We have had a few posts at YARGB about the global warming controversy. I want to consider some information about Ross McKitrick’s article What is the Hockey Stick Debate About? In this article McKitrick addresses several factors that do not seem to get enough play in the media when discussing global warming. I think that the article is somewhat technical, although not beyond the reach of most readers interested in devoting some time to this. I want to summarize his points in a less technical manner. Many of his concerns involve statistics. I have a pretty good background in statistics, having consulted with many psychology students doing dissertations, and I am actively engaged in psychological research. This does not make me infallible, but I have some expertise to lend to that aspect of his thesis. A counterargument to McKitrick is contained on the Real Climate site.
1) The now famous Hockey Stick graph appears to show the earth’s climate as very stable from the years 1000-1900, and then veers upward sharply in the 20th century. Interestingly the projections for the next hundred years all have a clearly linear appearance (i.e., close to being straight lines going up at various angles). It has been ubiquitous and, according to McKitrick, the primary evidence in crafting the Kyoto recommendations.
2) Scientists try to figure out past climate various through various means including temperature proxies, or substitutes, as well as ground borehole data. Older studies had shown a medieval warm period, based on borehole evidence (i.e., literally drilling holes into the earth).
This warm period is problematic for adherents of global warming. The Mann data, upon which the hockey stick is based, used multiple proxies for temperature. This study indicated no such medieval warm period. Stephen McIntyre found a number of errors in the data, and he could not recreate the principle components (a statistical technique that seeks to simplify a vast data array into the most essential components). According to the author Mann refused to provide them with the computer code he used to create the statistical analyses published in Nature.
3) McKitrick suggests that Mann made errors in putting the statistics together. This resulted in a bias being created for a Hockey Stick-like trend in the 20th century. He suggests that the Mann study actually “data-mines” for hockey sticks.
4) Many of thee proxies used in the Hockey Stick study are very problematic, according to McKitrick. In fact experts that study some of these proxies (called bristlecone pine) have clearly indicated that they should not be used as climate proxies. It so happens that these proxies falsely show large temperature increases in the 20th Century (contradicting other data gathered in geographic proximity to the bristlecone pines). Drop that data and the Hockey Stick goes with it.
5) In the Hockey Stick study, there were insufficient proxies used to calculate many of the temperatures in the medieval period. Extrapolations were used (i.e., guesses, albeit sophisticated). Remove the poor data sources and redo the analysis and the medieval warm period reappears and greatly influences the look of the overall graph.
You can look at this comparison between the two studies. Now, there are arguments back and forth, and this can be very technical information. However, how many journalists have the expertise to report on this and do so fairly? Obviously this doesn’t settle the matter, but climatology is extremely complex, but so is statistical modeling, in any discipline. And with any statistics, GIGO is always relevant.