The underlying cause of global warming is population growth.
Total human population of the earth went from about 600 million people in the year 1700 to about 1.2 billion in 1850, as the industrial revolution took off. The curve accelerated, reaching 2.5 billion in 1950. Since then, industrialization spread from the Europeanized West to East Asia and elsewhere; reaching 6 billion in 1999, and 8 billion in 2022.
The acceleration in global warming is closely related to the trajectory of population growth since 1950. All the well-known human causes of warming in the atmosphere and the ocean are by-products of many more people on the earth: air pollution from automobiles, aircraft and fossil-fuel energy; cutting down forests and paving over green areas for buildings and transportation; expansion of agriculture producing methane gases. As world population continues to expand to a projected 9 or 10 billion in the next half-century, these processes can realistically be expected to continue to drive up global temperature.
The ultimate solution to global warming is to reverse the population explosion. This does not mean just stopping world population from growing further. Projections vary, but all of them are huge numbers: 6, 8, or 10 billion people still put tremendous pressure on the ecology. They will continue to generate a high global temperature, compared to the comparatively cool temperature of 1950 when the population was a third of today's.
Reversing population growth is difficult to do by direct government policy. Coercive population controls are regarded by many people as unethical. Instead, there has emerged a spontaneous trend towards the reduction of population: below-replacement fertility rates.
Fertility rates per woman were high during the long historic era of high infant mortality; and in most industrialized countries with scientific health measures birth rates eventually fell to around the replacement level of approximately 2.1 per woman. But anything above replacement level would not bring down the total population. The new trend, visible since about 2000 in many parts of the industrialized world, below-replacement birth rates, does have the potential not just to stabilize the growth of world population, but to reduce it. The underlying cause of global warming, if this trend continues, will be reversed.
Fertility rates are now about 1.6 in the USA, northern and western Europe; 1.3 in Italy and southern Europe, and in Japan. South Korea is down to 0.8; China to 1.2-- even after abolition of the one-child policy, people have become accustomed to having a very small family.
Most of the Western Hemisphere, Europe, and East Asia are below replacement level. The high growth areas that remain are Africa at 4.2, and the Islamic world generally.
News on below-replacement birth rates is usually viewed with alarm by economists and politicians. A steady flow of cohorts of new adults spurs spending, especially in the family-formation life phase. An aging population spends less. And any decline in sales tends to have negative ripple-effects. The capitalist business model is based on expectations of growth; even falling short of projected growth tends to drive down investment and stock values. An extended period of below-replacement birth rates foreshadows economic contraction.
Governments around the world have been offering incentives for women to have more children; but these have had little effect. (Japan and Hungary are two examples.) Having children is not just an economic decision; it is a life-style choice; and it is affected by cultural changes and career patterns that governments have little power to control.
Economic contraction may be the price to be paid for stopping global warming. Disastrous effects of rising ocean levels * cannot be stopped merely by keeping global temperature at its current level; it would require reversing global warming. If the below-replacement birth rate pattern continues to exist, in the wealthier parts of the world; and spreads elsewhere -- just as most other customs have spread -- we may see a world population decline within the next 50 years. An increasingly aging population in this respect would be less damaging to the world ecology; because old people do not consume as much in new products and energy use.
* "... the global mean sea level will rise up to 10 feet over the next 2,000 years [i.e. out to the year 4020], if warming is limited to 1.5 degrees Celsius. With a 2-degree Celsius increase, seas could rise up to 20 feet... according to the World Meteorological Organization." Associated Press, Feb. 15, 2023.
Economists and politicians can hardly be expected to be happy about falling birth rates as a solution to global warming. But thus far, most other fixes have not had much effect. Below-replacement birth rates are a spontaneous human reaction to life under present social conditions. Whether ultra-low birth rates will continue to trend downwards is an open question. We may regard this as a choice of evils. But since population growth has been the most important driver of global warming, we should welcome a reversal of this trend, whatever its economic effects.
The falling fertility rates of the last few decdes are a gift to world ecology. It is almost as if, after 70 years of population boom and global warming, the human part of our ecology has spontaneously evolved a solution. Not immediately, since everything in the process of evolution takes time. But on the horizon of the foreseeable future, the world will survive and perhaps prosper.
