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

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

RCP8.5 does not provide a physically consistent worst case BAU trajectory that warrants continued emphasis in scientific research. Accordingly, it does not provide a useful benchmark for policy studies. Ritchie and Dowlatabadi 2017

When my son was 12 years old he was 5 feet tall. By the time he was 16 he was 6 feet — growing a whole foot taller in just 4 years. Having a math degree, I felt like I should create a quantitative scenario of his future growth to ensure that everything was OK.

I quickly became alarmed. Based on my calculations, I estimated that he would be 9 feet tall by age 28, growing at a rate of a foot every four years!

I spoke to a doctor, who said to make sure he got a balanced diet, good rest, and regular check-ups. We did all of these things and they worked!

My son topped out at 6’ 2” and, thanks to my alarming growth scenario and quick intervention, the worst case was avoided — he did not grow to 9 feet tall.

What is wrong with this story?

My original 9-foot-tall scenario was never plausible. The dynamics it describes are not how things work. So even though we took good care of my son, that is not the reason he did not grow to 9 feet tall. The scenario was implausible from the start, and my self-described heroic role in averting that scenario is an incorrect reading of that history.

If you understand this little analogy, then you understand current responses from some climate scientists to the retirement of RCP8.5. No, RCP8.5 did not become implausible because of climate policy. Today, I explain why.

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.⁴

There is another important conclusion to reach here: The entire set of RCP and SSP scenarios are contaminated by the “return to coal” theory. That means that any use of these scenarios as somehow representing a probablistic forecast of the future is simply flawed. However, the IPCC and others routinely make this mistake.

Second, the assumed dramatic expansion of coal consumption necessarily required additional implausible assumptions, notably the increasing use of coal-to-liquids (CTL) to replace petroleum and a slowdown of technological improvements in wind and solar technologies.

Here as well, the assumed increase in CTL was not averted due to climate policies. It represented a flawed assumption of RCP8.5 that was necessary to meet its coal growth assumption. The assumed increase in CTL was implausible at the time RCP8.5 was created.

Third, the assumed coal expansion required still more implausible assumptions. For example, RD17 explain that the assumption of a dramatic expansion of coal energy necessarily required other energy technologies to stagnate in their technological and economic improvements:

A coal-dominant energy system in RCP8.5 results from coal investment costs that continually decline, while the learning curve for solar, wind and nuclear power remain static.

For example, the figure below shows that the RCPs each required that solar PV experience a slowdown in improvements in module costs, as compared to the rate observed historically. In the figure, the RCPs are initiated in 2011, the year of their release.

Notes: Solar PV module cost ($/W) vs. cumulative installed capacity, log-log scale. Dashed lines show the cost trajectory implied by each scenario’s assumed learning rate from the 2011 reference point. Observed costs (solid blue) fall below every scenario’s projection, including the most aggressive (RCP2.6). Sources: Fraunhofer ISE; IRENA; NREL. Learning rates: Way et al. (2022); Lafond et al. (2018). Scenario LR assumptions: AR5 WG3; Rubin et al. (2015).

Those who attribute RCP8.5’s implausibility to “trends in the costs of renewables” are correct that these costs diverged from what the scenario projected. But they have causation backwards: those cost declines were the continuation of a fifty-year trend that suggested a strong baseline.

The scenario’s failure to capture the continuing trend in declining solar costs was simply the flip side of the implausible coal expansioon assumption. Solar needed to make way for evermore coal.

In Burgess, Ritchie, Shapland, and Pielke 2021 we examined all AR5 baseline scenarios against actual CO₂ emissions across 2005 to 2045, combining observations and near-term energy system model projections. We documented the differences between the RCPs and SSPs that are discussed in this post, as you can see in the figure above from that paper.

In that paper we concluded:

Recent (post-2005) trends and energy outlook projections (to 2040) of global CO₂ emissions are substantially lower than projected by baseline scenarios used in the IPCC’s Fifth (AR5) and Sixth (AR6) Assessment Reports, and are well off-track from widely-cited high-emission marker scenarios such as RCP8.5. We show that this divergence owes largely to per-capita GDP and carbon intensity growth slower than projected in baseline scenarios. The gap between observed and projected carbon intensity is very likely to continue to increase throughout the 21st century due to the implausible assumptions high-emission scenarios make about future fossil-fuel expansion (Ritchie and Dowlatabadi 2017).

The fact that RCP8.5 was implausible has been well established in the scientific literature since 2017. Further evidence has refined that view in the years since.

What happened after the RCPs were released in 2011 — Paris, the renewables revolution, expansion of US shale — is the unfolding history of the world continuing not to be plausibly characterized by RCP8.5. The scenario did not become implausible. The evidence that it was implausible simply became undeniable as the real world and the RCP8.5 world continued to diverge.

It is important that the community understands how the RCP mess came about and take steps to ensure that it does not happen again.

I fully understand both the need to save face and to portray climate policies as being more effective than they actually have been. However, if we do not accurately understand the reasons for the RCP8.5 debacle then we risk repeating those mistakes.

Before you go: Please click that “❤️ Like” button. More likes means more readers of THB. Thanks!

Comments, questions, discussion, debate — All welcome!

Note: The annual subscription for THB increases to $100 on June 1.

Don’t wait — Lock in your support at a lower level!

President Trump Tweeted (Truthed?) about RCP8.5 over the weekend. Surely that originated with THB breaking the news on RCP8.5. He didn’t get everything right, but how many Substacks get White House attention?

Read more:

• Today—>Hausfather, Peters, Forster: On the death of RCP8.5

• Today—>Burgess: Thank you for your attention to this (RCP8.5) matter

• SRES: Nakićenović, N. & Swart, R., eds. (2000). Special Report on Emissions Scenarios. IPCC/Cambridge University Press.

• Van Vuuren, D.P. et al. (2026). ScenarioMIP-CMIP7. Geoscientific Model Development, 19(7), 2627–2656.

• Hausfather, Z. & Peters, G. (2020). Emissions — the ‘business as usual’ story is misleading. Nature, 577, 618–620.

• Hausfather, Z. (2025). An assessment of current policy scenarios and the reduced plausibility of high-emissions pathways. Progress in Physical Geography, 49(1).

• Ritchie, J. & Dowlatabadi, H. (2017). Why do climate change scenarios return to coal? Energy, 140, 1276–1291.

• Burgess, M.G., Ritchie, J., Shapland, J. & Pielke, R. (2021). IPCC baseline scenarios have over-projected CO₂ emissions and economic growth. Environmental Research Letters, 16(1), 014016.

• Way, R., Lafond, F., Lillo, F. & Farmer, D. (2022). Empirically grounded technology forecasts and the energy transition. Joule, 6(9), 2057–2082.

• Riahi, K. et al. (2011). RCP 8.5 — A scenario of comparatively high greenhouse gas emissions. Climatic Change, 109, 33–57.

• Global Carbon Project (Friedlingstein et al., annual). Global Carbon Budget.

• Energy Institute (2024). Statistical Review of World Energy.

1. It is worth noting that Van Vuuren led the 2011 RCP project that gave us the now implausble extreme scenarios. His defense of RCP8.5 is of course also a defense of his own role (with many others, of course) in its creation.

2. Zeke adopts a much more moderate position in his co-authored post today. You can see my response to their post here.

3. Here is Michael Mann (not a scenario expert): “The good news is emissions are now tracking below older “business-as-usual” pathways because of climate policy in spite of Trump.”

4. The new HIGH “what if?” scenario in the ScenarioMIP ensemble continues to employ the return-to-coal hypothesis, as acknowledged by its creators: “Another plausibility question relates to the volume of fossil fuel reserves and resources. Clearly, the cumulative amount of fossil fuel use in the High emission scenario is considerably larger than the estimated total reserves (known deposits that are extractable at current prices and technologies) (Bauer et al., 2016; Rogner, 1997). However, it is also considerably lower than total resources estimates (estimates of undiscovered deposits and/or those not recoverable at current prices) meaning that future technologies and price trends could make the resource trend possible.”