Somebody recently posted another article on this topic, but the thread got derailed due to the controversial status of the author. I think this subject is important enough to merit another shot at a good thread, so I submit this article by Michael Liebreich of BloombergNEF and the Cleaning Up podcast.
The TLDR is that RCP 8.5/SSP5-8.5 are not going to be included in the next IPCC assessment report. 8.5 was a model of future emissions misleadingly called the "business as usual" pathway that projected what would happen if coal emissions continued to increase exponentially, as they were doing in the early 2000s. But that was never going to happen, and positing it has misled the public and undermined trust in climate science.
Here are some highlights from the article.
Firstly, 8.5 was not only implausible for energy economy reasons, but was likely physically impossible:
>By 2017, there was no justification to continue using RCP 8.5 for any purpose. Justin Ritchie and Hadi Dowlatabadi published a puplished a paper showing there weren’t enough recoverable coal reserves on the planet to follow RCP8.5 even if you wanted to.
Secondly, so much work was based on this implausible projection that even well-respected scientists clung to it well after it had been debunked:
>I ended up blocked by Dr Katharine Hayhoe after asking her about the U.S. National Climate Assessment, which derived the financial cost of climate action by comparing costs under RCP 8.5 (a scenario with a 2100 global population of 12.3 billion) with costs under RCP 4.5 (a scenario with a 2100 global population of 8.7 billion).
People's tendency to confuse emissions with temperatures and other impacts is another source of confusion:
>The RCPs are pathways of concentrations. If we are seeing temperatures running ahead of those predicted based on CO2 concentrations, or impacts running ahead of those predicted based on temperatures, then that is what we must research. We can’t just pretend that we will be in world of 1,100 ppm of CO2 by 2100, when the current trajectory takes us to 540 ppm. That’s not science.
Clinging to this unrealistic model gave actual climate villains justification to disregard climate science altogether:
>In the end it came to the attention of President Trump, who called out RCP 8.5 by name in his directive on Restoring Gold Standard Science, saying: “agencies have used Representative Concentration Pathway (RCP) scenario 8.5 to assess the potential effects of climate change in a “higher” warming scenario. RCP 8.5 is a worst-case scenario based on highly unlikely assumptions like end-of-century coal use exceeding estimates of recoverable coal reserves.”
This is obviously not to say that we are not in a climate emergency. It is to underscore the importance of basing our words and actions on the best available science, resisting the temptation to head straight for the most alarmist predictions:
>The tragedy of the #RCP85isBollox debacle is that the climate community spent a two decades telling people we were heading for 4°C to 7°C of warming, and quantifying the benefits of action by comparing RCP 8.5 with RCP 4.5 (looking at you, editors and authors of supposedly authoritative U.S. National Climate Assessments). The reality is that we are already doing better than RCP 4.5. and are heading for 2.5°C to 3°C of warming*, but still facing a very dark future of climate disruption. The need for urgent action to bend the curve down towards 2°C and below.
>We need to learn the lessons. It is not just that it is intellectually flawed to exaggerate science. It’s morally wrong to do so in order stir up fear in pursuit of public policy objectives, no matter how desirable those objectives are.