The Real Costs of Implausible Scenarios

The New Zealand climate minister tells every council to stop planning around RCP8.5

“It is quite interesting that even these rather abstract scenarios have such consequences as well in daily decisions”

— RCP scenario architect upon learning that they harm real people in 2026

The retirement of implausible baseline climate scenarios is now starting to have real-world consequences as decision makers realize they’ve been making decisions that have had unnecessary real-world, negative impacts on their constituents and clients.

In a letter shared 1 July 2026 with local communities across New Zealand (reproduced below), the country’s Minister of Climate Change, Simon Watts, has asked public officials to reconsider using RCP8.5 and other retired scenarios in their planning decisions.

Councils, the letter states, have used these implausible emissions scenarios in a way that,

“results in unnecessary costs being imposed on ratepayers, businesses, and communities.”

The letter also instructs councils that they “should not automatically” continue to treat these scenarios

“as the central or most likely future when making decisions that affect infrastructure investment, land use, property rights, or rates.”

A first hand look at what the impact of RCP8.5 looks like in New Zealand can be seen in Managed Out, documentary on “managed retreat” from the coasts filmed before the minister's letter.

On the Kapiti Coast, residents describe hazard notations stamped onto their property files that are based on a consultant's report built on futures projected under RCP8.5. One local resident complained:

"They're using RCP 8.5 which is the most extreme outcome. House prices are on the way down, insurance prices are sky-rocketing, because they have put notations on all of our LIM reports [Land Information Memorandum, which assesses hazards] which uses the extreme modelling."

A former Wellington city councilor in the film argues that the problem lies in:

"the use of the IPCC 8.5 model that is supposed to be used only for stress-testing and not for policy-making."

These residents are experiencing the unintended consequences of the use of a scenario that scientists built originally for exploratory earth system modeling to inform global climate negotiations turning up in the price of their homes.

Using RCP8.5 to predict coastal hazards in New Zealand — or anywhere else — and thus serve as the basis for home pricing and insurance is simply a misapplication of science (that may have had valid uses in research in other contexts).

How things went cattywampus

A baseline scenario — also called business as usual or a reference scenario — is according to the IPCC, a “scenario used as starting or reference point for a comparison between two or more scenarios.”

Climate policy analyses, including cost-benefit studies, typically start with a baseline scenario as a representation of where-we-are-headed and then compare that to other scenarios in which policy is introduced within the scenario. Such scenarios are typically called policy (or mitigation) scenarios.

Such a comparison, in principle, allows decision makers to evaluate the expected effects of implementing a policy versus not implementing that policy.

Based on the IPCC’s official AR6 baseline scenarios — RCP8.5, SSP5-8.5, SSP3-7.0 — New Zealand councils had drawn hazard lines across property titles, adjusted infrastructure plans, and shifted insurance and lending exposure, with real economic consequences for New Zealanders.

Thanks to the correct decision by the CMIP7 scenario committee, we now know that all of these decisions and consequences have been based on a scenario now officially recognized as implausible.

In a recent interview with Alistair Harding in New Zealand, Detlef van Vuuren — the Dutch scientist who helped to lead the development of the 2011 RCP scenarios (including RCP8.5) as well as the new 2026 CMIP7 scenarios that officially retired RCP8.5 — was asked if it was ever a fair description of RCP8.5 and the like as business-as-usual (BAU) or a no-mitigation future.

His answer:

“No, that was always a wrong way to describe it.”

And yet, as has been carefully documented here at THB, the set of three baseline scenarios has — for fifteen years — been characterized as a BAU or baseline or reference scenarios throughout climate research and policy. That designation trickled down to land use decisions in coastal New Zealand.

Van Vuuren now says that we all got that wrong from the start.

What happened?

Confusion from the beginning

In the original 2011 paper announcing the RCPs to the world, Van Vuuren et al. 2011 (VV11) describes the four RCP scenarios in a way that has been lost to history:

• one very low mitigation scenario (RCP2.6),

• and three that its Table 4 classifies as baselines:

  • RCP4.5 a "very low baseline,"

  • RCP6.0 a "medium baseline,"

  • and RCP8.5 a "very high baseline emission scenario."

You read that right — originally, there were supposed to be three baseline scenarios, spanning a very wide range, not just RCP8.5.

As baselines, RCP4.5, 6.0, and 8.5 were meant to represent the range of baselines found across the literature on scenarios. RCP8.5 was at the upper extreme of this range and RCP4.5 was at the lower extreme — both were equally valid as baselines.

At the same time, RCP4.5 and RCP6.0 were simultaneously also supposed to represent policy/mitigation scenarios, which are not baselines. They were thus intended to fulfill two conflicting roles in representing the literature, as shown in the table below, from VV11.

Three scenarios were intended to represent both baseline and policy scenarios across the broader scenario literature. What could go wrong?

But when it came time to select the specific scenarios from the IAM literature to characterize the RCPs, the scenarios actually chosen to fill the 4.5 and 6.0 slots were policy scenarios, not baseline scenarios. Thus the RCP4.5 and 6.0 scenarios had their own baselines, but they were well above 4.5 and 6.0 levels of radiative forcing. Only RCP8.5 was from an IAM baseline — a world with no climate policy at all.

You can see this in the figure below from Pielke and Ritchie 2021. Confusion is the only predictable result and that is exactly what happened.

For anyone who took the RCP pathways as they were constructed and presented, rather than as the broader range they were meant to represent as described in VV11 — the set read as a single baseline and three policy scenarios.

The only unambiguous no-policy reference in the framework was RCP8.5.

The scenario community was at the time well aware of the confusion this might cause. For instance, the image below is a slide from a presentation given by Richard Moss, another leader of the RCP development process, warning that RCP8.5 was not a reference scenario for the three other RCPs — another detail lost to history.

Despite the warning, tens of thousands of papers have been published that pair RCP8.5 as a reference scenario with the other RCPs as policy scenarios (typically RCP4.5 and RCP2.6). In particular RCP4.5 was often treated as representative of the consequences of aggressive climate policy,¹ when under the RCP framework, it should have been treated just like RCP8.5 — as a baseline.

Without correction from people who knew better — which includes the many people who have participated in scenario development the entire time — the misinterpretation of RCP8.5 as the only baseline scenario in the set took off and came to dominate how the RCPs (and SSPs) have been used for the past 15 years in both science and policy.

As one unnamed Dutch scientist told De Volkskrant in 2020:

“You simply do not realize that the RCPs can start a life of their own.”

Scenarios were flawed — which is why a wide range matters

It is now widely understood that the RCP and SSP scenarios were flawed from the start, based on reliance on a narrow set of assumptions — the details of which I won’t revisit today. However, the story here is much more complicated than just that.

The figure below shows how the official scenario baseline range of CMIP and IPCC changed from VV11 (original RCP) to CMIP5 — to IPCC AR5 (2014) — to CMIP6 and AR6 (2016-2021) — to CMIP7 (2026).

Baseline scenario ranges from the RCPs (2011) to CMIP7 (2026). In a post next week I’ll discuss the CMIP7 scenarios, now in their final form, in more detail.

Two things to highlight:

• Over more than a decade, from the development of the RCPs through IPCC AR6, the baseline scenario range became both more extreme and narrower;

• The original VV11 range largely (but not entirely) spans the new CMIP7 range.² Were the CMIP7 update being done off of VV11, the changes would fairly be considered a slight readjustment, not the massive change that you can see from AR6 to CMIP7.

The reason that the VV11 range largely lines up with CMIP7 is not because the individual scenarios held up.

The ranges largely overlap because VV11 included a very wide range of baselines, reflecting fundamental and irreducible uncertainties and ignorance — two characteristics of our understandings of the real world that scenario planning is, in principle, designed to help us grapple with.

A wide range of assumptions is what would have allowed the 2011 framework stay useful even though RCP8.5 later collapsed.

The lesson here is that the value of a scenario set lies in the breadth of assumptions it spans, not in the correctness of any single pathway. Incorporating a range of assumptions into scenarios is the hedge against the flaws of the details of any individual scenario, because the builders cannot know in advance which of their assumptions will hold up and which will not.

Even though the implausible baselines have been retired — appropriately — it seems that the importance of a diversity of assumptions across scenarios has not been appreciated by the CMIP7 scenario creators.

Correcting course is just starting

Returning to New Zealand and the RCR Van Vuuren interview — he expresses sympathy for those in the country whose fortunes have been compromised because of the unwitting use of RCP8.5 as a baseline scenario for current land use planning and regulation (emphasis added):

If I was owning a house somewhere and there is a risk in 2150, I think it [RCP8.5] is useful information. But I would expect that my mortgage being out 30 years or so, that it would be completely unfair that the value of my property would be so much determined by something that is uncertain and way out.

Van Vuuren suggests that, from his perspective, the RCP scenarios were never really intended to be used in local decision making:

The way that I normally use these scenarios is in climate negotiations. It is quite interesting that even these rather abstract scenarios have such consequences as well in daily decisions . . .

Interesting, indeed.

The mistakes made in New Zealand in using RCP8.5 as a scenario depicting an expected future, a forecast even, is understandable as a cascade of confusion and conflicting intent for the role of scenarios in climate research. However that cascade has had material consequences for real people in real places, reaching far beyond New Zealand.

With the RCPs, the scenario community pulled four scenarios from a literature of many thousands. That literature spanned an enormous range of possible futures, and appropriately so, as the future is a fuzzy place. The small group of four scenarios was asked to do double duty — to represent futures that we might be headed to without any consideration of climate policy, and futures that required considerable climate policy interventions. The intent was to “jump start” earth system modeling, not to serve as the basis for decisions all around the world spanning the local to the global scale.

Along the way, as mistakes were made, no one spoke up or pointed out the unfolding errors of scenario design and application.

We now know — and have known since 2017 thanks to Ritchie and Dowlatabadi — that the single anointed baseline was fatally flawed. Out in the real world (as opposed to model land) RCP8.5 was variously interpreted as a projection of our most likely future or even as a forecast and thus wired into consequential decisions.

Had the climate community recommended that scenario planning be based on a wide range of baselines, from 4.5 to 8.5, some may still have chosen the most extreme of the set but others would have had the chance to consider other baselines. By considering that wide range, the recalibration of that range would have likely been less consequential than the huge change in expectations that many are now experiencing.

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1. RCP4.5 is not climate policy success, but has often been treated as such, such as in the 2018 U.S. National Climate Assessment. I have discussed this elsewhere— another error of scenario use.

2. Not shown here, but the SRES baseline range was even wider than that of VV1.