When my son was 12 years old, he was four feet tall. By the time he was 16, he was five feet — growing a whole foot taller in just four years. Having a math degree, I felt like I should create a quantitative scenario of his future growth to ensure that everything was OK. [some emphasis, links added]
I quickly became alarmed. Based on my calculations, I estimated that he would be nine 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 nine feet tall.
What is wrong with this story?
My original nine-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 nine 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 policymakers responded with the implementation of climate policies that have now made RCP8.5 implausible.
These claims imply that the world was once headed for a ~4.8°C temperature increase by 2100, and now it is ~2.7°C — 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: that 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 support 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 about a 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.
The Honest Broker is written by climate expert Roger Pielke Jr and is reader-supported. If you value what you have read here, please consider subscribing and supporting the work that goes into it.
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