The high-end future emissions scenario – RCP8.5 – was never intended to be the most likely “business as usual” outcome. We take a deep dive into the creation of RCP8.5 and its subsequent (mis)use over at @CarbonBrief: carbonbrief.org/explainer-the-… 1/23(!)
Energy system modelers have created a range of scenarios to examine what might happen in a world where no additional climate policies are adopted. These range from a RCP6.0-type scenario (~3C warming by 2100) to RCP8.5 (~5C). RCP8.5 is the worst case available, not BAU: 2/x 
Lets take a brief look at the history of the development of the RCPs. After the IPCC AR4 came out in 2007, there was a widespread desire to update the old SRES scenarios – developed in the late 1990s – to better reflect current technological and socioeconomic conditions. 3/x
However, the IPCC AR5 was scheduled to come out in 2013, and climate modellers would need scenarios to use in their models by 2010. Given the relatively short period to generate new scenarios, researchers developed a “parallel approach" nature.com/articles/natur… 4/x
Rather than starting with detailed socioeconomic storylines to make emissions and climate scenarios, as had been the case with the SRES, they started by creating scenarios of future “radiative forcing” not associated with any particular socioeconomic or emissions scenario. 5/x
Unfortunately, the development of the socioeconomic pathways took much longer than originally foreseen, and the RCPs were never turned into fully integrated scenarios in time for the publication of the AR5. 6/x
This left them as useful tools for modelling different potential climate outcomes, but lacking any consistent socioeconomic assumptions that would allow researchers to examine the likelihood of different no-policy baseline and mitigation scenarios. 7/x
For example, Moss and colleagues specifically state that “RCP8.5 cannot be used as a no-climate-policy reference scenario for the other RCPs because RCP8.5’s socioeconomic, technology and biophysical assumptions differ from those of the other RCPs.” 8/x
This parallel process contributed to quite a bit of confusion regarding the relative likelihood of and assumptions underlying the different emissions scenarios used to generate the RCPs. It is likely one of the factors that gave rise to the use of RCP8.5 as “business-as-usual” 9/
One change introduced during the development of RCP scenarios was to combine no-mitigation “baseline” scenarios with mitigation scenarios where climate policy drives emission reductions. Prior efforts like the SRES had mostly only considered baseline scenarios 10/
By contrast, out of the four IAM runs used to create the RCP scenarios, only RCP8.5 was a “baseline” scenario that included no policy-driven mitigation (though RCP6.0 was consistent with some baseline scenarios even the particular IAM run used included mitigation). 11/
The paper outlining the RCPs says “RCP8.5 should be seen as a high emission scenario” while “RCP6.0 can be interpreted as either a medium baseline or a high mitigation case”, suggesting there was no intention for RCP8.5 to be a more likely BAU vs RCP6. link.springer.com/article/10.100… 12/
RCP8.5 is described as being around the 98th percentile of overall radiative forcing in all published scenarios in the literature, and around the 90th percentile for no-policy baseline scenarios. 13/
So, the scenario used to generate RCP8.5 was around the highest of the available baseline. While it was by no means considered impossible, it was also not considered to be more likely than any other baseline scenario – the vast majority of which resulted in lower emissions. 14/
While Riahi et al call RCP8.5 "a high-emission business as usual scenario" in their paper, Riahi told Carbon Brief that "I wished I would have been clearer with what I meant by business as usual in that paragraph." link.springer.com/article/10.100… 15/
Riahi meant that RCP8.5 was a high emissions case among the range of baseline no-policy scenarios in the literature, not a business as usual outcome would necessarily result in emissions that high. 16/
The high-end nature of the RCP8.5 relative to other no-policy baselines was poorly communicated to the wider scientific community, and RCP8.5 – being the only available RCP based on an IAM with no climate policy included – came to be widely referred to as “business as usual" 17/
There has been a serious debate in the academic literature involving @jritch and others about whether RCP8.5's fossil fuel use assumptions are even possible. More broadly, energy system modelers seem to feel its not a particular likely outcome: 18/
At the same time, even if emissions used to create RCP8.5 are unlikely, that doesn't mean there aren't reasons to include scenarios with that level of forcings. There are possible carbon cycle feedbacks that could result in RCP8.5-level forcing under lower emission scenarios. 19/
For example, when CMIP5 models are run using RCP8.5 emissions (rather than proscribing concentrations/forcings), they end up around 8.75 w/m^2 forcing rather than 8.5: journals.ametsoc.org/doi/full/10.11…. These feedbacks are still relatively poorly captured in current models 20/
RCP8.5 also gives a higher signal-to-noise ratio for the detection of significant changes in the climate system, and is useful for consistency with past modeling efforts 21/
That said, it is important for the research community to stop referring to RCP8.5 as if it were the sole (or most likely) no-policy "business as usual" outcome, and explore a wider range of possible baselines in climate impact studies 22/
The new 7 w/m^2 forcing scenario in CMIP6 should help, as it fills a gap between the low end of baseline scenarios (RCP6) and the high end (RCP8.5). 23/
To clarify here (as twitter makes nuance difficult at times), each RCP scenario still had an underlying IAM run that generated its emissions scenario:
Also, John Nielsen-Gammon (@climatexas) pointed out most of these details five years ago, before twitter debates on RCP8.5 were in vogue: climatechangenationalforum.org/what-is-busine…
