What is global warming? Most people would answer this seemingly simple question with something like the following (see the article at wikipedia.org):
Global warming is the observed increase in the average temperature of the Earth’s near-surface air and oceans in recent decades and its projected continuation. […]
One would think that all scientist agree on this definition. However, actually they don’t.
Some scientist would say that it does not even make sense. The June 2007 issue of the Journal of Non-equilibrium Thermodynamics includes a paper of Christopher Essex (U. of Western Ontario), Ross McKitrick (U. of Guelph) and Bjarne Andresen (Niels Bohr Institute), with the interesting title:
In this paper it is argued that the concept of a “global temperature” is thermodynamically as well as mathematically meaningless. First of all, you cannot just add local temperatures on the Earth and then take the average to define a single “global” temperature of the Earth. Secondly, the average is not canonically defined. For example, taking a box of air with temperature 0 degrees and an identical one with temperature 100 degrees would lead to an arithmetic average of 50 degrees (add the two numbers and divide by two). However, the geometric average in this case, obtained by multiplying the two numbers (in degree Kelvin) and taking the square root is 46 degrees. Thus claims of distaster – or not – maybe a consequence of the averaging method used.
So, what is Global Warming? Can it be defined in a sound way, both from a physics and mathematics viewpoint?
Update: The climate-friends at RealClimate.org thinks that this paper is irrelevant.