Why the Worst Worst-Case Climate Scenario is Going Away (too Slowly)
A long overdue conversation!
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Few concepts in climate science have been more deeply hidden in the weeds, and more consequential, than the scenarios for planet-heating emissions long used by climate modelers and policy makers to figure out our warming future and what to do about it.
The recent elimination of the worst worst-case scenario, known in the field as RCP8.5 (the “worst” in several senses of the word) has gained new attention thanks to social media posts by President Trump (with heaps of help from his executive assistant) and his White House team:
But the beginning of the end for RCP8.5 came nearly a decade ago, in a keystone 2017 paper by Justin Ritchie and Hadi Dowlatabadi at the University of British Columbia asking this question in its title: “Why do climate change scenarios return to coal?”
Their prime conclusion:
RCP8.5 and other ‘business-as-usual scenarios’ consistent with high CO2 forcing from vast future coal combustion are exceptionally unlikely. Therefore, SSP5-RCP8.5 should not be a priority for future scientific research or a benchmark for policy studies.
The big debate in the climate world now is over whether the end of this worst-case warming outcome was the result of policy and the explosive advance of renewable energy or because it was always wrong.
Updated after the show: There’s been so much action around this debate that I reached out to key figures, and we had a fascinating and productive pop-up discussion. The discussants were Zeke Hausfather of Stripe and Berkeley Earth, Matt Burgess of the University of Wyoming, Roger Pielke Jr., the emeritus University of Colorado prof who’s now at the American Enterprise Institute and Richard Tol, a longtime climate economist at the University of Sussex. See relevant posts by three of these researchers below.
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Reading
Matt Burgess responded to President Trump’s social media posts hailing the death of RCP8.5 on his great Guided Civic Revival Substack:
Zeke Hausfather and some longtime collaborators just posted a deep look at what’s going on with this scenario and the discussions around it:
Roger Pielke Jr., who’s been challenging the emissions scenario and the way it has been used (and abused) for many years. Here’s his new post defending his stance that RCP8.5 was always wrong and another when the news broke. There’s more on The Honest Broker.
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Also read the long analysis of scenarios in science written in 2021 by Roger Pielke and Justin Ritchie for Issues in Science and Technology: How Climate Scenarios Lost Touch With Reality.
A central idea:
The emissions scenarios the climate community is now using as baselines for climate models depend on portrayals of the present that are no longer true. And once the scenarios lost touch with reality, so did the climate, impact, and economic models that depend on them for their projections of the future. Yet these projections are a central part of the scientific basis upon which climate policymakers are now developing, debating, and adopting policies.
And play my song Liberated Carbon to provide a soundtrack while you read: