The key feature of my consumption model is the strong correlation between consumption and population, where “consumption” is defined as the total mass processed by humans. I’ve assumed that the mass consumed per year is proportional to the global ecological footprint as measured by the World Wildlife Fund (WWF), which has increased exponentially over time. Population as a function of consumption appears to be a downward facing parabola: that is, it rises to a peak and then drops off at the same rate. The rapid rise of consumption results in a faster approach to the population peak and the other side of the consumption-population curve, thus appearing as a population crash.
This mathematical description, if accurate, explains the “what,” but not the “how” or “why” behind it. If converting mass in the environment can yield more people, how can converting more mass yield less people? The simplest explanation I can think of is the analogy of someone generating a lot of waste in the process of building a house, growing food, and so on, but eventually overwhelming the trash collectors who haul the toxic waste away and reprocess it. Eventually the waste overwhelms the people as well.
Conservation biologists have a list of factors they use to explain how the populations of other species are lowered by human activity, which is summarized by the acronym HIPPO: Loss of habitat (“H”), introduction of invasive species (“I”), pollution (“P”), human population (the second “P”), and hunting or over-harvesting (“O”). All of these factors are no doubt behind the other correlation I found, between increasing consumption and a decreasing Living Planet Index, which indicates the size of the populations of other species. HIPPO may also help explain how our own population is poised to drop.
No matter how far we attempt to distance our fate from a reliance on Nature, humans are still dependent on other species for a wide range of services, not the least of which include cleaning the air and water, and providing and processing food. As other species die off, they can no longer provide those services. Without their services, we must pick up the slack, and our technology is not quite up to the task.
But HIPPO may have more direct effects on us, especially (as I’ve indicated) through pollution. Toxins that poison us or cause or exacerbate diseases such as cancer, and carbon dioxide emissions that change the climate (which adversely affects food production, introduces invasive disease-bearing species to environments ill-equipped to deal with them, as well as increasing the intensity of storms and droughts) are perhaps the most significant ways that pollution can reduce the human population. Over-harvesting may also play a role, in the form of more wars and violence, which could be traced to increased stress as members of our population interfere with each other’s lives more and more, while competing for fewer resources (loss of habitat).