HAS: the RCP8.5’s assumed CO2 emissions are a long way from exponential.
How did CO2 emissions come into this?
As usual I have not been clear. I am in full agreement with you about ham and eggs: if we had ham we could have a ham and egg sandwich, if we had eggs. If your point there is that there are many assumptions underlying the processes conjectured to be driving rising CO2, then yes, absolutely.
I am not interested in any of them.
My candidate for the reference point closest to RCP8.5 pays no attention to ham, eggs, volcanoes, aliens, or CO2 emissions, none of which can be shown to be exponential.
In particular the CDIAC estimate of CO2 emissions since 1750 is far from exponential, showing a CAGR on the order of 15% during the latter half of the 19th century but considerably less than that during the 20th century. Furthermore the CAGR of CO2 emissions has been wandering all over the place throughout the 20th century and on into this one.
Just to rub this in, if CO2 does have more than one exponential driver, then unless they all differ from each by at most a mere constant factor their net effect cannot be exponential because it is mathematically impossible for the sum of distinct exponentials to be an exponential.
What I’m proposing is to look at one thing, and one thing only: the CO2 concentration itself, ignoring all possible drivers of it, exponential or not.
Its excess over 280 ppmv has been growing very nicely at a straightforward exponential rate whose CAGR, in stark contrast to that of CO2 emissions, has held remarkably steady at 2.2% over the past half century.
If we knew more about the drivers we could forecast a departure from this steady CAGR.
But since the drivers present us with a complicated story that is hard to piece together, and even those pieces do not take into account the terrestrial and ocean sinks, climate feedbacks, etc., I propose ignoring all the possible drivers—ham, eggs, volcanoes, aliens, and the rest—and replacing the problematic notion of “business as usual” with the very simple concept of continued exponential growth of excess CO2.
Whose simple formula is 1.022^(y − 1795). (Add 280 to get total atmospheric CO2, good for the last few thousand years.)
