Average temperatures

In the debate about 'global warming' you often hear the concept of 'average global temperatures' being discussed.

But when Victoria has heat waves with temperatures over 45 degrees C and places near London have freezing cold and snow in February 2009, an 'average' doesn't seem to make much sense.

The question is 'What do 'global temperatures' or 'average global temperatures really mean?

Is it valid to talk about an 'average' temperature for the earth when temperatures vary wildly from place to place, from season to season and form year to year?

In the following article, researchers from Denmark and Canada analyse the issue of 'average temperatures', give various options for calculating it and highlight flaws in the concept.

Researchers Question Validity Of A 'Global Temperature'
"ScienceDaily (Mar. 18, 2007) - Discussions on global warming often refer to 'global temperature.' Yet the concept is thermodynamically as well as mathematically an impossibility, says Bjarne Andresen, a professor at The Niels Bohr Institute, University of Copenhagen, who has analyzed this topic in collaboration with professors Christopher Essex from University of Western Ontario and Ross McKitrick from University of Guelph, Canada...."