Now let’s imagine another illustrative scenario in connection with ourjar of bacteria. The time is 11:30 PM: one-half hour before the end. Thejar is one-eighth full. A thoughtful member of the culture projects thefuture and decides that more uninhabited resource-laden jars mustbe discovered in short order if the culture is to continue its trajectory.Imagine for a second the disbelief expressed by probably the vastmajority of other inhabitants: the jar is far from full, and has served for141 generations—a seeming eternity. Nonetheless, this explorer returnsreporting three other equal-sized food-filled jars within easy reach. Ahero’s welcome! How much longer will the culture be able to continuegrowing? What’s your answer?

The population doubles every ten minutes. If the original jar is filledat 12:00, the population doubles to fill the second jar by 12:10. Anotherdoubling fills all four by 12:20. The celebration is short-lived.Now we draw the inevitable parallels. A planet that has served us forcountless generations, and has seemed effectively infinite—imponderablylarge—makes it difficult for us to conceive of hitting limits. Are wehalf-full now? One-fourth? One-eighth? All three options are scary, todifferent degrees. At a 2% rate of growth (in resource use), the doublingtime is 35 years, and we only have about a century, even if at 1/8 fullright now.3

In relation to the bacteria parable, we’ve already done a fair bit ofexploring. We have no more jars. One planet rhymes with jars, but it ishostile to human life, has no food, and is not within easy reach. We haveno meaningful outlet. And even if we ignore the practical hardships,how much time would a second planet buy us anyway for uninterruptedgrowth? Another 35 years?

The sun deposits energy at Earth’s surface at a rate of about 1,000 W/m2(1,000 Watts per square meter; we’ll reach a better understanding for these units in Chapter 5). Ignoring clouds, the projected area intercepting the sun’s rays is justA�πR2⊕, whereR⊕is the radius of the earth, around 6,400 km. Roughly a quarter of the earth’s surface is land, and adding it all up we get about30×1015W hitting land. If we put solar panels on every square meter of land converting sunlight to electrical energy at 20% efficiency we keep 6×1015W. This is a little over 300 times the current global energy usage rate of 18 TW. What an encouraging number! Lots of margin. How long before our growth would get us there? After one century, we’re 10 times higher, and 100 times higher after two centuries. It would take about 2.5 centuries (250 years) to hit this limit. Then no more energy growth.

In summary, decoupling and substitution are touted as mechanismsby which economic growth need not slow down as energy and otherresources become constrained. We can make money using less of theresource (decoupling) or just find alternatives that are not constrained(substitution), the thinking goes. And yes, this is backed up by loadsof examples where such thingshavehappened. It would be foolish toclaim that we have reached the end of the line and can expectnomoregains from decoupling or substitution. But it would be equally foolishto imagine that they can produce dividends eternally so that economicgrowth is a permanent condition.