Let’s assume, just for a moment, that the human population on earth never stops growing. That must lead to disaster unless humans find a way to travel to new planets — certain of my resources are finite no matter how you look at them. So, being optimistic for a moment, let’s discard that potential future and consider instead a future in which population growth halts and even declines. What might that look like?
From a non-human perspective, the impact of the human population can be felt in terms of land and water use (to create food or harvest food), waste products, and the use of non-renewables to create everything from fertilize to the ubiquitous stuff that humans seem to love so much. Even with a stable population, some of these impact could still be quite severe. For example, pollutants that take a long time to be absorbed back into the natural order of things would continue to build up even with a decline in population. However, this scenario is certainly better than continued growth.
Another question that must be asked, of course, is how drastic the decline is, and what its impact is. For example, rapid die off on a large scale could result in positive things (such as an opportunity for flora and fauna to recover land) but also negative (imagine an abandoned nuclear power plant, war, or other possible consequences). Since this post is attempting an optimistic viewpoint, let us assume for the moment a not-too-rapid, controlled decline followed by a leveling out when the population reaches a level that can more easily be sustained by my currently groaning body.
What might that look like? According to the Global Footprint Network, the 2007 footprint of the human population was 1.5 earths . This would mean that (assuming no technological advances), the human population needs to decline by 33% (approximately) from the 2007 levels (6.67 billion), or by 2.5 billion people from the current 7 billion person population.
Over what time period might this take place? If the world population declined by 250 million people per year for 10 years, or 25 million per year for 100 years, we might achieve that goal. This is a very large number (a the list of anthropogenic disasters on Wikipedia has WWII as the worst, with a death toll of between 40 and 70 million over 5 years, or between 2% and 3% of the world population at the time) .
Although it is hard to find many attempts to model the impacts of population decline on humans themselves, there are some research articles on the topic. In particular, one consequence of controlled population decline is an excess of elderly individuals. According to Bloom et al , the relationship between fertility decline (which leads to population decline when births per year are less than deaths per year, and also to an aging population) and the size of the workforce is positive in the short run (meaning an increased percentage of the population is of working age) but negative in the long run (as the number of elders rises). According to the authors, migration has a relatively small effect on this. The paper concludes that population decline “may reduce output per capita” after a certain point.
An apparently open question is how controlled but large population decline (or even controlled but small population decline) would impact economic growth. Factors worth considering include how it might push off the end date of certain finite resources, how it might effect “dematerialized” growth, what institutions might be affected by the decline period (for example, how would a declining population of young people affect educational institutions?), what policies would best support a population with a relatively small workforce (compared to the number of aging adults) and so on. Additionally, it would be worth considering how such a decline might affect key resources (for example would it imply food scarcity without a shift in where people live?). An optimistic model of population decline (meaning one that does not assume causal disasters) would ideally help to shed light on which forms of decline might be more likely to lead to conflict within and among nations (if any). In examining all of these variables, it would also be fruitful to explore at what rate decline is most manageable, and to test these assumptions against existing examples of populations that have experienced decline.
Other people’s posts about population decline: [Wikipedia][Regions of Canada][Japan][Projected changes in Europe and impact of immigration][Russia]