“Population biology was a radical new field back in the early 20th century. Rather than just collecting statistics to describe animal populations, a few ambitious researchers like Alfred Lotka wanted to create basic mathematical models of things like predators and prey to predict the evolution of their linked populations. Predators (like wolves) eat prey (like bunnies) so they can make more wolf babies, thereby increasing the wolf population. Bunnies do a fine job of reproducing on their own, but if too many are eaten, their population numbers suffer. Today, population biologists, ecologists, and their compatriots use mathematical models to study everything from the spread of disease to the propagation of invasive species. The approach has even found its way to the study of human civilizations, including their collapse in places like Easter Island.

“We used these tools to build a simple model for the evolution of a civilization with its planet. In our approach, the exo-civilization’s population and the planetary environment are braided together by energy use and its consequences. The planet gives the civilization energy resources. The civilization consumes them to do the work of civilization building. As a civilization harvests more power from the planet, its capacities soar. That includes the ability to make and feed more babies. This link between available energy (in the form of food for simple organisms) and rising birth rates is fundamental to population biology. And for human civilization the steep rise we’ve seen in population is closely tied to fertilizer involving fossil-fuel use. So greater energy will, in the beginning, mean bigger populations. But there’s no free lunch from a planetary perspective. Using all that energy has to result in feedback on the planet. That’s what we earthlings are just starting to see with climate change. If global warming gets really nasty, everything from energy harvesting to food production is going to get severely stressed and our large human population won’t be sustainable. That’s why our exo-civilization models linked rising planetary impacts with population declines. It was all pretty straightforward, requiring no assumptions about alien economics, sociology, or any other science-fiction ideas.

“But to allow for some choice on the part of the exo-civilization we also included a basic switch describing how the civilization could respond to changing planetary conditions. For the sake of simplicity, we imagined that the planet had just two kinds of energy resources. One had a high planetary impact (like fossil fuels). The other had low impact (like solar energy). In some models we allowed the civilization to switch from to one to the other as things got bad.

“So, what did the model tell us? We saw three distinct kinds of civilizational histories. The first—and, alarmingly, most common—was what we called “the die-off.” As the civilization used energy, its numbers grew rapidly, but the use of the resource also pushed the planet away from the conditions the civilization grew up with. As the evolution of the civilization and planet continued, the population skyrocketed, blowing past the planet’s limits. The population, in other words, overshot the planet’s carrying capacity. Then came a big reduction in the civilization’s population until both the planet and the civilization reached a steady state. After that the population and the planet stopped changing. A sustainable planetary civilization was achieved, but at a high cost. In many of the models, we saw as much as 70 percent of the population perish before a steady state was reached. In reality, it’s not clear that a complex technological civilization like ours could survive such a catastrophe…”