I don’t know the answer to this question: I am not an expert in this area, and I haven’t the time or resources to track down the data to discover the answer. I even wonder if there exists controlled experimental data. It must be the case that the objections I make below are well known and have been considered by those who regularly use tree rings as proxies for temperature. I’m just hoping that some regular reader might know where to go on this matter.
Ulf Büntgen and a pal or two saw their “2500 Years of European Climate Variability and Human Susceptibility” published recently in Science (thanks to reader Sylvain Allard for bringing this to my attention). The abstract begins:
Climate variations have influenced the agricultural productivity, health risk and conflict level of preindustrial societies. Discrimination between environmental and anthropogenic impacts on past civilizations, however, remains difficult because of the paucity of high-resolution palaeoclimatic evidence. Here we present tree ring-based reconstructions of Central European summer precipitation and temperature variability over the past 2500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration.
The admission is “paucity of high-resolution palaeoclimatic evidence”, meaning no direct measures of temperature and precipitation exists. Thus, the reliance—I emphasize the word—on tree-ring data as a stand-in for what was not measured.
Then comes the key: “Recent warming is unprecedented”. Is it? How can we know? Well, by examining the reconstructions of temperature using tree rings: that is, by building statistical models of temperature as functions of tree-rings.
My question is this. Suppose, ceteris paribus, that temperature changes rapidly year on year. I leave “rapidly” undefined, as I do how the temperature changes (more in summer than in other seasons? equal change through all seasons? etc.; each possibly would presumably influence the way trees reacts to temperature). Can trees keep up with rapid temperature change?
My guess, based, it’s true, on vague biology, would be that the tree ring responds to this temperature change linearly when the year-on-year temperature change is slow, and it responds to the temperature change (say) logarithmically when the year-on-year temperature change is fast. That is, when temperature change is too quick, the tree can’t catch up and doesn’t respond as quickly to extremes. Tree rings would then, in effect, be a low pass filter on temperatures.
If that is true, then any reconstruction of temperature based on tree rings would always show less variability than would actual temperature measurements. The past would necessarily look calmer than the present, so to speak. Reconstructions would have more hockey sticks than at the Joe on a Friday night in January.
Now, if you knew how trees responded to rapid change, then you could of course incorporate this knowledge into your reconstruction models. But this removes these models from the land of simple regressions, and almost certainly, and unless the researcher is very careful, the results will be too certain in times of rapid change (the confidence or credible intervals should widen considerably when the regime switched from linear to logarithmic).
Then, too, we have the difference between estimates of the parameters of these models versus these models’ estimates of the actual observables (temperature). Use of the former—which is all you see in classical statistics—guarantees over-certainty.
Controlled experimental data would answer the question. Grow a strand of trees, paying attention to the ceteris paribus, then change temperature slowly, then rapidly and see what happens. Of course, since tree rings are laid down annually (I stand ready to be corrected here), this experiment will take some time.
You might then try to look to the wild where we have simultaneous (say) thermometer-based temperatures and tree rings, which must have been done. The problem here is observational bias. Chances are overwhelming that that ceteris paribus bit will not have been understood properly. I say this because it rarely is. This difficulty isn’t strong enough to bar these kinds of experiments, but it is sufficiently forceful such that we should always look at our results with some skepticism. Especially if our goal is to forecast temperatures changes of fractional degrees.
As I have said, all this is surely well known; thus this post is more a way for me to organize my thinking than any kind of review. I await enlightenment from you.