Climate Science Voyage of Discovery. III. History of the RCP 8.5 Controversy

Steve Wilent pointed our attention to what is known as the 8.5/BAU (does RCP 8.5 represent Business as Usual, as many scientific papers say?) controversy a while back. I thought I’d post this Twitter roll (hopefully you can click to it) by Oliver Geden because it includes the sociology of science perspective. so often missing from climate science discussions. Also it seems like a gradual introduction to acronyms, and has links to many other papers of interest.

If you have trouble following the Thread reader, please let me know.

What is a Representative Concentration Pathway? You can read definitions, which aren’t necessarily very helpful. I like this article by Zeke Haufather because it tells the story of how the RCP’s were developed and used, leading to:

However, its position as the only non-mitigation scenario considered in the IPCC AR5 along with relatively poor communication between energy modelling and climate modelling communities led to a widespread misperception both in the media and in the academic literature that RCP8.5 was the expected “business as usual” outcome in a world without any future climate policy.

While worst-case outcomes are important to take into account, particularly given the uncertainties in the magnitude of carbon cycle feedbacks, it is important that they not be considered in isolation. Taking the range of possible baseline outcomes from 6.0 to 8.5 W/m2 forcing would provide a more realistic set of scenarios for studying climate impacts in a no-policy future.

On the other hand, though, if models and scenarios estimate so many things, how can we be sure that their estimates are accurate? From the Ritchie paper:

This paper finds climate change scenarios anticipate a transition toward coal because of systematic errors in fossil production outlooks based on total geologic assessments like the LBE model. Such blind spots have distorted uncertainty ranges for long-run primary energy since the 1970s and continue to influence the levels of future climate change selected for the SSP-RCP scenario framework. Accounting for this bias indicates RCP8.5 and other ‘business-as-usual scenarios’ consistent with high CO2 forcing from vast future coal combustion are exceptionally unlikely. Therefore, SSP5-RCP8.5 should not be a priority for future scientific research or a benchmark for policy studies.

It’s also clear that “poor communication” between energy modelling and climate modelling communities and the difficulty of communicating what they are doing to other scientific communities, including the impacts folks, as well as the media, may be a real problem.

Geden also refers to the BECCS debate, which seems to me another discipline gap – modelers are going to assume BECCS but there is no link to people who might say it’s feasible or not in a particular place (Geden’s slide 6). When he says “the STS community will have a field day.” He means the science and technology studies community, who study, among other things, how people and groups work to produce knowledge.

Roger Pielke, Jr. tied this back to the discipline of scenario planning in his article here.

Scenarios of the future have long sat at the center of discussions of climate science, impacts and adaptation and mitigation policies. Scenario planning has a long history and can be traced to the RAND Corporation during World War 2 and, later (ironically enough) Shell, a fossil fuel company. Scenarios are not intended to be forecasts of the future, but rather to serve as an alternative to forecasting. Scenarios provide a description of possible futures contingent upon various factors, only some of which might be under the control of decision makers.

The climate community got off track by forgetting the distinction between using scenarios as an exploratory tool for developing and evaluating policy options, and using scenarios as forecasts of where the world is headed.

To summarize there are three separate ideas. 1. RCP 8.5 was never feasible, as it relied on using a great deal more coal that (other scientists think) doesn’t exist. 2. Lotsa coal is not business as usual. 3. the RCPs are scenarios and are not comparable to each other.
As I recall, the idea of scenario planning was to look at a broad range of futures without likelihoods and pick things to do that make sense under a range of scenarios. Not to pick the worst ones and model the extent of further bad things. Let’s do an example. Denver Water used to do scenario planning. Suppose they had a most extreme scenario- climate change dries up supply and the population grows. What to do? The point is to think about it. Not to model and write news stories about how we won’t have any water. It’s really about what you do with the info of “what could possibly happem” not how you derive it, nor to what detail.

5 thoughts on “Climate Science Voyage of Discovery. III. History of the RCP 8.5 Controversy”

    • I noticed this sentence..
      “There is a scientific debate about whether RCP 8.5 is a plausible and accurate representation of the concentrations of atmospheric carbon that would be reached on the business as usual path.”

      I’d add, there is a non-scientific debate about what is the “business as usual” path. It seems to me that first you would have to have all communities agree on what is business as usual (given that everything is in flux) and then and only then could scientists try to estimate the concentrations of atmospheric carbon.

      I actually don’t see a reason to spend much effort ($$$$) attempting to determine what would be biz as usual. We could pick a bunch of concentrations in the future and compare them without even having this disagreement.

  1. Another article on RCP8.5 today, from ClimateWire, “Worst climate scenario probably won’t happen, scientists say.” Of course, many media reports focus on the worst-case scenario, whether or not they realize that they are, in effect, misleading the audience.


    According to some experts, presenting the public with unreasonable climate scenarios could hurt global efforts to address climate change. It could suggest that global climate targets, like the goals of the Paris Agreement, are less achievable than they actually are.

    That’s one concern outlined in a comment published yesterday in Nature by University of California, Berkeley, climate scientist Zeke Hausfather and climate policy expert Glen Peters, research director at the Center for International Climate Research in Norway.

    The scenario in question — dubbed Representative Concentration Pathway 8.5, or RCP8.5 — is often referred to in climate studies as a “business-as-usual” trajectory. In other words, it’s framed as the outcome the world will experience if current conditions are allowed to continue into the future, with no future climate policy enacted.

    But that’s not actually the case, the authors argue.

    “We still have a huge lift to go to get warming below 2 degrees,” Hausfather said in an interview with E&E News. “But at the same time, we’ve started to exclude some of the truly catastrophic outcomes of high-end warming.”

    The worst-case scenario, RCP8.5, was originally intended “to explore an unlikely high-risk future,” the authors say. The dangerous outcomes outlined in this scenario would require extreme emissions of future greenhouse gases, the sort that would probably have to be driven by massive expansions in global coal consumption over the coming decades.

  2. More on RCP8.5. A Comment in Nature posted on January 29: “Emissions – the ‘business as usual’ story is misleading,” in which the authors say, “Stop using the worst-case scenario for climate warming as the most likely outcome — more-realistic baselines make for better policy.”


    RCP8.5 was intended to explore an unlikely high-risk future. But it has been widely used by some experts, policymakers and the media as something else entirely: as a likely ‘business as usual’ outcome. A sizeable portion of the literature on climate impacts refers to RCP8.5 as business as usual, implying that it is probable in the absence of stringent climate mitigation. The media then often amplifies this message, sometimes without communicating the nuances. This results in further confusion regarding probable emissions outcomes, because many climate researchers are not familiar with the details of these scenarios in the energy-modelling literature.


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