For some reason, Now It Can Be Told:

You’ve probably never heard of the term “RCP 8.5” — the highest-emission scenario used by climate scientists to project the planet’s future. But if you’ve read about climate change, you’ve seen the numbers and nightmarish outcomes it produced: 4°C of warming by 2100, sometimes 5°C, sea level rising multiple feet, parts of the planet too hot for humans.

Those numbers shaped a decade and a half of climate journalism, including a lot of my own when I covered climate change at Time magazine. I didn’t always know — and didn’t always communicate — that the scenario behind the most apocalyptic, attention-getting findings was largely an attempt to imagine how bad things could get, not a true forecast. But I wasn’t alone. RCP 8.5 was a frequent background presence in climate journalism.

So are we to conclude that author Bryan Walsh sometimes did know and yet failed to communicate that he was writing the equivalent of a Hollywood script?

As for why he’s telling the tale now, it’s a domino effect:

Last month, though, the scientists who built that scenario formally retired it. In a paper published in Geoscientific Model Development, Detlef van Vuuren and more than 40 co-authors eliminated RCP 8.5 from the scenarios that will feed into the Intergovernmental Panel on Climate Change’s (IPCC) Seventh Assessment Report, which is due in 2029. Based on falling clean-energy costs, climate policy, and recent emissions trends, the highest-emissions pathway had become, in their words, “implausible.”

Walsh still says things will be bad, just not as bad. But why would we trust that prediction?

Was RCP 8.5 ever realistic? One camp of experts, led by climate scientist Zeke Hausfather and energy modeler Glen Peters, argues that RCP 8.5 was plausible in 2011, but was taken off the table by genuine policy and technology progress. The other camp, led by Roger Pielke Jr., argues that the rate of global decarbonization has been roughly linear for decades. That would mean we didn’t actively avoid RCP 8.5; it was just never realistic to begin with. Both camps agree on what counts, though: RCP 8.5 should be gone, and the planet is still on track to warm between 2.5° and 3° by 2100.

Walsh seems to be saying, in most of the article, that if we could predict the policy we could predict the climate. But I have always thought that’s a case of hubris. There are too many variables and too many unknowns interacting in too complex and too poorly-understood a fashion. And that’s even if you assume that scientists and journalists are always acting in good faith, which is – as they say – somewhat implausible.