one. That means you churn butter ten times as fast, harvest cotton ten times as fast, and mill grain ten times as fast as your non-slave owning peers.

That, in a nutshell, was the economic case for slavery — not a moral justification, of course, but simply the reality of the pre-fossil fuel era. Economic well-being has always depended substantially on access to energy, and when all energy is derived from muscles, economic success depends on co-opting the muscles of others. That’s why virtually all economically powerful nation states prior to 1800 depended substantially on coercive labor.

How much energy was available to the (free) population of the antebellum South? In 1860, the U.S. had just under 4 million slaves, working for about 8.5 million free residents of the south and border states, or 0.47 slaves/free southerner. However, only one third of southern families owned slaves. So let’s multiply our 0.47 by 3, to yield 1.4 energy-slaves per slave-owning family member. There were also almost 3.4 million horses, mules, and oxen in the south as of 1860, or 0.4 pack animals per free person. A horse can do the work of about 7 people, so figure an average slaveholder had 1.4 + (7 x 0.4) or 4.2 energy slave-equivalents per free person. Taking southern society as a whole, the free population had (3.4 x 7 + 4)/8.5, or 3.27 "energy slaves" a piece.

Now consider the present day. The U.S. population is 311 million. We use 100 quadrillion Btus ("quads") of energy/year. Since a human requires 2,200 calories of food per day (1 food calorie = 3.97 Btus), our current energy use amounts to 100 energy slaves per capita.

That is the extraordinary legacy of industrialization and fossil fuel extraction. We get 30 times the energy access of an 1860s plantation owner … and human rights, too.

A 30-fold increase in per capita energy access, spread over 150 years, implies a compound annual growth rate of 2.3 percent. During the same period, U.S. population grew tenfold while GDP grew from just under $4.4 billion to $14.5 trillion. Put another way, our per capita GDP grew from $138 to $47,057 — or 3.96 percent per year.

These (admittedly crude) calculations suggest that increased per-capita energy access may explain as much as 60 percent of the growth in per-capita GDP over the last 150 years.

Since those 150 years saw the fastest rates of growth in living standards and population in all of human history, this raises an obvious if unsettling question: Do the last two centuries reflect our species finally living up to our economic growth potential … or are they an anomaly driven by a temporary, unsustainable ramp up in mining activity?