This is a post that aims to go through the evolution of the “Hockey Stick” from 1990 to the present day. It naturally misses out parts of the story, which deserve far more analysis, simply to keep the post short. Comments that expand on the bits I’ve omitted are welcome!
What is the “Hockey Stick” and who cares?
One of the key areas of controversy relating to climate change and the body that synthesises all the science – the Intergovernmental Panel on Climate Change (IPCC) – is the so-called “Hockey Stick” graph that first appeared in the IPCC in 2001.
The graph is important because it tries to reconstruct large-scale temperatures for the past 1000 years or so to put the current warming in context.
Lots of people have spent many hours trying to assess or discredit the graph and the science behind it:
- There have been several official (and controversial) inquires and reports on the science and the scientists. Two of the most well-known are the NRC Report and the Wegman Report.
- The Climate Audit blog, and its many followers, have been picking at the science, the raw data and the method that produced the “Hockey Stick” for a long time.
- The Bishop Hill blog has many posts on the “Hockey Stick” and the man behind that blog has even written a book about the graph (I’ve not read the book but I’d like to review it for this blog soon).
So if it’s so important, how did the “Hockey Stick” get here and where did it go? Let’s have a look…
IPCC First Assessment Report (FAR) – 1990
The temperature reconstruction of the last 1000 years or so in the FAR was little more than a best guess. The figure shown below from the report (you can find it on page 202 of the FAR (big pdf)) was even labelled as a “schematic” diagram and had no scale on the temperature axis:
It’s a composite overview of the evidence available in 1990 from ice cores, tree rings, historical records and other so-called “proxy” measures of temperature. This field of research was in its infancy so the schematic wasn’t highlighted much in the report.
[UPDATE: Having looked through the IPCC FAR again it doesn't actually say how this schematic was constructed. Looking at Jones et al. 2009 ("High-resolution palaeoclimatology of the last millennium: a review of current status and future prospects" The Holocene, 19, 3-49) it seems that it was compiled from a series of publictions by H. H. Lamb and was only based on temperature records associated with Central England, so I doubt very much that any ice core data was used! The key message from Jones et al. that casts serious doubt over the schematic is: "At no place in any of the Lamb publications is there any discussion of an explicit calibration against instrumental data, just Lamb’s qualitative judgement and interpretation of what he refers to as the ‘evidence’". The schematic also failed to make it into the 1992 IPCC Supplementary Report as it was decided that more data was required and it was not representative of a large area.]
Indeed, the reason for including this plot at all in the FAR is probably summed up by this quote:
“So it is important to recognise that natural variations of climate are appreciable and will modulate any future changes induced by man”
However, this plot is still referred to by a lot of people as the temperature at the “present day” end of the graph is not the highest value on the plot.
Given that it’s essentially a sketch, I’m surprised that people read much into this plot.
For example, the usually meticulous Science of Doom was, in my opinion, off the mark with his analysis of the development of the science here, skipping straight from the First to the Third Assessment Report version to imply that something underhand was going on. This doesn’t represent the scientific progress properly. So, we’ll look here at the parts of the story that SoD missed out.
[The SoD post also doesn't show the IPCC FAR version of the plot (he uses one from a 1993 textbook that has a temperature scale) and he points to the Wegman report as the point of reference for analysis of the "Hockey Stick", which is perhaps not the best source. Indeed, the Stoat blog has recently examined Wegman's analysis of this plot and the conclusions are not supportive.]
IPCC Second Assessment Report (SAR) – 1995
So where did the science go between the FAR and SAR?
It seems that it went backwards; the SAR reconstruction only goes back 600 years and not 1000 years like the FAR.
Here’s the relevant plot from page 175 of the IPCC SAR (another big pdf):
Why does it only go back 600 years? Well, here’s a quote from the SAR:
“Prior to 1400 data are insufficient to provide hemispheric temperature estimates.”
Ok, to be fair on the IPCC, there is now a temperature scale, which is a big improvement. Also, the IPCC has recognised what they do not know enough about the climate prior to 1400 AD and removed that part of the plot. I suppose you could read this as the start of a conspiracy to “cover-up” the Medieval Warm Period but there is no evidence for that. [For example, here's a recent example of interpreting a decent paper poorly to reach the conclusion you want regarding the MWP.]
The report also says:
“A recent analysis, using tree-ring density data, has attempted to reproduce more of the century time-scale temperature variability in this region (Briffa et al, 1995). This shows that the 20th century was clearly the warmest in the last 1000 years in this region, though shorter warmer periods occurred, for example, in the 13th and 14th centuries.”
So the state of the art science in 1995 was not particularly clear but it does give an indication of where it was going…
IPCC Third Assessment Report (TAR) – 2001
Here’s where the story really takes off…
This plot is a composite of all the “best” proxy climate data available at the time of writing the TAR – you can find the plot and lots of background information here on pages 130-136 of the TAR.
It is most strongly linked to Michael Mann of Penn State University and it was this version that was dubbed the “Hockey Stick” (because it looks like an Ice Hockey Stick, I thought that was worth mentioning in case UK readers are wondering why it doesn’t curl around at the end!)
As hinted at in the SAR, there was a lot of new work looking at these reconstructions between the SAR and the TAR so it goes back further and includes regions of uncertainty.
But this was still quite new science. If it could be trusted then it would be an important addition to the TAR. [But it wouldn't be the only or most important part of the report and not a fundamental result that supports the rest of the science, that's not how science works. To steal an analogy, science is more like a jigsaw than a house of cards.]
The IPCC clearly came to the conclusion that this plot was trustworthy and delivered this verdict:
“Taking into account [the] substantial uncertainties, Mann et al. (1999) concluded that the 1990s were likely to have been the warmest decade, and 1998 the warmest year, of the past millennium for at least the Northern Hemisphere.”
This did not go down well with some people (and the language was toned down in the subsequent IPCC report).
The controversy is quite well documented, Wikipedia is as good a starting place as any. More recently, some of the discussion between the IPCC scientists that appeared in the incomplete email record that was taken from the University of East Anglia’s Climatic Research Unit in 2009 has further fueled this controversy. [The "hide the decline" email being the most obvious and relevant example, although this has been misinterpreted and blown out of all proportion - RealClimate give a good account of the context.]
The raw data, the methodology and the statistical tools used to produce the graph have all been examined in great depth. Everyone from bloggers to the US Senate have been interested in it. It must be one of the most intensely scrutinised graphs ever produced.
But it’s only a graph, so why all the fuss?
In my opinion, using relatively new science to advise policy makers on issues that affect the whole population’s way of life is bound to throw up problems, especially if you don’t like what the message is. But that was the situation that the IPCC was in and they were probably right to stress the importance of this graph – it was new science but none of the investigations into it have landed a killer blow.
Indeed, maybe all the scrutiny would help the science develop faster and become more reliable.
So did it?
IPCC Fourth Assessment Report (AR4) – 2007
Here is the most recent IPCC “Hockey Stick”-type plot. Its background information can be found here in the AR4.
Despite all the attacks on the 2001 “Hockey Stick”, an improved version was included in the 2007 IPCC report. The report discussed the peer-reviewed criticisms of the methodology, which the IPCC deserve credit for.
Questions still remain over the statistical methods used in this graph and this does not make the discussion very accessible but here is a very quick attempt.
The main conclusion from several of the relevant inquiries was that the statistical methods were not ideal but they did not change the result (i.e. the shape of the graph) in any significant way. To give a specific example, Lord Oxburgh’s review of the science concluded that the climate scientists should collaborate more with statisticians. This point is hard to disagree with.
Some people, however, still believe that the statistical methods are a terminal issue for the “Hockey Stick”. Here’s Bishop Hill in 2008 on the stats (from a very “sceptical” point of view). He also reported on a couple of papers that aimed to refute the major criticisms of the methods and their tortured journeys to get into the IPCC AR4 (although, having published in both GRL and CC myself, this story doesn’t sound as remarkable as Bishop Hill spins it!) And here’s a defence of the “Hockey Stick” methods from RealClimate in 2005.
This argument is going to continue and the science will continue to improve.
The thing that strikes me as odd, though, is that most of the criticism aimed at the “Hockey Stick” is still aimed at the 2001 version. I suppose that there is more ammunition to attack this version with – the data was very new, the methods were new, it was high-profile and the CRU emails give new opportunities to quote mine. I get the feeling that some of the “sceptic” community have put nearly all their eggs in this basket and it therefore needs to continue to attack the 2001 version.
In the meantime, the science has moved on and improved. Indeed, the latest Mann et al. version of the work in PNAS has been questioned in the same journal and the response by Mann et al. showed suggested that some of those criticism were quite strange.
IPCC Fifth Assessment Report (AR5) 2013
So what’s going to happen next?
I assume that the work of some of the key players may have been slowed a little because of all the inquiries they’ve had to deal with.
However, as the field has developed other groups will have taken on the challenge and there are now more groups than ever (with new ideas and perspectives) working on these issues. Indeed, the IPCC has increased the prominence of this type of work for their next report – the outline for AR5 includes a whole chapter (Chapter 5) on palaeoclimatology – it was a chapter sub-section in AR4 and TAR.
This is good news and hopefully we’re getting closer to truth, which is what the scientists wanted all along.
Mann ME, Zhang Z, Hughes MK, Bradley RS, Miller SK, Rutherford S, & Ni F (2008). Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proceedings of the National Academy of Sciences of the United States of America, 105 (36), 13252-7 PMID: 18765811