I hope you’ll watch and share this illuminating and constructive face-to-face discussion with four key analysts of the scenarios for emissions of planet-heating gases that indirectly have shaped, and sometimes misshaped, climate policy discourse and decisions (and media coverage).
My guests were Richard Tol, Zeke Hausfather, Matt Burgess, and Roger Pielke Jr.
As I discussed here recently, the worst-case emissions track, a k a RCP8.5, is being dropped from the basket of plausible futures used in modeling studies that underpin core conclusions in the periodic reports from the U.N. Intergovernmental Panel on Climate Change. (Read the April paper describing the revisions to scenarios, which the authors explain came about because “underlying projections…were somewhat outdated by the time climate model output became available… Also, over time, critiques emerged about the plausibility of the most extreme scenarios.”)
My guests, like most researchers dug in on this area of climate research, largely agree that the highest emissions scenario is deeply implausible. The debate is mostly on whether that is a new development — the result of the enormous progress in cutting the cost of renewable energy — or because the scenario was never plausible. (I favor the second camp.)
As I mentioned in yesterday’s curtain raiser post, a key 2017 paper led by Justin Ritchie of the University of British Columbia laid out the near impossibility of mining and burning the amount of coal required to get such high emissions. Click back for that paper and other background:
Here’s Hausfather, who works at the company Stripe and at the research lab Berkeley Earth (and of course co-writes The Climate Brink):
Even though Roger and I like to spar online a lot, there’s a couple of things that I think we’d all agree are true today. One is that RCP 8.5 was never a particularly plausible baseline scenario. And there was a mistake by the community to refer to it as business as usual.
I like to call it a mistaken game of telephone. No, I don’t think there was a bad intent there.
I think that a lot of climate scientists didn’t necessarily read the energy modeling literature, which made it pretty clear back in 2011 when RCP 8.5 was formally published that it was intended to represent the 90th percentile of baseline outcomes in the literature at the time,not a particular likely outcome….
I think there’s sort of two separate questions that we’ve been debating a lot here. One is, how plausible was the scenario ever? And I think we all agree it was pretty implausible. And the second is, has the world actually moved away from higher emissions scenarios?
And that latter question, I think the answer is probably yes.
Driven in part by, as Richard pointed out, the falling cost of clean energy, which is mediated obviously by some policy actions taken by various countries, including the US, China and others. Exactly where we might have been headed, I think is a really tough question to answer.
Roger Pielke acknowledged that clean energy tech has advanced tremendously but said his key metric is the overall rate of decarbonization of the economy, and — wheteher you look at the United States or the planet as a whole — that rate has remained a straight declining line decade after decade. I showed that decarbonization curve from one of his Substack posts and he said:
My hypothesis is a lot of the wonderful things that have happened, and it’s true. Technologies have advanced, costs have gone down. But they are basically maintaining a trend that’s been in place long before Paris, long before Kyoto, long before the Rio Conference.
So we’re calling climate policy the sort of things that we used to call other names. There are some differences around the world. If we want to have deep decarbonization at a rate faster than this linear decline, this curve is going to have to have an inflection point. It’s that simple.
When RCP8.5 “escaped the courtyard of science”
Roger Pielke explained (and no one disagreed) that one of the biggest harms from the exreme warming scenario came as it became used outside of the world of climate simulation - something he and Justin Ritchie explored in depth in Issues in Science and Technology in 2021 (as well as in this paper: “Distorting the view of our climate future: The misuse and abuse of climate pathways and scenarios.”) Here’s the main point:
At some point, Justin and I in our paper, our incredibly long, boring paper on the history of all this, we quote a researcher who says, we didn’t realize that RCPs could take on a life of their own. these scenarios escaped from the courtyard of science and went out in the real world.
And it turns out they’re used in policy, they’re used in regulation, they’re used in global banking, they’re used in insurance, rate setting and risk assessment. I don’t think it’s an exaggeration to say that everyone in the planet is in some way touched by these scenarios. And so the institutions of science really didn’t keep up with the growing importance of climate change as a topic and climate institutions. And so here we are in 2026. And like Richard said, better late than never, but it would have been better if it happened 10 or 15 years ago.
There’s much, much more from Hausfather, Pielke and Matt Burgess and Richard Tol.
We also discussed how research conclusions centered on the extreme warming scenario long provided just the kind of dramatic fodder that climate-focused journalists need. I brought up the pull of “narrative capture” — my shorthand for the dynamic where internal or external dynamics cause people to cling to validating visions of a problem (the climate crisis) even when data shift.
I’m on the run all week and this is where AI tools are incredibly useful. Here’s the Gemini summary of some other points (you can ask the AI on YouTube to do this kind of task). I asked: “Distill the key observations and assertions made by the guests in this discussion of the value and limits of developing scenarios for human activities that could worsen or lessen global warming. Pick out half a dozen key moments and create time stamps.”
Here’s Gemini:
This discussion examines the history, misuse, and eventual retirement of the RCP 8.5 emissions scenario, which was widely but erroneously labeled as “business as usual.” The panelists — Richard Tol, Zeke Hausfather, Matt Burgess, and Roger Pielke Jr. — argue that while scenarios are useful for scientific modeling, they have often been misinterpreted by the media and policymakers, leading to flawed risk assessments.
Key Moments and Insights:
* The “Business as Usual” Misconception: (16:07-17:20) Zeke Hausfather explains that the climate community made a mistake in referring to RCP 8.5 as “business as usual,” noting it was originally intended to represent a 90th-percentile baseline, not a likely outcome.
* Institutional Lags in Science: (13:10-14:50) Richard Tol discusses the necessity of updating scenarios more frequently than the current decadal cycle, pointing out that scientific modeling cycles often struggle to keep pace with the urgent needs of policy and global risk assessment.
* Policy Evaluation vs. Scenario Reliance: (46:51-47:20) Roger Pielke Jr. asserts that evaluating actual policy—whether it accelerates or reverses emission curves—is significantly more important than relying on static, hypothetical emissions scenarios.
* The “Franken-Scenario” Problem: (51:06-51:44) Matt Burgess criticizes the practice of combining extreme high-emission scenarios with slow-growth economic assumptions, which creates “Franken-scenarios” that produce misleading results about climate vulnerability and adaptation.
* The Media’s Role in Alarmism: (59:00-1:00:43) The panel discusses how sensationalist headlines are often incentivized by journals and media outlets, making it easier to publish studies that claim climate change is “worse than we thought” compared to more nuanced assessments.
* From Scenarios to Warming Levels: (1:13:30-1:14:43) Zeke Hausfather suggests the community should pivot away from specific emission scenarios entirely, instead focusing on impacts associated with specific global warming levels (e.g., 2°C or 3°C worlds) to better communicate risk and uncertainty.
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Thank you Martha Morningsong, Bruce Maslack, Zach Keeshin, and many others for tuning into my live video! Join me for my next live video in the app.
Postscript
Dan Miller and Leon Simons had an informative conversation with Detlef van Vuuren, the lead author of the April paper laying the groundwork for the changes in emissions scenarios.
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