Where is humanity going? How realistic is a future of fusion and space colonies? What constraints are imposed by physics, by resource availability, and by human psychology? Are default expectations grounded in reality? This textbook, written for a general-education audience, aims to address these questions without either the hype or the indifference typical of many books. The message throughout is that humanity faces a broad sweep of foundational problems as we inevitably transition away from fossil fuels and confront planetary limits in a host of unprecedented ways—a shift whose scale and probable rapidity offers little historical guidance. Salvaging a decent future requires keen awareness, quantitative assessment, deliberate preventive action, and—above all—recognition that prevailing assumptions about human identity and destiny have been cruelly misshapen by the profoundly unsustainable trajectory of the last 150 years. The goal is to shake off unfounded and unexamined expectations, while elucidating the relevant physics and encouraging greater facility in quantitative reasoning. After addressing limits to growth, population dynamics, uncooperative space environments, and the current fossil underpinnings of modern civilization, various sources of alternative energy are considered in detail— assessing how they stack up against each other, and which show the greatest potential. Following this is an exploration of systemic human impediments to effective and timely responses, capped by guidelines for individual adaptations resulting in reduced energy and material demands on the planet’s groaning capacity. Appendices provide refreshers on math and chemistry, as well as supplementary material of potential interest relating to cosmology, electric transportation, and an evolutionary perspective on humanity’s place in nature.

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.

The human penchant for blaming and even demonizing "others" might lead to resource wars in the face of hardships imposed by limited resource availability. Such a path is lamentable on many levels, not least of which is that precious resources and energy would be channeled toward destructive acts rather than using them to build a better future. Are humans capable of mounting a transformative effort of global cooperation on a scale even greater than that of, say, World War II if we are not fighting an enemy other than ourselves and our own resource demands? Can we identify a precedent in which human societies have done so in the past at a large/relevant scale?

The first step in avoiding these pitfalls is awareness of the roles that human personality and psychology play in these problems, as this section has attempted to point out. One thing that became evident to the author on the basis of the Do the Math survey was that like seems to attract like: the communication style of the blog was a magnet for those of the same or adjacent types. But the message had startlingly little grip on the S crowd—especially the population-dominant xSFx types (second column in Figure 18.2). Perhaps a concerted effort to recruit all personality types to communicate important messages will better reach a broader audience, in terms that are more resonant with recipients.

But maybe our trajectory is pretty typical. By the time an intelligent species arises, the many millions or billions of years of life leading up to that event may ubiquitously lead to deposits of fossil fuels. The first species smart enough to utilize the planet’s fossil fuels does so with reckless abandon. Because evolution does not skip steps, we should not expect to find a species wise enough to refrain from rapid fossil fuel use emerging before a species who is just smart enough to use them, but not smart enough not to. So the “intelligent” species short-circuits the battery in a blaze of glory that may even involve baby-step excursions into space before either climate change or other resource/planetary limitations removes the fossil fuel source that made it all possible. Lacking wisdom and foresight, solid plans are not in place to handle the withdrawal, which does not go well and leaves the species in a crippled lower-tech state. Rebuilding from the ashes is then much less likely to explode without that one-time elixir that made it all possible the first time. A simple, and possibly quite satisfying life may await, but it may not involve traveling or communicating across space for others to learn of our existence.