Luke — re Spencer's CO2 sensitivity value, ie empirically measured 2XCO2
of 0.5 - 0.6 C fits with several other authors who've likewise found it
seems to fall in the range 0.4 - 0.6 C. I thought OK that is interesting, then worked it
out myself by two more different ways (one by SST's, one by difference) and yep, it
seems to be in that
ballpark.