A temperature can be defined only for a homogeneous system

I am very skeptical of the entire AGW movement to centralize power and primitize lives. Even so, the tone of this article also raises my skeptical hackles. From ‘Global Temperature’ — Why Should We Trust A Statistic That Might Not Even Exist? from the I & I Editorial Board.

None-the-less, they are addressing a central issue (one among many) which rarely gets discussed. How are we measuring AGW and are the measurements reliable, extensive and useful? They are not. We don't have much reliable data in terms of territorial coverage or in temporal terms.

I&I are getting at the more fundamental issue of exactly what is the metric we are choosing to use?

The United Nation’s Intergovernmental Panel on Climate Change is quite certain Earth will be in trouble if the global temperature exceeds pre-industrial levels by 1.5 degrees Celsius or more. But how can anyone know? According to university research, “global temperature” is a meaningless concept.

“Discussions on global warming often refer to ‘global temperature.’ Yet the concept is thermodynamically as well as mathematically an impossibility,” says Science Daily, paraphrasing Bjarne Andresen, a professor at the University of Copenhagen’s Niels Bohr Institute, one of three authors of a paper questioning the “validity of a ‘global temperature.'”

“The temperature obtained by collecting measurements of air temperatures at a large number of measuring stations around the globe, weighing them according to the area they represent, and then calculating the yearly average according to the usual method of adding all values and dividing by the number of points.”

But a “temperature can be defined only for a homogeneous system,” says Andresen. The climate is not regulated by a single temperature. Instead, “differences of temperatures drive the processes and create the storms, sea currents, thunder, etc. which make up the climate”.

While it’s “possible to treat temperature statistically locally,” says Science Daily, “it is meaningless to talk about 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.”

Fair enough.

Here is the aspect which I don't think I have seen before.

A few years after the University of Copenhagen report was published, University of Guelph economist Ross McKitrick, one of the report’s authors, noted in another paper that “number of weather stations providing data . . . plunged in 1990 and again in 2005. The sample size has fallen by over 75% from its peak in the early 1970s, and is now smaller than at any time since 1919.”

“There are serious quality problems in the surface temperature data sets that call into question whether the global temperature history, especially over land, can be considered both continuous and precise. Users should be aware of these limitations, especially in policy-sensitive applications.”

A 75% reduction in land based temperature measurement? Hmmm. Not sure what to make of that.

Satellite heat measurement came in to place in the 1970s. If the decline in land-based measurement systems is real, might it simply be a displacement of land based instruments by satellite measurement? I would guess that is the case but it would be interesting to know.

And it is still probably a problem, for several reasons. We need land-based measure to validate and calibrate the satellite measures. We still need to geographical coverage because large portions of the globe are only partially measured. There are mismatches between land and satellite measurements which still need reconciling (unless that has been resolved since the last time I looked at this a few years ago.) If you get rid of one of the two measures, you may be losing information because you still don't know the source of the difference.