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It is not just the hajj, and it is not just Mecca. In the sugarcane region of El Salvador, as much as one-fifth of the population—including over a quarter of the men—has chronic kidney disease, the presumed result of dehydration from working the fields they were able to comfortably harvest as recently as two decades ago. With dialysis, which is expensive, those with kidney failure can expect to live five years; without it, life expectancy is measured in weeks. Of course, heat stress promises to assail us in places other than our kidneys, too. As I type that sentence, in the California desert in mid-June, it is 121 degrees outside my door. It is not a record high.

This is among the things cosmologists mean when they talk about the utter improbability of anything as advanced as human intelligence evolving anywhere in a universe as inhospitable to life as this one: every uninhabitable planet out there is a reminder of just how unique a set of circumstances is required to produce a climate equilibrium supportive of life. No intelligent life that we know of ever evolved, anywhere in the universe, outside of the narrow Goldilocks range of temperatures that enclosed all of human evolution, and that we have now left behind, probably permanently.

How much hotter will it get? The question may sound scientific, inviting expertise, but the answer is almost entirely human—which is to say, political. The menace of climate change is a mercurial one; uncertainty makes it a shape-shifting threat. When will the planet warm by two degrees, and when by three? How much sea-level rise will be here by 2030, by 2050, by 2100, as our children are leaving the earth to their children and grandchildren? Which cities will flood, which forests will dry out, which breadbaskets will become husks? That uncertainty is among the most momentous metanarratives that climate change will bring to our culture over the next decades—an eerie lack of clarity about what the world we live in will even look like, just a decade or two down the road, when we will still be living in the same homes and paying the same mortgages, watching the same television shows and making appeals to many of the same justices of the Supreme Court. But while there are a few things science does not know about how the climate system will respond to all the carbon we’ve pumped into the air, the uncertainty of what will happen—that haunting uncertainty—emerges not from scientific ignorance but, overwhelmingly, from the open question of how we respond. That is, principally, how much more carbon we decide to emit, which is not a question for the natural sciences but the human ones. Climatologists can, today, predict with uncanny accuracy where a hurricane will hit, and at what intensity, as much as a week out from landfall; this is not just because the models are good but because all the inputs are known. When it comes to global warming, the models are just as good, but the key input is a mystery: What will we do?

The lessons there are unfortunately bleak. Three-quarters of a century since global warming was first recognized as a problem, we have made no meaningful adjustment to our production or consumption of energy to account for it and protect ourselves. For far too long, casual climate observers have watched scientists draw pathways to a stable climate and concluded that the world would adapt accordingly; instead, the world has done more or less nothing, as though those pathways would implement themselves. Market forces have delivered cheaper and more widely available green energy, but the same market forces have absorbed those innovations, which is to say profited from them, while continuing to grow emissions. Politics has produced gestures of tremendous global solidarity and cooperation, then discarded those promises immediately. It has become commonplace among climate activists to say that we have, today, all the tools we need to avoid catastrophic climate change—even major climate change. It is also true. But political will is not some trivial ingredient, always at hand. We have the tools we need to solve global poverty, epidemic disease, and abuse of women, as well.

It was as recent as 2016 that the celebrated Paris climate accords were adopted—defining two degrees of global warming as a must-meet target and rallying all the world’s nations to meet it—and the returns are already dispiritingly grim. In 2017, carbon emissions grew by 1.4 percent, according to the International Energy Agency, after an ambiguous couple of years optimists had hoped represented a leveling-off, or peak; instead, we’re climbing again. Even before the new spike, not a single major industrial nation was on track to fulfill the commitments it made in the Paris treaty. Of course, those commitments only get us down to 3.2 degrees; to keep the planet under 2 degrees of warming, all signatory nations have to significantly better their pledges. At present, there are 195 signatories, of which only the following are considered even “in range” of their Paris targets: Morocco, Gambia, Bhutan, Costa Rica, Ethiopia, India, and the Philippines. This puts Donald Trump’s commitment to withdraw from the treaty in a useful perspective; in fact, his spite may ultimately prove perversely productive, since the evacuation of American leadership on climate seems to have mobilized China—giving Xi Jinping an opportunity and an enticement to adopt a much more aggressive posture toward climate. Of course those renewed Chinese commitments are, at this point, just rhetorical, too; the country already has the world’s largest footprint, and in the first three months of 2018 its emissions grew by 4 percent. China commands half of the planet’s coal-power capacity, with plants that only operate, on average, half of the time—which means their use could quickly grow. Globally, coal power has nearly doubled since 2000. According to one analysis, if the world as a whole followed the Chinese example, it would bring five degrees of warming by 2100.

In 2018, the United Nations predicted that at the current emissions rate the world would pass 1.5 degrees by 2040, if not sooner; according to the 2017 National Climate Assessment, even if global carbon concentration was immediately stabilized, we should expect more than half a degree Celsius of additional warming to come. Which is why staying below 2 degrees probably requires not just carbon scale-back but what are called “negative emissions.” These tools come in two forms: technologies that would suck carbon out of the air (called CCS, for “carbon capture and storage”) and new approaches to forestry and agriculture that would do the same, in a slightly more old-fashioned way (bioenergy with carbon capture and storage, or “BECCS”).

According to a raft of recent papers, both are something close to fantasy, at least at present. In 2018, the European Academies’ Science Advisory Council found that existing negative-emissions technologies have “limited realistic potential” to even slow the increase in concentration of carbon in the atmosphere—let alone meaningfully reduce that concentration. In 2018, Nature dismissed all scenarios built on CCS as “magical thinking.” It is not even so pleasant to engage in that thinking. There is not much carbon in the air, all told, just 410 parts per million, but it is everywhere, and so relying on carbon capture globally could require large-scale scrubbing plantations nearly everywhere on Earth—the planet transformed into something like an air-recycling plant orbiting the sun, an industrial satellite tracing a parabola through the solar system. (This is not what Barbara Ward or Buckminster Fuller meant by “spaceship earth.”) And while advances are sure to come, bringing costs down and making more efficient machines, we can’t wait much longer for that progress; we simply don’t have the time. One estimate suggests that, to have hopes of two degrees, we need to open new full-scale carbon capture plants at the pace of one and a half per day every day for the next seventy years. In 2018, the world had eighteen of them, total.