I want to say a word about what a proxyindicator is. And isn’t.
I noticed that the term is not in some, perhaps many, dictionaries, so I guess this leaves me free to do what I want with it! But wait, the term “proxy” is of course in the dictionary. It is an ancient short version of the word “procuracy” which is the authority to act for another. Thus, a proxy vote. Proxyindicator (or proxy indicator) is a term widely used in climate science though it is used in many other fields as well to refer to a measurement that is indirect, or more accurately, that stands in for the direct measurement. For instance, the number of women in positions of authority in various national governments is a proxy indicator for perceptions of equality of women in those various nations.
I first heard the term in a class … the first class I ever took that had anything to do with human evolution. The instructor was John Barry, and he referred to Proxy Indicators of ancient climate. And it is in this manner that I’ve mostly heard and used the term since then. I quickly add, those in the know usually abbreviate it to the short form “proxy.”
And the word is misunderstood in a very important way that I’d like to address.
Consider temperature. Right now, I can see that the temperature is 73 degrees F because my gnome toolbar says so. I was just in the car and it said 77 degrees there, on the dashboard, and when I drove by the bank a moment ago it said 74 degrees. Our house thermometer (everyone has some sort of thermometer, right?) says 76, and projects it on the ceiling along with he time. The thermometer itself is outside on the porch in a plastic bag in the shade, and communicates with the laser projecting alarm clock via radio waves.
Those are all thermometers that give a direct measurement of the temperature, right? Of course, a temperature measurement related to paleoclimate would not be a daily measure, but perhaps an average over a year’s time. And a proxy indicator would be some indirect measurement of that, like which species of gerbil lives under the porch or which kind of plankton we might find in the aquarium.
In this sense, the temperature is measured directly with the various thermometers, and a proxy for temperature would be some ecological measure that might be quite general or even inaccurate. But a proxy could also be chemical. For instance, I could take the temperature every day for a year and average it out, then I could look at the isotopes of the biofilm that is constantly forming on the curb outside my house. In different temperatures, perhaps different isotopes act differently, so the average temperature from all my thermometers and the characteristics of the biofilm’s isotopic imposition would be linked using a model. The model would have to do with how chemicals work in different temperatures while the thermometers are giving a direct measurement of the actual, real temperature.
The proxy is in fact something giving an indirect measurement of temperature, via the ecological limits of a plant or animal or a chemical reaction that works differently in different temperatures. But this is also true of the direct measurement.
Well, very few thermometers use gerbils or plankton, but they do all use an indirect measurement. Most thermometers in the past used the expansion and contraction of a liquid, such as mercury or alcohol, in a tube. The temperature reading is not a reading of temperature at all. It is a measure of volume. The volume at any given moment is an indirect effect of the actual temperature in the same way that isotopic measurements are an indirect effect of actual temperature. Another kind of measuring device uses the differential expansion and contraction of different kinds of metals. A bi-metallic strip bends to different degrees depending on temperature, and that can be an indirect measurement of the actual temperature of the air. Such a thermometer measured bent-ness of metal, not temperature.
It is not the case that these mechanical proxies (liquid expansion or bi-metallic strips) are automatically more accurate than the other methods. Up at the cabin, I can estimate the annual temperature and humidity by noting that white pines are the predominant tree, pike, walleye and bass the predominant large fish, which frogs and toads are common, that there are loons but not tundra swans nesting, and so on and so forth. I could also estimate the annual temperature by observing the bear thingie. The bear thingie is a stake with a wooden bear on it, and the bear is holding a thermometer in one hand and a rain gage in the other, and there is a wind indicator sticking out of its head. It is outdoor Kabin-Kitsch and it is designed to give you basic weather data and be cute at the same time. When I use my knowledge of North American Ecology to estimate the annual temperature of the area around the cabin, I come up with about 40 degrees F. When I look at the climate data for the region I find that the average temperature is about 40 degrees F. The trees and fish and birds told me the temperature quite accurately. When I look at the bear holding the thermometer, I get a different number. According to the bear, the average temperature at the cabin over the year is 125 degrees F. Yes, it is true that the thermometer that bear is holding has been broken for a year. But it makes the point: It is not measuring the temperature. At the moment, it is measuring some very bent metal. When it was working it was measuring some less bent medal.
There is another kind of temperature indicator that is usually considered to be direct but is, again, not a direct measurement of temperature. It uses physics. Various materials conduct electricity. In most cases, the actual amount of conductivity depends on other factors like temperature. So, some materials can be used to estimate temperature by running electricity through them at a constant rate, then seeing how much gets through. The difference from time to time will be mainly due to temperature changes.
Now, I ask you, is a person who believes that physicists don’t’ really understand fission (and thus can’t use Potassium-Argon dating to estimate the age of old fossils) or who doesn’t think that mathematicians know how to make climate models (and can thus reject most climate related research) also going to believe that we understand the way electricity varies with temperature as it passes through a conductive substance? No, of course not! That would be entirely inconsistent. You can’t reject evolutionary science, climate science, vaccine science, or any other science you somehow find inconvenient or contrary to your beliefs, and math too boot, and then blithely accept the same methods of understanding the world when it comes to the thermometers in your clock, in your car, or in your bear.
All measurements of temperature are proxies. And whatever you were thinking about some scale of believability or applicability across different kinds of proxies may very well be incorrect.