Back in September, I said:
But I do have to wonder about average global temperature and stuff like that. Taking a mountain of data and boiling it down to just one value is a effort that requires huge assumptions and no one should be comfortable relying on a single metric for such a complex system.
Now, I don’t know how they calculate such things exactly. But I do know what when the average temperature for a given day for a given location is taken, they find the average of the high and low temperatures for the day. Of course that’s not the average at all. It may be close, but there’s room to be way off.
From ScienceDaily:
“It is impossible to talk about a single temperature for something as complicated as the climate of Earth”, Bjarne Andresen says, an an expert of thermodynamics. “A temperature can be defined only for a homogeneous system. Furthermore, the climate is not governed by a single temperature. Rather, differences of temperatures drive the processes and create the storms, sea currents, thunder, etc. which make up the climate”.
He explains that while it is possible to treat temperature statistically locally, it is meaningless to talk about a a global temperature for Earth. The Globe consists of a huge number of components which one cannot just add up and average. That would correspond to calculating the average phone number in the phone book. That is meaningless. [...]
If temperature decreases at one point and it increases at another, the average will remain the same as before, but it will give rise to an entirely different thermodynamics and thus a different climate. If, for example, it is 10 degrees at one point and 40 degrees at another, the average is 25 degrees. But if instead there is 25 degrees both places, the average is still 25 degrees. These two cases would give rise to two entirely different types of climate, because in the former case one would have pressure differences and strong winds, while in the latter there would be no wind.
The article goes on to mention “that there are many ways of calculating average temperatures.”
Depending on the averaging method used, the same set of measured data can simultaneously show an upward trend and a downward trend in average temperature. Thus claims of disaster may be a consequence of which averaging method has been used, the researchers point out.
What Bjarne Andresen and his coworkers emphasize is that physical arguments are needed to decide whether one averaging method or another is needed to calculate an average which is relevant to describe the state of Earth.
