In the case of Mann v Steyn, the defendants brought a statistician to the stand, Abraham J. Wyner of the University of Pennsylvania. Wyner, along with Blakely B. McShane of Northwestern University (“M&W”), wrote an article questioning Michael Mann’s hockey stick theory of global temperature.
The article was considered by the editors of the journal Annals of Applied Statistics significant enough to warrant a special issue. The issue included M&W’s article, 12 reviews, and a rejoinder. Of the 12 reviews, I would characterize six as complimentary, one as neutral, and five as critical.
Among the five that were critical was the review by Mann and two coauthors. The fact that Mann sues people who criticize him suggests he is rather thin-skinned. A second of the five makes political arguments.
Two others of the five critical reviews look at the statistical technique employed by M&W to question Mann’s construction of the history of global temperature for the past thousand years. M&W, in their rejoinder, do not entirely disagree with the criticism of the statistical technique they employed.
Another criticism that I describe as neutral is that M&W, like Mann, construct a temperature proxy that does not allow for spatial variation. See Loehe and McCulloch (2008) for a model that constructs a history of global temperature allowing for spatial as well as temporal variation.
A key characteristic of Mann’s hockey stick theory is that global temperature was more or less constant for about eight hundred years and then started rising with the burning of fossil fuels. The long period of approximately flat temperature is the shaft of the hockey stick, and the short period of rising temperature is the blade of the hockey stick.
The dichotomous nature of the hockey stick argues that the cause of rising temperature is the introduction of something new into the planet’s climate system about two hundred years ago; i.e., carbon dioxide. The hockey stick theory of global temperature, to use the unscientific terminology of climate alarmists, denies that there has been natural variation in global temperature during the past thousand years.
The hockey stick has been massively contracted by a variety of direct observations as well as by a variety of proxy data. An example of direct observation is the revelation of former Viking settlements in Newfoundland and elsewhere upon the recent retreat of glaciers. The revelation of former Viking settlements indicates that the earth had been relatively warm at certain times during the past thousand years, Then, got colder, so that the glaciers advanced and covered some Viking settlements, which cold period we call the Little Ice Age, and subsequently got warmer. A proxy that doesn’t show this change in temperature isn’t a reliable proxy.
Mann relied on tree rings for his proxy data. More so, he selected only a certain period during which he correlated his tree rings with observed temperatures (the thermometer being a relatively recent invention). The selection of only certain years when both the proxy data and temperature readings were available, has come to be known as “hide the decline” (meaning hide the decline in the correlation). This selection, or shall I say “manipulation” of the data suggests something fishy is going on. Why is the correlation of tree rings with observed temperatures sometimes not reliable? And, if the correlation is sometimes unreliable, how can the construction of the history of temperature with tree rings be reliable?
Mann’s hockey stick appeared in the third assessment of the IPCC and, so, gained something like an imprimatur, or an official approval. A Nobel Peace Prize was awarded to Al Gore and the IPCC. Subsequently, Mann’s hockey stick has been contradicted and summer sea ice in the Arctic Ocean hasn’t disappeared. But, as a political movement, the green agenda got underway.
Looking at M&W’s article, they showed that Mann’s data could support any number of theories of the history of temperature, and that tree rings are no better at predicting future temperatures than a naive forecast. Concerning the latter, it was pointed out that there are unavoidable problems with forecasting when you have a small sample; and, considering the ergodicity of climate data (along with many other time series), Mann, M&W and everybody else has a problem. Hence, it is very useful to have some direct observations of temperature in the past, such as I discussed with regard to Viking settlements.
Informed by the knowledge that there is natural variation, we can compare the construction of the history of global temperatures by candidate proxies to certain directly-observed phenomena of the past, as well as to temperature readings since the invention of the thermometer. For example, does the construction include a Little Ice Age?
With regard to Mann, it is not unusual for a pioneer in a field to make mistakes. Indeed, flawed research often gains attention specifically to correct mistakes.
The problems with Mann’s hockey stick, together with the revelation of attempts by climate alarmists to suppress what they called climate skepticism and the “pause” during the early 21st Century, contributed to a swinging of the political pendulum against the alarmist agenda. The end of the pause contributed to another swinging of the political pendulum in favor of the alarmist agenda. Ultimately, the continuing increase in global temperature, and whether the increase is accelerating, will be persuasive, and not arguments over past temperatures.