Understanding The Great Climate Model Scenario Debate

Written by Dr Roger Pielke Jr

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Last week saw a major debate in climate science jump from Twitter discussions among experts into the broader scientific community, where it has ignited a sometimes heated and undeniably consequential discussion.

I’ve received many requests to help explain this issue for the non-expert. Here I’ll take a first step in meeting this challenge.

The issues associated with scenarios in climate research are complex and technical, but also incredibly important for both the future of climate science and policy – they are far too important to be left only to experts.

At issue is the proper role of scenarios in projecting future changes in climate, the associated impacts on society and environment and the economics, policy, and politics of adaptation and mitigation.

In short, the entire climate issue is wrapped up with scenarios. That is why this debate is so important.

The jump from a relatively narrow discussion among experts to broader legitimacy in scientific discussions resulted because of a commentary published in Nature by Zeke Hausfather, of The Breakthrough Institute, and Glen Peters, Center for International Climate Research, in Oslo, Norway.

An interesting aspect of the sociology of science is how important but uncomfortable debates in plain view do not seem to fully exist until they are legitimized by certain authorities.

Please read their commentary and then return here where I’ll take a step back and try to explain in greater detail what they are talking about.

More than a decade ago, the climate research community developed a set of new scenarios for future research and assessment in support of policymaking.

Different users of scenarios need different things from them. For instance, “Climate models require data on the time-evolving emissions or concentrations of radiatively active constituents, and some have additional requirements for information about the time-evolving paths for land use and land cover.”

For purposes of this column, we can lump together these users’ needs under the umbrella of a “concentration” pathway that describes how radiative forcing evolves over some time period, such as to 2100.

At the same time, researchers who are looking at the potential impacts of climate change on society and the environment or economics and policy need not just projections of changes to the physical climate system, but characterizations of the world in which those changes occur – including things such as population size and distribution, economic growth and inequality, urbanization, energy system production and consumption, land use and more.

For purposes of this column, we can call this needed data a “socioeconomic pathway,” evolving over some time period, such as to 2100.

The creation of concentration and socioeconomic pathways as part of the new scenarios was initially planned to be independent of each other.

But in practice, this was only partially possible, because the concentration pathways were expected to be “plausible” meaning that they had to be associated with socioeconomic pathways that conceivably could occur in the future.

So in order to develop scenarios that could be used by all users, socioeconomic pathways needed to be developed to fill the gap until the official process for developing socioeconomic pathways reached completion.

With the development of concentration and socioeconomic pathways placed on independent tracks (and expected to be reconciled later), it turned out that the concentration pathways were completed before the socioeconomic pathways.

These pathways were named “Representative Concentration Pathways” and four were chosen at radiative forcing levels of 2.6, 4.5, 6.0 and 8.5 watts per meter squared.

It was only more recently that the original socioeconomic pathway process was completed and then integrated with an updated generation of radiative forcing scenarios.

These new scenarios are called “shared socioeconomic pathways” (SSP) and are focused on radiative forcing levels of 1.9, 2.6, 3.4, 4.5, 6.0, 7.0 and 8.5 watts per meter squared.

The scenario at issue in the Hausfather and Peters commentary is RCP8.5, the most extreme scenario of the original RCPs as well as the newer SSPs.

Prior to the creation of the SSPs, because the official socioeconomic pathway process was not completed, there was not a full integration of the concentration and socioeconomic pathways.

Instead, the community relied on individual integrated assessment modeling teams to generate what are called “marker” scenarios to represent various socioeconomic pathways leading to each forcing level of the RCPs.

The “marker scenario” created for RCP8.5 explained that it “depicts thus a relatively conservative business as usual case.”

It is not clear to what degree the socioeconomic pathway of this marker scenario was subjected to any evaluation with respect to plausibility, much less its characterization as “business as usual.”

When the parallel processes originally envisioned for the creation of concentration and socioeconomic pathways were finally brought back together, there was clear evidence that the earlier characterization of the RCP8.5 marker scenario as “business as usual” was far off the mark.

In fact, the entire family of RCP8.5 scenarios was problematic according to the results of the SSP process, “8.5 watts per meter squared] can only emerge under a relatively narrow range of circumstances.”

Justin Ritchie, of the University of British Columbia, puts the critique in more colorful terms:

“We would really need a society that worshipped fossil fuels to follow [RCP8.5-SSP5], like the ‘Church of Oil’ from [Margaret] Atwood or that guy who was a Western US coal magnate who thought it was humanity’s purpose to get all the coal out because CO2 was good for the atmosphere.”

This complex situation is obviously ripe for confusion. When people refer to RCP8.5 they could be referring to a number of things:

Regrettably, more than a small part of the debate and discussion following the publication of Hausfather and Peters’ commentary has involved people talking past each other based on different understandings of what “RCP8.5” actually means in practice.

For instance, some physical scientists use RCP8.5 to refer only to a concentration pathway, while social scientists and economists focus instead on RCP8.5 as representing the associated socioeconomic pathways.

The critique that Hausfather and Peters offered, and the more technical critiques in the peer-reviewed literature and online discussions, are focused on all of the socioeconomic pathways underlying all incarnations of RCP8.5, old and new or marker and reference.

The problem that they identify is that a radiative forcing of 8.5 watts per meter squared cannot be created with associated plausible socioeconomic pathways: “the world imagined in RCP8.5 is one that, in our view, becomes increasingly implausible with every passing year.”

This echoes the conclusion of Ritchie and Hadi Dowlatabadi, of the University of British Columbia, who wrote in 2017, “RCP8.5 should not be a priority for future scientific research.”

The specific details of the critiques of the socioeconomic pathways underlying RCP8.5 go beyond today’s column, but center on unrealistic projections of expanded global coal consumption:

“Emission pathways to get to RCP8.5 generally require an unprecedented fivefold increase in coal use by the end of the century, an amount larger than some estimates of recoverable coal reserves.”

Just because a particular socioeconomic pathway is judged to be unlikely or implausible doesn’t mean that a concentration pathway of 8.5 watts per meter squared (or even higher) has no utility for exploratory scientific research.

There is much that can be learned through exploratory research.

However, confusion and, potentially, flawed science, results from conflating exploratory physical science research with a socioeconomic pathway with little plausibility to offer projections of climate, cost-benefit analyses, or other research that’s dependent upon an associated socioeconomic pathway.

But the deeper problem with RCP8.5 is not that it is simply implausible. It is that this scenario has been placed at the center of climate research.

In their Nature commentary last week, Hausfather and Peters argue that RCP8.5 “has been widely used by some experts, policymakers and the media as something else entirely: as a likely ‘business as usual’ outcome.”

The mischaracterization of this scenario, they argue, is found in “a sizeable portion of the [academic] literature” and is amplified by the media, resulting in “further confusion” among climate researchers.

Hausfather and Peters implore that, “we must all — from physical scientists and climate-impact modelers to communicators and policymakers — stop presenting the worst-case scenario as the most likely one.”

I have in previous columns here discussed how the misuse of RCP8.5 has shaped the public discourse on climate change towards the apocalyptic, has influenced the focus of major scientific assessment reports, and reflects a deeper misunderstanding of the roles of scenarios in supporting policy.

Because RCP8.5 has become so fundamental to a wide range of climate research, assessment and policy analyses, it will take many years for the community to address its mischaracterization and misuse, assuming that action is taken to chart a new course.


Roger Pielke Jr. has been a professor at the University of Colorado since 2001. Previously, he was a staff scientist in the Environmental and Societal Impacts Group of the National Center for Atmospheric Research. He has degrees in mathematics, public policy, and political science, and is the author of numerous books. (Amazon).

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Comments (19)

  • Avatar

    Andy Rowlands

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    These RCP figures and the use of the words ‘radiative forcing’ assume they so-called ‘greenhouse gas theory’ is actually correct do they not?

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      JDHuffman

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      That’s correct, Andy. Pielke is a Warmist, while he claims he’s not. He believes CO2 can heat the planet. He has zero background in the relevant physics, but he wants to be included. He wants some of the revenue, without having a clue about the actual science, or a care about the truth.

      His post here is a perfect example of his endless rambling. People need to learn about such types.

  • Avatar

    Andy Rowlands

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    I’m rather surprised, I had thought RP was on ‘our side’. He would be what is unflatteringly termed a ‘lukewarmer’ then I presume? I correspond fairly frequently with Greg Wrightstone on social media. He also believes CO2 causes in his words ‘a small amount of warming’, and we have discussed it and politely decided to agree to disagree. Greg is in the rather unenviable position of being attacked by alarmists and skeptics. He is on the IPCC list of expert reviewers, probably because they see his stance on CO2 as meaning he is on their side, but he says both the modest warming and increasing CO2 are beneficial, and dismisses the doom-and-gloom predictions, so gets called a ‘denier’.

    • Avatar

      JDHuffman

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      Andy, what I’ve learned, in my years of following this hoax, is that “Lukewarmers” are NOT on the side of truth. Typically, they have some vested interest in keeping the hoax going. Often their income, or a portion of their income, comes from AGW funding, either directly or indirectly.

      In my experience, they will turn rabid, if you confront them with facts. Spencer censors real Skeptics.

      • Avatar

        Andy Rowlands

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        Fair comment JD. All I can say is my own personal experience, I get on well with Greg. Several people have said Roy Spencer can be acidic in his comments. I’ve had two conversations with him, one on email from his website and one on Fb, and he was perfectly civil with me, but I fully acknowledge that may not be the case for everyone.

        • Avatar

          JDHuffman

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          It may be that it was you that was the civil one, Andy. You could probably get along with many.

          But just try to explain to Spencer the reasons CO2 cannot heat the planet. You will see a different side of him….

        • Avatar

          Jerry Krause

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          Hi Andy,

          Listen to JD, my comments are now censored by Spencer.

          Have a good day, Jerry

          • Avatar

            Andy Rowlands

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            You Jerry, and JD, may well be absolutely right in what you say, I have no reason to doubt either of you. Roy is another ‘lukewarmer’ and I suspect he, and John Christy, are taking this stance because they are both dependent on their funding and their jobs. At least I hope that is the case. I’ve discussed this at length with John at PSI, and we suspect once they retire they will become much more openly skeptical.

  • Avatar

    Matt

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    Hi Andy, J.D; and Jerry and bystanders who are logarithmically loosing their innocence.

    When I glance at the “concentration” pathway referenced in the eleventh paragraph of this article. I fear and despair.

    ” The practical appeal of the radiative forcing concept is due, in the main, to the ASSUMPTION that there exists a general relationship between the global mean forcing and the global mean equilibrium surface temperature response (i.e., the global mean climate sensitivity parameter, λ) which is similar for all the different types of forcings.” (my bold)

    The Radiative Forcing Of Climate Change paper states that the Radiative Forcing hypothesis is an assumption???

    The paper also states it is a complex issue. To simplify an allegedly complex issue the first step is to eliminate ASSUMPTIONS and the second step is to separate out that which is known, individually identifying and quantifying those separately known constituent factors of the not so complex problem.

    My assumption is there is a mass of fools putting the cart before the horse. Surely one does some science to validate or refute assumptions. Could somebody please rant a stream of profanities on my behalf.
    Matt

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      Rosie Langridge

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      Please see my questions below.

      Also, if this is pure make-believe, can you give me an idea of who made it up, how they got it established as reputable science worthy of being discussed, when did it become accepted, which (formerly?) reputable scientific periodicals print this kind of make-believe, and why Joseph Postna even needs to write his paper?
      Alternatively, I’ve not been following the debates, and have I got completely the wrong end of the stick?
      Thanks

      • Avatar

        JDHuffman

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        The CO2 hoax would take a long time to explain. Basically, the science is terribly flawed, but is covered up by years and years of “science papers”. Many of the big names don’t even understand the basic physics. Roy Spencer has even claimed that putting on a sweater or coat means that “cold” can warm “hot”! He has no clue about thermodynamics, or how stupid that sounds. He’s just one example.

        Established instutions of “science” are terribly corrupt. Funding is much, much more important to them than actual science, or truth. They control input with censorship.

  • Avatar

    Rosie Langridge

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    Be grateful for clear answers, no joshing or cryptic remarks.

    This article by Dr Roger Pielke – is it scientific nonsense?
    Is this set of ‘theories’ exactly what Joseph Postma is arguing against in his paper saying that there is no radiative greenhouse effect?

    • Avatar

      Matt Holl

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      Hi Rosie,

      The art of listening is to ask questions. Congratulations.
      I refer you to three separate lectures on youtube each of which I have watched a number of times. I have spotted a minor error here and there but no deception.

      “the power of truth-dr patrick moore”

      “inconvenient lie day one-dr tim ball”

      “inconvenient lie day one- dr will happer”

      Three wise men.
      Kind regards. Matt

    • Avatar

      Matt

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      Hi Rosie
      This will provide an easier result.

      Patrick Moore – The Power of Truth – YouTube

      • Avatar

        Rosie Langridge

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        Thanks, I’ll do that, but (bearing in mind a strong science background) what about this stuff on this page from Pielke – is it garbage or is it sound scientific analysis and enquiry?

        • Avatar

          JDHuffman

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          Rosie, Pielke’s education is in “political science”. He knows NOTHING about the relevant physics of Earth’s energy balance. My opinion is that he’s only in the debate to make money. He offers nothing by rambling drivel, like his article here. He has effectively claimed that CO2 can heat the planet. He would describe himself as a “Lukewarmer”. I would describe him as an “Attention Seeker”.

          He know nothing about physics, but he knows a lot about propaganda, and how to make money from it.

          • Avatar

            Matt

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            Thank you J.D.

          • Avatar

            Rosie Langridge

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            Thanks all. ‘Scientific’ garbage it is. I thought it couldn’t be me losing my senses.

  • Avatar

    James McGonn

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    The reality is that meteorologists don’t give a shit about the facts of the atmosphere. Thermodynamics is just one of a number of approaches that they absolutetely refuse to consider. You guys are just scratching the surface.

    Meteorology’s theoretical thinking is just marketing. It has evolved over the years to coincide with what the public generally assumes. Consequently they are not going to do anything that would risk revealing to the public that it actually is nonsensical.

    Meteorology is not an actual science and it has actually never been anything but a long, continuing conversation about a subset of physical phenomena.

    Meteorology doesn’t debate. They don’t do experiments. And they are extremely careful and even skilled at using imprecise terminology so that debate and experiments never threaten their percieved credibility.

    The fact that you guys have isolated yourselves by conceptually focussing on the one area of science you understand comprehensively–thermodynamics–makes it easy for them to dismiss you.

    There is a larger puzzle of truth that needs to be put together one piece at a time. You guys have discovered one of these pieces. Congratulations. Now you all need to stop waving your hands in the air declaring that you have the whole puzzle. You don’t. You have one piece!

    Meteorology has failed to understand storms
    https://anchor.fm/james-mcginn/episodes/Meteorology-has-failed-to-understand-storms-e91i9b

    James McGinn / Genius

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