Things I took for granted: tree rings are accurate temperature proxies

The biggest hurdle I took in my quest to understand the global warming story was a graph called the Hockey Stick. It represents the temperatures over the last 1,000 years. I believed what I was seeing and saw this as a proof of the current anthropogenic global warming. I found it everywhere and was done by scientists, so naively I thought it had to be correct. If temperatures stayed stable for about 1,000 years and the last hundred years took of like a rocket, how much proof do one need to have, considering that carbon dioxide was a greenhouse gas and we emit loads of it into the atmosphere.

Tree-rings showed how warm or cold the climate was, I had no real problem with that, I learned that in primary school. The higher the temperature, the wider the tree-ring. The lower the temperature, the smaller the tree-ring. Just count and measure them and you are done. This was confirmed by scientists who declared that Bristlecones pines were good proxies for temperatures of the past because they live long.

There was the word: proxies. Thousand years ago there were no thermometers. The temperatures back then were measured not by instruments that measure temperature, but by something that is influenced by temperatures. That is a proxy. Temperatures have an influence on the width of tree-rings. True, but are they good indicators of past temperatures?

Looking at the background. Trees are complex organisms. They react on temperature, sure, but also on a bunch of other things. Beside temperature they react on:

• precipitation
• nutrients
• disease
• wind
• sunlight
• competition with other trees
• competition with animals
• local variations
• concentration of carbon dioxide in the air
• events like storms, lighting,…
• and probably many, many more…

That makes it different from thermometer. Thermometers measure temperature via the expansion/contraction of a substance, which is representative for the current temperature. There is a direct relation between the expansion/contraction and the temperature. Hence the ability to measure temperature.
On the other hand, temperature has an influence on the width of tree rings, but this is not direct. The tree rings translate the temperature signal, but also those other signals. The temperature signal is diluted in the other signals, there will be a lot of noise in the tree rings. Getting rid of the noise and distilling only the temperature signal will not be possible if nothing is known about the other signals.

There are other things: datasets from trees are sparse. There aren’t that many very old trees. How to compare with modern datasets with real thermometers? Temperature readings of thermometers are read every day at least two times, sometimes a reading every hour. Tree rings one a year. Tree-rings depend on the tree and the period it lived.
A lot of fuss about sparse data, where did I hear that before?

Back to my own story: the Hockey Stick was the difficult hurdle to take. It was difficult because I began to realize that the media brought one sided information about the climate, but wasn’t that far to realize that it was necessary to be critical, not just assume that something was true because the majority thinks so.

Why did I took it for granted? It is a combination of several things:

• I just looked at the graph and what it meant seemed obvious. I even saw it as proof that humans were causing global warming.
• I trusted science. I had no reason to believe the scientists were biased in any way or that the information that reached me was one-sided.
• The graph was found everywhere I searched for historical temperature data. I had no reason to think this could be one-sided information, it seemed straight forward.
• It was presented as something evident: there was no doubt about this. For example scientists state that “tree rings are a good proxy” because they live long. I didn’t realize that he probably meant a proxy in time, not necessarily a reliable proxy for temperature. Big difference.
• The basis looks simple and straight forward: warmer: bigger rings. colder: smaller rings. It didn’t seem rocket science. What I forgot to take into account was that a tree is a living thing and reacting on the many influences in its environment. It was brought too simple, but a little bit of thinking would have discovered flaws in the reasoning.

When I think back about this period, I ask myself the big question: how could I ever believed tree rings are thermometers in disguise? How could I ever have believed this stuff?

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What difference does it make anyway?

In the previous two post I looked at the original hockey stick and its very last incarnation. Both hockey stick shapes seemed to be artifacts of the methods used, not of the underlying data. But, you could say, “Even if this uptick doesn’t follow from the data in those two graphs, we measure surface temperatures more than a century and the way the temperatures go is up. If the hockey stick graph doesn’t tell the story, the measurement data surely do! So what difference does it make anyway?”.
I seen this remark popping up at several discussions. At first I was puzzled by such statements, but now I think it fails to take into account what is really at issue. Let’s look into it in more detail.

There are two data sets in play here. The first is the proxy data set, which consists of proxy data like tree rings (Mann’s hockey stick) or ocean sediment core data (Marcott’s hockey stick). The second set is the instrumental record which consists of temperature measurements with thermometers.

Proxy data is NOT real temperature data. Previously I assumed it was, because I knew that for example in a good year the rings of a tree will be wider than in a colder year. Although this is definitely true, it is also true that there are other influences on tree rings like moisture, nutrition, diseases, pests, competition with other plants/trees, interactions with wildlife, weather events and who know how many other elements that are important in the health of that tree. In that sense, the width of the tree ring is not only dependent on temperature, but also on these other influences. This means the temperature signal is diluted in the proxy data and not directly comparable with real temperature data. What could be said is that the conditions for that tree were better or worse during time, not necessarily that temperatures went up or down. This proxy data will consist of a temperature signal, but it will be noisy data (the temperature signal probably is a big part of it, not necessarily a constant part).

Thermometers on the other side have a very good temperature signal. When the temperature goes up, the substance they contain (alcohol, mercury, metal) will expand. When they cool, that substance will contract. The higher the temperature, the bigger the expansion. The lower the temperature, the bigger the contraction.
After the measurements, it becomes more complicated with issues like the UHI (Urban Heat Island) effect on the measurements and the further processing of this data (do they really are representative for global or Northern Hemisphere temperatures), but that is a different story altogether.

Another issue in this comparison is the resolution. For example, the Marcott hockey stick has a resolution of more than 300 years. The instrumental record has a resolution that is much higher and could be described with a resolution of one day. Even if we bring that to a year, even 10 or 20 years, it is a much higher resolution than the proxy data set. If the instrumental record data were somehow put behind the proxy data and treated the same way as the proxy data, it would been barely 1 (one) measly point and probably not even placed high in the graph either.

As far as I know, there is no dispute that the world has being warming since 160 years. Temperatures are being measured for some time now and although we are now in a flat-lined region, generally the trend since 1850 was upwards. But that is not what these two hockey stick graphs were trying to say here. The issue they want to prove is that the last century is unusually warm compared to previous eras. According to their statements it hasn’t happened in let’s say the last 1,000 (Mann’s hockey stick) or 11,300 years (the Marcott hockey stick) and therefor it has the human fingerprint all over (because of humans emitting more and more CO₂ into the atmosphere).

Let’s keep focus on what is really being said here. At issue in the hockey sticks is the uniqueness of the warming, not the fact that it warmed. We already know it warmed, but we don’t know if this didn’t happen before and the data given by these two studies are not sufficient to base that conclusion on. Even if it would have happened in the past, these methods will not be able to show this. When this uniqueness within the long time frame doesn’t follow from the data, it makes no sense to prove this with the incredibly short data set we have with the instrumental record.

Another issue that came to light with the Marcott paper: making the claim that the last 100 years are unprecedented (in the press release) and later saying the non-robustness of the last 100 years doesn’t matter because the instrumental record could well prove it (in the FAQ), is not really honest. The claim made was exactly about that non-robust data, when in reality the data of the graph was not saying much about the last 100 years, even seem to conclude that this data is useless for this current period. If the available evidence doesn’t support a claim, then one shouldn’t make that claim.

Returning from this to the initial question: what difference does the non-correctness of the last part of the hockey stick graphs make, because we know the earth has warmed the last 160 years anyway? As seen above, that is a false premise because that was not the thing that the hockey sticks wanted to prove anyway. But there is more to it than that and it was the statements about the Marcott paper that let me to notice this. The initial question diverts the attention from the strong statements that were made in the press. Just let me turn the question around: if the proxy data has to be tortured in order to get it into a hockey stick shape, how much signal of our current temperatures is there really in the proxy data set? To put it in other words: how much really is this an “independent” confirmation of our current temperatures anyway?

In the end, does it matter? For those who have read the papers, probably not. If they saw the articles in the press, they could put this into context. But it does matter for the laymen who only got to see the articles in the press and were yet again confirmed in their beliefs, without realizing that the papers themselves didn’t warrant those conclusions at all.

Warning: may contain traces of science

The Marcott Hockey stick

Almost two months ago, when this blog was just starting and the first few posts were put online, the media breathlessly reported the publication of a new paper by Marcott, Shakun, Clark and Mix. In the paper there was a graph that was quite impressive. It showed the temperatures of roughly the last 11,300 years (the Holocene). This graph showed an increase of temperatures until 10,000 years ago, then a plateau that lasted about 5,000 years, then a gradual decrease of the temperatures until the last hundred years or so, when temperatures suddenly went completely through the roof.

To be honest, I was not really impressed, I remembered well The Hockey Stick and the current paper showed something rather similar, but over a larger time frame. It was presented in the media as independent research that came to the same conclusion. There was a difference though: the Marcott paper was well documented. This would make it easier for skeptical souls to analyze the whole thing.

Some of the claims that were made in this press release (my bold):

…

The analysis reveals that the planet today is warmer than it’s been during 70 to 80 percent of the last 11,300 years.

…

What that history shows, the researchers say, is that during the last 5,000 years, the Earth on average cooled about 1.3 degrees Fahrenheit–until the last 100 years, when it warmed about 1.3 degrees F.

…

“What is most troubling,” Clark says, “is that this warming will be significantly greater than at any time during the past 11,300 years“.

…

Wow, these are heavy claims. The 11,300 years statement was repeated no less than six times throughout the press release. They surely wanted to rub this in! This was the crux of the press release. Leave this out and it would be unremarkable and rather bland press release, nothing newsworthy.

When the press release broke out I didn’t care about it too much. I had the impression that this was a statistical construct, just as the original hockey stick was. Other people with much more knowledge of statistics probably would dissect the whole thing to pieces. Nothing for me to worry about. For those who want to have more background on how this hockey stick was crafted statistically, this series at ClimateAudit has the analysis of Stephen McIntyre in all its finest details. I will not go in too much details, I only will mention a few things that struck me in passing by.

It was as expected: in the underlying data there was no uptick. Also, Shaun Marcott used the same data in his thesis and the corresponding graph showed no uptick either. What changed in the meanwhile so that with the same data an uptick would surface?

McIntyre found that the proxy data were re-dated et voilà: after the re-dating suddenly there was an uptick. So: no re-date → no uptick. Re-date → uptick. The uptick seemed dependent on the re-dating of the proxy data.

When the paper came more and more under fire, a FAQ was published on the realclimate website. It is a very interesting read. This is what it said about the uptick (my bold):

Our global paleotemperature reconstruction includes a so-called “uptick” in temperatures during the 20th-century. However, in the paper we make the point that this particular feature is of shorter duration than the inherent smoothing in our statistical averaging procedure, and that it is based on only a few available paleo-reconstructions of the type we used. Thus, the 20th century portion of our paleotemperature stack is not statistically robust, cannot be considered representative of global temperature changes, and therefore is not the basis of any of our conclusions.

and

Any small “upticks” or “downticks” in temperature that last less than several hundred years in our compilation of paleoclimate data are probably not robust, as stated in the paper.

and later in the answers on questions of the readers:

The most recent points are affected strongly by proxy dropout and so their exact behavior is not robust.

and:

They specifically state that this reconstruction is not going to be useful for the recent period – there are many more sources of data for that which are not used here – not least the instrumental record

That’s puzzling. In the press release we were repeatedly told that the temperatures of the last 100 years were significant higher than over the last 11,300 years and now we get to hear that:

• If there was a warming trend in another part of the Holocene similar to that of the last hundred years, this method wouldn’t even be able to detect it.
• The last 100 years of the graph is not robust and one cannot derive anything conclusive from it.
• The conclusion in their paper is stated differently than that of the press release. In the paper they state that the last hundred years are not robust and therefor are not included in any of their conclusions. But they “forgot” to mention this fact in the press release and shouted out the relevance of this last hundred years with certainty. This is very misleading. It tricks readers of the press release (it will be read by much more people than the paper) into believing the paper’s conclusion is that the last hundred years are definitely warmer, when this wasn’t the conclusion at all.
• It was stated that although the reconstruction was not useful for the recent period, other sources like the instrumental record are. But this is not the issue here. The reconstruction in the paper has an uptick, the press release refers several times to the same uptick. It is being emphasized in the media as independent confirmation of other upticks. Yet it is not robust and not useful for the period it is told it is significant for. If this is true, then this paper is no independent confirmation of the 20th century warming and it should not be presented as such. If they compared to other sources like the instrumental record in stead of the Marcott uptick, fine, but then they should have said so in the press release. They did not and even emphasized the “finding” of the paper as if it was significant.

To continue with this last point: the final date of the reconstruction is 1940. That doesn’t make much sense. If this is really the case, they found an uptick starting 100 years before 1940. But CO₂ only got traction in the 1950s, so if the last proxies were (re)dated 1940 and they really found an increase, they couldn’t possibly attribute this to anthropogenic CO₂. This means that they found an unprecedented huge natural temperature increase from the end of the Little Ice Age until the 1940s! That is exactly the opposite what they suggested in the press release. Which blows their statement (about CO₂)…

It’s the only variable that can best explain the rapid increase in global temperatures.

…straight out the water.

Apparently they aimed for maximum shock effect. As a layman, after looking at this, I have many questions. How trustworthy is/are the author(s) of the press release? Why the huge disconnect between the conclusion of the paper and the statements of the press release? What is it that they really were trying to communicate here? Obviously not the science.

The making of The Hockey Stick

The Hockey Stick (MBH98)

My story from believer to skeptic – part 3
You might see Part 1 or
Part 2 first if you haven’t already.

In Part 2 I explained I started to realize that the global warming alarmism was just a gross exaggeration, not always backed by the observations. But the alarmist side still had a convincing argument. The last hurdle to take was the temperature chart of the last 1000 year. I came to know the name of this chart: The Hockey Stick. It was made by Michael Mann, Raymond Bradley and Malcolm Hughes in 1998 (MBH98) and was extended in 1999 (MBH99). It represented temperatures of the Northern Hemisphere over the last 1000 years. It showed a slowly rising of the temperature (the shaft) until the last -let’s say 100 years- when it went completely through the roof (the blade).

This chart was been found everywhere I looked for proof of human fingerprint of the warming due to CO₂. I still was really naive at that time and I believed this graph was correct because it popped up about everywhere. How could this be incorrect after so many people eyeballing it? I compared it to the system of Open Software in which bugs gets removed by maximum exposure to many people that get access to it. The more people look at it, the quicker the bugs can be found and eliminated. So my thoughts were, the more that Hockey Stick figure gets reviewed, if there were errors in it, the quicker they would be found.

It was confusing. If this graph really was true, then my new understanding about climate didn’t even matter, then there was a very good reason for sounding the alarm. It was clear for all to see. In the time frame in which man was developing rapidly, temperatures went through the roof. It couldn’t be more clear.

In the meanwhile it became January 2009, about three months after I started my quest. At that time I became a regular visitor of a site called ClimateAudit. It is run by a Canadian mathematician Stephen McIntyre. I didn’t realize it, but he was an important factor in the investigation of the hockey stick graph. Together with Ross McKitrick (a Canadian environmental economist of the University of Guelph) he investigated the graph and they found things didn’t add up.

The Wegman report

One day, when I visited the blog, I checked the links bar on the left in stead of directly start reading the articles. There I stumbled on a link to the Wegman Report (left link bar, under “Links”). From previous searches I remembered vaguely that it had something to do with the Hockey Stick. I downloaded the report and started to read. I was baffled. The more I read it, the more I realized the Hockey Stick probably was based on faulty assumptions, bad statistics and probably a too close related group of scientists. This was the final drop in a bucket almost full.

To come back to the report. Edward Wegman is a statistician (Center For Computational Statistics, George Mason University) and he made this report for U.S. Congress in 2006. Some issues of the many parts that shook me (my bold):

The controversy of Mann’s methods lies in that the proxies are centered on the mean of the period 1902-1995, rather than on the whole time period. This mean is, thus, actually decentered low, which will cause it to exhibit a larger variance giving it preference for being selected, as the first principal component. The net effect of this decentering using the proxy data in MBH98 and MBH99 is to produce a “hockey stick” shape. Centering the mean is a critical factor in using the principal component methodology properly. It is not clear that Mann and associates realized the error in their methodology at the time of publication. Because of the lack of full documentation of their data and computer code, we have not been able to reproduce their research. We did, however, successfully recapture similar results to those of MM [McIntyre & McKitrick]. This recreation supports the critique of the MBH98 methods, as the offset of the mean value creates an artificially large deviation from the desired mean value of zero.

After MBH99, Stephen McIntyre and Ross McKitrick [MM03] published their critique of the 1998 paper, citing calculation errors, unjustified truncation or extrapolation of source data, obsolete data, geographical location errors and incorrect calculation of principal components. They also claimed that using the MBH98 methodology and the Northern Hemisphere average temperature index for the period 1400-1980 shows that temperatures in the 15th century exceeded those of the late 20th century. In particular, they claim that MBH98’s incorrect usage of PCA alone resulted in the well-known “hockey stick” shape.

Wow, this was heavy! The hockey stick didn’t seem to be withstand scrutiny well. As far as I could understand it, the uptick at the end was not derived from the data itself, but was formed from an incorrect usage of methodology, combined with the use of a proxy that doesn’t seem to be a good proxy for temperature in the first place. Stephen McIntyre and Ross McKitrick went even further and tested the methodology itself. They took red noise (a kind of random data) and combined it with the Bristlecone data. They found that this created hockey sticks in 99% of the cases. Oops.

More info

This story of the investigation of the hockey stick is broader than the Wegman report. But this report was the trigger for looking into the issue in more detail. The best summery of this story is written by Marcel Crok in NatuurWetenschappen & Techniek: a very clear and well researched NWT article in pdf format. Best to look at this pdf before continue, in order to be able to situate the impact it had on me.

Even more background information (on this page is a link to a pdf file with the background information).
Finally, the Hockey Stick Project page with the complete time line and responses to criticism on their investigation.

The impact

Why this report (and the papers of McIntyre and McKitrick on the Hockey Stick) had such an impact on me?

• It showed that my assumption that the graph was properly eyeballed didn’t hold. It took a lot of time and effort of at least one person (and probably three persons or more) to get hold of the methods and data to be able to review it. It took me by surprise that the information that proved anthropogenic global warming was not organized or documented. This didn’t make much sense to me. How could that very important data not being available for all to investigate? Is this how scientists behave when they have crucial data that is important for explaining “the most important problem of human kind”!?!? The eyeballing I presumed was only the peer review that was apparently done within a small group of closely related people. We should expect more scrutiny of data that affect policy decisions.
• The graph depended heavily on statistics, but when statisticians reviewed this paper, they found many errors in it, not detected by the (much celebrated) peer-review process. This got me thinking about what this peer review was all about. In this case it definitely wasn’t the gold standard it was told to be.
• It was the first time that I noticed the political side behind climate science (the graph was prominent in the 2001 IPCC report).

The final result is that I am not much impressed anymore with someone showing a hockey stick result as proof of global warming and there have been many since.

Ending

And hey, we know the temperatures in the 20th century went up. What’s the big deal? Sure, temperatures went up in last century, but that was not the claim. The claim was that the temperatures of the 1990s were the highest of the last 1000 years, but this doesn’t follow from the data.

I couldn’t see this in the past, because I didn’t look at the data and just believed what others told me (others who obvious didn’t look at the data either). This is a viscous circle. People read that CAGW is true, don’t check the data and pass the message on. In this way it looks as if this is an universal truth, because the same message is mentioned everywhere.

Stephen McIntyre and his blog certainly had a huge influence on me. What influenced me most was the expression to check things oneself and in the end this is what I began to do. That was one of the reasons why I started this blog and adopted this blog name.