Pielke Jr. Explains the Continued Retreat from High Carbon Emissions Scenarios

   

     Climate impact scientist Roger Pielke Jr. explains the recent CMIP-7 scenario retirements due to implausibility.  CMIP-7 concluded in April that the RCP8.5, SSP5-8.5, and SSP3-7.0 scenarios were “implausible.” Since these have been included in most or all past climate science, this is big news. So-called ‘business-as-usual’ scenarios will now show much less global warming. How much less?

“The table below shows just how large these changes are in gigatons (Gt) of carbon dioxide emitted from the burning of fossil fuels to 2100 — both in terms of the highest cumulative emissions across the scenario set (in the first bullet below) and in terms of the scenarios designated as reference or current policy (second bullet below):

     The Medium scenario is considered to be most likely and in line with current projections. The low-medium scenario is possible with stronger decarbonization efforts. The carbon emissions of the most likely Medium scenario are nearly a third of those projected in the SSP 5-8.5 scenario.

     Pielke Jr. notes:

“While MEDIUM is the only scenario framed as “current policy” in the CMIP7 set — at 3C in 2100 — it sits above other “current policy projections,” which you can see in the table below. In addition, the MEDIUM-to-LOW scenario at 2.3C in 2100 is just about at the lower end of the projections that assume that national pledges are met.”

     The temperature ranges given in the different projections that are considered plausible run from 2.2 degrees Celsius to 3.0 deg. C. 

     He also explains that the High scenario should not be treated as a business-as-usual scenario, but one in which policies are adopted that increase emissions.

“Thus, the range from MEDIUM-to-LOW to MEDIUM — with medians of 2.3C to 3.0C in 2100 — can serve usefully as the equivalent of a baseline or reference range. The HIGH scenario, at ~3.4C in 2100, is explicitly designed as an exploratory scenario to illustrate a world in which policies are adopted that increase emissions. CMIP7 explains:

“Clearly, this scenario is not a “business-as-usual” scenario nor the no-policy reference scenario for the other scenarios.”

“This point needs to be emphasized. HIGH is not a baseline or reference scenario and should not be treated as such. I’m sure this warning will be ignored by some.”

     As he shows below, the most likely Medium scenario means that annual global emissions are projected to stabilize as they are currently close to doing, and begin to drop in the 2030s. That dropping is accelerated in the Low-Medium scenario. 

     With the new changes, we should see graphs in future IPCC reports that look much different. He notes that the current and pledged policies range is now below what RCP4.5 was presented as achieving with aggressive policy. Below, he gives his thoughts on the implications of the new scenario plausibility determinations, including some uncomfortable political maneuverability for those who favor more catastrophic scenarios and who want accelerated decarbonization.

 

 

References:

 

The Retreat Continues: Updated again, lower again. CMIP7's highest emissions scenario now has less than half the CO₂ of RCP8.5. Roger Pielke Jr. The Honest Broker. July 15, 2026. The Retreat Continues - by Roger Pielke Jr.

The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7). Detlef P. Van Vuuren, Brian C. O'Neill, Claudia Tebaldi, Benjamin M. Sanderson, Louise P. Chini, Pierre Friedlingstein, Tomoko Hasegawa, Keywan Riahi, Bala Govindasamy, Nico Bauer, Veronika Eyring, Cheikh M. N. Fall, Katja Frieler, Matthew J. Gidden, Laila K. Gohar, Annika Högner, Andrew D. Jones, Jarmo Kikstra, Andrew King, Reto Knutti, Elmar Kriegler, Peter Lawrence, Chris Lennard, Jason Lowe, Camilla Mathison, Shahbaz Mehmood, Zebedee Nicholls, Luciana F. Prado, Qiang Zhang, Steven K. Rose, Alex C. Ruane, Marit Sandstad, Carl-Friedrich Schleussner, Roland Seferian, Jana Sillmann, Chris Smith, Anna A. Sörensson, Swapna Panickal, Kaoru Tachiiri, Naomi Vaughan, Saritha S. Vishwanathan, Tokuta Yokohata, Marco Zecchetto, and Tilo Ziehn. European Geosciences Union. April 7, 2026. GMD - The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7)