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Just two years ago, amid global fanfare, the Paris climate accords were signed — initiating what seemed, for a brief moment, like the beginning of a planet-saving movement. But almost immediately, the international goal it established of limiting global warming to two degrees Celsius began to seem, to many of the world’s most vulnerable, dramatically inadequate; the Marshall Islands’ representative gave it a blunter name, calling two degrees of warming “genocide.”

The alarming new report you may have read about this week from the UN’s Intergovernmental Panel on Climate Change — which examines just how much better 1.5 degrees of warming would be than 2 — echoes the charge. “Amplifies” may be the better term. Hundreds of millions of lives are at stake, the report declares, should the world warm more than 1.5 degrees Celsius, which it will do as soon as 2040, if current trends continue. Nearly all coral reefs would die out, wildfires and heat waves would sweep across the planet annually, and the interplay between drought and flooding and temperature would mean that the world’s food supply would become dramatically less secure. Avoiding that scale of suffering, the report says, requires such a thorough transformation of the world’s economy, agriculture, and culture that “there is no documented historical precedent.” The New York Times declared that the report showed a “strong risk” of climate crisis in the coming decades; in Grist, Eric Holthaus wrote that “civilization is at stake.”

If you are alarmed by those sentences, you should be — they are horrifying. But it is, actually, worse than that — considerably worse. That is because the new report’s worst-case scenario is, actually, a best case. In fact, it is a beyond-best-case scenario. What has been called a genocidal level of warming is already our inevitable future. The question is how much worse than that it will get.

Barring the arrival of dramatic new carbon-sucking technologies, which are so far from scalability at present that they are best described as fantasies of industrial absolution, it will not be possible to keep warming below two degrees Celsius — the level the new report describes as a climate catastrophe. As a planet, we are coursing along a trajectory that brings us north of four degrees by the end of the century. The IPCC is right that two degrees marks a world of climate catastrophe. Four degrees is twice as bad as that. And that is where we are headed, at present — a climate hell twice as hellish as the one the IPCC says, rightly, we must avoid at all costs. But the real meaning of the report is not “climate change is much worse than you think,” because anyone who knows the state of the research will find nothing surprising in it. The real meaning is, “you now have permission to freak out.”

As recently as a year ago, when I explored worst-case scenarios for climate change, alarmism of this kind was considered anathema to many scientists, who believed that storytelling that focused on the scary possibilities was just as damaging to public engagement as denial. There have been a few scary developments in climate research over the past year — more methane from Arctic lakes and permafrost than expected, which could accelerate warming; an unprecedented heat wave, arctic wildfires, and hurricanes rolling through both of the world’s major oceans this past summer. But by and large the consensus is the same: We are on track for four degrees of warming, more than twice as much as most scientists believe is possible to endure without inflicting climate suffering on hundreds of millions or threatening at least parts of the social and political infrastructure we call, grandly, “civilization.” The only thing that changed, this week, is that the scientists, finally, have hit the panic button.

Because the numbers are so small, we tend to trivialize the differences between one degree and two, two degrees and four. Human experience and memory offers no good analogy for how we should think about those thresholds, but with degrees of warming, as with world wars or recurrences of cancer, you don’t want to see even one.

At two degrees, the melting of ice sheets will pass a tipping point of collapse, flooding dozens of the world’s major cities this century. At that amount of warming, it is estimated, global GDP, per capita, will be cut by 13 percent. Four hundred million more people will suffer from water scarcity, and even in the northern latitudes heat waves will kill thousands each summer. It will be worse in the planet’s equatorial band. In India, where many cities now numbering in the many millions would become unliveably hot, there would be 32 times as many extreme heat waves, each lasting five times as long and exposing, in total, 93 times more people. This is two degrees — practically speaking, our absolute best-case climate scenario.

At three degrees, southern Europe will be in permanent drought. The average drought in Central America would last 19 months and in the Caribbean 21 months. In northern Africa, the figure is 60 months — five years. The areas burned each year by wildfires would double in the Mediterranean and sextuple in the United States. Beyond the sea-level rise, which will already be swallowing cities from Miami Beach to Jakarta, damages just from river flooding will grow 30-fold in Bangladesh, 20-fold in India, and as much as 60-fold in the U.K. This is three degrees — better than we’d do if all the nations of the world honored their Paris commitments, which none of them are. Practically speaking, barring those dramatic tech deus ex machinas, this seems to me about as positive a realistic outcome as it is rational to expect.

At four degrees, there would be eight million cases of dengue fever each year in Latin America alone. Global grain yields could fall by as much as 50 percent, producing annual or close-to-annual food crises. The global economy would be more than 30 percent smaller than it would be without climate change, and we would see at least half again as much conflict and warfare as we do today. Possibly more. Our current trajectory, remember, takes us higher still, and while there are many reasons to think we will bend that curve soon — the plummeting cost of renewable energy, the growing global consensus about phasing out coal — it is worth remembering that, whatever you may have heard about the green revolution and the price of solar, at present, global carbon emissions are still growing.

None of the above is news — most of that data is drawn from this single, conventional-wisdom fact sheet. In fact, nothing in the IPCC report is news, either; not to the scientific community or to climate activists or even to anyone who’s been a close reader of new research about warming over the last few years. That is what the IPCC does: It does not introduce new findings or even new perspectives, but rather corrals the messy mass of existing, pedigreed scientific research into consensus assessments designed to deliver to the policymakers of the world an absolutely unquestionable account of the state of knowledge. Almost since the panel was convened, in 1988, it has been criticized for being too cautious in its assessment of the problem — a large body of temperamentally cautious scientists zeroing on those predictions they can all agree on (and which, they may have hoped, policymakers might find workable). The panel’s Wikipedia page has separate subsections for “Outdatedness of reports” and “Conservative nature of IPCC reports.”

Which is why it is so remarkable that the tone of this report is so alarmist — it’s not that the news about climate has changed, but that the scientific community is finally discarding caution in describing the implications of its own finding.

They have also, thankfully, offered a practical suggestion, proposing the imposition of a carbon tax many, many times higher than those currently in use or being considered — they propose raising the cost of a ton of carbon possibly as high $5,000 by 2030, a price they suggest may have to grow to $27,000 per ton by 2100. Today, the average price of carbon across 42 major economies is just $8 per ton. The new Nobel laureate in economics, William Nordhaus, made his name by almost inventing the economic study of climate change, and his preferred carbon tax is $40 per ton — which would probably land us at about 3.5 degrees of warming. He considers that grotesque level “optimal.”

But a carbon tax is only a spark to action, not action itself. And the action needed is at a scale and a speed almost unimaginable to most of us. The IPCC report called it unprecedented. Other activists often see one precedent, in all of human history, citing the model of how the United States prepared for World War II, and calling for a global mobilization of that kind — all of the world’s rivalrous societies and nationalistic governments and self-interested industries organized around the common pursuit of a stable and comfortable climate as though warming was an existential threat.

It is. And the World War II mobilization metaphor is not hyperbole. To avoid warming of the kind the IPCC now calls catastrophic requires a complete rebuilding of the entire energy infrastructure of the world, a thorough reworking of agricultural practices and diet to entirely eliminate carbon emissions from farming, and a battery of cultural changes to the way those of us in the wealthy West, at least, conduct our lives. And we need to do all of that in two, or possibly three, decades. As a comparison, simply the last phase of the recent three-stop extension of New York City’s Second Avenue subway line took 12 years. All told, from the first groundbreaking, the project took 45 years.

That is not to say it’s over or we’re doomed. Stalling warming below four degrees is better than surpassing it, keeping temperatures below three is better still, and the closer we get to two degrees the more miraculous. That is because climate change isn’t binary, and doesn’t just kick in, full force, at any particular temperature level; it’s a function that gets worse over time as long as we produce greenhouse gases. How long we continue to is, really, up to us, which is to say it will be determined in the province of politics, which is to say public panic like that produced by the IPCC report can be a very productive form of policy pressure.

There are also those far-fetched alternatives I mentioned — carbon capture and solar geoengineering — but each is far from workable at the moment and, even in theory, come with really scary drawbacks. But even if the technology becomes dramatically cheaper and more efficient over the next few years, you would need to build them out across the globe, as well — whole plantations sucking carbon almost everywhere on the planet. It will take quite a long time to build those, in other words, even if they worked, and we simply don’t have that many years left to act.

A few weeks ago, as the IPCC report loomed, I had lunch with a prominent climate scientist who’d been involved in earlier reports and has done considerable work on local preparedness as well. I asked if he thought New York would eventually build a sea wall or surge barrier to protect the city from sea-level rise and flooding. Yes, he said, Manhattan will be protected, at any cost. But major infrastructure projects like these take decades — typically about 30 years. Even if we began building today, he said, the barrier would not be finished in time to save Howard Beach and other parts of southern Queens and Brooklyn. Soon enough, he said, you’ll see the city adjust accordingly — halting new infrastructure projects there, eventually pulling back from even quotidian maintenance like sewer repairs and generally signaling to current residents that they will not be able to leave behind their homes, when they die, to their children. And of course a sea wall to protect New York only encloses the narrows of New York Harbor, leaving all of Long Island exposed.

This is just the threat from sea level, and just one (very rich) metropolitan area. The world is much bigger than that, but so is climate change. It is also very fast, with more than half the carbon humanity has ever emitted into the atmosphere having come in just the last 25 years, since Al Gore published his first book on climate change. Monday’s IPCC may seem like a dramatic departure, and it is. But there is going to be much more like it coming. So long as we continue to squander what little time we have, the news will only get worse from here.