The River Parable teaches us that when we see babies floating down a river we must search upstream to find where and why they are falling in, and act up there to address the fundamental problems. Our world is waking up to the disaster that is climate change, but a key upstream cause is unsustainable population growth. Too many people being born need water and food in places they don’t exist. People build economies that demand for more energy, minerals and other natural resources than our ecosystems can sustainably produce. We have been seeing our babies in the river for years, but have refused to act upstream.

For decades now we have witnessed massive increases in many human populations, global warming, epidemics of unusual infections, shifts in water distribution and the disappearance of ecosystems, forests, food and other natural resources. Over the earth’s history, nature developed and inflicted population controls on all species. These natural checks and balances kicked in whenever local population densities increased beyond limits we don’t fully understand. Lemmings famously commit group suicide by jumping off cliffs into the sea when their food supplies dwindle. Disease outbreaks like the Black Plague, smallpox, and HIV decimated swollen human populations and altered history in dramatic ways. Famines and droughts occur, predators increase, wars break out, the weak are culled and the surviving populations, stronger, rebuild. These are “natural” cycles.

Now, though, nature is telling us that we are in terrible trouble, and we are just beginning to feel her wrath. Over the last few centuries, human scientific knowledge and technology have become so effective at thwarting nature’s attempts to control our rising populations, countering disease outbreaks, and altering food and water supplies, that our human population has spiraled out of control. We are creating physical and social imbalances of enormous proportions. Our success has put our world, not just ourselves, on a rollercoaster toward crisis. Malthus had it right long before his warnings became a global reality.

Expanding human population is perhaps the greatest challenge of our century, or the millennium. The world population will rise from 7 billion to 9 billion by 2050. 230 million children are born each year in the world, but much of this population explosion is not what humans want. Among young women ages 19 years and below, more than 80% of pregnancies were unintended. 40% (90 million) pregnancies each year are not only unintended but “unwanted”, the result of abuse or war or conceptions by parents not interested and often unable to care for the children adequately for social, financial and resource reasons.

Of course, many of these children die, approximately 10 million each year according to UNICEF. One child dies of hunger every 10 seconds. 1.5 million children die of causes related to inadequate water, sanitation and hygiene. How many of these fall into the “unintended” or “unwanted” category is unknown, but it is likely that at least the same 40% do, if not many more, since 90% of all annual births occur in the developing world where access to, understanding and acceptance of effective birth controls is poor, and access to health care such as vaccines, fresh water, and adequate nutrition is still low.

Current birth control methods can be effective, but they only work when available and used with absolute consistency. In the US, it is estimated that only 5% of unplanned conceptions result from contraceptive failure. 52% of unintended pregnancies result from couples not using contraception in the month the woman got pregnant, and 43% result from inconsistent or incorrect contraceptive use.

Global warming is at this moment the world’s most discussed physical challenge. Leaders from 195 countries forged the Paris Agreement in hopes of slowing carbon emissions, one important cause. Why is global warming happening, why have CO2 emissions risen so rapidly? Wise observers have recognized the fundamental origin of the problem – overpopulation of humans and their overwhelming demands on our world.

Humans are making more babies than the earth can sustain. These babies grow up to need more, want more and demand more of everything. The 2015 UN’s Sustainable Development Goals (4) list 19 challenges, such as food, water, energy, habitation, “women’s rights”, poverty, etc., that we must address in order to achieve a healthy and sustainable world. Population control is not specifically named as one of them. Perhaps the UN felt it was untouchable. But it is the fundamental issue, integral to attaining every one of the goals, and perhaps the most powerful possible intervention that could help us achieve all 19.

We are at a moment of tremendous risk but great opportunity, and we are actually in a position to respond. Increasingly effective medical science and technology are two major culprits of our current disaster scenario, but they can also help us back off the edge. Population control is a complex process burdened by millennia of biological, political, social and religious baggage. We need a very broad approach, a sea change in global planning, dedication, and funding of research, infrastructure and resources to reverse the population explosion.

From a scientific perspective, a great deal hinges on our success in controlling two early biological steps, the fertilization and implantation of an ovum in a mother’s body. Anything before those moments is considered sexual behavior, and anything after that could be construed as murder.

We have reasonably effective barrier methods for both men and women to prevent sperm from meeting egg, but to be effective, one or both of the partners must have access to and pro-actively use the condom. According to last year’s World Bank’s “Recommendations for Reproductive, Maternal, Newborn and Child Health programs”, basic birth control materials could be provided to every woman in the world for approximately $5 per person. But full and effective cooperation of every individual would be required.

Over the last decade, reproductive endocrinologists have developed implantable, hormone-releasing rods which can prevent pregnancy for up to three years, but this technology requires minor surgical implantation and removal. Hormone-releasing IUDs can provide conception control for up to five years. However, now endocrinologists and geneticists are developing the knowledge and technology necessary to give women day to day, voluntary, reversible control of their fertility.

Male humans can produce about 1000 sperm a second, so they have a monumental supply available. Either that sperm must be physically blocked from meeting the female’s egg, or the man’s sperm count must be decreased to tiny numbers. But this is not something that can be turned on and off at a moment’s notice. Real time or “reversible” male contraceptive methods currently depend on restraint (abstinence), timely barrier use (condoms), or withdrawal. Surgical blockage of the vas is quite simple but is not yet easily reversible. Hormonal interruption of ejaculation requires specialized expertise and is not easy to reverse either. New approaches are being developed, but none have yet proven fully safe or effective. And men, in moments of passion, often do not care enough about the consequences of sex to act to prevent conception, or worse, may want conception against the woman’s will.

It is essential, then, to give all women the power to achieve effective and absolute choice in their decisions about family size and timing of childbirth. In this age of overpopulation (not to mention tremendous human rights inequalities), this is a personal, biological and social justice imperative. It will change the power dynamics in families and cultures, and start to reverse environmental and social degradation. Population control takes place in a woman’s body, and world population control must begin and probably end there. It will take advancement of our current scientific effort, but more importantly, a revolution of social and political will.

Many societies have demonstrated unintentional zero or negative population growth. Modern Japan is a dramatic example. It was not intentional, but rather caused by biological (population age) and societal (marriage, work and income demographic) factors. Other countries have seen decreases in their populations, but usually due to wars, population flight, famine or drought. Often this has occurred in the face of high birth rates but counterbalancing infant and adult mortality.

We know that international public health efforts can move countries to act on behalf of their public health. Global public health campaigns have reached all corners of the globe to vaccinate every child against illnesses such as smallpox, polio and measles. We have convinced warring armies to stop fighting temporarily to achieve this. Education campaigns have spread tools and techniques to help every baby breathe, prevent and treat HIV, and modify deeply held traditional beliefs like putting dung on umbilical stumps and performing female genital mutilation.

Many world leaders have made reversing climate change one of their top national agendas, but many also admit that achieving such goals will be impossible in the face of huge projected population increases. It is time that a parallel campaign is initiated to bend back the curve of world population growth. We are scientifically close to having the medical means to voluntarily prevent fertilization, thus preventing the creation of all unwanted children. Many women do not have control of the outcomes of consensual intercourse, much less of abusive sexual activity. Individual fertility control will create the opportunity for women to “opt in” and thus reduce the vast number of unplanned and unwanted births. This will represent a major human rights breakthrough, enable a major step toward controlling the overall rate of population growth, and change the trajectory of climate change.

Sean Palfrey, MD

Cambridge, Mass