My population-consumption model provides some insight into the minimum we must do to avoid population loss if we cannot increase the amount of available resources.
One option, a variant of the ideal world scenario, keeps the population constant at its current level. It involves increasing capacity (the amount of resources that are renewable) by 44 percent from 11.5 billion to at least 16.6 billion hectares (1/3 of the Earth’s surface area) at an annual rate of at least 0.2 percent (comparable to what the rate was in the 1960s), and keeping per capita consumption practically constant. The core population growth rate, historically 2.0 percent, would need to fall to 1.1 percent.
The longer we wait to change our behavior, the harder it will be to avoid population loss because in the interim the population will have grown and we will have consumed more resources. My model projects that if instead of this year we wait until 2011, we will need to triple the rate at which we must add capacity (0.6 percent annually) to a maximum that has crept up nearly 5 percent to 17.4 billion hectares. Also, the core population growth rate will need to increase to 1.3 percent.
There is a considerable amount of sensitivity of population change to the values of these variables and the interactions between them. For example, the exponent of the year in the function for per capita consumption must be very slightly negative (between a hundredth of a billionth and a billionth) to avoid any population loss. Because this results in a reduction in per capita consumption over time (albeit a very small one) the core population growth must compensate (if we had zero core population growth in the above example starting this year, the population would fall by 70 million people). If the core population growth increases (in an attempt to elevate the population) then the maximum capacity and rate of capacity increase must also rise to avoid a reduction in total resources.
Based on my analysis, it makes sense as a general rule to increase capacity as much and as fast as possible (more than half, at a rate of at least one-quarter percent annually), while keeping per capita consumption constant and the live birth rate above 1.1 percent.