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Understanding The Great Climate Model Scenario Debate

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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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