As you probably know, the IPCC established four Representative Concentration Pathways (RCPs) for their 5th Assessment Report. The number after each "RCP" is the assumed amount of radiative forcing, in watts per square meter, from all greenhouse gases in the year 2100.
The pathways cover many variables all across the board, and are the inputs climate modelers use to do their calculations. Here are the actual numerical values of all the parameters that go into each RCP.
So which pathway will the world follow, at least approximately? Which pathway are we on so far?
I'm going to simplify this by looking only at atmospheric CO2 concentrations, because this is a blog, not a journal paper.
The RCPs don't begin to differ until about 2009 (and go all the way to 2500, when RCP 8.5 assumes CO2 would be 1,962 ppm), and then differ slowly, but by 2018 we have this:
I got the CO2 values for the year by taking the annual CO2 radiative forcing (RF) from NOAA's Global Radiative Forcing results and using the equation
to calculate C, which here means CO2 and C0=278 ppm. (The latter is the value the RCPs take for their first year, 1765.)
So we're between RCP 6.0 and RCP 8.5. To find an actual value, I linear interpolated between their values for each year to get our actual RCP value:
The values were small in the beginning because there wasn't much variance between RCP 6.0 and RCP 8.5 in the beginning -- the difference is now 4.5 ppm CO2 -- and because our path was below RCP 6.0 in the beginning. But we made up for it and now the value is stabilized, for now at least, at 6.8.
So we're at RCP6.8.