Pielke Jr. - Good Riddance to RCP8.5 Extreme Climate Scenario and Why the Coupled Model Intercomparison Project (CMIP) is the World’s Most Consequential Scientific Advisory Committee

       This post summarizes two related posts by climate impact scientist Roger Pielke Jr. He first notes that the international committee that determines the climate scenarios for the Intergovernmental Panel on Climate Change (IPCC) reports has just put out a new list of scenarios, and the RCP8.5 scenario has now been eliminated. In the past, it had often been referred to as the ‘business as usual’ scenario. In fact, they have eliminated the three most extreme scenarios. He explains:

“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. 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.”

     Apparently, it has become widely acknowledged that those scenarios are no longer plausible (not that they ever really were). Pielke Jr. does not agree that these scenarios became implausible due to lower-cost renewables or the emergence of climate policy, as some have stated. He mentions there are tens of thousands of papers that continue to be published that use those implausible scenarios. The new scenarios, the CMIP7, are shown below, compared to the past extreme ones that have now been removed due to implausibility. The two highest dotted line scenarios will now be gone. Note that the new maximum emissions for the year 2100 are now projected at 71 Gt CO₂/yr, far below the SSP5-8.5 maximum of 128 Gt CO₂/yr.

     He also notes that the 2100 projections made the unlikely assumptions that the 2100 global population would be near 13 billion, well above any contemporary demographic projection (the highest are typically around 11 billion), and a five-fold expansion of global coal use, which is very unlikely. He does think that the new CMIP high scenario is still likely implausible, though less so than the eliminated ones. As he says, “the plausibility vacuum remains.”

“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.”

“Science is self-correcting. What matters now is what happens next.”

     In his next post, Pielke Jr. talks about the importance of the committee of the World Climate Research Program that determines the Coupled Model Intercomparison Project (CMIP) scenarios. Those scenarios are used by many countries in their national climate assessments. He emphasizes the importance of these scenarios:

“It is no exaggeration that the CMIP climate projections influence trillions of dollars in investment and regulation. They are, in functional terms, among the most consequential 21st century scientific products designed to inform policymaking, economics, and regulation. They are not just about science, but about science advice to policymakers in government, business, and civil society.”

     Below, he notes some of the main sources of the scientists on the committee:

“The CMIP scenarios are developed by a community of integrated assessment modellers numbering perhaps two hundred people worldwide, working in roughly fifteen institutions, and concentrated heavily in two: the International Institute for Applied Systems Analysis (IIASA) outside Vienna, and the Potsdam Institute for Climate Impact Research (PIK) in Germany.”

     The PIK is led in part by earth-system scientist Johan Rockström, whom I have criticized for his planetary boundaries assumptions, which I found to be too catastrophist, not realistic enough, and perhaps too ideological. Pielke Jr. notes that:

“Rockström has explicitly connected the “planetary boundaries” concept to the neo-Malthusian ideas of Dennis and Donella Meadows of the Club of Rome (Limits to Growth).”

     Thus, as he seems to suggest, PIK could have strong biases to produce results consistent with their ideological orientation. He also says that the scientists on the committee are too heavily from Europe and North America.

“The same individuals who lead the institutions that sit at the center of the production of CMIP7 marker scenarios are also among the leading public proponents of a very particular policy-relevant framing of what those scenarios should imply in policy.”

     Below, he explains that typical scientific advisory groups have stated procedures and expectations, but CMIP7 does not seem to adhere to any similar structure. He does note that recently, they seemed to have acknowledged the gap in good governance.

     In contrast, the U.S. has specific formats and rules for scientific advisory boards enshrined in the Federal Advisory Committee Act of 1972 (FACA). Under FACA, each scientific advisory committee must publish its mission and membership rules. In the EU, such committees are required to submit an annual Declaration of Interests covering financial holdings, employment, consultancies, and research funding. The World Health Organization has similar rules for members of its advisory committees.

“The contrast between the governance of these important science advisory organizations with how CMIP scenarios are produced — without a public charter, without published Terms of Reference for the body that selects them, without standardized declarations of interest, with no transparency on how participating experts are selected, and with no opportunity for public input to proposed products — indicates that institutions of climate research created to inform policymakers needs to significantly improvements in their governance.”

 

 

References:

 

                   

The World's Most Important Science Advisory Committee: Climate research has a serious governance gap. Roger Pielke Jr. The Honest Broker. May 4, 2026. The World's Most Important Science Advisory Committee

RCP8.5 is Officially Dead: The most significant development in climate research in decades. Roger Pielke Jr. The Honest Broker. April 29, 2026. 🚨RCP8.5 is Officially Dead - by Roger Pielke Jr.