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This is not good, but indifference is unfortunately nothing new when it comes to climate. Projecting future warming is a foolish game, given how many layers of uncertainty govern the outcome; but if a best-case scenario is now somewhere between 2 and 2.5 degrees of warming by 2100, it seems that the likeliest outcome, the fattest part of the bell curve of probability, sits at about 3 degrees, or just a bit above. Probably even that amount of warming would require significant negative-emissions use, given that our use of carbon is still growing. And there is also some risk from scientific uncertainty, the possibility that we are underestimating the effects of those feedback loops in natural systems we only poorly understand. Conceivably, if those processes are triggered, we could hit 4 degrees of warming by 2100, even with a meaningful reduction in emissions over the coming decades. But the track record since Kyoto implies that human shortsightedness makes it unproductive to offer predictions about what will happen, when it comes to emissions and warming; better to consider what could happen. The sky is literally the limit.

Cities, where the world will overwhelmingly live in the near future, only magnify the problem of high temperature. Asphalt and concrete and everything else that makes a city dense, including human flesh, absorb ambient heat, essentially storing it for a time like a slow-release poison pill; this is especially problematic because, in a heat wave, nightly reprieves are vital, allowing bodies to recover. When those reprieves are shorter, and shallower, flesh simply continues to simmer. In fact, the concrete and asphalt of cities absorb so much heat during the day that when it is released, at night, it can raise the local temperature as much as 22 degrees Fahrenheit, turning what could be bearably hot days into deadly ones—as in the Chicago heat wave of 1995, which killed 739 people, the direct-heat effects compounded by broken public health infrastructure. That commonly cited figure only reflects immediate deaths; of the many thousands more who visited hospitals during the heat wave, almost half died within the year. Others merely suffered permanent brain damage. Scientists call this the “heat island” effect—each city its own enclosed space, and hotter the more crowded it is.

Of course, the world is rapidly urbanizing, with the United Nations estimating that two-thirds of the global population will live in cities by 2050—2.5 billion new urbanites, by that count. For a century or more, the city has seemed like a vision of the future to much of the world, which keeps inventing new scales of metropolis: bigger than 5 million people, bigger than 10, bigger than 20. Climate change won’t likely slow that pattern by much, but it will make the great migrations it reflects more perilous, with many millions of the world’s ambitious flooding into cities whose calendars are dotted with days of deadly heat, gathering in those new megalopolises like moths to a flame.

In theory, climate change could even reverse those migrations, perhaps more totally than crime did in many American cities in the last century, turning urban populations in certain parts of the world outward as the cities themselves become unbearable. In the heat, roads in cities will melt and train tracks will buckle—this is actually happening already, but the impacts will mushroom in the decades ahead. Currently, there are 354 major cities with average maximum summertime temperatures of 95 degrees Fahrenheit or higher. By 2050, that list could grow to 970, and the number of people living in those cities and exposed to that deadly heat could grow eightfold, to 1.6 billion. In the United States alone, 70,000 workers have been seriously injured by heat since 1992, and by 2050, 255,000 are expected to die globally from direct heat effects. Already, as many as 1 billion are at risk for heat stress worldwide, and a third of the world’s population is subject to deadly heat waves at least twenty days each year; by 2100, that third will grow to half, even if we manage to pull up short of two degrees. If we don’t, the number could climb to three-quarters.

In the United States, heat stroke has a pathetic reputation—a plague you learn about from summer camp, like swimming cramps. But heat death is among the cruelest punishments to a human body, just as painful and disorienting as hypothermia. First comes “heat exhaustion,” mostly a mark of dehydration: profuse sweating, nausea, headache. After a certain point, though, water won’t help, your core temperature rising as your body sends blood outward to the skin, hoping desperately to cool it down. The skin often reddens; internal organs begin to fail. Eventually you could stop sweating. The brain, too, stops working properly, and sometimes, after a period of agitation and combativeness, the episode is punctuated with a lethal heart attack. “When it comes to extreme heat,” Langewiesche has written, “you can no more escape the conditions than you can shed your skin.”

Hunger

Climates differ and plants vary, but the basic rule of thumb for staple cereal crops grown at optimal temperature is that for every degree of warming, yields decline by 10 percent. Some estimates run higher. Which means that if the planet is five degrees warmer at the end of the century, when projections suggest we may have as many as 50 percent more people to feed, we may also have 50 percent less grain to give them. Or even less, because yields actually decline faster the warmer things get. And proteins are worse: it takes eight pounds of grain to produce just a single pound of hamburger meat, butchered from a cow that spent its life warming the planet with methane burps.

Globally, grain accounts for about 40 percent of the human diet; when you add soybeans and corn, you get up to two-thirds of all human calories. Overall, the United Nations estimates that the planet will need nearly twice as much food in 2050 as it does today—and although this is a speculative figure, it’s not a bad one. Pollyannaish plant physiologists will point out that the cereal-crop math applies only to those regions already at peak growing temperature, and they are right—theoretically, a warmer climate will make it easier to grow wheat in Greenland. But as a pathbreaking paper by Rosamond Naylor and David Battisti pointed out, the tropics are already too hot to efficiently grow grain, and those places where grain is produced today are already at optimal growing temperature—which means even a small warming will push them down a slope of declining productivity. The same, broadly speaking, is true for corn. At four degrees of warming, corn yields in the United States, the world’s top producer of maize, are expected to drop by almost half. Predicted declines are not quite as dramatic in the next three biggest producers—China, Argentina, Brazil—but in each case the country would lose at least a fifth of its productivity.

A decade ago, climatologists might have told you that although direct heat undermined plant growth, the extra carbon in the atmosphere would have the opposite effect—a kind of airborne fertilizer. The effect is strongest on weeds, though, and does not seem to hold for grain. And at higher concentrations of carbon, plants grow thicker leaves, which sounds innocuous. But thicker leaves are worse at absorbing CO2, an effect that means, by the end of the century, as much as 6.39 billion additional tons in the atmosphere each year.

Beyond carbon, climate change means staple crops are doing battle with more insects—their increased activity could cut yields an additional 2 to 4 percent—as well as fungus and disease, not to mention flooding. Some crops, like sorghum, are a bit more robust, but even in those regions where such alternatives have been a staple, their production has diminished recently; and while grain breeders have some hope that they can produce more heat-tolerant strains, they’ve been trying for decades without success. The world’s natural wheat belt is moving poleward by about 160 miles each decade, but you can’t easily move croplands north a few hundred miles, and not just because it’s difficult to suddenly clear the land occupied now by towns, highways, office parks, and industrial installations. Yields in places like remote areas of Canada and Russia, even if they warmed by a few degrees, would be limited by the quality of soil there, since it takes many centuries for the planet to produce optimally fertile dirt. The lands that are fertile are the ones we are already using, and the climate is changing much too fast to wait for the northern soil to catch up. That soil, believe it or not, is literally disappearing—75 billion tons of soil lost each year. In the United States, the rate of erosion is ten times as high as the natural replenishment rate; in China and India, it is thirty to forty times as fast.