In the fall of 2015, several artists in the Caribbean appeared in a film titled “1.5 Stay Alive,” featuring a series of homespun music videos. The name of the project referred to a campaign to limit the rise in average global temperature to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial times. Since the 2009 United Nations climate talks, in Copenhagen, the consensus among scientists and policymakers has been that two degrees Celsius should be the limit; any further temperature increase would be catastrophic. But, for the citizens of small island countries and other vulnerable places in the tropics and the Arctic, even two degrees of warming would be a death sentence. Their communities would be inundated, and eventually destroyed. This could happen by the time the children in the film reached middle age.

Later that year, at the climate talks in Paris, “1.5 to Stay Alive” had become a rallying cry for the leaders of the Alliance of Small Island States, which includes countries like the Bahamas, the Maldives, and the Marshall Islands. As a result, the final Paris Agreement declared that, while warming shall not surpass two degrees Celsius above pre-industrial levels (the target now commonly cited), countries should pursue “efforts to limit the temperature increase to 1.5 degrees C.” The Alliance of Small Island States also asked the Intergovernmental Panel on Climate Change (I.P.C.C.)—the scientific body that informs the climate policies of the United Nations’ member states—to prepare a special report on the specific impacts of global warming of 1.5 degrees, along with ways the world could feasibly keep the temperature from rising further. “We needed literature on the conditions of the 1.5-degree world, so this spurred a tremendous amount of new research,” William Solecki, a climate scientist at Hunter College, in New York, and one of the report’s lead authors, told me. “As soon as the ink was drying on the Paris Agreement, we were off.”

Last night, in Incheon, South Korea, after a week of deliberation, the I.P.C.C. released the new findings. The summary tells a nightmarish tale—one much worse than any of those in the I.P.C.C.’s previous reports—surveying the climate-change impacts we’re already experiencing with one degree of warming, and the severity of the impacts to come once we surpass 1.5 degrees of warming. Ten million more people would be exposed to permanent inundation, and several hundred million more to “climate-related risks and susceptible to poverty.” Malaria and dengue fever will be more widespread, and crops like maize, rice, and wheat will have smaller and smaller yields—particularly in sub-Saharan Africa, Southeast Asia, and Central and South America. Security and economic growth will be that much more imperilled. “Robust scientific literature now shows that there are significant differences between 1.5 and 2 degrees,” Adelle Thomas, a geographer from the Bahamas and also one of the report’s lead authors, told me. “The scientific consensus is really strong. It’s not just a political slogan: ‘1.5 to stay alive.’ It’s true.”

The report marks the start of the I.P.C.C.’s latest assessment cycle, the sixth since the organization was formed by the U.N. Environment Program and the World Meteorological Organization, in 1988. Its importance is hard to overstate. The thirty-three-page summary for policymakers—which is based on more than six thousand cited studies, and written by ninety-one authors from forty different countries—is a collective scream sieved through the stern, strained language of bureaucratese. Unique ecosystems will vanish and species will go extinct by the thousands. With two degrees of warming, three times as many insects (eighteen per cent), and twice as many plants (sixteen per cent) and vertebrates (eight per cent), will lose their geographic range, when compared with warming of 1.5 degrees. Nearly all the coral reefs (more than ninety-nine per cent) will be dead, including the Great Barrier Reef, an ecosystem some twenty-five million years old, which is visible from space and is already in severe decline. The global annual catch from marine fisheries will decrease by three million tons. The likelihood of a sea-ice-free Arctic summer will increase from once per century to once per decade. “The next few years are probably the most important in our history,” Debra Roberts, a co-chair of the I.P.C.C. Working Group II, said. (There are three working groups: one focussed on the physical science of climate change; the second on impacts, adaptation, and vulnerabilities; and the third on mitigation.)

To keep warming at 1.5 degrees, governments and private businesses must make unprecedented changes—on a sweeping global scale—in energy systems, land management, building efficiency, industrial operations, shipping and aviation, and city-wide design. Within the next decade, human-caused carbon-dioxide emissions need to fall forty-five per cent below 2010 levels. By 2050, net carbon-dioxide emissions must equal zero. “It’s a goal that we can aspire to, but maybe not meet,” Jennifer Francis, a climate scientist at Rutgers University who studies Arctic warming and its impacts on global climate, said. “So it’s useful, even if it isn’t all that realistic.”

“Human activities,” the report’s authors note, have already caused the global mean temperature to increase as much as 1.2 degrees Celsius above pre-industrial levels, with greater warming in certain regions, and particularly in the Arctic. Warming has already led to weather extremes such as heat waves and precipitation of increased intensity and frequency. If warming continues at its current rate, it could reach 1.5 degrees by the time a child who is now a toddler starts high school. The longer countries take to reduce energy consumption and transition to renewable energy sources like solar and wind, the more they will have to rely on technologies such as carbon removal, which are currently too expensive, experimental, and small-scale to do the job.

“You have to think in terms of the survival of human society,” Benjamin Horton, a British geographer who is currently leading the creation of a sea-level-rise adaptation plan for Singapore and who will serve as an editor for a later segment of the I.P.C.C.’s sixth assessment report, said. “It’s not only the magnitude of change, it’s the pace at which it changes,” he said. The rate of sea-level rise accelerates once the West Antarctic Ice Sheet hits its tipping point—likely set to occur somewhere between 1.5 and two degrees of warming, if it hasn’t occurred already—when physics demands the whole sheet will irreversibly disintegrate. At that point, Horton said, “You can’t do anything about it. It is very hard to grow an ice sheet, but very easy to melt.”

As things stand now, even if every country met the commitment it made in the Paris Agreement, the temperature would still increase to three degrees Celsius above pre-industrial levels by the end of the century. If the world continues burning fossil fuels and emitting greenhouse gases at the current rate, it could rise by four degrees—a fact that the Trump Administration, which withdrew the United States from the Paris Agreement, in June, 2017, acknowledged with language buried deep in an August draft report issued in support of eliminating Obama-era fuel-economy rules. Representatives from the Trump Administration were in Incheon and had to approve the conclusions of the report. Whether President Trump will respond to its findings remains to be seen. But Adelle Thomas told me that during the draft process, while authors were reviewing more than forty thousand comments submitted by different nations, “there was a real difference between developed countries and non-developed countries.” The former, she said, “wanted to soften the report, and take out anything that specifically mentions island states, challenging why small developing islands should be considered a special case.”