If we want the population sizes to be reduced or at least stabilized by the mid twenty first century, we must make the necessary changes in the total fertility rates before the end of the twentieth century. We live in a time of short time horizons. It will be difficult to convince people to act now to change course, when the full results of the change may not be apparent in those people's lifetimes. The higher the standard of living one wishes to sustain, the more urgent it is to reduce population size. Fifth Law Sustainability requires that the size of the population be less than or equal to the carrying capacity of the ecosystem for the desired standard of living.
Sustainability requires an equilibrium between human society and stable ecosystems.
The rate of destruction of ecosystems increases as the rate of growth of the population increases. Population growth rates less than or equal to zero are necessary, but are not sufficient, conditions for halting the destruction of the environment. The individuals who promote growth are motivated by the recognition that growth is good for them. In order to gain public support for their goals, they must convince people that population growth and growth in the rates of consumption of resources are also good for society. When large efforts are made to improve the efficiency with which resources are used, the resulting savings are easily and completely wiped out by the added resource needs that arise as a consequence of modest increases in population.