Climate Expert Asks: What Is a ‘Worst-Case’ Climate Scenario?

The implausibly extreme and hugely popular climate scenario RCP8.5 made it into President Trump’s executive order last week on “Restoring Gold Standard Science.” [emphasis, links added]

Ironically, the Trump administration’s characterization of RCP8.5 did not quite reach the “gold standard,” and maybe not even a “bronze standard. “

The EO states:

[Federal a]gencies have used Representative Concentration Pathway (RCP) scenario 8.5 to assess the potential effects of climate change in a “higher” warming scenario. RCP 8.5 is a worst-case scenario based on highly unlikely assumptions like end-of-century coal use exceeding estimates of recoverable coal reserves. Scientists have warned that presenting RCP 8.5 as a likely outcome is misleading.

RCP8.5 is not simply “highly unlikely” — it is falsified, meaning that its emissions trajectory is already well out of step with reality.

We showed this conclusively in Burgess et al. 2021, from which the annotated figure below comes:

The gap between the black arrow (RCP8.5) and the blue arrow (reality) indicates that RCP8.5 is not just unlikely, but impossible — it is already wildly wrong.

Since we published that paper, that gap between RCP8.5 and reality has only grown larger (stay tuned on that!).

It is an interesting thought experiment to ask what it would take for the real world to “catch up” to RCP8.5. Setting aside the real-world plausibility of such a “catch-up” scenario, as a matter of simple math, starting in 2025 that new scenario would have to be much more aggressive than RCP8.5, and thus even more extreme.

If RCP8.5 is implausible, then a new catch-up-to-RCP8.5 scenario would necessarily be even more implausible.

Climate scientist Zeke Hausfather dismissed the Trump administration’s claims about RCP8.5 by stating that the research community has moved on, and in the process, he was no doubt sub-Tweeting THB (which under no circumstances is to be engaged directly!):

Its bizarre, particularly coming after there has been a real shift in [RCP8.5] use post-AR6 due to recognition of the changing likelihoods of high emissions scenarios. Its almost like science is self-correcting.

Zeke’s nothing-to-see-here is wrong.

The IPCC AR6 came out in 2021. From 2018 to 2021, Google Scholar reports 17,000 articles published using RCP8.5. From 2022 to 2025, Google Scholar reports 16,900 articles published using RCP8.5. Some shift.

The Trump administration’s characterization of RCP8.5 is not quite right, but its focus on its continued misuse is also not wrong.

With RCP8.5 out of date and implausible, it raises an important question: What then is a “worst-case” climate scenario for use in policy?

In discussing this with colleagues, reading a bazillion RCP8.5 papers, and chatting with AI models trained on such information, I’ve concluded that a very common definition of “worst-case scenario” is simply circular: The most extreme scenario developed by the IPCC-affiliated scenario community and made available for research purposes.

In labeling a scenario as “worst case,” the climate community has never made a systematic, scientific effort to assess either plausibility or, at the other extreme, whether there might be even more extreme scenarios that might be plausibly “worser.”

Worst-case scenarios are important for policy and planning because they allow for stress-testing of proposed and enacted decisions against low probability but plausible outcomes that policymakers might wish to consider in robust policymaking, resilient to a highly uncertain future.

However, consideration of an impossible or extremely improbable scenario could lead to wasted effort, misplaced resources, and poorly informed decision-making.

Careful consideration of worst-case scenarios is thus crucially important and must mean more than simply the most extreme scenario I have at my disposal.

In 2009, climate scientist Steve Schneider wrote an essay in Nature describing a “worst-case scenario” with a 2100 carbon dioxide concentration of 1,000 parts-per-million (ppm) and approaching a temperature increase of 7 degrees Celsius.

Schneider’s “worst case” was even more extreme than RCP8.5, which had yet to enter the scene, and seemed to be based on the fact that 1,000 is a nice round number, close to the most extreme scenario then available (A1FI).

But why stop at 1,000 ppm and 7°C by 2100? Surely 1,500 ppm and 10°C would be much worse? I could go on, inventing ever-more-worser scenarios. But this exercise would obviously be useless from the perspective of reliably informing decision-making.

Then there is also the matter of “worse” with respect to what? As Mike Hulme has argued:

“[T]here are some futures beyond 1.5 degrees C (or even 2 degrees C) that are more desirable than other futures which do not exceed these warming thresholds. We should not mistake one set for the other.”

Is a worst-case climate scenario defined by temperatures alone or, alternatively, human outcomes like health, wealth, equity, and so on? Who gets to decide?

Consider that RCP8.5 has large temperature increases, but in a world that is assumed to be fantastically wealthy. Is that a better or worse case than a scenario with massive global poverty and inequity, but much lower change in temperatures?

It turns out that defining a “worst-case scenario” is not a bloodless, technical exercise, but a deeply value-laden process that must recognize that different people will value different outcomes differently.

That makes the characterization of a “worst-case outcome” inevitably political, and the product of discussion, disagreement, debate, and negotiation. There are many legitimate perspectives on what constitutes “worst case.”

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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Top image: Official White House Photo by Molly Riley