Is the famous global warming "hockey stick" graph, the one Al Gore used in his movie, actually fatally flawed, and why is the "spaghetti" graph an acceptable replacement?
Complicated question, but Reddit's scientist panel was unable to answer it, and none of my friends can talk me through the whole thing.
As most people involved in this debate probably know, Stephen McIntyre did a statistical analysis on the MBH98 "hockey stick" graph and claimed a number of flaws. Two of these which were verified as real errors by a U.S. congressional report are:
(1) The method used to calculate MBH98 generates hockey sticks even with random data. Quoting Wikipedia
: "The report stated that the MBH method creates a hockey-stick shape even when supplied with random input data (Figure 4.4), and argues that the MBH method uses weather station data from 1902 to 1995 as a basis for calibrating other input data."
(2) The data used as the basis of this method is scientifically flawed and in any case irreproducible. Wikipedia again: "The report said that MBH method creates a PC1 statistic dominated by bristlecone and foxtail pine tree ring series (closely related species). However there is evidence in the literature, that the use of the bristlecone pine series as a temperature proxy may not be valid (suppressing "warm period" in the hockey stick handle); and that bristlecones do exhibit CO2-fertilized growth over the last 150 years (enhancing warming in the hockey stick blade)."
In response to (1), the blog RealClimate, which is run by the "M" in MBH, posted an "unmanipulated
" chart which still included the bad data from (2).
In response to (2), a paper was written by Wahl and Ammann recalculated the data, but still used the method of (1). On page 63 of their paper
, you can see a number called r2
, which calculates the trustworthiness of the model for making predictions; at points on the graph, r2
Graduate student Linah Ababneh
failed to reproduce the bristlecone series. Nevertheless, RealClimate says for some reason that "including these data improves the statistical validation over the 19th Century period and they therefore should be included
When you combine (1) and (2), the graph no longer resembles a hockey stick at all. To their credit, both RealClimate and Wahl and Ammann have shown this graph in their work, but both of them say that their model is better, for a reason I can't understand. So, question number 1: I've explained this to the best of my knowledge, but I can't find what I'm missing. Is a method with r2
=0.000 really that awesome? I could probably draw random squiggly lines with better predictive ability than that.
My second question is a lot simpler: both RealClimate and Wikipedia have now phased out the hockey stick graph for a spaghetti graph
. Wikipedia says, "It is unknown which, if any, of these reconstructions is an accurate representation of climate history". Well, that much should be obvious. Why is there such variation, and doesn't that produce an untrustworthy margin of error? Why are there graphs out there that look like this
I don't mind if your answer is just a link to somewhere as long as it is discussing one of these two points.
Previous good general discussion here.
(In case anyone is worried for my salvation, regardless of the answer to this question I do think anthropogenic climate change can have a huge effect on the Earth, but I believe that it could be negative or positive
depending on how people go about it.)