Since the first Earth Day in April 1970, the challenge of climate change has only worsened. In this piece from our Spring 2016 issue, the former editor of the New England Journal of Medicine discusses the collision of global warning, excess carbon-based consumption in rich regions, destitution in poor ones, overpopulation, and new forms of epidemic.

Zika, the mosquito-borne virus that is spreading rapidly in South America and heading north toward the U.S. as summer comes, shows how a previously isolated and sporadic illness can suddenly become a frightening pandemic because of the combined effects of global warming and overpopulation. Carried by the Aedes aegypti mosquito, Zika apparently arose in Uganda in the 1940s and occurred only episodically until 2015, when it began to spread explosively in Brazil, mainly in densely crowded urban areas.

Like other mosquitoes, which are vectors for many diseases, Aedes aegypti thrives in a warm climate, and, as nearly all experts now agree, the world's climate is steadily growing warmer because of human activity. In addition, this particular mosquito has evolved to live in close proximity to humans, and breeds in small amounts of water in human trash, such as bottle caps and plastic containers. The transmission of the virus occurs in both directions-from mosquitoes to people and from infected people back to mosquitoes. The more densely packed the population, the more easily Zika spreads, and that is particularly so in slums where there are few screens or air conditioners, and even mosquito repellent is rare-and where trash collection is even rarer. Thus, Zika is the poster child of a pandemic resulting from both climate change and overpopulation.

Nearly everyone now acknowledges that global warming is real and caused by human activity. There are very few "deniers" left, except among paid consultants to oil companies and on the Republican side of the aisle in Congress. Since the Industrial Revolution, carbon emissions have grown right along with population and the use of fossil fuels. The resultant increase in greenhouse gases, which trap heat in the atmosphere, causes the climate to warm, and sea levels to rise as glaciers melt. Atmospheric carbon dioxide, the most important of the greenhouse gases, reached a record 398 parts per million (ppm) in 2015, up from 285 ppm in 1850. Much of the carbon dioxide is absorbed by the oceans, which causes them to become more acidic and threatens the marine food chain on which we all depend. Droughts are more frequent and deserts are expanding. Floods and severe storms are also more frequent as the atmosphere warms.

But the cause of global warming is not just our "carbon footprint"-that is, the amount of greenhouse gases emitted per capita-but the number of humans contributing to it. The world population is now more than 7.3 billion, compared with 2.5 billion in 1950, when I was growing up, and 1.3 billion in 1850 during the Industrial Revolution. It will reach about 9.5 billion in 2050. Yet, while there is much discussion of climate change, very little is said these days about population growth. It seems almost to have been ruled off the table as a legitimate topic, even though it is an essential part of the equation.

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(Photo: AP/Andre Penner)

How many people can the planet support? The carrying capacity for any species is defined as the maximum number that can be sustained indefinitely, and in the case of humans is usually said to be about ten billion, albeit with a wide range of estimates. But humans are not just any species; we are increasingly divided into rich and poor, both within and across countries, and the effects of overpopulation are seen unevenly, and well before any theoretical carrying capacity is reached. For nearly all of human history, the risk has instead been under-population-the lack of communities large enough to foster human progress, and even at times, the threat of extinction. We didn't reach the first billion until about 1800. But with better sanitation and living standards, especially since the Industrial Revolution, global population grew rapidly, with shorter and shorter doubling times. In addition to fossil fuels, we are now exhausting other natural resources, as well as despoiling the environment in trying to extract them. And we have created what is known, somewhat misleadingly, as the "great Pacific garbage patch" by dumping into the oceans vast amounts of discarded plastic containers, which tend to break into small particles that remain suspended in certain regions just beneath the water's surface.

IN 1798, THOMAS MALTHUS famously predicted that population growth would soon lead to mass starvation. After he was shown to be stunningly wrong, not much public attention was given to the subject until 1968, when Paul R. Ehrlich published his best-selling book, The Population Bomb-at a time when the global population was a mere 3.6 billion. Like Malthus, Ehrlich predicted imminent mass starvation, and argued for stringent population control. But in the last half of the 20th century, remarkable technological improvements in agriculture-the "green revolution"-greatly increased food supply, and Ehrlich's predictions, like those of Malthus, were off the mark.

The fact that dire predictions had proved wrong may have been one reason that overpopulation largely disappeared from public discourse in the 1980s. The decade saw a renewed confidence in technology to solve nearly any problem. And with growing economic inequality, it was easy for wealthy populations to conclude that they were immune to the effects of overpopulation. There was also an element of "political correctness," in that the problems of overpopulation were mainly the problems of poor people in poor regions of the world, and many in the developed world felt that family size is a private matter and it was unseemly to suggest that disadvantaged populations should have fewer children. (No one had anything good to say about China's one-child policy, quite apart from the methods used to achieve it.)

Moreover, the 1980s was the decade when climate change first became widely recognized, not only by scientists, but increasingly by the larger public, and that concern supplanted concern about overpopulation. In 1988, James E. Hansen, Director of NASA's Goddard Institute for Space Studies, testified before Congress on the dangers of global warming, and about the same time, the World Meteorological Organization established the Intergovernmental Panel on Climate Change. The focus shifted from population to carbon emissions, even though they were, and are, related.

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It was as though if we could all cut down on the use of fossil fuels and be better stewards of the environment, the total number of people wouldn't matter. But it does matter, of course.

There is a limit, even though one can argue what that is, and how much suffering people on the fringes should endure before we recognize it.

The threats, then, are twofold and interrelated: First, the number of people, and second, the way we live. However, there is an inverse and paradoxical relationship. In general, areas of the world with the fastest population growth are those with the smallest per capita carbon footprint and consumption of resources. But poorer regions are hardly going to be satisfied to remain that way. Underdeveloped regions of the world aspire to the life of affluent regions, which means that even as their birthrates decline, which they inevitably do with economic development, their environmental footprint will grow. With poverty still widespread, their populations will also continue to grow. Even if we stabilized the population at its current level, it is likely that consumption per capita would continue to increase because of rapid, often uncontrolled, development-exactly as happened in China and is happening in India.

A grave effect of overpopulation and climate change is the scarcity of clean water, either to drink or for sanitation. Much of the available water is used for irrigation, and as conditions become warmer and more arid, more water is required for crops. The scarcity is particularly acute in North Africa and the Middle East, but we can see it also in the American Southwest and southern California. For example, the Colorado River, the source of much of the water there, is depleted before it can reach its original outlet to the Sea of Cortez. The rivers and aquifers of Africa are similarly becoming exhausted. The larger the population, the worse the problem.

One result is mass migration in search of water and arable land, and this probably underlies some of the unrest in North Africa and the Middle East. Migrants are especially vulnerable to starvation and violence of all kinds, in addition to disruption of education, childhood immunizations, and other health care. Even while much of the earth is growing more arid, some places are now experiencing disastrous floods. But that water is of little use, because of contamination with sewage. The rainfall in the growing megacities is also largely wasted, since it doesn't reach the soil and is quickly contaminated.

(Photo: AP/Keystone/Salvatore Di Nolfi)

These effects are bad enough, but what may be most threatening is epidemic disease. The slums of the new, rapidly growing megacities are breeding grounds for disease. In Lagos, Nigeria, for example, a city of some 21 million people, about two-thirds of the inhabitants live in slums. Contamination from sewage causes cholera, but there is also the likelihood of the spread of other infectious diseases that have previously been contained in small geographic areas or by seasonal cool weather. For example, for many years Ebola outbreaks have occurred sporadically in isolated villages, but did not reach epidemic proportions until there were large and mobile populations.

Before Zika, several other pandemics-defined as worldwide epidemics-have appeared in recent years. They arose in one part of the world, but because of crowding and easy travel, they were able to spread widely. A new disease (or new to the broader world) called SARS (severe acute respiratory syndrome) arose in southern China in 2002 and was carried to some two dozen countries by infected travelers. Another apparently new disease called MERS (Middle East respiratory syndrome) arose in Saudi Arabia in 2012, but spread to many countries, including the United States, and to South Korea, where it is still causing serious, sometimes fatal, illness.

So far, we have not had a pandemic on the scale of the 1918 flu pandemic or the bubonic plague of the 14th century.

But conditions are ripe for it. Probably the most likely cause would be an influenza virus, transmitted to humans from birds or other animals. In a recent op-ed about Zika in The New York Times, Michael T. Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota, wrote, "Even more than these viruses, we should be afraid of a planet-wide catastrophe caused by influenza." Flu viruses mutate often, and can shift in both their host targets and their virulence. Bird flu, which is often fatal, is not readily transmissible between humans, but that could change. Moreover, because transmissible flu viruses are airborne, they spread easily and quickly from person to person. Most important, people can spread the virus before they have symptoms, unlike the case with other diseases such as Ebola. Thus, someone can get on a plane feeling quite well, but still spread the virus to everyone around him by talking or coughing.

The 2011 film Contagion illustrated the dangers very well: A flu-like virus causes a deadly pandemic, starting with a young Minneapolis woman (Gwyneth Paltrow) who had just returned from a trip to Hong Kong. At the end of the film, we learn that she was probably infected by shaking hands with the chef at a restaurant. His hands were contaminated with the blood of a pig that had been infected by a bat that dropped a piece of banana into the pig pen. Parts of the film were unrealistic, but this idea of the origin of a flu epidemic was not. The ability to monitor or contain such outbreaks is limited, particularly in sub-Saharan Africa. As Keiji Fukuda, then the assistant director-general for health security at the World Health Organization (WHO), said in 2013, "The world is not ready for a large, severe outbreak."

TO DEAL WITH THE TWIN threats of overpopulation and climate change, we will need to get busy. Small, incremental efforts will not be enough. Much is made of the fact that as living standards and urbanization increase, and as women in particular become better educated, fertility rates drop. In fact, in some countries, including Japan, Russia, much of Eastern Europe, Germany, Italy, and Spain, population is either static or declining. But population growth is not evenly distributed. These small declines in the wealthy countries will be more than offset by continued growth in other parts of the world. The population in Africa, for example, is expected to double by 2050-from about 1.2 billion to 2.5 billion. Although the rate of growth worldwide has slowed in recent years, it has not reached zero, so the population will inexorably grow, albeit more slowly. The United Nations Department of Economic and Social Affairs predicts continued growth for the remainder of this century, with a projected population of about 11 billion in 2100.

What can be done? There are two non-coercive and constructive ways to bring the rate of population growth to zero or less. First, we need to provide enough economic security to families in developing countries to reduce the incentive to have large numbers of children. It is often assumed that providing better birth control is the answer, and that may be partially true. But it is likely that many families want a large number of children because they need them for farm labor or to contribute otherwise to family income, and also to provide for their parents when they reach old age. Unlike families in the developed world, these families see children as a "profit center," not a "cost center."

People need to be protected against illness, extreme poverty, and the infirmities of old age to be willing to have fewer children.

They need a minimum social safety net.

Second, we need to make stronger efforts to ensure that girls are educated. The evidence is overwhelming that maternal education, regardless of income, correlates with smaller family size. It is essential, then, to focus on the education of girls and more generally the status of women, for moral reasons as well as population control.

In 1994, when the global population was 5.6 billion, the International Conference on Population and Development met in Cairo and issued a lengthy Programme of Action that was widely heralded and adopted by a special session of the U.N. General Assembly in 1999. It was notable for its strong, and I believe warranted, emphasis on human welfare and the status of women. In fact, it was a veritable Christmas list of all the things that make life worth living, even including a fulfilling sex life within an egalitarian marriage. But it said little about the harms of overpopulation, nor how to bring about, and pay for, the undeniably better world it called for. And it included this in its opening statement: "The implementation of the recommendations contained in the Programme of Action is the sovereign right of each country, consistent with national laws and development priorities, with full respect for the various religious and ethical values and cultural backgrounds of its people, and in conformity with universally recognized international human rights." Anyone who has read Katherine Zoepf's January 11 New Yorker article "Sisters in Law," about the legal status of women in Saudi Arabia, or the February 5 New York Times article by Pam Belluck and Joe Cochrane about female genital cutting in Indonesia, where it is performed on nearly half of Indonesian girls, will see the problem with the deference to sovereign rights, religious and ethical values, and cultural backgrounds.

(Photo: AP/Sipa/Andre Larsson)

Even though bringing population growth under control is crucial, global warming and the wanton consumption of natural resources are still primarily caused by the way we in the developed world live. It is still our carbon footprint that is doing most of the damage. Three years ago, New York Times reporter Elisabeth Rosenthal wrote an article titled "Your Biggest Carbon Sin May Be Air Travel." It concerned the industry-inspired U.S. law that forbids American airlines from participating in the European Union Emissions Trading System, which charges airlines for excess carbon emissions generated by flights in or out of European airports. She pointed out that for many Americans, air travel is probably the largest contributor to their individual carbon footprint. And she added: "It is for me. And for people like Al Gore or Richard Branson who crisscross the world, often by private jet, proclaiming their devotion to the environment." It is for me, too. Air travel emissions account for about 5 percent of global warming, according to Rosenthal, but that fraction is projected to rise. Unless there are required changes in our habits that apply to everyone, analogous to the rationing of gasoline for cars during World War II, Al Gore and I will probably continue to live pretty much the way we have, with only small changes at the margins. People will embrace restrictions in the way they live only if they are shared.

To tackle the problem seriously means to tackle it at the national level, and paradoxically to do so by modifying our allegiance to nationhood. We all breathe the same air, and depend on the same oceans. Because we have no international body with sufficient authority, we will have to rely on nation-states to join together to modify their behavior for the good of the planet, and that means blunting the super-patriotism that afflicts most countries. Just as the formation of the U.S. required the 13 colonies to modify their sense of sovereignty for the greater good, so must the countries of the world do that for the sake of the planet. Yet, some European countries are now actually arguing for increasing population growth, because they see the aging and decline of their population as a national threat. They would like to create more young people to support the old ones, and generally to grow their way out of their problems. For similar reasons, China has announced an end to its one-child policy. But no country is alone on the planet. Not only would these countries add to the global problems, but even within their own borders, the policy simply delays the effects of an aging population for another generation.

To provide enough security to families in developing countries to reduce their incentive to have large numbers of children will take money, not only from the governments of these countries, but even more from developed countries.

The concept of a subsidy for basic needs is not new. Some European countries are currently considering providing a small income to all their residents for that reason.

A similar scheme could be set up by which wealthy nations contribute to a global fund to ensure basic needs for impoverished regions of the world. Contributions could be based on a small percentage of GDP. A portion might be earmarked to support education for girls.

(Photo: AP/Hannibal Hanschke/DPA)

Developed countries also need to subject themselves to the same constraints we ask of developing regions. Large families could become socially unacceptable in the same way that cigarette smoking gradually became less acceptable in the U.S. But the real job for affluent countries is to rein in overconsumption, profligate waste, and the use of fossil fuels. This requires a transition that is far beyond anything now being seriously discussed in mainstream politics or global diplomacy. The much-celebrated U.N. Climate Change Conference, held in Paris late last year, merely pledged participating countries to work toward a goal and revisit the subject in five years. Proposals to accomplish a serious transition to a sustainable economy are invariably countered by massive lobbying by business elites and more general objections that this will cost jobs and limit economic growth. We need a shift that radically reconceives prosperity and how we define it. To survive as well-functioning, civilized communities in a static global population, there will inevitably have to be some redistribution of wealth, both within countries and across them. We might have to make do with less-certainly with less as traditionally understood-and distribute it more equitably. Just as the notion of the supreme nation-state needs modification, so, too, does our devotion to unfettered capitalism and the grail of GDP growth. While politically, my solutions are a nonstarter, that could change. They are certainly more palatable than pandemics, starvation, and wars as a means of population control and resource allocation, and we could come to that realization fairly suddenly.

I am very much aware that I have not laid out a political road map-that is, a route to building a mass movement and the leadership to deal adequately with the problems. I simply don't know how that is to be done, and I'm pessimistic that it will be. After all, our Congress can't pass even the simplest, most uncontroversial legislation, and much of the rest of the world is not only ungovernable, but committed to tribal warfare of one sort or another. But I do know that we cannot continue as we are now, and that small efforts at the margins are not enough. My purpose is to convey a sense of urgency and the reasons for it.

The first step is to begin talking candidly about the issues. Overpopulation cannot continue to be the problem that dares not speak its name. Humans are not just fouling their nest, but crowding it beyond its capacity. Only when both problems are taken seriously and become part of respectable discourse will we be able to move ahead on the steps necessary to deal with the self-destructive way we are treating our planet.