Humanity’s consumption behaviors consequently did and do matter, and in this arena, all people have not been created equal. Greenhouse gas release has been linked overwhelmingly, at least up until recently, to the high-consumption habits of the industrial nations. As a result, in an ethical outrage as big as all outdoors, the coming shifts in climate and sea level will most harm the world’s poor, who are least responsible for the atmosphere’s composition, and will least harm the wealthy, who bear the biggest responsibility.
What part can the size of the human race play in finding a happy ending to this morality play? Population scenarios cannot directly address the inequity in emissions patterns—but they are far from unimportant.
Countries with the highest emissions per capita tend to have smaller families on average, whereas those with low emissions per capita tend to have larger ones. Americans, for example, consumed 8.6 tons of oil or its commercial energy equivalent per capita in 2007, according to data kept by British Petroleum; Indians consumed just 0.4 ton per capita. (These figures somewhat distort the gap because they exclude biomass and other noncommercial forms of energy, for which data are unreliable.)
So while India gained 17 million people in that year and the U.S. gained three million, by this simplified math the U.S. growth in population counted for the equivalent of an additional 25.6 million tons of oil consumed, whereas India’s much greater growth counted for only 6.6 million additional tons. With such large disparities, the climate would be better served if the Americans emulated Indian consumption than if India emulated U.S. population.
End of story? For a variety of reasons, not quite. Population is not a contrasting force to consumption but something very close to its parent. Alone, each of us has no significant impact on the planet, even when our collective behavior overwhelms its natural processes. Historically, population has grown fastest when per capita consumption is modest. Later, consumption tends to explode on the base of a population that is large, but it is by then growing more slowly. Throughout the 19th century, the U.S. population grew at rates typical of Africa today. That century of rapid growth helped to make 21st-century America (with 307 million people now) a consumption behemoth.
The same one-two punch of population growth followed by consumption growth is now occurring in China (1.34 billion people) and India (1.2 billion). Per capita commercial energy use has been growing so rapidly in both countries (or at least it was through 2007 on the eve of the economic meltdown) that if the trends continue unabated the typical Chinese will outconsume the typical American before 2040, with Indians surpassing Americans by 2080. Population and consumption thus feed on each other’s growth to expand humans’ environmental footprint exponentially over time.
Moreover, because every human being consumes and disposes of multiple natural resources, a birth that does not occur averts consumption impacts in every direction. A person reducing her carbon footprint, conversely, does not automatically use less water. A wind turbine displaces coal-fired electricity but hardly prevents the depletion of forests (now disappearing in the tropics at the rate of one Kentucky-size swath a year) or fisheries (at current depletion rates facing exhaustion by the middle of the century). But unlike wind turbines, humans reproduce themselves. So every smaller generation means that the multipliers of consumption linked to population also shrink on into the future.
Because most environmental challenges emerge on scales of decades and centuries, population growth packs a long-term wallop. With respect to saving the planet, over a few short years it is hard for smaller families to beat sharp reductions in per capita consumption. Since the early 1990s, however, published calculations have demonstrated that slower population growth over decades yields significant reductions of greenhouse gas emissions even in countries where per capita fossil-fuel consumption is modest.