Comments on: An Exchange at Scientific American

By: TCO

TCO — Tue, 20 Sep 2005 05:00:38 +0000

Do the bristlecones still produce a hockeystick curve with a “simple average” or a normal centered reconstruction vice the PCA cherrypicking method?

I agree that Nature should have peer-reviewed this Corrigendum given all the controversy and Mann’s defensiveness. And they should have published MM’s comment in some form. But MM still had a big effect and should not get discouraged. And they did the right thing by going to another peer-reviewed journal (GRL).

By: Bruce Alan

Bruce Alan — Sun, 31 Jul 2005 16:51:53 +0000

Thanks for mentally connecting CO2 fertilized growth rates as a significant factor in the fabrication of the hockey stick with the bristle cone data. I had just considered increased growth rate data as a factor to eventually balance CO2 emissions and offset the previous cries of gloom on plant growth with increasing CO2 levels. May I suggest that you "remind" the audience periodically, with 1-2 choice links that show the dramatic effect on plant growth and if possible, bristle cones. It might help newcomers and surfers passing by, perhaps a few ardent GWers. I found this link useful, but think a more "GW neutral" reference would be nice.

By: Michael Neibel

Michael Neibel — Sat, 25 Jun 2005 01:28:05 +0000

Thanks John. It did help. I’m going to look up the concepts you mentioned and study them.

By: Steve McIntyre

Steve McIntyre — Fri, 24 Jun 2005 23:39:49 +0000

One of my main points is to provide a very lowbrow explanation of the hockey stick. I don’t think that linearity/nonlinearity are the issues: the issue is the bristlecones. These have a hockeystick pattern, which is imparted to the temperature reconstruction because of non-robust statistical procedures. So the question is then – what causes the hockey stick in bristlecone pine growth? The answer is almost certainly 20th century fertilization by CO2 primarily, perhaps by nitrates as well. The issue IMHO is not nonlinearity, but simply leaving out a factor. Steve.

By: John Simon

John Simon — Fri, 24 Jun 2005 22:49:12 +0000

Addendum – I had a brain fade in the previous post. OLS is Ordinary Least Squares but it is also a linear estimator, sorry. (It’s BLUE (Best Linear Unbiased Estimator) if you really want to know)

By: John Simon

John Simon — Fri, 24 Jun 2005 22:46:32 +0000

Making linear assumptions is fairly common in statistics. The trick is making sure that the assumption is not ‘too bad’.

Allow me to explain somewhat. Most of statistics does linear regression (the workhorse is OLS – Ordinary Linear Regression). Some tricks can be done to ensure that this regression is correct even for non-linear functional forms. For example, taking logs of the data is one such trick Y=XY can be transformed into the linear regression log(Y)=log(X)+log(Y) (you can have exponents on the X and Y but I didn’t want to unnecessarily complicate this example).

Another trick/assumption is that you can linearise any non-linear function by taking a first order Taylor expansion. The point with this is that the approximation is ‘locally’ correct. However, depending on how important the other terms in the Taylor expansion are ‘local’ may be a very small area. Thus, in the present context, the question is whether the true temperature/proxy function is near enough to linear over the relevant range. Relevant is a rather vague term but at one level if you are talking about observations within the bounds of the data set then you are on safer ground than if you are extrapolating to values outside the bounds of your data set.

Hope this helps in some small way…

By: Michael Neibel

Michael Neibel — Fri, 24 Jun 2005 19:26:14 +0000

I feel like a fish out of water becaue I am not a scientist of any kind. But I’ve been studying philosophy for quite awhile and have been getting better at spotting an invalid concept when I see one. When I saw Mann’s HS, in my mind, I knew I was looking at flawed science on the premise that there can’t be a linear explanation for a non-linear system and that is how the HS appeared to me. I had no clue why the HS was wrong. I only knew that it could not be right and would advise others (if they asked) to disregard it as folly.

Now I know that non-linear data points can be plotted on a chart to reveal a linear line if the number of such points is sufficiently small, and that adding more data points makes the line disappear revealing the true non-linear nature of the data. My question is: is there anything in the nature of statistics whereby a nonlinear system has a linear explanation and still be valid/

By: Peter Bickle

Peter Bickle — Fri, 24 Jun 2005 11:35:52 +0000

David Appell is [such a strong Mann supporter - SM: I've toned this language down from post; I don't want to parse posts] it does not matter how much is said, he will alwats say the HS is gospel. Sure the earth is warming, but it has been a lot hotter over the eons.

By: Michael Jankowski

Michael Jankowski — Fri, 24 Jun 2005 03:03:39 +0000

The “it does not matter” cry has spread beyond the potential errors of MBH98 and/or the proxies and reached the hockey-stick itself. Now, “it does not matter” to many people if the MWP was real and warmer than today…you just have to believe that GHG emissions warmed the planet in the 20th century towards devastation and that comparing today to the MWP is apples vs oranges. It certainly makes for a convenient out should the hockey stick meet a fateful end.

By: Richard Lewis

Richard Lewis — Thu, 23 Jun 2005 22:30:04 +0000

The hockey stick not merely broken, but shredded? An interesting hypothesis is offered here:

http://service.spiegel.de/cache/international/spiegel/0,1518,357366,00.html