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bolido_sentimental  2yr ago (text post)   3861 thread views

Small-Scale Question Sunday for September 25, 2022

Do you have a dumb question that you're kind of embarrassed to ask in the main thread? Is there something you're just not sure about?

This is your opportunity to ask questions. No question too simple or too silly.

Culture war topics are accepted, and proposals for a better intro post are appreciated.

Posted because I didn't see Zorba post one today. Feel free to delete if that's an issue.

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I've read several times here that the climate forecast often described by news articles as "business as usual" (RCP8.5) is based on outdated assumptions and that actual emissions are already well below that forecast.

The explanations seem reasonable, but when I've tried to look for data to confirm / quantify it I havent had any luck.

Does anyone have a good source with numbers for forecast emissions from RCP8.5 (and possibly other forecast models) directly compared against actual emissions since the forecast was made?

I've been DIYing this for some years; I don't have the links handy but the CMIP5 model results and the HADCRUT global temperatures are publically available.

There is indeed significant divergence; https://imgur.com/a/krYXyKu.

IIRC RCP4.5 was a better match -- but if you look at the modelling assumptions actual emissions have been more in line with the RCP8.5 assumptions. Which makes me think that we don't actually have a great idea of what we are modelling.

This doesn't prevent climate activists from using the RCP8.5 assumptions as a bailey ("we are on track for +4 degrees C by 2100") while still retreating to the motte ("nonono, you can't just pick the worst-case scenario just because gross carbon emissions keep going up") when presented with graphs like the above.

https://www.nature.com/articles/d41586-020-00177-3 comes from a quick google (duckduckgo actually but its the first google result too) (no idea anything about the topic myself) -

RCP8.5 was intended to explore an unlikely high-risk future2. 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.

Sorry for snapping at you but that was one of the examples I was thinking about when I wrote the post. It makes a lot of assertions but is very short on actual numbers. There's a chart but it's on a 100 year scale and far too zoomed out to read off the emissions numbers for any given year. And maybe I'm just not reading the references properly but I looked up a few of them and couldn't find anything in there that directly says how many gigatonnes of CO2 were emitted vs how many were forecast by RCP8.5.

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