The Retreat Continues

Updated again, lower again. CMIP7's highest emissions scenario now has less than half the CO₂ of RCP8.5.

“It’s just a flesh wound. ‘Tis but a scratch!”

Back in April, THB broke the news to the world that the only three official reference scenarios used for the past 15 years in much of climate research, assessment and policy had been retired. The CMIP7¹ conclusion that RCP8.5, SSP5-8.5, and SSP3-7.0 were “implausible” was huge news.

Today, I share more big news about how official climate scenarios continue to evolve.

CMIP7 has just published the final versions of its new scenarios as they have emerged from integrated assessment models (IAMs), updating the placeholders that accompanied their April announcement. The final versions — once again — dramatically reduces the cumulative carbon dioxide emissions from fossil fuels from the April placeholders.

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):

• SSP5-8.5, 7,380—> CMIP7 HIGH 1.0, 4,349—> CMIP7 HIGH FINAL, 3,438

• SSP5-8.5, 7,380—> CMIP7 MEDIUM 1.0, 3,098—> CMIP7 MEDIUM FINAL, 2,528

The bar chart below shows just how large the reduction of projected emissions has been.

You can see in the table above that each integrated assessment model associated with each new scenario has been identified, however, the ability to share the details of each scenario awaits the publication of those IAM runs and associated papers. ScenarioMIP explains that this embargo holds until September 1:

The data can be used only for scientific purposes aimed at the production of peer-reviewed publications. The posting of the scenario data and/or analyses based on the data on social media or other dissemination (in the public or the press) is thus strictly prohibited.

I will of course respect this embargo and this post — and any other discussions of the new scenarios here at THB — will rely only on the publicly available data archive accompanying Van Vuuren et al. 2026.

Based on the publicly available data accompanying the CMIP7 paper describing the new scenarios, I can provide an analysis of the new scenarios based on Pielke et al. 2022.

The figure below shows the annual trajectory of emissions for the retired reference scenario SSP5-8.5, along with several of the new CMIP7 scenarios. CMIP7 HIGH is intended not as a reference, but as an exploratory scenario assuming increased greenhouse gas emissions, “resulting from deep political, technological, and structural deviation from current trends.”

The figure below — based on Pielke et al. 2022 — shows how the new CMIP7 scenarios compare to the AR5/SSP scenario database in terms of cumulative carbon dioxide emissions and implied 2100 global average temperature. Note that these temperature estimates are from the emulator used by Van Vuuren et al. 2026, and have yet to be run in earth system models, so are preliminary.²

For the MEDIUM scenario, CMIP7 explains:

The Medium emission scenario is a benchmark that shows the consequences of the current policy situation (as of 2025) and trends continuing over the century. In the scenario, it is assumed that policy effort is continued at the current level. As for other scenarios, also this scenario should not be considered as a “most likely” scenario.

For the MEDIUM-to-LOW scenario CMIP7 explains:

The Medium to Low emission scenario describes a future in which strengthened mitigation efforts are delayed, falling short of the levels required to meet the objectives of the Paris Agreement. . . . The underlying logic of the scenario is that political constraints limit rapid near-term action; however, these lessen over time as the magnitude of observed climate impacts increases and mitigation costs decline with technological progress.

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.

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.

What is the significance of the new emissions scenarios? Some thoughts:

• The massive scale-back of projected future carbon dioxide emissions means, all else equal, less future changes in climate;

• As a consequence, there will surely be enormous political pressures placed on the climate science community to create projections with greater changes in climate, based on factors other than carbon dioxide emissions;

• If so, then we should expect significant challenges to scientific understandings that have long been stable, such as related to the carbon cycle, climate sensitivity, aerosols, and other greenhouse gases;

• Any such evolution of understandings may well be scientifically justifiable and legitimate, but a political overlay will create challenges;

• Meantime, tens of thousands of studies reliant on the implausible scenarios need to be reconsidered carefully, as the new reference range undershoots the 4.5 scenarios that were often presented — and misrepresented — as climate policy success;

• The range from MEDIUM-to-LOW to MEDIUM spans ~4.1–5.7 W/m², so the current and pledged policies range now sits below what RCP4.5 was presented as achieving with aggressive policy;

• None of this affects the huge magnitude of the challenge of accelerating the decarbonization of the global economy, which is not affected by scenarios;

• Climate research, assessment, advocacy, policy, and politics all need to catch up to the altered view of the climate future.

However things evolve, climate science and policy are now in a new era.

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Paid subscribers can find sources and data for the figures and tables in the Excel file below.