I was able to remove most of the remaining error in my theoretical consumption model by using a power function (rather than a linear function) for per capita consumption as a function of time. Consumption (ecological footprint) and population now match the data from 1961 through 2003 to an average of no more than 0.6 percent (oscillating to a peak 15 percent for footprint and a peak 3 percent for population). In addition, a new feature of the model reflects people’s resistance to reducing consumption, effectively keeping population from increasing at the expense of per capita consumption: If per capita consumption drops, population will drop proportionally to compensate for it.
The modified model projects that for business as usual the world’s population will peak in 2020 at a value of 7.3 billion and drop to a stable value of 1.7 billion by 2132. Of the 530 billion hectares of resources currently available, all but a renewable amount (capacity) of 11 billion hectares will remain, with an average per capita consumption of 6.5 hectares. Ideality (the average of lifespan in years and happiness in percent), currently 64, will rise to a stable value of 74 for the survivors. This option is likely the one that results in the highest ideality.
If capacity and resources don’t grow, the option resulting in the best outcome in terms of population and ideality ends up effectively preserving ideality (per capita consumption and ideality are constant). It is attained by reducing growth in per capita consumption (by decreasing the exponent of time in the power function to about 1/17,000 of its current value) and having only 0.05 percent population growth. In this scenario we lose more than one quarter of the population over the next 60 years, with subsequent losses becoming very gradual (the population drops below 4.9 billion by the end of the next century).
By increasing capacity without increasing resources, we could grow our population to a maximum of about 190 billion people while maintaining our current ideality. This would be a monumental task, since it would require an annual capacity increase of 5.5 percent to 42 times its current value (11 billion hectares to 480 billion hectares). This option is what I’ve previously referred to as the “ideal world scenario.”
If we increase both capacity and the amount of resources, then we can grow the population even higher, but again ideality won’t change. The trajectory of population growth is strongly dependent on how fast resources can be reached and consumed, and less so on how fast we can increase capacity (the latter basically operating to keep the population from decreasing when the maximum speed is reached). By my calculations, we could spend at least the next six thousand years devouring just the mass in the Solar System, with a population growing beyond one thousand trillion people.