Found an interesting article published in the Journal of Non-Equilibrium Thermodynamics (June 2006) on the measurement of global temperature. I copied and pasted the conclusion if you want to skip over all of the fancy-ass science stuff.

Does a Global Temperature Exist?
**Conclusion**
There is no global temperature. The reasons lie in the properties of the equation of state

governing local thermodynamic equilibrium, and the implications cannot be avoided by substituting

statistics for physics.

Since temperature is an intensive variable, the total temperature is meaningless in terms

of the system being measured, and hence any one simple average has no necessary meaning.

Neither does temperature have a constant proportional relationship with energy or other

extensive thermodynamic properties.

Averages of the Earth’s temperature field are thus devoid of a physical context which

would indicate how they are to be interpreted, or what meaning can be attached to changes

in their levels, up or down. Statistics cannot stand in as a replacement for the missing physics

because data alone are context-free. Assuming a context only leads to paradoxes such as

simultaneous warming and cooling in the same system based on arbitrary choice in some

free parameter. Considering even a restrictive class of admissible coordinate transformations

yields families of averaging rules that likewise generate opposite trends in the same data,

and by implication indicating contradictory rankings of years in terms of warmth.

The physics provides no guidance as to which interpretation of the data is warranted.

Since arbitrary indexes are being used to measure a physically non-existent quantity, it is

not surprising that different formulae yield different results with no apparent way to select among them.

The purpose of this paper was to explain the fundamental meaninglessness of so-called

global temperature data. The problem can be (and has been) happily ignored in the name of

the empirical study of climate. But nature is not obliged to respect our statistical conventions

and conceptual shortcuts. Debates over the levels and trends in so-called global temperatures

will continue interminably, as will disputes over the significance of these things for the human

experience of climate, until some physical basis is established for the meaningful measurement

of climate variables, if indeed that is even possible.

It may happen that one particular average will one day prove to stand out with some

special physical significance. However, that is not so today. The burden rests with those

who calculate these statistics to prove their logic and value in terms of the governing dynamical

equations, let alone the wider, less technical, contexts in which they are commonly

encountered.