Is population an important variable in carbon emissions? Sure. But it's far from the only variable and by no means does it determine carbon emissions.
Take China, a country where growth trends for population and emissions have diverged widely in recent decades. China's population growth has slowed markedly since the early 1980s, when Beijing began enforcing a strict one-child policy. Average annual population growth was 1.25 percent between 1987 and 2000, but between 2000 and 2014 it was only 0.56 percent—half as high in the later period as in the earlier period. But carbon emissions tell a different story. Between 1987 and 2000, annual growth in carbon dioxide emissions was 4.75 percent. Between 2000 and 2014, the annual rate of growth increased to 12.7 percent—well over twice as high in the later period as in the earlier one.
This is all consistent with an equation known as the Kaya identity, which states that carbon dioxide emissions are the product of four factors. Human population is one of these factors. The other three are gross domestic product per capita; energy intensity (the amount of energy used per unit of gross domestic product); and carbon intensity (the amount of carbon dioxide produced per unit of energy used). The "other three" factors can ultimately be summarized in one number—emissions per capita—a number that can decline when people's behavior changes and nations achieve social progress. Social progress and economic growth can, as demonstrated by experiences in East Asia, Western Europe, and North America, reduce population growth rates. So the best way to limit carbon dioxide emissions is through altering behavior—not through limiting population.
Management and governance. Carbon dioxide emissions are a kind of pollution, but people nonetheless have a right to emit carbon dioxide as they go about their lives. Unfortunately, individual people have strong incentives to produce more carbon pollution than is in everyone's collective interest—that is, the "tragedy of the commons" comes into play. Lowering carbon dioxide emissions per capita, then, requires good management of people's emissions behavior, which in turn requires that low-carbon infrastructure and good carbon governance be established.
Low-carbon infrastructure includes, in addition to public transportation systems such as subways and high-speed railways, low-carbon energy generation systems such as wind and solar farms and electricity grids that link customers with low-carbon energy. In recent years, much of this has been established in Sichuan province, the area in southwestern China where I was born. For many centuries the farming families in Sichuan had burned biomass for cooking—but in the early 1990s, Sichuanese people began purchasing coal to burn in their stoves. Sichuan's coal, unfortunately, is generally of lower quality than the coal produced in most other Chinese provinces—its sulfur content is higher and its heating value is lower. In March 2014, however, drilling began at the giant Longwangmiao natural gas field in southern Sichuan. A pipeline soon began to transport gas from the field to nearby cities, towns, and villages. Natural gas is more expensive than coal or biomass, but farmers enthusiastically embraced natural gas for cooking because natural gas is cleaner and gas stoves make it very easy to adjust the flame—a very important issue in Sichuanese cuisine. The gas pipeline and other infrastructure elements have helped bring about an energy revolution for Sichuan's farming families—a revolution that, because natural gas entails much lower carbon dioxide emissions than coal, reduces emissions per capita.
But for climate mitigation, establishing good governance is as important as improving infrastructure. Low-carbon infrastructure is of little value if consumers are unable or unwilling to use it. Inconvenience and high costs can undercut the attractiveness of low-carbon infrastructure—if subways and high-speed railway systems are too expensive, for example, customers will shy away from them. Certain entities might even intentionally make it inconvenient to use low-carbon infrastructure. For example, the electricity grids in China are monopolized by two state-owned enterprises—the State Grid Corporation of China and the China Southern Power Grid Company. These companies prefer the stability of coal-fired electricity and other types of thermal electricity to intermittent energy sources such as wind. In China's currently sluggish economic environment, thermal electricity can meet nearly all electricity demand, and the grid companies are intentionally making it difficult to connect wind power to the grid. Under such circumstances it becomes very difficult, if not impossible, for customers to consume green electricity. So even if low-carbon infrastructure is perfect, utilization rates will be low if governance and management are poor.
Reducing emissions per capita depends among other things on supportive tax policy, pricing, and financing. For example, governments can "internalize" the otherwise externalized costs of high-carbon energy sources by levying carbon taxes on coal and oil. By implementing carbon taxes and providing subsidies for renewable energy, governments can support the development of low-carbon energy sources such as hydrogen-based power and wind and solar energy.
Getting educated. But is it possible to limit or reduce per capita emissions as societies get richer? Certainly, rich people are able to consume more of everything, including high-carbon energy. On the other hand, the recently concluded Paris climate agreement, in its requirement that developed countries provide developing countries with $100 billion a year for climate mitigation and adaptation, incorporates the idea that poor countries will be able to reduce their carbon emissions if they have more money on hand. But in today's China, a real threat exists that carbon dioxide emissions will increase because of poverty. China's economic growth is slowing. Many Chinese people are increasingly unable to pay extra for low-carbon energy and local governments are increasingly unable to subsidize such energy. Indeed, some government entities and poor consumers are returning to cheap, high-carbon energy sources such as coal. The weakening economy is harming air quality in China and also reducing ambitions for climate mitigation. The jury is still out on how economic fortunes will influence carbon emissions going forward.
Beyond building low-carbon infrastructure and establishing good governance, one more element is necessary for cutting carbon emissions—improving education, especially for girls. Good education allows people to understand the importance and urgency of reducing carbon emissions. In advanced societies with good education systems, consumption of high-carbon energy may eventually become taboo. Providing good education to girls in particular—again, as experiences in East Asia, Western Europe, and North America demonstrate—will allow women to pursue careers, delay childbearing, and give birth to fewer children. This process will naturally place limits on global human population and, albeit in an indirect way, help reduce carbon dioxide emissions.