Skip to comments.GLOBAL WARMING BOMBSHELL: Hockeystick Broken
Posted on 01/13/2005 4:20:13 PM PST by neverdem
A prime piece of evidence linking human activity to climate change turns out to be an artifact of poor mathematics.
Progress in science is sometimes made by great discoveries. But science also advances when we learn that something we believed to be true isn't. When solving a jigsaw puzzle, the solution can sometimes be stymied by the fact that a wrong piece has been wedged in a key place.
In the scientific and political debate over global warming, the latest wrong piece may be the "hockey stick," the famous plot (prominently displayed by the IPCC report, 2001), published by University of Massachusetts geoscientist Michael Mann and colleagues. This plot purports to show that we are now experiencing the warmest climate in a millennium, and that the earth, after remaining cool for centuries during the medieval era, suddenly began to heat up about 100 years ago--just at the time that the burning of coal and oil led to an increase in atmospheric levels of carbon dioxide.
I talked about this at length in my December 2003 column. Unfortunately, discussion of this plot has been so polluted by political and activist frenzy that it is hard to dig into it to reach the science. My earlier column was largely a plea to let science proceed unmolested. Unfortunately, the very importance of the issue has made careful science difficult to pursue.
But now a shock: independent Canadian scientists Stephen McIntyre and Ross McKitrick have uncovered a fundamental mathematical flaw in the computer program that was used to produce the hockey stick. In his original publications of the stick, Mann purported to use a standard method known as principal component analysis, or PCA, to find the dominant features in a set of more than 70 different climate records.
But it wasn't so. McIntyre and McKitrick obtained part of the program that Mann used, and they found serious problems. Not only does the program not do conventional PCA, but also it handles data normalization in a way that can only be described as mistaken.
Now comes the real shocker. This improper normalization procedure tends to emphasize any data that do have the hockey stick shape, and to suppress all data that do not. To demonstrate this effect, McIntyre and McKitrick created some meaningless test data that had, on average, no trends. This method of generating random data is called "Monte Carlo" analysis, after the famous casino, and it is widely used in statistical analysis to test procedures. When McIntyre and McKitrick fed these random data into the Mann procedure, out popped a hockey stick shape!
That discovery hit me like a bombshell, and I suspect it is having the same effect on many others. Suddenly the hockey stick, the poster-child of the global warming community, turns out to be an artifact of poor mathematics. How could it happen? What is going on? Let me digress into a short technical discussion of how this incredible error took place.
In PCA and similar techniques, each of the (in this case, typically 70) different data sets have their averages subtracted (so they have a mean of zero), and then are multiplied by a number to make their average around that mean to be equal to one; in technical jargon, we say that each data set is normalized to zero mean and unit variance. In standard PCA, each data set is normalized over its complete data period; for the global climate data that Mann used to create his hockey stick graph, this was the interval 1400-1980. But the computer program Mann used did not do that. Instead, it forced each data set to have zero mean for the time period 1902-1980, and to match the historical records for this interval. This is the time when the historical temperature is well known, so this procedure does guarantee the most accurate temperature scale. But it completely screws up PCA. PCA is mostly concerned with the data sets that have high variance, and the Mann normalization procedure tends to give very high variance to any data set with a hockey stick shape. (Such data sets have zero mean only over the 1902-1980 period, not over the longer 1400-1980 period.)
The net result: the "principal component" will have a hockey stick shape even if most of the data do not.
McIntyre and McKitrick sent their detailed analysis to Nature magazine for publication, and it was extensively refereed. But their paper was finally rejected. In frustration, McIntyre and McKitrick put the entire record of their submission and the referee reports on a Web page for all to see. If you look, you'll see that McIntyre and McKitrick have found numerous other problems with the Mann analysis. I emphasize the bug in their PCA program simply because it is so blatant and so easy to understand. Apparently, Mann and his colleagues never tested their program with the standard Monte Carlo approach, or they would have discovered the error themselves. Other and different criticisms of the hockey stick are emerging (see, for example, the paper by Hans von Storch and colleagues in the September 30 issue of Science).
Some people may complain that McIntyre and McKitrick did not publish their results in a refereed journal. That is true--but not for lack of trying. Moreover, the paper was refereed--and even better, the referee reports are there for us to read. McIntyre and McKitrick's only failure was in not convincing Nature that the paper was important enough to publish.
How does this bombshell affect what we think about global warming?
It certainly does not negate the threat of a long-term global temperature increase. In fact, McIntyre and McKitrick are careful to point out that it is hard to draw conclusions from these data, even with their corrections. Did medieval global warming take place? Last month the consensus was that it did not; now the correct answer is that nobody really knows. Uncovering errors in the Mann analysis doesn't settle the debate; it just reopens it. We now know less about the history of climate, and its natural fluctuations over century-scale time frames, than we thought we knew.
If you are concerned about global warming (as I am) and think that human-created carbon dioxide may contribute (as I do), then you still should agree that we are much better off having broken the hockey stick. Misinformation can do real harm, because it distorts predictions. Suppose, for example, that future measurements in the years 2005-2015 show a clear and distinct global cooling trend. (It could happen.) If we mistakenly took the hockey stick seriously--that is, if we believed that natural fluctuations in climate are small--then we might conclude (mistakenly) that the cooling could not be a natural occurrence. And that might lead in turn to the mistaken conclusion that global warming predictions are a lot of hooey. If, on the other hand, we reject the hockey stick, and recognize that natural fluctuations can be large, then we will not be misled by a few years of random cooling.
A phony hockey stick is more dangerous than a broken one--if we know it is broken. It is our responsibility as scientists to look at the data in an unbiased way, and draw whatever conclusions follow. When we discover a mistake, we admit it, learn from it, and perhaps discover once again the value of caution.
Richard A. Muller, a 1982 MacArthur Fellow, is a physics professor at the University of California, Berkeley, where he teaches a course called "Physics for Future Presidents." Since 1972, he has been a Jason consultant on U.S. national security.
However, your decision that the paper has a "devastating" number of errors is meaningless. Unless you have suddenly become a scientific reviewer in the field which doesn't seem to be the case. The purpose of this thread is to attack the hockey stick. That has been successful as you have done little to defend it. You have (to reiterate) raised useful concerns about Soon and others who attack the hockey stick, but you have presented little to support the hockey stick except some repetition of Mann's statistics.
The "complexity" of those statistics (which you first used as a sword and then as a shield) makes it impractical to critique them. But there are two ways in which Mann has been critiqued here:
First you have cast doubt on the use of temperature proxies by everyone but Mann and Mann's use is the weakest of anyone's (let's not get distracted by the 98 paper, it only looks at LIA and the 99 paper is sorely lacking in data).
Second we have presented ample evidence from temperature oriented proxies that the MWP was at least as warm as today. You have poked a few holes in that assessment but have not come close to refuting it. You have yet to provide an indication that there was cooling during MWP to counterbalance the substantial warming observed in N America and Europe. That warming greatly exceeds today's climate and with the addition of other global warming evidence clearly breaks the hockey stick.
Fine with me. I haven't found the paper so if you have a link please post it.
It's better to debate these source papers than reviews like Mann's or Soon's so we don't get caught up in "what he said" games. I am interested in the facts not in reparsing Soon's words. But try to keep in mind there are a lot of papers representing a lot of climate anomalies including a fair number pointing to warming for MWP equal to or greater than today's. You are more than welcome to point out any that point to less warming or cooling in the MWP anywhere on earth, but naturally those should be source analyses of temperature proxies. In other words reread WOSG's post 101 and answer his points without evading the issue like you did in post #105.
In other words you have yet to show ANY evidence that temperatures were cooler in the MWP anywhere than they are today.
Averaging 3 centuries to produce one number and comparing that to two decades of above-normal surface temperature measurements (with a well known 0.1 to 0.15 degree bias)?
Comparing a reference year in one study to reference years in others which use different measurements and calculate global average differently? Nice bit of 1900ism there.
There were certainly decades in the MWP that exceeded the extremes of the 1980's and 1990's, that's easy to establish from the long term averages being higher and what we observe about short term variations. You have yet to show a single study showing the MWP was cooler than the 1900's which would call that into question. On the other hand, I and the others have presented global temperatures proxies to show that the MWP was not isolated to the north Atlantic. Yet all you do is blame eurocentrism for the overemphasis on MWP and wave around your non-quantifiable (in terms of temperature offsets) ocean current local effects as the other cause.
On the other hand I admit it is easy to get caught up in Viking stories. But I will keep waiting for your evidence that MWP was cooler than the 1900's comparing averages to averages.
But suffice to say it's not thousands of reporting stations worldwide, weighted for area and averaged. Because of that fact, the extremes like 1998 won't show up. You are welcome to point to some error bars and repeat (but not explain) some statistics, and I will try to critique it.
Still waiting for some temperature proxy measurements showing average (about a half century or more) cooler conditions anywhere on earth during the MWP. So far the Vikings are winning.
I have looked at 3 so far, jones1998 shows warming anomalies in both hemis from 1960 onwards but the readme says that the 1961-1990 anomalies have been altered in a way I don't understand (yet).
Crowley2000 is also mentioned in Mann's paper and has data that has been thoroughly manipulated using a simulation to remove all forcing effects except those from greenhouse gases. There's no testing of the CO2 forcing hypothesis and therefore no proof of anything.
Evans2002 (mentioned by Soon) only goes back into the LIA, but shows many warmer decades than any in the 1900's. Unfortunately this is based on tree rings and there's no indication of the method used to determine temperature.
I have a lot more work to do, but so far the hockey stick looks like an artifact of assumptions and methodology and is not supported by the raw data.
Briffa1998 (files labeled 2001) was quoted by Soon as showing the 20th century to be warmer than any of the previous 5. The data is interesting though, it shows NH growing season average temperatures from tree rings as having most of that 20th century warmth in the 1930's and 40's.
So this seems to disprove part of your beliefs above, although you will no doubt cling tightly to them.
I believe that the MWP may well be warmer than the early part of the 20th century, roughly on par with the average temperature of the 20th century and slightly cooler than the recent decades (say 1985 onwards).
The data shows that there was warming in 1930's and 1940's. Your comment is at least leaning heavily.
But there has not been cooling since. There was a period of very slight cooling about 1940 - 1980, followed by a greater increase of warming. The warming since 1980 has been about 0.4C, lots of room for a MWP.
Nonetheless, it is likely that Crowley used the same data tweaking in both papers. The dataset I downloaded is tweaked and can't be used to compare current proxy measurements with past ones. I'll keep downloading data sets and posting conclusions for all the ones I download no matter what the results, but I can't draw conclusions when data has been manipulated.
0.4 C warming (subtracting the 0.1 to 0.15 degree measurement bias) is hardly unusual in the climate record. If you look at ANY of the raw data (untweaked) you will see one or two decades of dramatic warming all over the place including the LIA.
The dataset is from Crowley's paper, but it is the Crowley-Lowery dataset. That's what it says in the readme file. My appeal to probability seems sound in this case.