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.
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.
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 on 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.