IPCC projections about future climate are based primarily on the computer models of the earth’s coupled oceanic and atmospheric circulation system that I discussed earlier in Chapter 14. As we have seen, these models manage to reproduce industrial era temperature change only by tuning the way they parameterize – numerically approximate – certain crucial but poorly understood physical processes, in particular how clouds respond to a CO2-induced warming. The models’ one clear testable prediction about the present-day impact of rising CO2 – that tropical tropospheric temperatures should be rising more than surface temperatures – has not been consistently confirmed. And over the course of most of our present climate ear, the Holocene, work is conflicting on whether temperature has been rising along with CO2.