Light of the Stars

by Adam Frank

  The specific conditions on each planet will ultimately determine the mix of energy modes available to a civilization evolving there. Geothermal may be more favorable on some worlds, while wind may be more easily tapped on others. The main point for now is that the list above hits almost all the choices. Other than imagining exotic planets with special magnetic fields or continuous lightning conditions, what’s on the list above is all that exists. Adding new energy sources other than those we’ve listed requires inventing science fiction stories about discovering “new physics.”

  Step 5: Know the Impact. Since we can list the different energy sources available to a young civilization, we can also calculate the planetary impact of their use. If this sounds like science fiction, remember that way back in 1903, Svante Arrhenius carried out exactly this kind of calculation for the Earth and combustion (that is, the burning of fossil fuels). Arrhenius knew the composition of the Earth’s atmosphere, and he could calculate the impact of using coal. This impact was the production of CO2, and the change it produced was an enhanced greenhouse effect.19

  So, for civilizations powered by combustion, we already know how to model their impacts on their planets. All that’s needed is to account for the potential differences in their host planets’ properties, which will include things like the composition of the atmosphere and orbital location in the habitable zone.

  What about the impact of other energy sources? For some cases, the calculations have already been started. A study by scientists at the Max Planck Institute in Germany looked at the global effects of wind power. Wind turbines work by pulling energy out of smooth, large-scale flows of air and turning it into electricity. But in the process, they leave choppy, turbulent airflows downstream. The German group found that extracting energy from wind power on a scale massive enough to power our current civilization would leave a global imprint akin to mild global warming. Even wind, the darling of renewable-energy harvesting modalities, has a planetary cost (though far lower than fossil fuels).20

  Because we have a deep understanding of the physics and chemistry of each of the energy sources listed above, it doesn’t take a quantum leap in science to calculate how their use will produce feedbacks on a planet other than our own. For each energy source a civilization might harvest, we have the information necessary to calculate the associated planetary cost. With that capacity, we reach the final step in the path to our theoretical archaeology of exo-civilizations.

  Step 6: Turn the Crank. Given steps 1 through 5, we now have a recipe for calculating exo-civilization histories. We begin by creating a model for the interaction of a young civilization with its planetary environment. This model will come in the form of equations predicting how the civilization’s population and its host planetary systems change with time. As in the predator-prey model, the equations will be coupled. There will be an equation describing the change in the planetary systems (such as atmosphere) and an equation describing the changes in the civilization-building population. Each equation will have terms that describe the feedback from planet to civilization and civilization to planet. It’s worth mentioning that to do this job well, we’ll need more than just two equations, because we’ll probably need to track different resources and their use, along with their effect on the different planetary systems like oceans, ice, and so forth. But for now, we can stick with just “the planet” and “the civilization.”

  In general, the civilization will use its energy sources, and the waste from those energy sources will push on the state of the planetary systems. As the planetary systems shift based on the feedback, the civilization will either thrive or be stressed, as reflected in how their population changes. Because the coupling will be complicated, we won’t know what to expect until we’ve solved the equations making up the model.

  Doing this once doesn’t tell us very much. What we are interested in is Drake’s final factor: the average lifetime of civilizations. In order to calculate an average, we will have to run our models many times for many different kinds of planets. In a sense, by running the experiment in civilization building over and over, we will create our own mini-version of the universe. Some of the model runs will begin with planets that are close to the inner edge of their stars’ habitable zones, where they’ll be particularly susceptible to enhanced greenhouse warming. Some will be farther out. Some of our model runs will have planets with atmospheres that have less oxygen than ours, while others will have more oxygen. Some will begin with civilizations using wind power, and others will begin with civilizations using geothermal. You get the picture.

  In the end, we will have to “turn the crank” and run tens of thousands of models, each with different starting conditions. That might seem like a lot of work, but modern computers are fast.

  PATHS TO PROGRESS, ROADS TO HELL

  Carrying out a theoretical archaeology of exo-civilizations correctly will be demanding. It will require input from fields as diverse as atmospheric science, geology, energy science, and ecology. To create realistic models, we’ll have to get the physics, chemistry, planetary science, and ecological interactions right in terms of what we build into the models. That is going to be a long and interesting project.

  But even as we build our way toward that goal, we can take some initial steps now. These first explorations can give scientists the lay of the astrobiological exo-civilization landscape. In the fall of 2016, a team of us went on this kind of scouting mission. The result was simultaneously thrilling, hopeful, and possibly a little depressing.

  Our team included Marina Alberti, an urban ecologist from the University of Washington. A native of Italy, her passion is how evolution is already responding to the Anthropocene. Marina studies urban environments and how new species are being created in the midst of our vast project of planetwide city building. Axel Kleidon was also part of the effort. Axel is also an innovative thinker who works at the Max Planck Institute for Biogeochemistry, developing new ways to look at the Earth as a single thermodynamic system, like a giant, planetwide steam engine. Finally, there was Jonathan Carroll-Nellenback. Jonathan was my graduate student years ago and now works with me as a senior computational scientist at the University of Rochester. His talent for theoretical work is pretty remarkable. Sometimes I’ll bring Jonathan a problem in the morning, and by the next day he’ll bring it back, fully solved and displayed in beautiful graphics.

  Together, we formulated a model for the evolution of a civilization with its planet. The equations were pretty simple. We weren’t trying to capture the details of Earth or of any other specific planet. Instead, our aim was to describe the interaction of civilizations and planets in the most general way possible, which would serve as a first step toward doing something more detailed and realistic.

  In our approach, the population and the environment were linked via an energy resource. The planet supplied the energy resource, and the civilization used the energy resource. Greater energy use meant a larger population on the one hand, and greater environmental change on the other. Greater environmental change lowered the planet’s carrying capacity for the civilization, which should lead to lower populations.

  Along with these features, we also included a specific mechanism to describe how the civilization might respond to changing conditions on its planet. For the sake of simplicity, we imagined that the planet had just two kinds of energy resources. One resource had a high planetary impact (as fossil fuels do), while the other had a low impact (as solar energy does). Here, high and low impact reflected the degree to which using the energy source forced the planetary environment to change.

  Once the planetary environment was pushed past some predefined point, the civilization switched energy resources. You can think of this in terms of the planet’s temperature. Once the planetary temperature rose to the specified value, the civilization stopped using the high-impact energy source and switched to the low-impact source.

  Using this strategy in the models gave us a specific and simple way to boil down th
e civilization’s sociology. We didn’t want to try and model how they’d recognize and act on their Anthropocene. Instead, it came down to the planetary temperature that finally gets the civilization to do something. Since that was just an input, we could change it from one run to another and see how history played out for “smart” civilizations and “dumb” ones. Either the civilization acted early, when their planet’s temperature had just started to rise, or they acted late. While we couldn’t model the sociology of how they made that choice, we could model the choice’s physical consequences. Would acting early save them? Would anything save them?

  So, what did the model tell us?

  Our exploration of the exo-civilization/planet system yielded three distinct trajectories. The first—and, alarmingly, most common—was what we called “the die-off.” As the civilization used its energy resource, its numbers grew as expected (see page 196, graph A). But the use of the resource pushed the planetary environment away from its initial state. As the evolution of the coupled civilization/planet system continued, the population rose sharply beyond what the environment could sustain. The population, in other words, overshot what the planet could support. A big reduction in the civilization’s population followed, until both the planet and the civilization had reached a steady state. After that point, neither the population nor the planet changed anymore. A sustainable planetary civilization was achieved, but at a considerable cost.

  In many of the models, we saw as much as 70 percent of the population die before a steady state was reached. Imagine seven out of every ten people you know perishing because of global climate catastrophes. It’s not clear how large of a die-off a complex technological society could handle without falling apart. During the period of the Black Death in the fourteenth century, Europe lost between 30 and 50 percent of its population, but managed to revive. Medieval Europe, of course, wasn’t highly technological in the modern sense, nor as isolated as a planet in space would be.

  The second trajectory class we found was one we called the “soft landing” (see page 196, graph B). The population grew and the planet changed, but the models showed a smooth transition to a steady state after an early switch to the low-impact energy resource. Eventually, the civilization came into equilibrium with its planet without a massive die-off.

  Four kinds of trajectories for exo-civilizations and their planets discovered from mathematical models.

  The final class of trajectory was the most worrisome: full-blown collapse. As in the die-off models, the population initially grew swiftly. In this case, however, the speed of planetary change pulled the planet’s carrying capacity down so fast that the population plummeted all the way to extinction.

  One of the most remarkable aspects of this class was that the collapse was inevitable. One would think that switching from the high- to low-impact energy source would make things better. But for some trajectories, it didn’t matter. If we used only the high-impact resource, the population reached a peak and then quickly dropped to zero (graph C). If we allowed the civilization to switch to the low-impact version of an energy resource, the collapse was only delayed. The population would start to fall, then appear to stabilize, and finally, suddenly, rush downward to extinction (graph D).

  The collapses that occurred even when the civilization did the smart thing demonstrate an essential point about the modeling process: it can surprise you. Because the equations representing the model are complex, unexpected behavior can happen. These are consequences you wouldn’t have thought of if you hadn’t done the work of cranking out the solutions.

  Only after you study the behavior seen in the models do you understand what happened. Remember that our simplified models were tracing the development of a civilization and its planet together. In the case of the delayed-collapse trajectories, we were finding scenarios that showed us that switching from a high- to low-impact energy resource won’t matter if the change is made too late. Even though the civilization in our model recognized its entry into an Anthropocene-like transition and switched energy sources to make things better, the planet was already heading into new climate territory. Once the ball got rolling, the planet’s own internal machinery took over. It wasn’t coming back to the original climate state, and it took the civilization down with it as it ran away into a new state.

  In these cases, the planetary environment’s own dynamics were the culprit. Push a planet too hard, and it won’t return to where it began. We know this can happen, even without a civilization present, because of what happened with Venus and its runaway greenhouse effect. Our models were showing, in generic terms, how a civilization could push a planet into a different kind of runaway through its own activity.

  The work that Jonathan, Marina, Axel, and I did showed us some of the basic ways a civilization and its planet might change together. It was good that we saw that long-term sustainable versions of the planet-plus-civilization system were possible. But the warnings were there as well. The self-perpetuating feedbacks that drove some civilizations to collapse, even after they made the smart choices, were particularly sobering.

  THE FINAL FACTOR

  It’s reasonable to ask what this archaeology of exo-civilizations really tells us about reality. Aren’t these models just mathematical toys? Isn’t it true that we have not a single instance of a civilization other than our own to make comparisons with? Answering these questions will help us see what can be gained by taking exo-civilizations seriously as subjects of scientific inquiry. It will also help us see what’s at stake for us as we try to use this astrobiological perspective to understand our choices about our own project of civilization.

  Models and Reality: The trajectory of the Anthropocene shown with real data for world energy consumption, CO2 concentration, and global population for the last 10,000 years.

  It is absolutely true that models and reality are two different things. A model is a simplification, like a skeleton without the muscle and skin. But looking at just a skeleton will tell you a lot about the animal. That is how we know about dinosaurs. More to the point, as we move forward, our models will be based on ever more sophisticated versions of what we already know about how planets work. They are, and will be, built on ever-stronger skeleton frames of physics and chemistry—in other words, the laws of planets. In that way, they are far more than mere imaginative toys.

  The models allow us to go beyond fiction. By relying on the laws of planets, they capture key aspects of reality. That means they have their own logic. They have their own stories to tell us that we would not see without them. It’s one thing to argue over what you think will happen when a civilization on a distant planet becomes technologically sophisticated. Your friend might have a different opinion, and that’s an all-night argument waiting to happen. But it’s something entirely different to spin up the math and let it see into the complexities that elude us. Instead of mere opinion, we can let the model show us how the universe might behave. The realistic constraints models place on their stories give those stories scientific value. It grounds them in the realm of the possible.

  All the research we’ve explored in this chapter constitutes just a first step. It’s an outline of what this kind of enterprise will look like as we devote more time and effort to the endeavor. The stories we’ve told here are just the first of many, and they will grow more precise as our understanding increases.

  The next step will be to build far more realistic models and use them to explore a much wider range of realistic cases. After running these models for hundreds of thousands of different situations, we will have the simulated trajectories—the histories—of hundreds of thousands of inhabited worlds.

  A planet that lives close to the inner edge of its habitable zone might be so highly sensitive to runaway greenhouse warming that its civilization barely has time to progress before it faces its own version of the Anthropocene and collapses. Another world, farther out from its star, may be less sensitive to planetary change but have a civilization that refuses to re
cognize the change until the die-off has already begun. A different species on a different world could manage to build its project of civilization using only lower-impact forms of energy and make a gentle soft landing to a sustainable state that lasts thousands of millennia.

  What part of these stories matters to us? The answer to that question is simple: Drake’s final factor. With trajectories for millions of simulated planets and civilizations in hand, we can calculate an average lifetime. How long, on average, does a civilization last?

  Consider, for a moment, what that single number would tell us.

  If the average lifetime of exo-civilizations is two hundred years, then we are in big trouble. If we find most model civilizations collapse after just a few centuries, the implication would be that civilizations like ours just don’t work well on a planetary scale. A short average lifetime would mean that the universe doesn’t do sustainable civilizations. The lesson would be that we humans are threading the eye of a needle with the Anthropocene and don’t have much room for error in our choices. In that case, it may already be too late.

  If the average lifetime of civilizations emerging from our models were tens of thousands of years, that would be good news. It would mean it’s not too hard for any civilization to make it through the bottleneck of an Anthropocene.21 There would be lots of different strategies for reducing our impact on the planetary systems that work. It would mean we have lots of wiggle room. We could make mistakes and still recover.

  In this way, a single number from our archaeology of exo-civilizations—the average civilization lifetime—would have profound implications for our own future and our actions in the present. It would let us see what might be coming. And with that knowledge, our understanding of the choices we face would become deeper, richer, and be based on some wisdom.