The latest UN report is clear: Climate change is here, it’s a crisis, and it’s caused by fossil fuels.

A new United Nations–led report from hundreds of climate scientists around the world makes it clear: The human-driven climate crisis is now well under way. Earth is likely hotter now than it has been at any moment since the beginning of the last Ice Age, 125,000 years ago, and the world has warmed 1.1 degrees Celsius, or nearly 2 degrees Fahrenheit, since the Industrial Revolution began—an “unprecedented” and “rapid” change with no parallel in the Common Era. What’s more, the recent spate of horrific heat waves, fire-fueling droughts, and flood-inducing storms that have imperiled the inhabited world are not only typical of global warming, but directly caused by it.

Climate change has arrived, in other words, and it will keep getting worse until humanity reduces its greenhouse-gas pollution to zero, which can be accomplished only by dethroning oil, coal, and gas as the central energy sources powering the global economy.

But the speed of that transition matters—and preventing every last ton of carbon pollution, and averting every additional tenth of a degree of warming, will not only lessen the harm over the next few decades, but resound for centuries and even millennia to come.

These are the conclusions of the newest assessment report of the Intergovernmental Panel on Climate Change, a UN-sponsored body that has periodically released a synthesis of current climate science since its founding in 1988. The group’s reports tend to punctuate the otherwise slow immiseration of climate change; its previous synthesis report, released in 2013, helped inform international climate policy, including the writing of the Paris Agreement.

This is its sixth report and its most definitive. The group’s findings must be agreed to by 195 countries; this famously makes it more conservative than some scientists believe is prudent. But compared with previous reports, there is little restraint here. In its strongest statement of culpability ever, the IPCC declared that humanity is “unequivocally” responsible for climate change. “In past reports, we’ve had to make that statement more hesitantly. Now it’s a statement of fact,” Gregory Flato, a vice chair of the group that authored the report and a senior research scientist within the Canadian government, told me.

Some of the worst impacts of climate change can still be avoided. “There are still emissions pathways that would lead us to limiting warming to 1.5 degrees, but they require deep, rapid cuts in greenhouse-gas emissions,” Flato said. “That leaves a glimmer of optimism that we could limit warming to levels like that.” But it would require much more expedient action from the United States than is contemplated in, say, the bipartisan infrastructure bill that Congress is currently considering.

The report is 3,949 pages long and synthesizes the past eight years of advances in climate science; altogether, it cites some 14,000 studies. It would be folly to try to summarize all that work here. But the bottom line is that climate science, and the cataclysm of climate change, has lurched into the present tense. Where scientists once warned of disasters in the distant future, now they strive to understand what has already happened—and what is too late to save. Here are four takeaways:

1. Climate change is now a fact of modern life—and it will only get worse.

Climate change has been happening now nearly since before it first became a public issue. When James Hansen, the head of climate science at NASA, first warned Congress about climate change in 1988, he framed it in what might be called the tentative present tense, saying that the agency could now say “with a high degree of confidence” that global warming was under way.

And it was. As the new report notes: “Each of the last four decades has been successively warmer than any decade that preceded it since 1850.” Yet in the past few years, global warming has moved from a statistical property to an ambient condition of modern life. A mega-drought seems to grip the American West without end. A series of wildfires have passed, like a baton, from one part of the world to another, going from California to the Amazon to Australia to Greece to California again. And then there was the morning, a few weeks ago, when Americans on the East Coast and in the Midwest woke up, thousands of miles away from any wildfire, and smelled smoke in the air.

“We’re reaching a point where the impacts of climate change are becoming too hard to ignore for many people,” Zeke Hausfather, a climate scientist and a co-author of one of the report’s chapters, told me.

For the first time, the report establishes that those extreme events are happening because of climate change. Scientists’ ability to attribute individual events to the warming atmosphere is the “biggest advance” the field has seen in the past decade, Ben Cook, a climate-science professor at Columbia University, told me.

“Every inhabited region across the globe” has seen a well-documented increase in heat waves, heavy rain, or drought, the report says. Human activity is also behind the demise of glaciers since 1990, the hemorrhaging of the Greenland ice sheet, and the decline of snow cover in the Northern Hemisphere since 1950, the report says. No part of the world has been left untouched by humanity’s prodigious carbon pollution.

2. Sea-level rise will be worse than once thought—and could occur quickly and catastrophically.

In the past decade, climate scientists have arrived at more pessimistic views about sea-level rise, and those views are reflected in this report. Most researchers now believe that the oceans will rise roughly half a foot more than once projected. In a relatively optimistic “intermediate” emissions scenario, for instance, the IPCC once projected that oceans would rise about one and a half feet by 2100. The new report finds that just under two feet is more likely, and two and a half feet is not out of the question.

The authors could not eliminate from their models the small chance that some of the largest glaciers in West Antarctica could catastrophically collapse this century. In that scenario, humanity could see more than six and a half feet of sea-level rise by 2100 and perhaps as much as 16 feet of sea-level rise by 2150.

3. Sea-level rise is also essentially irreversible.

If humanity successfully learns how to remove carbon from the atmosphere, some of the impacts of climate change, such as ocean acidification and the rise in land temperatures, may be reversible.

But some will not. Sea-level rise is chief among them. “Once you have melting under way, it’s very hard to rein it in, even if you go full-scale into reversal of global warming,” Kim Cobb, a climate scientist at Georgia Tech and a co-author of the report, told me. The slow increase in sea levels could continue for millennia.

“When I say those words, it almost chokes me up. It scares the crap out of me, frankly,” Cobb said. “This is a horrific long-term consequence to the decisions we’ll be making this decade on our watch.”

4. The climate is now changing on political time.

If climate change is happening now, then its time scales—which once seemed distant—are suddenly ticking by at the speed of the political or business calendars. An earlier draft of this report cautioned that the world could see more than 1.5 degrees Celsius of warming by the early 2030s. Although that language was removed because researchers could not guarantee that a fluke event, such as a once-in-a-century volcanic eruption, would not briefly cool the planet and delay the inevitable for a few years, the broad point remains. The IPCC now warns that the world is likely to exceed 1.5 degrees Celsius of warming by 2040 even if humanity cuts carbon pollution as rapidly as is plausible. In fact, the agency estimates that enough greenhouse gas is already in the atmosphere today to raise the planet’s temperature by 1.5 degrees Celsius—only the cooling effects of smog and other forms of conventional air pollution are keeping temperatures depressed.

But humanity may still avoid warming the planet by 1.5 degrees Celsius by 2100. On all pathways, the world’s temperature will increase more than 1.5 degrees Celsius by the middle of the century; the only question is whether it then begins to cool down or keeps going up. Current policies suggest that the planet is set for 3 degrees Celsius, or more than 5 degrees Fahrenheit, of warming by the end of the century.

“We’re already seeing extreme rainfall, heat waves, and droughts that are all implicitly or explicitly tied to climate change—and this is just a 1-degree world,” Cook said. “I would not want to live in a 4-degree world. And a 3-degree world … would be quite challenging.”

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