GLOBAL WARMING BOMBSHELL: Hockeystick Broken

MIT Technology Review ^ | 15 October 2004 | Richard Muller

[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.

[glock rocks]

Awesome! Bookmarked.

[Yelling]

If that is a Daly article, then he mixed up the two papers that Mann produced on the topic. I don't doubt John's commitment to his site and views but his science was pretty shaky at best.

[Yelling]

I am not aware of your source for the "it implies temperatures are higher in 20th century than at any time in 1000 years" so I may be off here. However I have always understood it to show that the warming in the latter part of the 20th century is where the warming is showing. Some "skeptics" used to early part to show it wasn't warmer.

In regards, to Mann's response, if Mann did nothing then people would say that M&M are right because Mann can't refute them (that happened with the initial article). So by your analysis, he is wrong if he doesn't reply or is defensiveness if he is.

[Yelling]

In regards to your link Here Comes the Sun, you may want to check out some of the comments on Veizer and Shaviv's work. Here is a good one to start with: Reply to V & S

[yoe]

(World Data Center for Paleoclimatology)

Climate changes through the centuries mapped by trees are a fascinating study and likely, the most accurate.

[yoe]

"Abstract A compilation of paleoclimate records from lake sediments, trees, glaciers, and marine sediments provides a view of circum-Arctic environmental variability over the last 400 years. From 1840 to the mid-20th century, the Arctic warmed to the highest levels in four centuries. This warming ended the Little Ice Age in the Arctic and has caused dramatic retreats of glaciers, melting of permafrost and sea-ice, and alteration of terrestrial and lake ecosystems. Although significant warming, particularly after 1920, was likely due to increases in atmospheric trace-gases, the initiation of the warming in the mid-19th century suggests that increased solar irradiance, decreased volcanic activity, and feedbacks internal to the climate system played roles."

http://www.ngdc.noaa.gov/paleo/sciencepub/front.htm

[WOSG]

"I am not aware of your source for the "it implies temperatures are higher in 20th century than at any time in 1000 years" so I may be off here."

You only have to (A) look at the graph and (B) look at how the results were represented in places like IPCC reports t osee this. Where is the well-known 'medeival warming period' in the graphs? Why did they disappear?

[WOSG]

"If that is a Daly article, then he mixed up the two papers that Mann produced on the topic."

You havent bothered reading the article. He's not attacking Mann per se, but examining the actual temperature records to see how they fit these 'hockey stick' reconstruction generally; others have taken Mann and 'run with it' in particular the IPCC. His key figure is not the Mann figure but the US assessment report that is based on 'hockey stick' reconstructions (Fig 5). The climate data he presents contradicts the hockey stick constructions.

versus data such as ...

Or evidence such as:

" They [Hong et al, studying Chinese climate] estimate the temperature between 1100 and 1200 AD to have been some 2°F warmer than it is today. This matches the Medieval Warm Period and is confirmed by plant remains from species that normally found in southern China. They ascertain very cold temperatures between 1550 and 1750, matching the Little Ice Age. They also see a solar connection in these climate changes. A carbon 14 solar proxy correlates with the oxygen 18 temperature proxy. In other words, sun caused Chinese climate changes."

Or his quote from a South African: "The climate of the interior of South Africa was around 1°C cooler in the Little Ice Age and may have been over 3°C higher than at present during the extremes of the Medieval Warm Period. "

This data is not reflected in the 'hockey stick' reconstructions, even though Daly points out many climate data points showing a multi-degree swing from medieval warming period to 'little ice age', and showing present-day temps *under* that medeival warming period...

Mann shifted both proxies used and time scales (50-year smoothing to 10-year smoothing) and a 'hockey stick' results. How much is an artifact of the smoothing and proxy differences vs something real? How do you account for the disappearance of a well-known climate trend in his reconstuctions, ie, the Medieval warming period?

Coup de grace: Daly's article points out this choice quote from the US National Assessment: "New studies indicate that temperatures in recent decades are higher than at any time in at least the past 1,000 years." ... Go Mann's website and he calls that one of the "myths" of climate change theory. What gives? Mann recognizes that the science doesn't support that conclusion, stated in these 'assessment reports' that in turn are 'based' on Mann's work. Yet Mann makes it sound like skeptics are attacking strawmen, and not the very real over-generalizations that these politicized reports engage in. If Mann says this is a 'myth' is the U.S. assessment report wrong???

[WOSG]

Thanks... what is GISP2 data? What data points/proxies were used to get that temp data?

[WOSG]

I don't need to say "implies" se previous reply ... U.S. Assessment took Mann's work and on that basis (incorrectly) stated directly:

"New studies indicate that temperatures in recent decades are higher than at any time in at least the past 1,000 years. (Overview p.11)" See
http://www.greeningearthsociety.org/Articles/2000/hockey.htm

I leave it to you to decide if this was unwarranted inference or not based on Mann's work. The graph seems stark, but then the U.S. assessment took out the wide 'error bars' in Mann's original report...

Mann's "Myth #3":
MYTH #3: The "Hockey Stick" studies claim that the 20th century on the whole is the warmest period of the past 1000 years.
http://www.realclimate.org/index.php?p=11

"Abstract: The data set of proxies of past climate used in Mann, Bradley and Hughes (1998, “MBH98” hereafter) for the estimation of temperaturefrom 1400 to 1980 contains collation errors, unjustifiable truncation or extrapolation of source data, obsolete data, geographical location errors, incorrect calculation of principal components and other quality control defects. We detail these errors and defects. We then apply MBH98 methodology to the construction of a Northern Hemisphere average temperature index for the 1400-1980 period, using corrected and updated source data. The major finding is that the values in the early 15th century exceed any values in the 20th century. The particular hockey stick chape derived in the MBH98 proxy construction -- a temperature index that decreases slightly between the early 15th century and early 20th century and then increases dramatically up to 1980 -- is primarily an artefact of poor data handling, obsolete data and incorrect calculation of principal components. "
http://www.uoguelph.ca/~rmckitri/research/trc.html

Thus the M+M critique is not simply the PCA, but the underlying data... which is what others (eg Daly) were questioning too ... There are better proxies than tree rings, apparently.

And Mann and M+M are IN AGREEMENT THAT LATE 20TH CENTURY IS NOT WARMEST PERIOD IN LAST 1000 YEARS, both contradicting the 'assessment' reports.

We may have the horrible conclusion that global warming, even if real, is simply bumping the world up slightly to a temperature that the world enjoyed in 1100AD, when the Vikings colonized Greenland.

[WOSG]

"Mann takes his science seriously and dreads the consequences of global warming."

The two are wholly uncorrelated if not anti-correlated.

"He's countering bad arguments in a way he feels the public can understand."

Um, he's going outside peer-reviewed forums to say stuff like "hey, this wasn't accepted via peer review so it's no good"... hmmmmm. I can understand that, and smirk at the irony of it.

"Specifically, he says MM's statistical analysis is flat out wrong."

Wow, what a shocker, scientist doesnt recant his own work when it's critiqued. I too can't follow the statistics to know who's right, but there is more beyond the statistics to question. See my other post, viz. Daly article. Direct data does show a trend, but in many cases not hockey-stick like.

The peers did *not* support him on the merits so much as fail to see the merit of the critique in a particular journal. A critique like that has to be bullet-proof, but it cant be tossed out-of-hand on that basis.

[Yelling]

GISP2 - Greenland Ice Sheet Project - more here: http://www.gisp2.sr.unh.edu/GISP2/

[Hardastarboard]

But I just watched "Water World". Global warming is real. I saw the effects in a movie.

[Yelling]

In fact, I have read the article in a fair amount of depth. I did (do) have respect for John's dedication so I feel that it deserved that much.

However as well as reading the article, I also like to read the references that he links to. As well as the hockey stick, you showed two graphs there. The first on temperatures from the Sargasso Sea, the second from West African SST.

Looking at the Sargasso Sea graph we can see that it comes from a paper called The Little Ice Age and Medieval Warm Period in the Sargasso Sea by Keigwin. However Daly seemed to leave out this work (by the same author): Slope water current over the Laurentian Fan on Interannual to Millennial Time Scales. This is interesting since the records in that paper show a record opposite to the Sagas Sea record (warming during the little ice age and cooling during the MWP) and lead the authors to conclude that the temperature changes are due to current changes, possible related to the NAO.

The second diagram is from this paper: Coherent High- and Low-Latitude Climate Variability During the Holocene Warm Period (found here ). The authors conclude that there were changes in the wind patterns and that ... "The strengthened monsoonal wind field evidently supported stronger upwelling as documented by the greatly increased G. bulloides abundances (Fig. 2).".

So according the the author of the first paper the Sargasso Sea changed temperature because of a shift in ocean currents. According to the second a shift in winds caused more upwelling along the coast and thus cooler water temperature. So, neither supports the idea that global temperatures were warmer during the MWP or cooler during the LIA. Yet Daly uses them to show that. Bad science at the best, fraud at the worst.

In regards to Daly's understanding of the papers that Mann wrote, the "Coup de grace" is that he only references Mann's 1999 paper, NOT Mann's 1998 paper (unless his reference 17 is supposed to, but he never refers to reference 17 in the text - you see I did read it). Mann's 1999 paper is only good for the Northern Hemisphere yet 1/2 of Daly's data sets are from the Southern Hemisphere.

[Yelling]

I am not following you here. The Myth 3 you reference (by Mann) says the following:

" On the other hand, in the context of the long-term reconstructions, the early 20th century appears to have been a relatively cold period while the mid 20th century was comparable in warmth, by most estimates, to peak Medieval warmth (i.e., the so-called "Medieval Warm Period"). It is not the average 20th century warmth, but the magnitude of warming during the 20th century, and the level of warmth observed during the past few decades, which appear to be anomalous in a long-term context"

So he is saying that the latter decades are where we see warming that is greater than other times during the last 1,000 years. This appears to be what your quote from the US Assessment says.

In regards to McKitrick's work, what you quote is from his early paper on it. After Mann published to correct some errors in his references, M&M have looked mostly at the PC analysis. After according to McKitrick's analysis, the start of the Little Ice Age is warmer than current times.

Oh, and by-the-way, as per my comments above, Mann and McKitrick do not agree "THAT LATE 20TH CENTURY IS NOT WARMEST PERIOD IN LAST 1000 YEARS"

Anyway, that was fun. I look forward to your next post addressing the points I raised.

Regards,
Yelling

[Straight Vermonter]

Thanks... what is GISP2 data? What data points/proxies were used to get that temp data?

Greenland Ice Sheet Project 2.

[WOSG]

Mann is contradicting the U.S. assessment directly.
He calls the statement they made a MYTH.

what is 'warming'? ... a trend. do not confuse the rate of change with the temperature level itself. You claim to be reading this stuff carefully, so reread the sentence.

There was a lot of 'warming' in the 20th century, from a base lower than average temperatures seen 800 years ago. That does NOT support U.S. assessment claims of 'highest temperature'. Read Mann's verbiage, it's practically Clintonesque in *implying* warmth (high temperature) but not saying it. "warming" does not equate to "high temperature".

Thus ...

"Mann and McKitrick do not agree "THAT LATE 20TH CENTURY IS NOT WARMEST PERIOD IN LAST 1000 YEARS" "

Who doesn't agree with this statement?
Who wants to claim that present-day earth really is warmer than back in the days the Vikings colonized Greenland?

[WOSG]

"If that is a Daly article, then he mixed up the two papers that Mann produced on the topic."

That's not right. He quoted the northern hemisphere paper and cited it as northern hemisphere data. Again, you miss the point, his target was the U.S. assessment conclusions.

[WOSG]

"Looking at the Sargasso Sea graph we can see that it comes from a paper called The Little Ice Age and Medieval Warm Period in the Sargasso Sea by Keigwin."

Fine, yes. Here is the abstract, which buttresses what Daly was using it for ...

"Sea surface temperature (SST), salinity, and flux of terrigenous material oscillated on millennial time scales in the Pleistocene North Atlantic, but there are few records of Holocene variability. Because of high rates of sediment accumulation, Holocene oscillations are well documented in the northern Sargasso Sea. Results from a radiocarbon-dated box core show that SST was approximately 1deg C cooler than today approximately 400 years ago (the Little Ice Age) and 1700 years ago, and approximately 1 C warmer than today 1000 years ago (the Medieval Warm Period). Thus, at least some of the warming since the Little Ice Age appears to be part of a natural oscillation."

... thus supporting Daly's point.

" However Daly seemed to leave out this work (by the same author): Slope water current over the Laurentian Fan on Interannual to Millennial Time Scales. This is interesting since the records in that paper show a record opposite to the Sagas Sea record (warming during the little ice age and cooling during the MWP) and lead the authors to conclude that the temperature changes are due to current changes, possible related to the NAO."

You are confused. Why does referencing one author's work require you to reference another? How does authors' conclusion in a separate paper matter?
And since when does the fact that currents and winds changed over the same time scale contradict temperature change?
You seem confused about cause-and-effect here.
You seem to think that current changes and wind change somehow disprove that the climate changed on a big scale prior to the 'hockey stick' era ... of course the opposite is true. Both indicate that there are larger scale changes in the climate that result from 'natural oscillations' as the Keigwin paper puts it. Thus, even if the Sargasso sea temperature were due to an oscillation in sea currents, it fits Daly's hypothesis better.

And if you dismiss this data point, you'd miss the point that Daly was making: these observations were not in one local area ... this was ONLY ONE DATA POINT! He presents many of them, and they all line up with a clear story of wider variations across many different locations than the 'hockey stick' seems to allow for.

Even if the sargasso sea were due to current changes, those current change themselves were due to wider and longer climate variations than the 'hockey stick' showed. Add in Kenyan lake, China, Tasmania, South Africa etc. etc. not to mention the european data, and you have a trend.

"So according the the author of the first paper the Sargasso Sea changed temperature because of a shift in ocean currents. According to the second a shift in winds caused more upwelling along the coast and thus cooler water temperature. So, neither supports the idea that global temperatures were warmer during the MWP or cooler during the LIA."

Your reading comprehension is pretty poor, I must say.
The first paper you mention, in its abstract says that termperatures were measure 1 degree C warmer than today ...
The second paper YOU CITE says: "The most recent of these events was the Little Ice Age, which occurred between 1300 to 1850 A.D., when subtropical SSTs were reduced by 3 to 4 C. " ... so papers that speak directly of these wide-range temperature reductions and increases over the past 1000 years somehow contradicts Daly's attempt to MAKE THE SAME POINT about temperature variation?

Of course, feel free to argue that these data points, and a dozen more like it, dont 'prove' the variations really exist. Fine. But global warming skeptics can say the same thing of the hockey stick itself and about the current claimed 'warming'... 'heat island', "CO2 fertilization" and plenty of other effects to pour doubt onto climate measurements ... even more so on the 'climate models' which are woefully defecient in data to 'prove' much.

That latter citation shows the changes in temperature and the changes in solar radiation as well that may cause it, over the time scale of thousands of years. Why would it be obvious that solar changes affect us on some time scales (day/night, the 4 seasons, 10,000 years) but you would dismiss that impact for other time scales (ie the 1000 year time frame)?

If you look at Daly's whole article, he points out how solar activity, both radiation levels and sunspot activity, indicate the 'forcing' that led to the measured long-scale temperature variations in the last 1000 years.
Daly was able to correlate the solar activity both with the reported temperature changes over 1000 years while ALSO explaining much of the 20th century temperature changes.
The data is both more direct and more consistent with wider data sets than the dubious "flat line" over 1000 years that the hockey stick shows.

"In regards to Daly's understanding of the papers that Mann wrote, the "Coup de grace" is that he only references Mann's 1999 paper, NOT Mann's 1998 paper (unless his reference 17 is supposed to, but he never refers to reference 17 in the text - you see I did read it). Mann's 1999 paper is only good for the Northern Hemisphere yet 1/2 of Daly's data sets are from the Southern Hemisphere."

Non sequitor. See my other reply on this. Daly was not attacking a specific paper, but the 'hockey stick' myth as a whole and especially as constructed in the U.S. assessment (and then in IPCC report), which was based on Mann's work, but degraded it by extrapolating it wrongly and constructing hypotheses unsupported by even what Mann had done.

[Yelling]

You said. “Mann is contradicting the U.S. assessment directly. He calls the statement they made a MYTH. “
I don’t agree with how you interpret the statements.

The US Assessment says: “studies indicate that temperatures in recent decades are higher than at any time in at least the past 1,000 years." Mann says
“the level of warmth observed during the past few decades, which appear to be anomalous in a long-term context.” What he calls a myth is that some claim his work calls the 20th century the warmest, but that is clearly not so. Only the recent decades are.

Mann does not word things the way I would but he is quite clear. Also, you make the point "warming" does not equate to "high temperature". I assume that by this you are saying that warming refers to the rate which I can agree with. But Mann uses the word “warmth” when describing the recent decades, NOT “warming”. Again, that is quite clear.

You ask who doesn't agree with the following? "THAT LATE 20TH CENTURY IS NOT WARMEST PERIOD IN LAST 1000 YEARS". Just about every climatologist disagrees with that statement. The late 20th century is warmer than previous times during the last 1000 years (on a global scale).

You also ask: “Who wants to claim that present-day earth really is warmer than back in the days the Vikings colonized Greenland?”. Again, I would say most climatologists say that. Don’t confuse local with global.