OK, I'll have a look at the links on Muller et al. that you've cited elsewhere.

So, my question actually was "not simple"  (at least compared to forcings) ... but still, it's striking how reluctant any of the Prime Movers of climate alarmism seem to be in addressing it.

As best as I can discern, the GMT is derived from (a) consideration of time series analyses of temp records at about 2500 weather stations around the world, plus (b) interpolations / extrapolations / Bayesian estimation of temps at other locations, where there are no weather stations (thus the ultimate summation contains numbers that no one has ever measured); (c) considerable massaging / filtering / trimming of the raw data by (presumably) standard statistical techniques to separate 'signal' from 'noise', and (d) couching the result in terms of temperature 'anomalies' (= differences), because the concept of an 'absolute' local temperature is very ambiguous, due to the homogeneity issue.

Am I getting warmer?