The international committee responsible for the official scenarios that feed into climate modeling, which are the basis for most projective climate research and the assessments of the Intergovernmental Panel on Climate Change (IPCC), has just published the next generation of climate scenarios. [some emphasis, links added]
Big news: The new framework has eliminated the most extreme scenarios that have dominated climate research over much of the past several decades — specifically, RCP8.5, SSP5-8.5, and SSP3-7.0.
This is an absolutely huge development in climate science that will have lasting impacts across research and policy.
The future is not what it used to be.
Today’s post commends the researchers who have brought climate scenarios more in line with current understandings, but also raises some significant continuing issues with the scenarios.
Let’s get started…
The new scenarios come from the Coupled Model Intercomparison Project (CMIP) — a project of the World Climate Research Programme (WCRP), cosponsored by the World Meteorological Organization, the International Science Council, and UNESCO’s Intergovernmental Oceanographic Commission.
Under CMIP, now in its seventh iteration, sits another little-known committee responsible for developing the scenarios that Earth system models use to project future climate.1
That committee — called ScenarioMIP — just published the new scenario framework that will underpin the IPCC’s Seventh Assessment Report (AR7) and much of the research that it will draw upon.
In a paper released earlier this month, Van Vuuren et al. (VVetal26) introduce a new set of seven scenarios.
The authors write of the obsolete high-end emissions scenarios (emphasis added):
“For the 21st century, this range will be smaller than assessed before: on the high-end of the range, the CMIP6 high emission levels (quantified by SSP5-8.5) have become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends.”
Read that again — the high-end scenarios are implausible.2
I disagree that the implausibility of the high-end scenarios resulted from the falling costs of renewables or the emergence of climate policy, but that is a debate for another day.
What matters today is that the group with official responsibility for developing climate scenarios for the IPCC and broader research community has now admitted that the scenarios that have dominated climate research, assessment, and policy during the past two cycles of the IPCC assessment process are implausible: They describe impossible futures.
Tens of thousands of research papers have been — and continue to be — published using these scenarios, and a similar number of media headlines have amplified their findings, and governments and international organizations have built these implausible scenarios into policy and regulation.
We now know that all of this is built on a foundation of sand.
What changed
The new CMIP7 ScenarioMIP framework offers seven scenarios spanning a range from “VERY LOW” through “HIGH.” The current naming convention drops the radiative-forcing target labels of the SSP era.
There is no “8.5” scenario, and no “7.0” scenario, but as I’ll show below, each scenario has a radiative-forcing level in 2100.
I ran the available new scenarios (HIGH, MEDIUM, LOW, and VERY LOW) through the FaIR-calibrated and constrained ensemble that Sanderson and Smith (2025) used to characterize the CMIP7 set (FaIR v. 2.2.0 as described in their README file).
I then ran each of the five tier-1 SSPs through the same emulator with identical parameters to ensure that the results are apples-to-apples. The full methodology, data, and code are in the appendix to this post.
The headline results follow.
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