I read this, too. I understand what they're saying, but I think they're overstating the case -- or at least making it sound more simple than it is.
For one thing, phone numbers are not a measure of anything, and exchange rates is a bad example because we're talking about conversion rates between multiple arbitrary measures of productivity or wealth. Temperature doesn't measure a rate, it measures a state.
Don't get me wrong -- I'm a huge AGW skeptic -- but this argument really goes nowhere on its own.
I agree that it is difficult to nearly impossible to take some definitive measurement -- but that's true of any system - like say your car engine. Does your car engine have an "engine" temperature? Even when it's running, your engine has cooler parts and warmer parts. The same argument "no global temperature" would apply to the engine, but nobody would seriously argue that the engine doesn't heat up when you start it in the morning.
That being said, it does touch on something that most lay people don't get, and that's that when we are talking about "Global Temperature" we are talking about an estimated average, and we're also talking about, over tens of thousands of years, a few degrees difference. This few degrees difference often means that the margin of error overwhelms the changes we do see, and there's the whole apples to oranges argument on top of that.
Different estimation methods will yield different estimates. Tree ring analysis is one method. Ice core analysis is another. Sediment analyisis is yet another. And, of course, there's the good ole "average your thermometer readings" method.
Using these methods, we can get a pretty good idea of periods the relative amplitude of long-term temperature and climate variability, but to say that we can tell you excactly what the "global temperature" was at any given time given any combination of these is a bit of a stretch.
If we had thermometers in fixed places at a fixed height all over the world in areas unmolested by artificial local effects (such as shade trees growing over them or urban heat island encroachment) and all agreed that the average reading of those thermometers gave us an average surface temperature -- and that average went up over time, we could say that the surface is warming.
We do have satellites that can figure an average temperature in a column of air -- we're better equipped today to estimate the earth's thermal energy balance with more observations in some sort of meaningful fashion. But we've only been able to do that for maybe the past 30 or so years. To tack data like that side by side with ice core analyses can be very misleading. Analyses such as this (along with extra smoothing of the proxy data to get rid of the midieval warm period) is what gave us the famous, purposely alarming "hockey stick" graph in the 1990's.
If we defined what we want to call "global temperature" and everyone agreed that what they are talking about when they use the term is in terms of that definition -- we'd have something meaningful. We would still have to be careful about just what it is that we can or can't infer from it.
Mostly what I'm cautioning against here is skeptics arguing that we can't say the earth is warming because there's no "global temperature". When you dig into the argument, you do start to see some of the traps in saying that "we know what it is" and especially in saying "we know what it was" -- when "it" (global mean temperature) probably doesn't have an official, narrow definition. But on its face it's like saying that we can't say our car engine is heating up because there's no "engine temperature".
On the other hand, because the problems involved in defining and/or taking that temperature, especially with the small amplitude of the changes and the large margin of error, we should take people's so-called measurements with a sustantial amount of salt.
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