According to the United Nations Population Fund, human population grew from 1.6 billion to 6.1 billion people during the course of the 20th century. (Think about it: It took all of time for population to reach 1.6 billion; then it shot to 6.1 billion over just 100 years.) During that time emissions of CO2, the leading greenhouse gas, grew 12-fold. And with worldwide population expected to surpass nine billion over the next 50 years, environmentalists and others are worried about the ability of the planet to withstand the added load of greenhouse gases entering the atmosphere and wreaking havoc on ecosystems down below.

In 1970, when worldwide greenhouse gas emissions had just begun to transgress the sustainable capacity of the atmosphere, the world population was about 3.7 billion; today it’s about 6.1 billion — an increase of 86 percent.

In that same period, worldwide emissions from fossil fuels rose from about 14 billion tons to an estimated 29 billion tons — an increase of 107 percent.

Population growth is not the direct cause of global warming, burning fossil fuels is.

Where some of the confusion comes from is that CO2 emissions are reasonably well correlated to population.

It’s not a one-to-one relationship, but there is a solid relationship. For a couple of centuries, more people meant a great deal more CO2 which more closely tracked gross domestic product on a one-to-one ratio. As countries became richer, that was reflected in their GDP and also in their reduction in fertility. More GDP equals flattening population but still increasing CO2 emissions.

“Population, global warming and consumption patterns are inextricably linked in their collective global environmental impact,” reports the Global Population and Environment Program at the non-profit Sierra Club. “As developing countries’ contribution to global emissions grows, population size and growth rates will become significant factors in magnifying the impacts of global warming.”

According to the Worldwatch Institute, a nonprofit environmental think tank, the overriding challenges facing our global civilization are to curtail climate change and slow population growth. “Success on these two fronts would make other challenges, such as reversing the deforestation of Earth, stabilizing water tables, and protecting plant and animal diversity, much more manageable,” reports the group. “If we cannot stabilize climate and we cannot stabilize population, there is not an ecosystem on Earth that we can save.”

Many population experts believe the answer lies in improving the health of women and children in developing nations. By reducing poverty and infant mortality, increasing women’s and girls’ access to basic human rights (health care, education, economic opportunity), educating women about birth control options and ensuring access to voluntary family planning services, women will choose to limit family size.