Human Population Growth and Climate Change

The largest single threat to the ecology and biodiversity of the planet in the decades to come will be global climate disruption due to the buildup of human-generated greenhouse gases in the atmosphere. People around the world are beginning to address the problem by reducing their carbon footprint through less consumption and better technology. But unsustainable human population growth can overwhelm those efforts, leading us to conclude that we not only need smaller footprints, but fewer feet.

Portland, Oregon, for example, decreased its combined per-capita residential energy and car driving carbon footprint by 5 percent between 2000 and 2005. During this same period, however, its population grew by 8 percent.

A 2009 study of the relationship between population growth and global warming determined that the “carbon legacy” of just one child can produce 20 times more greenhouse gas than a person will save by driving a high-mileage car, recycling, using energy-efficient appliances and light bulbs, etc. Each child born in the United States will add about 9,441 metric tons of carbon dioxide to the carbon legacy of an average parent. The study concludes, “Clearly, the potential savings from reduced reproduction are huge compared to the savings that can be achieved by changes in lifestyle.”

One of the study’s authors, Paul Murtaugh, warned that: “In discussions about climate change, we tend to focus on the carbon emissions of an individual over his or her lifetime. Those are important issues and it's essential that they should be considered. But an added challenge facing us is continuing population growth and increasing global consumption of resources. . . . Future growth amplifies the consequences of people's reproductive choices today, the same way that compound interest amplifies a bank balance."

The size of the carbon legacy is closely tied to consumption patterns. Under current conditions, a child born in the United States will be responsible for almost seven times the carbon emissions of a child born in China and 168 times the impact of a child born in Bangladesh.

The globalization of the world economy, moreover, can mask the true carbon footprint of individual nations. China, for example, recently surpassed the United States to become the world’s leading greenhouse gas emitter. But a large portion of those gases is emitted in the production of consumer goods for the United States and Europe. Thus a large share of “China’s” greenhouse gas footprint is actually the displaced footprint of high-consumption western nations.

The United States has the largest population in the developed world, and is the only developed nation experiencing significant population growth: Its population may double before the end of the century. Its 300 million inhabitants produce greenhouse gases at a per-capita rate that is more than double that of Europe, five times the global average, and more than 10 times the average of developing nations. The U.S. greenhouse gas contribution is driven by a disastrous combination of high population, significant growth, and massive (and rising) consumption levels, and thus far, lack of political will to end our fossil-fuel addiction.

More than half of the U.S. population now lives in car-dependent suburbs. Cumulatively, we drive 3 trillion miles each year. The average miles traveled per capita is increasing rapidly, and the transportation sector now accounts for one-third of all U.S. carbon emissions.

Another one-fifth of U.S. carbon emissions comes from the residential sector. Average home sizes have increased dramatically in recent decades, as has the accompanying footprint of each home. Suburban sprawl contributes significantly to deforestation, reducing the capacity of the planet to absorb the increased CO2 we emit. Due to a dramatic decrease in household size, from 3.1 persons per home in 1970 to 2.6 in 2000, homebuilding is outpacing the population growth that is driving it. More Americans are driving farther to reach bigger homes with higher heating and cooling demands and fewer people per household than ever before. All of these trends exacerbate the carbon footprint inherent in the basic energy needs of a burgeoning U.S. population.

Globally, recent research indicates that assumptions regarding declining fertility rates used by the Intergovernmental Panel on Climate Change to develop future emissions scenarios may be overly optimistic. While fertility rates have generally declined over the past few decades, progress has slowed in recent years, especially in developing nations, largely due to cutbacks in family planning assistance and political interference from the United States. And even if fertility rates are reduced to below replacement levels, population levels will continue to climb steeply for some time as people live longer and billions of young people mature and proceed through their reproductive years. Per-capita greenhouse gas emissions may drop, but the population bulge will continue to contribute to a dangerous increase in greenhouse gases in the atmosphere.

Time is short, but it not too late to stop runaway global warming. Economy-wide reduction of greenhouse gas emissions to a level that brings atmospheric CO2 back from 386 parts per million to 350 or less, scaling back first-world consumption patterns, and long-term population reduction to ecologically sustainable levels will solve the global warming crisis and move us to toward a healthier, more stable, post-fossil fuel, post-growth addicted society.