This, then, is the sustainability challenge—not to control population growth, but to find ways to improve well-being and satisfy basic consumption desires without repression or environmental degradation, especially to minimize pollution, biodiversity loss, and greenhouse gas emissions.

Women will choose to have fewer children—to reduce fertility—when they have higher status; more options; living standards that include higher levels of education, literacy, employment, health care, and savings; a higher average age of marriage; and safe contraceptive choices.

A region is typically considered overpopulated when it exceeds its carrying capacity, which is the number of people that that region’s resources (typically food) can support. But that estimate will depend on what those people are eating, and what they are willing to eat. It’s well known, for instance, that a vegetarian diet is easier to sustain than a carnivorous one. Sufficiency will also vary with our ever-changing ability to produce food.

About half the world’s population lives on less than $3/day; they cause very little climate pollution (only 15% of the global total). Those of us in the top 10% of global income (living on more than $23/day, or about $8400/year) are responsible for 36% of global carbon emissions.

Currently, higher income tends to be correlated with higher climate pollution.

I believe that overpopulation discourse undermines the kind of safe, voluntary family planning and reproductive health care that respects women’s autonomy. Too often it encourages coercive methods like involuntary sterilization.

We need to understand that population is likely to stabilize or even go down in the future as the younger generation ages and that momentum peters out. In the meantime the real challenge facing us is how to plan for a growing population in environmentally sustainable and socially equitable ways.

But what people don’t understand is that the demographic momentum built into these numbers has a lot to do with age distribution: there are presently a large proportion of people of reproductive age in the population, especially in the global south, and even if they have only two or fewer children, it means an absolute increase in population numbers.

the average family size now is about 2.5 children. Birth rates remain relatively high in some countries, particularly in sub-Saharan Africa, but this is mainly due to lack of investment in public health services, poverty eradication, education, women’s rights, etc. Elsewhere in the world many countries are facing population decline, with birth rates falling below replacement level fertility. In the U.S. right now women are having on average less than two children.

who is producing most of these CO2 emissions? And Oxfam came out with a study about four years ago that estimated the world’s richest 1% probably emit 30 times more than the poorest 50% of the planet.

Since the time of Thomas Malthus, people concerned about overpopulation have been worried about whether there’s enough food to go around.

The resources around us are elastic.

In the richer industrial countries women and their partners had long made choices about family size.These personal choices had serendipitous national effects, not least on economic development and the environment. In developing countries, millions of poor women are still waiting

In this era of climate change, the real challenge is to fundamentally transform inefficient, inequitable, and environmentally harmful systems of resource production, consumption, and distribution in order to sustainably accommodate a population of over 9 billion in 2050.

Today, the biggest barrier to an effective international climate policy is the failure of the Global North, in particular the U.S., to agree to a significant reduction in carbon emissions. Pinning the blame on overpopulation in the Global South plays into the politics of climate denial.

Focusing on women's fertility diverts our attention from the role of industrial agriculture, extractive industries, luxury consumption, and militarism in causing environmental degradation

Population alarmism threatens to erode the progress made since the 1994 Cairo conference in moving the family planning field away from top-down and coercive population control programs toward a focus on reproductive health and rights. In many countries, programs are still biased against poor women, who often receive disrespectful, bad quality services and are denied real contraceptive choice. When the message that controlling fertility is not only a demographic but an environmental mandate filters down to already prejudiced providers, it will only make services worse. A troubling sign is that the U.S. Agency for International Development is considering re-introducing incentives, including compensation payments for sterilization, into family planning programs

The main reason why global population is projected to increase to 9 billion by 2050, and possibly 10 billion by 2100 (a high projection that is disputed by demographers), is that currently there exists a large cohort of young people of reproductive age.

Women's rights are key. Fertility rates remain high where women's status is low. Less than one fifth of the world's countries will account for nearly all of the world's population growth this century. Not coincidentally, those countries—the least developed nations in sub-Saharan Africa, south Asia, and elsewhere—are also where girls are less likely to attend school, where child marriage is common, and where women lack basic rights.

Over the last half century, we've learned that the best way to slow growth is not through coercive "population control," but by ensuring that all people are able to make real choices about childbearing.

In the world's most "water poor" countries, population is expected to double by 2050.Slower growth is not a panacea for the world's water problems, but it could ease pressure on scarce resources and buy time to craft solutions.

In the UN's low projection, our numbers peak at 8 billion by mid-century, then decline to 6 billion by 2100. By contrast, the medium and high projections envision continued growth for the foreseeable future. According to the medium projection, the world's population would reach 10 billion by 2100; according to the high projection, nearly 16 billion.

As human numbers approach 7 billion, the question is, "where do where we go from here?"

Unintended pregnancies occur when women don't want to get pregnant but are not using contraception. Among the reasons for this unmet need for contraception are a lack of knowledge about contraception, difficult access to supplies and services, the cost of contraception, fear of side effects, and opposition from spouses and other family members. Family planning programs reduce these obstacles, thus reducing unintended pregnancies and birth rates.

This approach permitted women and men to control their reproductive lives and avoid unwanted childbearing.

If we ever succeed in eliminating the gross disparities that today characterize per-capita consumption levels, our success will make even more obvious the eventual necessity of a non-growing population for true sustainability.

There are roughly a thousand times more people on the planet today than at the dawn of agriculture.

Two main human forces have multiplied the scale of our interactions so that they are now beginning to overwhelm natural systems

Since the essence of sustainability is assuring that the activities of current generations do not threaten the well-being of future ones

Scale and change are fundamental determinants of environmental sustainability, and demographic trends are fundamental to human scale and change. If we ignore these trends or insist that there is no ethical way to affect their speed and direction, true sustainability will be as hard to reach as the end of a rainbow.