When birth and death rates converge, the world’s population will peak, plateau and then continue rising, or level off. The trajectory of each rate will determine which of these possibilities is in our future. A death rate increasing faster than the birth rate will lead to a decrease in population, and a birth rate increasing faster than the death rate will lead to an increase in population.
The existence of a finite resource limit ensures that the population will peak and then drop, because there won’t be enough resources for people to survive. If resources are replenished at a constant rate, the population will level off. If the amount of resources is effectively unlimited, the population will grow. A brief leveling followed by an increase of population would indicate a temporary period where resources are replenished followed by access to a larger amount of resources.
The slowing of population growth since its maximum in 1976 implies that we are approaching a finite resource limit. If the deceleration showed signs of easing off instead of getting greater, we might be forced to live with constantly replenished resources and the population would approach a constant value.
We can slow our approach to the resource limit by decreasing our per capita consumption of resources, reducing the birth rate, increasing the death rate, or any combination of these (preferably not the last one); but eventually we will reach the limit and our population will crash. If the current trend continues, the population growth rate will drop past zero in 2017.
Of the resources we could be running out of, the most obvious is energy. Based on several curve fits of energy production, I expect that production of petroleum and natural gas will likely peak in 2018 and 2020, respectively. To make up the difference beyond 2018, other energy sources would need to provide an additional 312 quadrillion Btu per year, which in terms of non-fossil sources is almost four times the projected production in 2018 and nearly six times this year’s projected production.
Although production of petroleum and natural gas would be peaking about the time that the word’s population reached its maximum, there would still be a few decades of reserves of fuel left (at the same rates). This implies that both population and energy are being limited by some other resource.
I’ve suggested that the resource we are actually running out of is natural capital, the infrastructure that provides food, water, temperature control, and a vast array of other things and services that until now we have gotten for free on a renewable basis. The “running out” includes not only consumption but degradation (typically by waste). Consistent with this explanation, my projection of the Living Planet Index, a measure of the populations of other species, falls to zero by 2014, highlighting the stress that other species (a major part of natural capital) are enduring.
There is no “replacing” other species, or finding some new reserve that we can import; at least within the few years we might have left. Reducing our consumption of everything may, as I’ve said, buy some time, but there is a limit to how little we can consume and still keep living. Based on my consumption model, if the average person had an ecological footprint of 0.1 hectare (1/4 acre, the world minimum according to the World Wildlife Fund), we would put off other species crashing for about 180 years and our own by about ten times that long. It is doubtful, however, that most of us would choose to live like an average citizen of Afghanistan.