By far, the most significant environmental cause that cities support is simple population control. People have more babies in the country, for a number of reasons. Economically, having more children makes sense in agrarian environments: more hands to help in the fields and around the house, without the space constraints of urban living. Rural life—particularly in the Third World—doesn’t offer the same ready access to birth control and family-planning clinics. Cities, on the other hand, trend in the opposite direction, offering increased economic opportunity for women, expensive real estate, availability of birth control. Those incentives have turned out to be so powerful that they have reversed one of the dominant demographic trends of the last few centuries of life on earth: the population explosion that has been the subject of countless doomsday scenarios, from Malthus to Paul Ehrlich’s influential early-1970s manifesto The Population Bomb. In countries that organized into modern metropolitan cities long ago, birthrates have dropped below the “replacement level” of 2.1 children per woman. Italy, Russia, Spain, Japan—all these countries are seeing birthrates around 1.5 per woman, which means that their populations will begin shrinking in the coming decades. The same trend is occurring in the Third World: birthrates were as high as 6 children per woman in the 1970s; now they are 2.9. As urbanization continues worldwide, current estimates project that the earth’s human population will peak at around 8 billion in 2050. After that, it’s a population implosion that we’ll have to worry about.
THIS IS THE WORLD THAT SNOW AND WHITEHEAD HELPED make possible: a planet of cities. We no longer doubt that metropolitan centers with tens of millions of people can be a sustainable proposition, the way Victorian Londoners worried about the long-term viability of their sprawling, cancerous metropolis. In fact, the runaway growth of metropolitan centers may prove to be essential in establishing a sustainable future for humans on the planet. That reversal of fortune has much to do with the shifting relationship between microbe and metropolis that the Broad Street epidemic helped set in motion. “Cities were once the most helpless and devastated victims of disease, but they became great disease conquerors,” Jane Jacobs wrote, in one of many classic passages from Death and Life of the Great American City.
All the apparatus of surgery, hygiene, microbiology, chemistry, telecommunications, public health measures, teaching and research hospitals, ambulances and the like, which people not only in cities but also outside them depend upon for the unending war against premature mortality, are fundamentally products of big cities and would be inconceivable without big cities. The surplus wealth, the productivity, the close-grained juxtaposition of talents that permit society to support advances such as these are themselves products of our organization into cities, and especially into big and dense cities.
Perhaps the simplest way to explain why Broad Street was such a watershed event is to borrow Jacobs’ phrase and say it this way: Broad Street marked the first time in history when a reasonable person might have surveyed the state of urban life and come to the conclusion that cities would someday become great conquerers of disease. Until then, it looked like a losing battle all the way.
Ultimately, the transformation that Broad Street helped usher in revolved around density, capitalizing on the advantages of dense urban living while minimizing the dangers. Crowding two hundred people per acre, building cities with populations in the millions sharing the same water supply, struggling to find a way to get rid of all that human and animal waste—this was a lifestyle decision that seemed fundamentally at odds with both personal and environmental health. But the nations that first organized themselves around metropolitan settlements—as turbulent as those transformations were—are now the most affluent places on the planet, with life expectancies that are nearly double that of predominantly rural nations. A hundred and fifty years after Broad Street, we see density as a positive force: an engine of wealth creation, population reduction, environmental sustainability. We are now, as a species, dependent on dense urban living as a survival strategy.
But the forecasts that predict a city-planet where eighty percent of us live in metropolitan areas are just that: forecasts. It is possible that this epic transformation could be undone in the coming decades or centuries. The rise of sustainable metropolitan environments was not a historical inevitability: it was the result of specific technological, institutional, economic, and scientific developments, many of which played a role in the extended story of Broad Street. It’s entirely possible that new forces could emerge—or old foes return—that would imperil this city-planet of ours. But what might they be?
It is unlikely that these antiurban forces will come in the form of some new incentive that lures people back to the countryside, like the fanciful dream of telecommuting prophesied by the futurists a decade ago, when the Internet was first entering mainstream culture.
There’s a reason why the world’s wealthiest people—people with near-infinite options vis-à-vis the choice of where to make their home—consistently choose to live in the densest areas on the planet. Ultimately, they live in these spaces for the same reason that the squatter classes of São Paulo do: because cities are where the action is. Cities are centers of opportunity, tolerance, wealth creation, social networking, health, population control, and creativity. No doubt, the Internet and its descendants will continue exporting some of these values to rural communities in the decades to come. But of course, the Internet will continue enhancing the experience of urban life as well. The sidewalk flaneurs get as much out of the Web as the ranchers do, if not more.
The two great looming threats of our new century—global warming and our finite supply of fossil fuels—may well have massively disruptive effects on existing cities in the coming decades. But they are not likely to disrupt the macro pattern of urbanization in the long run, unless you believe the environmental crisis is likely to end in some global cataclysm that sends us back to agrarian or hunter-gatherer living. Most of the world’s urban centers lie within a few dozen meters of sea level, and if the ice caps do indeed melt at the rate they are currently forecast to, many of our metropolitan descendants will be relocating by the midsection of the twenty-first century. But there’s no reason to think they’ll be relocating to rural or suburban areas. Most likely, they’ll simply retreat to higher ground, and new dense metropolitan areas will form around them. The wealthiest cities of the world will follow Venice’s lead and simply try to engineer their way around the problem. The poorest cities will follow New Orleans’ lead—at least so far—and just move to other nearby cities. Either way the poplation stays urban.
Neither does the end of oil foretell the end of cities. The reason why cities have taken on the “green” stamp of approval in recent years is not that they are literally green with foliage. (Air quality has improved markedly, and parks are better funded than ever, but they remain concrete jungles for the most part.) We now see cities as environmentally responsible communities because their energy footprints are so much smaller than other forms of human settlement. In a sense, the environmentalists are learning something that the capitalists learned a few centuries ago: There are efficiencies to urban living that outweigh all the annoyances. City dwellers spend less money heating and cooling their homes; they have fewer children; they recycle their waste more economically; and most important, they consume less energy moving around day to day, thanks to the shorter commutes and mass transit that density enables. “By the most significant measures, New York is the greenest community in the United States, and one of the greenest cities in the world,” The New Yorker’s David Owen writes. “The most devastating damage humans have done to the environment has arisen from the heedless burning of fossil fuels, a category in which New Yorkers are practically prehistoric. The average Manhattanite consumes gasoline at a rate that the country as a whole hasn’t matched since the mid–nineteen-twenties, when the most widely owned car in the United States was the Ford Model T. Eighty-two per cent of Manhattan residents travel to work by public trans
it, by bicycle, or on foot. That’s ten times the rate for Americans in general, and eight times the rate for residents of Los Angeles County. New York City is more populous than all but eleven states; if it were granted statehood, it would rank fifty-first in per-capita energy use.” In other words, a serious crisis of non-renewable energy resources is likely to accelerate the urbanization trend, not derail it.
None of this is intended to belittle the long-term problems caused by global warming and our dependence on fossil fuels. Both trends are likely to trigger disastrous consequences if left unchecked, and the sooner we get serious about solutions to both problems, the better. But in both cases, one of the primary solutions may well prove to be to encourage people to move to metropolitan areas. A warmer planet is still a city-planet, for better or worse.
Yet that doesn’t mean continued urbanization is inevitable. It just means that the potential threats will come from somewhere else. Most likely, if some new force derails our mass migration to the cities, it will take the form of a threat that specifically exploits density to harm us, just as Vibrio cholerae did two hundred years ago.
IN THE IMMEDIATE AFTERMATH OF THE 9/11 ATTACKS, MANY commentators observed that there was a certain dark irony in the technological method of the terrorists: they had used what were effectively Stone Age tools—knives—to gain control of advanced American machines—four Boeing 7-series planes—and then employed that technology as a weapon against its creators. But while the planes were clearly instrumental to the attack, the advanced technology that caused the greatest loss of life lay elsewhere: the terrorists also exploited the technical knowledge that enabled 25,000 people to occupy a building 110 stories high. (Consider that a dead-on collision with the five-story Pentagon produced only seventy-nine casualties on the ground.) The heat of jet fuel and the impact of a 400-mph collision were lethal weapons that morning, but without the terrifying potential energy released by those collapsing floors, the body count would have been lower by an order of magnitude.
The 9/11 attackers were, ultimately, exploiting the tremendous advance in the technologies of density that we have enjoyed since the birth of skyscrapers in the late nineteenth century. There were four hundred people per acre in Soho in 1854, in London’s most densely populated neighborhood. The Twin Towers sat on approximately one acre of real estate, and yet they harbored a population of 50,000 on a workday. That level of density offers a long list of potential benefits, but it is also an open invitation for mass killing—and, what’s worse, mass killing that doesn’t require an army to carry it out. You just need enough ammunition to destroy two buildings, and right there you’ve got a body count that rivals the ten years of American losses in the Vietnam War.
Density is the crucial ingredient often left out in discussions of asymmetric warfare. It is not merely that technology has given smaller and smaller organizations access to increasingly deadly weapons—though that is surely half the story—but that the patterns of human settlement over the past two hundred years have made those weapons far more deadly than they would be if one could somehow time-travel back to 1800 and set them off. Even if you could have hijacked an airplane back in John Snow’s day, you’d have been hard pressed to find an urban area crowded enough to kill a hundred civilians on the ground. Today, the planet is covered with thousands of cities that offer far more enticing targets. If terrorist-sponsored asymmetric warfare were the only threat facing human beings, we would be far better off as a species covering the planet with suburban sprawl and emptying the cities altogether. But we don’t have that option. So we’re either going to have to acclimate to a certain predictable presence of terrorist threats—the way the Victorian Londoners acclimated to the terrible plagues that would sweep through their city every few years—or we’re going to have to follow John Snow’s lead and figure out a reliable way to eliminate the threat.
Certain threats, however, may not be tolerable. One of the most menacing that the twenty-first-century city faces is a holdover from the Cold War: nuclear weapons. The doomsday scenarios are familiar enough: A megaton hydrogen bomb—too big for “suitcase bombs” but much smaller than today’s twenty-five-megaton state-of-the-art weapons—detonated at the site of the Broad Street pump vaporizes the entire area from the western edge of Hyde Park to Waterloo Bridge. A weekday attack would effectively wipe out the entire British government, reducing the Houses of Parliament and 10 Downing Street to radioactive ash. Most of London’s landmarks—Buckingham Palace, Big Ben, Westminster Abbey—would simply cease to exist. A wider zone extending out to Chelsea and Kensington and to the eastern edge of the old City would suffer 98 percent loss of life. Move a few miles farther out—up to Camden Town, out to Notting Hill or the East End—and half the population dies, with most buildings damaged beyond recognition. Anyone who happens to see the blast directly is blinded for life; most survivors suffer hideous radiation sickness that makes them envy the dead. As you move out from Ground Zero, the fallout leaves a vast wake of elevated cancer occurrences and genetic defects.
Then there are the secondary effects, the collateral damage. The entire government would have to be replaced overnight; the damage to the financial centers in the city would be catastrophic for the world economy. The detonation site itself would be uninhabitable for decades. Every resident of a major world city—every New Yorker and Parisian, every person in every street in Tokyo and Hong Kong—would find his habitat transformed: from safety in numbers to mass terror. The great cities of the world would start to look like giant bull’s-eyes: millions of potential casualties conveniently stacked up in easily demolished high-rises. One such attack would probably not impede the metropolitan migration—after all, Hiroshima and Nagasaki didn’t stop Tokyo from becoming the world’s largest city. But several detonations might well tip the balance. Turn our metropolitan centers into genuine nuclear targets and you risk a whole other kind of “nuclear winter”: a season of mass exodus unrivaled in human history.
It would be bad news, in other words. And this bad news is likely to arrive courtesy of a walk-on part on the world-historical stage, somebody driving a rigged SUV into Soho and pulling the trigger. There are 20,000 nuclear weapons in the world capable of inflicting this level of damage. That we know about. On a planet of more than 6 billion people, there have to be thousands and thousands of lost souls ready and willing to detonate one of those weapons in a crowded urban center. How long before those two sets intersect?
That driver with the rigged SUV isn’t going to be deterred by the conventional logic of détente-era nuclear politics. Mutually assured destruction isn’t much of a deterrent to him. Mutually assured destruction, in fact, sounds like a pretty good outcome. Game theory has always had trouble accounting for players with no rational self-interest, and the theories of nuclear deterrence are no exception. And once the bomb goes off, there’s no second line of defense—no vaccines or quarantines to block off the worst-case scenario. There will be maps, but they’ll be maps of incineration and fallout and mass graves. They won’t help us understand the threat the way Snow’s map helped us understand cholera. They will merely document the extent of the tragedy.
THE PERILS OF DENSITY GROW MORE EXPLOSIVE—OR MORE infectious, as the case may be—as the wages of fear are increasingly doled out in twenty-first-century currency: chemical or biological weapons, a freelancer virus or bacterium terrorizing the planet for no particular cause other than its fundamental drive to reproduce. When people still worry about the long-term sustainability of dense human settlement, it is more often than not these self-replicating weapons that conjure up the doomsday scenarios. Tightly bound networks of humans and microbes make a great case study in the power of exponential growth. Infect ten people with the Ebola virus in Montana and you might end up killing a hundred others, depending on when the initial victims were taken to the high-density environment of a hospital. But infect ten people with Ebola in downtown Manhattan and you could kill a million, or more. Traditional bombs obviously grow more
deadly as the populations they target increase in size, but the upward slope in that case is linear. With epidemics, the deadliness grows exponentially.
In September 2004, health officials in Thailand began a program of vaccinating poultry workers with the conventional flu shots that are routinely doled out in Western countries at the start of flu season every year. For months, health experts around the world had been calling for precisely this intervention. This, in itself, was a telling phenomenon. Conventional flu vaccines are effective against only the type A and type B strains of influenza—the kind that sidelines you for a week with a fever and a stuffy head, but that is rarely fatal in anyone except the very young or the very old. The risk of a global pandemic emerging from these viruses is slim at best, which is why, historically, public-health officials in the West have not concerned themselves with the question of whether poultry workers on the other side of the world have received their flu shots. The virus that the public-health officials were worried about—H5N1, also known as the avian flu—is entirely unfazed by conventional flu shots. So why were so many global health organizations calling for vaccines in Asia? If they were worried about avian flu, why prescribe a vaccine that was known to be ineffective against it?
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