Climate change, Elizabeth Warren said during CNN’s climate town hall on Wednesday night, “is the existential threat. It is the one that threatens all life on this planet.” Fellow presidential candidate Kamala Harris and former candidate Jay Inslee have used the phrase “existential threat” to describe climate change as well.

It’s reassuring to see candidates on the national stage take climate change seriously as the grave danger it is. And since the Democratic National Committee has declined to host a debate focused entirely on climate change, we may not get to see any of the candidates describe in more detail what climate scenarios they consider likely, and what exactly they mean by “existential threat.”

But there’s a standard meaning of that phrase: that it’s going to wipe out humanity — or even, as Warren implied Wednesday night, all life on our planet. Civilization will topple, and famine and natural disasters will pick off the survivors. Cockroaches will reign on Earth — or maybe they won’t, since there’s a mass insect extinction underway.

Scenarios like these have attracted intense attention lately. This spring, one of the most prominent arguments that climate change poses an existential threat went viral in Vice: “New Report Warns ‘High Likelihood of Human Civilization Coming to an End’ Within 30 Years.’”

The Vice story summed up a new report from the Breakthrough National Centre for Climate Restoration, an Australian think tank, arguing that the Intergovernmental Panel on Climate Change analysis of the impacts of climate change understates how much harm it’ll do, and that in reality we face something much worse, with runaway feedback effects amplifying the initial warming until the Earth is “largely uninhabitable.” It doesn’t actually argue that the world will end in 30 years, but it suggests we’ll reach the tipping point by then.

The story went up on Vice with an orange-tinged, haunting illustration of the Statue of Liberty submerged to the neck by rising seas. The post was shared more than 70,000 times on Facebook and left readers terrified, despairing, and sharing doubts about whether it’s ethical to have children.

The Breakthrough report — and the media coverage of it — frustrated many climate scientists. In a detailed response, six researchers argued that the report overstates the risks from climate change, and that subsequent reporting overstated it even further. The fact is that even the most pessimistic reports, evaluated responsibly, don’t suggest climate change will end human civilization, much less within our lifetimes. (Don’t stop saving for retirement.) Vice later altered the headline to “New Report Warns ‘High Likelihood Of Human Civilization Coming To An End’ Starting Within 30 Years’ — and even published a rebuttal.

But the Vice piece tapped into what’s actually been a long-running and sometimes contentious conversation about the climate crisis — specifically, about whether it’s merely just devastating or in fact an existential risk to humanity.

And it resonated with people in part because most mainstream research into climate change has focused — prompted by the IPCC — on scenarios with around 2 degrees of warming, less often on (fairly likely) scenarios with 3 or 4 degrees of warming, and infrequently on worst-case scenarios of climate disaster, leaving a lot of questions about those worst-case scenarios unanswered.* How likely are they? How catastrophic would they be? Scientists don’t all agree, and that uncertainty creates room for the most shocking stories to go viral.

Beneath the disagreement over climate risks is a disagreement over worldviews. From one perspective, quibbling over whether climate change will kill millions or billions is a waste of time when, in either case, we urgently need to act. But from another perspective, the difference is deeply significant; for example, it changes whether potential solutions that carry significant risks, like some forms of solar geoengineering, are warranted.

Another broad disagreement is whether alarmism makes our prospects of tackling climate change better or worse. As some people see it, we’re not doing nearly enough to fight climate change, so we’d better focus in on the worst-case scenarios in case that will be what it takes to finally spur people to action. Others, though, worry that alarmism, far from motivating people, leads to paralysis — too much despair about the future to even bother working on it.

So, yes, the Vice story did hype up the threat from the climate crisis, and it likely won’t be the last of those stories we’ll see.

The argument that climate change will kill us all

The expected effects of climate change, according to organizations like the IPCC and the World Bank, are fairly terrifying.

They suggest the planet’s climate will change fast enough to cause widespread droughts and famines, the spread of insect-borne diseases, the displacement of populations, and a worsening of severe poverty.

But here’s one thing they don’t predict: mass civilizational collapse.

Most models warn that as a result of climate change, the incredibly rapid progress humanity has been making in life expectancies and in ending extreme poverty will stall; we could even lose decades of the progress we’ve made. If extreme poverty gets as bad as it was in 1980 due to climate change, that will be an immeasurable humanitarian failure, and hundreds of millions of people will die. But the 1980s definitely did have human civilization, and the future in this version would too.

Another way of looking at it is that the predicted effects of climate change are very bad, but not in a cinematic way. Sea levels will rise, but not up to the Statue of Liberty’s neck (if all the ice in the world melted, sea levels would rise to approximately the statue’s waist). Lots of people will die, most of them low-income. It’s not surprising that this gets less viral attention than extreme, extinction-focused scenarios.

But that isn’t to say extreme scenarios are made up from nothing. Where do some people conclude that climate change might swallow up civilization itself?

Well, for one thing, lots of climate policy analysts agree that the IPCC is too optimistic. In particular, the IPCC has kept insisting that it’s still possible to keep warming under 2 degrees Celsius when at this point, that’s really unrealistic. As my colleague David Roberts put it:

Models have often included unrealistically low estimates of current and future emissions growth, unrealistically early peaks in global emissions, and unequitable estimates of emission curves in developing countries (implicitly assuming stunted development). ... Models routinely show 4 or even 6 percent annual reductions, a rate of emissions decline that has never been achieved by anyone, anywhere, ever, much less consistently over 50 years.

So it’s not surprising that some people got interested in more pessimistic models. What if we assume that we don’t get our emissions under control? What if we assume that there are severe “feedback cycles” where warming causes the release of carbon dioxide currently contained in the land and in the oceans, fueling further warming? And what if, instead of trying to model the most likely outcome, we look at outcomes that may only have a 10 percent chance of occurring but would be particularly disastrous if they did?

The Breakthrough report, authored by former fossil fuel executive Ian Dunlop and author David Spratt, for the most part summarizes cases for pessimism that have been raised in other papers and public statements.

It says, for example, “attention has been given to a ‘hothouse Earth’ scenario, in which system feedbacks and their mutual interaction could drive the Earth System climate to a point of no return, whereby further warming would become self-sustaining. This ‘hothouse Earth’ planetary threshold could exist at a temperature rise as low as 2°C, possibly even lower.”

“Our argument is in essence that on the present path, including the commitments in Paris, warming will be three or three and a bit degrees,” Spratt told me. “If you include climate cycle feedbacks, which are not included in the IPCC analysis, it’ll be effectively higher.” For both those claims, there’s significant published science backing him. Then he gets to the controversial bit: “Three degrees may end our civilization.”

For that claim, he cites climate scientist John Schellnhuber, who said in an interview early this year, “if we get it wrong, do the wrong things ... then I think there is a very big risk that we will just end our civilisation,” and UN Secretary General António Guterres, who has said “The problem is that the status quo is a suicide.”

It’s hard to know how to interpret remarks like those in an interview, but Spratt’s take is that the end of human civilization is not just a possibility but a likely outcome if we continue down our current path. Many people had no trouble believing it.

Scientists objected. Here’s what they said.

Six climate scientists reviewed the accuracy of the report and Vice’s write-up at Climate Feedback, a nonprofit that works to improve climate reporting by getting comments from scientists on striking claims in the press. Their responses were scathing.

“This is a classic case of a media article over-stating the conclusions and significance of a non-peer reviewed report that itself had already overstated (and indeed misrepresented) peer-reviewed science,” wrote Richard Betts, who chairs the department for climate impact research at the University of Exeter and leads the European Union project that studies the impacts of extreme global warming.

The Breakthrough report does indeed gather claims from other papers, climate leaders, and thinkers. But it selected many of the scariest and most speculative papers and presented them without being clear about how plausible they are.

And some of its most outrageous claims are just wrong. The report argues that if temperatures continue to rise, “fifty-five percent of the global population are subject to more than 20 days a year of lethal heat conditions beyond that which humans can survive.” That’d be terrifying. But Betts points out that this is based on the definition of a “deadly heat wave” from a paper that defined a deadly heat wave as one above a threshold where at least one person is expected to die (based on historical data). And some of the temperatures identified as deadly are as low as 86 degrees Fahrenheit (30 degrees Celsius) with high humidity — hot, but not what comes to mind from the phrase “lethal heat conditions beyond what humans can survive.”

“The report’s authors have merely read (or possibly seen without actually reading) a few of the scariest papers they could find, misunderstood (or not read properly) at least one of them, and presented unjustified statements,” Betts added.

“The scenario constructed in this report does not have a ‘high likelihood’ of occurring,” wrote Andrew King, a research fellow at the University of Melbourne.

Spratt says that it needn’t be likely to be an important focus. “Sensible risk management is to look at what are the worst feasible options and take actions to stop them occurring,” he told me. “In risk management, we ask what is the worst possible outcome and avoid it. We don’t assume that middle-of-the-road outcomes are the worst thing around, because that’d be disastrous.” That said, he agreed that much of the media coverage, including the viral Vice article, was “over-the-top and often misleading.”

But the scientists who reviewed the article didn’t just object to the headlines. They felt that the core claim — that 3 or 4 degrees of warming could destroy civilization — was also deeply unlikely. “While there is plenty of scientific evidence that climate change will pose increasingly existential threats to the most vulnerable individuals in society and to key global ecosystems,” wrote UCLA researcher Daniel Swain, “even these dire outcomes aren’t equivalent to the ‘annihilation of intelligent life,’ as is claimed in the report.”

One important thing here is that “suicide,” “catastrophic,” and “end of civilization” are all nontechnical terms, and people may have very different things in mind when they use them — especially if we’re looking at interviews rather than at papers.

I also talked to some researchers who study existential risks, like John Halstead, who studies climate change mitigation at the philanthropic advising group Founders Pledge, and who has a detailed online analysis of all the (strikingly few) climate change papers that address existential risk (his analysis has not been peer-reviewed yet).

Halstead looks into the models of potential temperature increases that Breakthrough’s report highlights. The models show a surprisingly large chance of extreme degrees of warming. Halstead points out that in many papers, this is the result of the simplistic form of statistical modeling used. Other papers have made a convincing case that this form of statistical modeling is an irresponsible way to reason about climate change, and that the dire projections rest on a statistical method that is widely understood to be a bad approach for that question.

Further, “the carbon effects don’t seem to pose an existential risk,” he told me. “People use 10 degrees as an illustrative example” — of a nightmare scenario where climate change goes much, much worse than expected in every respect — “and looking at it, even 10 degrees would not really cause the collapse of industrial civilization,” though the effects would still be pretty horrifying. (On the question of whether an increase of 10 degrees would be survivable, there is much debate.)

Does it matter if climate change is an existential risk or just a really bad one?

That last distinction Halstead draws — of climate change as being awful but not quite an existential threat — is a controversial one.

That’s where a difference in worldviews looms large: Existential risk researchers are extremely concerned with the difference between the annihilation of humanity and mass casualties that humanity can survive. To everyone else, those two outcomes seem pretty similar.

To academics in philosophy and public policy who study the future of humankind, an existential risk is a very specific thing: a disaster that destroys all future human potential and ensures that no generations of humans will ever leave Earth and explore our universe. The death of 7 billion people is, of course, an unimaginable tragedy. But researchers who study existential risks argue that the annihilation of humanity is actually much, much worse than that. Not only do we lose existing people, but we lose all the people who could otherwise have had the chance to exist.

In this worldview, 7 billion humans dying is not just seven times as bad as 1 billion humans dying — it’s much worse. This style of thinking seems plausible enough when you think about past tragedies; the Black Death, which killed at least a tenth of all humans alive at the time, was not one-tenth as bad as a hypothetical plague that wiped us all out.

Most people don’t think about existential risks much. Many analyses of climate change — including the report Vice based its article on — treat the deaths of a billion people and the extinction of humanity as pretty similar outcomes, interchangeably using descriptions of catastrophes that would kill hundreds of millions and catastrophes that’d kill us all. And the existential risk conversation can come across as tone-deaf and off-puttingly academic, as if it’s no big deal if merely hundreds of millions of people will die due to climate change.

Obviously, and this needs to be stressed, climate change is a big deal either way. But there are differences between catastrophe and extinction. If the models tell us that all humans are going to die, then extreme solutions — which might save us, or might have unprecedented, catastrophic negative consequences — might be worth trying. Think of plans to release aerosols into the atmosphere to reflect sunlight and cool the planet back down in the manner that volcanic explosions do. It’d be an enormous endeavor with significant potential downsides (we don’t even yet know all the risks it might pose), but if the alternative is extinction then those risks would be worth taking.

But if the models tell us that climate change is devastating but survivable, as most models show, then those last-ditch solutions should perhaps stay in the toolkit for now.

Then there’s the morale argument. Defenders of overstating the risks of climate change point out that, well, understating them isn’t working. The IPCC may have chosen to maintain optimism about containing warming to 2 degrees Celsius in the hopes that it’d spur people to action, but if so, it hasn’t really worked. Maybe alarmism will achieve what optimism couldn’t.

That’s how Spratt sees it. “Alarmism?” he said to me. “Should we be alarmed about where we’re going? Of course we should be.”

Swedish teenager Greta Thunberg has taken an arguably alarmist bent in her advocacy for climate solutions in the EU, saying, “Our house is on fire. I don’t want your hope. ... I want you to panic.” She’s gotten strong reactions from politicians, suggesting that at least sometimes a relentless focus on the severity of the emergency can get results.

So where does this all leave us? It’s worthwhile to look into the worst-case scenarios, and even to highlight and emphasize them. But it’s important to accurately represent current climate consensus along the way. It’s hard to see how we solve a problem we have widespread misapprehensions about in either direction, and when a warning is overstated or inaccurate, it may sow more confusion than inspiration.

Climate change won’t kill us all. That matters. Yet it’s one of the biggest challenges ahead of us, and the results of our failure to act will be devastating. That message — the most accurate message we’ve got — will have to stand on its own.

*Update: This piece has been updated to offer a more accurate description of what mainstream research on climate has focused on. H/t to David Wallace-Wells for his insight on this point.

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