EXRACT: The most significant development in climate research in decades…
“[T]he high-emissions RCP8.5 scenario has long been described as a “business-as-usual” pathway with a continued emphasis on energy from fossil fuels with no climate policies in place. This remains 100% accurate . . .“ — from 2021, Chris Field (co-chair of IPCC WG2 AR5) and Marcia McNutt (president of U.S. National Academy of Sciences, Engineering, and Medicine )
The international committee responsible for the official scenarios that feed into climate modeling that 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.
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 which will have lasting impacts across research and policy.
The future is not what it used to be.
The new scenarios come from the Coupled Model Intercomparison Project (CMIP) — a project of the World Climate Research Programme (WCRP), co-sponsored 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 with responsibility for developing the scenarios necessary for earth system models 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, a similar number of media headlines have amplified their findings, and governments and international organization have built these implausible scenarios into policy and regulation.
We now know that all of this is built on a foundation of sand…
Why this matters: these scenarios live in policy
The now-implausible upper-end scenarios — RCP8.5, SSP5-8.5, and SSP3-7.0 — are not just academic constructs used in esoteric research. They are embedded in the policies and regulations of most of the world’s largest economies, found across the world’s most important multilateral institutions, and used in the climate stress tests that govern hundreds of billions of dollars in bank capital.
National climate impact assessments in the United States, United Kingdom, Germany, Canada, Australia, Japan, and the Netherlands all use RCP8.5 or SSP5-8.5 as a reference scenario. The Network for Greening the Financial System framework, used by more than 140 central banks, has utilized a “Hot House World” scenario calibrated to RCP8.5 physical risk into the bank stress tests run by the European Central Bank, the Bank of England, the Reserve Bank of New Zealand, the Banque de France, and the US Federal Reserve. The World Bank’s Climate Change Knowledge Portal, which provides the climate diagnostics that feed into the Country Climate and Development Reports for more than 100 client countries, defaults to SSP5-8.5 and SSP3-7.0.
The abandonment of the high-end legacy scenarios by CMIP7 will need to propagate through this entire infrastructure. The policy machinery built on RCP8.5 and the other implausible scenarios is systemic.
What this means
The new CMIP7 ScenarioMIP framework represents a real course correction, but with more work to do. SSP5-8.5 is gone. SSP3-7.0 has a successor that is less extreme but arguably remains implausible. The middle of the set is more pessimistic than trajectories of current and announced policies. The plausibility vacuum at the heart of the architecture has yet to be addressed.
All this means that users of climate models and model output based on legacy scenarios will now face decisions about if and how they’d like to realign with the latest scientific understandings versus continuing to rely on outdated research.
Furthermore, there are no doubt many — hundreds if not thousands — of studies in the publication pipeline that depend upon the upper end scenarios. Editors and reviewers should ensure that they are properly characterized as exploratory and are not intended to be interpreted as projective.
We’ve known since 2017 that upper end climate scenarios are fatally flawed. Nine years later, that understanding has now become officially recognized. That is good news.
We can debate whether nine years is short or long for the overturning of scientific understandings with massive economic and policy implications. But today, that overturning is undeniable.
Science is self-correcting. What matters now is what happens next.
To read Professor Roger Pielke JR’s full article, which outlines more detail and critiques the newly adopted scenarios for the IPCC, please click HERE.