On the flip side, the more CO2 there is in the atmosphere, the faster plants grow, and the more they suck from the atmosphere. That’s a negative feedback system, and it tends to moderate changes.
The exact magnitude of the many possible feedback systems in the atmosphere is uncertain. As the Siberian permafrost melts, will it release gigatonnes of CO2? It is possible that even under more optimistic emissions scenarios, the feedback systems will push more carbon into the atmosphere, so we end up with the same CO2 concentrations as we would have done under RCP 8.5. And there’s another layer of uncertainty, which is “climate sensitivity”. It’s not precisely known how much warming a given increase in CO2 concentrations will cause; perhaps even if the amount of carbon in the atmosphere is relatively low, the warming will be greater (although that now looks less likely).
So really bad outcomes are possible even without RCP 8.5-level emissions. We’d have to get “incredibly unlucky” with feedbacks for the world to end up warming that much, says Hausfather, but we can’t eliminate the possibility.
As a result, there’s an ongoing row among academics about whether RCP 8.5 (and its equally pessimistic successor2) should be used at all. Roger Pielke Jr, a professor of public policy and environmental studies at the University of Colorado, Boulder, argues that it essentially has no place: that it’s based on an implausible scenario, even if you could end up somewhere similar because of uncertainties elsewhere. “Two wrongs don’t make a right,” he says.
But Richard Betts, a climate scientist at Exeter University and the Met Office, and one of the authors of the UK government’s climate change risk assessment, argues differently. We need to be aware of the risks of these 4°C worlds that we could – plausibly – still find ourselves in, he says. And there’s been lots of research done on worlds like that, mostly using RCP 8.5.
Gavin Schmidt, the director of NASA’s Goddard Institute, adds that using extreme scenarios allows modellers to tease out causality in a way that more narrow ones don’t. “It allows you to see non-linearity, allows you to look for thresholds,” he says. “You get a stronger signal to noise ratio.” The actual emission scenarios are less likely, he agrees, but studying them helps us understand the climate system better.
The trouble is that if a journalist sees two graphs in a study, one of a plausible RCP 4.5 world and one of a much more dramatic RCP 8.5 world, they’ll probably want to run the RCP 8.5 graph, “because it’s more impressive,” says Hausfather. “And they’ll say scientists predict this much sea level rise, or this many heat deaths. And it’s not accurate because you’d need to add ‘but only if we burn all our coal or get very unlucky’.”
There’s also a risk that RCP 8.5 will crowd out research into other areas. For instance, Our World in Data’s recent efforts to understand human impacts on biodiversity have been hampered because almost all the research into coral reef collapse has been carried out using RCP 8.5. Predictably enough, all the coral dies in that scenario. But what would coral reefs look like under 2°C of warming, or 3°C? We don’t really know.
(People have got in touch to suggest that recent studies have looked into the other pathways, and they do seem to show severe impacts on coral even at lower concentrations.)
The RCP 8.5 debate is heated. Pielke Jr and Schmidt in particular have a longstanding mutual animosity, and Schmidt feels Pielke Jr has “spent 20 years trying to elbow out scientists from the centre ground” on climate change. So it’s hard for a journalist to get involved without taking sides.
But it’s important, because the more we associate the worst-case scenario with business as usual, the more pessimistic the public debate will be. “It might demotivate people,” says Hausfather. “It’s much easier to see the Paris goals [of no more than 1.5°C warming] as achievable if you know we’re on course for a 3°C world, rather than a 5°C one.”
I disagree with Pielke Jr: RCP 8.5 has its place, because as Betts says, there’s a whole range of research into unlikely but plausible scenarios using it, and if you’re doing risk assessments, you need to look at outcomes that probably won’t happen but which would be disastrous if they did, so you can try to avoid them. At the moment the papers looking at those scenarios all use 8.5.
But on the other hand, if the IPCC puts out RCPs without explicitly saying which are the most likely, then policymakers and journalists will take whichever scenario most suits their needs, whether that’s pretending there’s no problem, or magnifying the problem for the sake of a headline. This isn’t the fault of climate modellers. But no one involved in the IPCC is explicitly saying “RCP 8.5 is pretty unlikely,” and that fateful phrase “business as usual” is still attached to it.
Hausfather has a solution for this: attach explicit percentage likelihoods to the different scenarios – say that RCP 4.5 is 45% probable, or RCP 8.5 is 5% probable. They’d be necessarily subjective but at least it would show that no one thinks they’re all equally likely, or that RCP 8.5 is the course we’re already on. Betts agrees.
There’s a risk that climate sceptics will leap on ideas like this, and say that climate scientists have systematically overstated the risk of climate change; Betts says that this already happens. But climate change is an unhedgeable risk, and we need to pay attention to even unlikely outcomes if they’re very bad, in the same way that you wouldn’t play Russian roulette even if it’s only a 17% chance of death. Besides, even the less dramatic 3°C worlds involve dangerous levels of sea level rise, heat waves, and millions of unnecessary deaths a year. You’d think that’d be worrying enough.
Every week, people see headlines, based on RCP 8.5 scenarios, that say things like three billion people could face “near-unliveable conditions” by 2070. Understandably, people are rattled; and some are even saying they’re not having children. People are more scared than they need to be.
Climate change is extremely bad. But we’re not in the “last chance saloon”, if that means that we face some inevitable catastrophe, or 5-1 down and facing ruinous defeat; and the politicians at COP26 are not guilty of facilitating a worse genocide than Hitler’s. Archbishop Welby apologised for that. But it’s an understandable mistake if he’s always being told that “business as usual” means a march to doom.
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