I frequently meet people who think that overpopulation will lead to some future disaster (i.e., a “population bomb”). This is frustrating mostly because fear mongering about overpopulation has been a favourite past-time of many academics for more than two centuries now. The two most famous examples of overpopulation fear mongering came from Thomas Malthus in the 19th century and Paul Ehrlich in the 20th century. Both academics predicted global catastrophes at dates that have come and gone without humans surpassing carrying capacity.
These scholars were not necessarily bad scientists improperly using the scientific method. The human population has been growing exponentially since 1650, which is the longest period of exponential population growth for any organism… ever. As ecologists and biologists know, strongly r-selected species that experience exponential population growth for even a few years reach their carrying capacity and then experience a population collapse. Malthus and Ehrlich reasoned that this was bound to happen to us as well.
The mistake of both Malthus and Ehrlich was that they didn’t realize that humans are a strongly K-selected species and the cause of our exponential growth was a-typical (i.e., human ingenuity enabled us to continually raise the carrying capacity by improving medicine, health, and agricultural practices). But if Malthus and Ehrlich are wrong, does anyone have a better understanding of the future of our population? Can our population keep exploding exponentially?
First off, several population models have been constructed over the past two decades, and they provide us with interesting data. But perhaps the most insightful study was recently published by a team of Spanish mathematicians at the Autonomous University of Madrid. They modelled human population trends from 1900 to 2010. This enabled them to extrapolate these trends and make predictions for the future. They concluded that the world population would stop growing by mid-century (2050) at around ~8-9 billion individuals. This prediction is in line with the majority of the United Nations low-estimate projections for future population growth.
These researchers were able to make such firm predictions because of important trends in population growth that many people may be unaware of. For example, although the human population reached 7 billion recently, the growth rate peaked in 1963 (2.2% growth) and has been slowing ever since (in 2011 it was 1.1%). Also, the total annual birth rate peaked in the 1980s and has been declining ever since. And finally countries in the developed world are already at (or below) replacement level fertility levels (2.33 children per woman). In fact, the global population itself is quickly reaching replacement level fertility levels (which explains why growth rates and total annual birth rates are declining) (figure below).
What is causing population growth to slow?
Our species is not slowing down population growth because we are reaching carrying capacity. We are slowing down population growth because of education, gender equality, the rural-to-urban transition, and birth control.
Human growth rate is directly correlated with affluence. The richer a country becomes the slower their population grows. This is because affluent countries provide better education for both men and women. When women are educated they are freed to participate in society and build careers (as opposed to being career mothers). Women in developed affluent countries tend to have 2-3 children (or 0-1 children) as opposed to 5-10 children. In fact, even in developed countries, the trend for women to have fewer and fewer children may be continuing. As a result, it would not surprise me if fertility levels were well below 2 and approaching 1 in many developed countries by the 2030s. And as countries throughout the world modernize and develop, equal access to education for females should continue to spread.
Another major cause of the slower population growth is the rural-to-urban transition. In 1900 every country had a predominantly rural population. In rural areas farming is the dominant (if not only) way to make a living. This mode of production provides children with an important function: they can work the farm. As a result, there is an economic incentive to have children. However, children are very expensive in urban settings, and they are never an economic benefit because they are in school until they can provide for themselves. This always leads to family size decreasing in urban settings. And urbanization as a global process is not going to be stopped. Even conservative estimates suspect that 60% of the world’s population will be living in cities by 2030. The U.N. projects that it will be around 70% by 2050 (again, that is a conservative estimate).
Considering that countries in the developed world have already urbanized, the majority of these rural-to-urban migrations will be in the developing world (figure below).
Finally, the invention of (and cheap and easy access to) birth control is something that changed the Western world forever. Once women were able to gain more control over their own reproduction family size started to decrease. There were fewer unwanted/unplanned children. That is why providing cheap and easy access to birth control world wide is so important. Combined with female education and rapid urbanization, these forces will allow all countries to join the developed countries with a fertility level of 2.3 (or lower).
Of course, all of this (equal access to education for females and continued rapid urbanization leading to decreased global fertility) is all dependent on current rates of economic development in the developed world. Without raising the standard of living for the global population, the population trends observed today will reverse. And in order to ensure that the developing world’s economic growth continues, we must ensure that we transition to a new energy economy and avoid major nation-state wars.
Statistician Hans Rosling has calculated that all of these scenarios are probable. I agree. If current economic development trends continue we should expect the average person’s income in India and China (for example) to reach the same levels of the U.K., U.S.A., and Japan by 2048. Check out Rosling’s TED talk on this: Asia’s rise – how and when. Also, as Peter Diamandis and Elon Musk have pointed out, a transition to a new energy economy is highly probable and will likely happen over the next 20 years. This transition (from fossil fuels to predominantly solar) will not only provide us with clean and renewable energy, but also more abundant and cheaper energy. Finally, as Steven Pinker explained in his recent book The Better Angels of Our Nature, we are living in the most peaceful time in human history. There have been no developed world nation state wars since 1945. I believe this trend should continue.
All of this makes it highly probable that between now (2013) and 2050 global population will plateau. Equal access to education for females and rapid urbanization will lead the way. The trends are clear: the more educated women are and the more urbanized populations are, the smaller their family size. So long as current economic development trends continue, a transition to new energy is fulfilled, and developed world state-conflict remains non-existent, those social trends driving reduced fertility will also continue.
Due to all of the factors involved in this population transition, it may seem like an unlikely situation. But the key is that all of these trends are very strong and well under way. It would take an extreme reversal of current trends for population not to stabilize. That is why the global population researchers from the Autonomous University of Madrid were able to make such a strong conclusion from their mathematical models about our future population decline.
All this means that on the scale of hundreds of years our population growth may actually look like a very steep sigmoidal curve. But of course, in 2050 our planet and species will look very different than it currently is. There is a limit to what our models can predict about the future population. It could be that the human population plateaus and stabilizes. However, in a world with more energy, more geopolitical stability, advanced A.I., and a larger extraterrestrial presence, our species demographics may begin to change in unexpected ways. For now, we may simply be relieved that Malthus and Ehrlich were wrong. We will not encounter a population bomb.
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