Two German shepherds kept as pets in Europe or the U.S. use more resources in a year than the average person living in Bangladesh. The world’s richest 500 million people produce half of global carbon dioxide emissions, while the poorest 3 billion emit just 7 percent. Industrial tree cutting is now responsible for the majority of the 13 million hectares of forest lost to fire or the blade each year — surpassing the smaller-scale footprints of subsistence farmers who leave behind long, narrow swaths of cleared land, so-called fish bones.

In fact, urban population growth and agricultural exports drive deforestation more than overall population growth, according to research from geographer Ruth DeFries of Columbia University and her colleagues. In other words, the increasing urbanization of the developing world — as well as an ongoing increase in consumption in the developed world for products that have an impact on forests, whether furniture, shoe leather or chicken fed on soy meal — is driving deforestation, rather than containing it as populations leave rural areas to concentrate in booming megalopolises.

The combination of increasing health, empowered women and falling birth rates may be the most important revolution to come out of the tumultuous 20th century.

So are the world’s environmental ills really a result of the burgeoning number of humans on the planet — growing by more than 150 people a minute and predicted by the United Nations to reach at least 9 billion people by 2050? Or are they more due to the fact that, while human population doubled in the past 50 years, we increased our use of resources fourfold?

Peak Humanity

2.6: The average number of children women are having todayFirst and foremost, human population growth peaked long ago, according to demographer Joel Cohen of Rockefeller University in New York and others. The peak growth rate — a little more than 2 percent per year — occurred somewhere between 1965 and 1970, when the world’s population was just 3.3 billion people, and has been dropping ever since, reaching a little over 1 percent today. In 1987, the number of people added to the planet each year topped out at 87 million, a number that is now down to roughly 78 million people per year. That means human population numbers will drop voluntarily for the first time ever in human history in the 21st century. A Baby Bust has replaced the Baby Boom.

The reason? Empowerment of women. A massive reduction in child mortality, combined with educated mothers pursuing their own advancement and in control of birth control, has helped to drop the average human brood from over five children per woman of childbearing age to just 2.6 per woman today. As journalist Fred Pearce writes in his new book, The Coming Population Crash: “The population bomb is being defused. By women. Because they want to.”

In fact, the combination of increasing health (especially a greater proportion of babies surviving to adulthood), empowered women and falling birth rates may be the most important revolution to come out of the tumultuous 20th century. Those of us born between 1930 and 2050 will be among the privileged few to have ever witnessed a doubling of global population. It took from the dawn of humanity to the 19th century to achieve 1 billion people on the planet — an achievement that now comes roughly every few decades. And the 21st century will likely belong to the old, as elders outnumber youth for the first (recorded) time in human history: Fewer than 10 percent of people alive today are under 4 years old, while those 60 and older now constitute more than 10 percent of the population. Birth rates in countries such as Germany have fallen so far that populations are already shrinking.

World map of population growth (2011)

POPULATION GROWTH RATE (2011) Births, deaths, immigration and emigration — all influenced by health, nutrition, education, prosperity and more — create different patterns of population change in different places. Source: World Bank.

Yet this demographic transition does not hold everywhere. While family planning has proven effective in the past in countries ranging from Thailand to Iran, funding for such programs has dwindled in recent years. Partially as a result, developing countries in eastern Africa — Kenya, Tanzania, Uganda and Zimbabwe — have seen populations begin to swell again in recent years.

Capacity Unknown

The real question is, how many people can the planet sustain? As Cohen notes in his book How Many People Can the Earth Support?, microbiologist Anton van Leeuwenhoek calculated a carrying capacity of roughly 13.4 billion people back in 1679, based on the population density of his native Holland and its size relative to the rest of the globe. Modern guesses are hardly more scientific, ranging from as few as 1 billion (recently proposed by James Lovelock as our likely number by 2100 thanks to catastrophic climate change) to as many as 1 trillion.