OK I found the formula. It is:
FV = D * ((1+r)^T - 1) / r , where:
FV = Cumulative carbon emissions in GtC over 93 yrs (2007-2100)
D = 1st yr emissions (7.0 GtC)
r = annual % increase of emissions
T = time in yrs (93 yrs)
So rather than the baseline IPCC number of 670 GtC over the remaining 21st century, the actual baseline emissions would be:
1857 GtC @ 2% annual growth
1396 GtC @ 1.5% annual growth
The EIA projects about an approx. 2% annual growth in hydrocarbon energy consumption over the next quarter century. If that continued over the remainder of the 21st century, the baseline number is 1857 GtC.
Hence the reduction required is NOT from 670 GtC down to 490 GtC, but from 1857 GtC down to 490 GtC.
That is a big difference. I'd be very interested in knowing which input value was modeled -- 1857 GtC or 670 GtC.
This affects everything -- climate modeling, how achievable the needed reductions are, etc.
That superficially looks like a 74% reduction in hydrocarbon energy consumption is required. But it's worse than that. You'd have to virtually eliminate hydrocarbon energy. Why?
Because no matter what technology or how ambitious the plan, it takes time to implement. Hence the 1st few decades you're still burning hydrocarbons at the current rate (inc'l annual increase). All that counts against the IPCC 21st century cumulative limit of 490 GtC.
That means in later decades of the 21st century, much greater reductions are needed than 74%. The entire globe would have to mostly run on fusion or something like that, otherwise you'll go over 490 GtC cumulative emissions. And even that results in atmospheric CO2 increasing to 450 ppm, significantly above current levels.