RCP8.5 was never a plausible “business-as-usual” (BAU) or no-policy baseline; it was a high-end exploratory scenario from the start, and evidence shows it was already diverging from reality based on its own assumptions, not primarily due to later climate policies.

RCP8.5 (developed around 2011) was designed as one of four Representative Concentration Pathways for climate modeling. It leads to ~8.5 W/m² radiative forcing by 2100, corresponding to high GHG concentrations and roughly 4–5°C warming (with a wide range).

RCP8.5 was designed as a high-end (roughly 90th percentile) exploratory scenario, not a central “business-as-usual” projection. Its divergence from observed trends stems primarily from unrealistic internal assumptions (especially massive coal expansion), evident well before major post-2015 policy accelerations or renewables cost drops fully took effect.

Recent developments (as of 2026) reinforce this: the CMIP7 ScenarioMIP framework for IPCC AR7 has effectively retired RCP8.5/SSP5-8.5 as a core scenario. The new “High” scenario sits lower (around RCP6-7 range), acknowledging that extreme high-end paths have become implausible based on renewables trends, emission data, and socioeconomics—not solely policy “success.”

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No, RCP8.5 Did Not Become Implausible Because of Climate Policy

Getting things right may be uncomfortable, but it is the only way forward

By Roger Pielke Jr.

No, RCP8.5 did not become implausible because of climate policy. Today, I explain why. The Honest Broker has the story.

In the past several weeks prominent climate researchers have defended RCP8.5 as a scenario that a decade ago plausibly described where the world was headed, but thanks to their warnings, the world’s policy makers responded with implementation of climate policies that have now made RCP8.5 implausible.

The implication of these claims is that the world was once headed for ~4.8C temperature increase by 2100 and now it is ~2.7C — a huge decrease.

For example:

• Detlef van Vuuren,1 lead author of the ScenarioMIP paper released last month, explained to The Australian that RCP8.5 had,

“become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends.”

• Zeke Hausfather, a climate scientist at Stripe and a frequent, friendly intellectual sparring partner of mine, also framed RCP8.5 as once plausible but now implausible due to climate policy successes:

“[I]t is incontrovertible that rapid cost declines, investment in, and deployment of clean energy technologies in the past 15 years have changed the plausible scenarios for fossil fuel use later in this century. These new scenarios reflect this success.”2

• Robert Vautard, Co-Chair of IPCC Working Group I for AR7, also framed the retirement of RCP8.5 as the result of successful climate policies:

“Previous “high scenarios” started in 2015 and assumed no climate policies, but there ARE now many climate policies in many countries, developed in particular with the Paris Agreement signed in 2016 (sic), and before. . . it shows that climate mitigation policies do consistently reduce global warming.”

Each of these framings rests on a common logic: RCP8.5 once described a plausible trajectory; subsequent policy progress and technology cost trends moved the world away from it; therefore the scenario became implausible.3

This story, were it true, would be incredibly convenient for the climate science community. Rather than introducing a flawed scenario to the world that dominated climate science and policy for more than a decade — and then stubbornly defending it — this retelling characterizes the climate science community as near-infallible and heroic.

This story is not true. RCP8.5, and other extreme scenarios, were never plausible.

Scenario plausibility is determined by what theory and evidence supports at the time a scenario is created, not simply by whether the world eventually moved toward or away from the projections that emerge from that scenario.

That means that a scenario that deviates from how the world actually evolved was not necessarily retroactively implausible at the time it was created. A scenario built on assumptions inconsistent with available theory and evidence is implausible at construction, regardless of whether subsequent events confirm or contradict its projections.

The Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) — published in 2000 — defined a scenario as:

“a coherent, internally consistent, and plausible description of a possible future state of the world.”

Scenarios are explicitly “not predictions.” But they must be consistent with theory and evidence.

Any projection built on scenario assumptions that contradict available theory and evidence is invalid from the start, regardless of what happens next. Further, scenarios are not predictions, and a family of scenarios does not describe a probability distribution of expected futures. Much wisdom on scenarios has been lost since IPCC SRES in 2000.

Below, I discuss three assumptions of RCP8.5 that made it implausible from the start (ignoring other implausible assumptions, like its incredible population growth rates):

• Reliance on a flawed theory for the dramatic expansion of coal energy;
• A corresponding rapid increase in coal-to-liquids, displacing petroleum;
• A necessary slowdown in technological improvements in solar energy technology.

Let’s take a look at each.

First, RCP8.5 required burning coal at an implausible rate. Coal is the most carbon intensive fossil fuel, and huge amounts needed to be burned to reach the high forcing level that was assigned to the most extreme RCP scenario.

Recall that under the RCPs radiative forcing levels for 2100 were chosen first, and only later were integrated assessment modelers assigned the task of figuring out how those levels could be reached. RCP8.5 required implausible socio-economic assumptions to meet its preassigned design criteria. Unfortunately, the scenario develoipment community yet to learn that socio-economics should come first.

The figure above, from Ritchie and Dowlatabadi 2017 (RD17), shows that RCP8.5 assumed ~8x increase in primary energy generation from coal. In fact, the entire envelope of assumed future coal consumption across the RCP and SSP scenarios shows an increase in coal — based on a single theory, characterized by RD17 as a “return to coal.”

RD17 explain that the MESSAGE integrated assessment model that generated RCP8.5 applied no constraint from geological reality, and simply assumed that the massive amounts of coal it required would be available. The scenario extrapolated 2000s Chinese coal growth rates and assumed the physical resource base would accommodate whatever the consumption assumption required irrepective of real-world constraints.

The figure below shows that coal consumption as a proportion of the global energy mix started declining around 2013, while RCP8.5 had it steadily increasing. The reason for the divergence between reality and RCP8.5 was not climate policy, but rather a false assumption baked into the scenario from the start.

RCP8.5 — and indeed all of the RCP and SSP scenarios — had a single point of failure in its assumption of a return-to-coal. This assumption alone settles the question of plausibility. A scenario requiring five times proven coal reserves is not plausible by any standard.

Read the full story here.